displacement hull catamaran

Advantages of Catamarans and Catamaran Hull Speeds Calculation

• Categories : Marine history
• Tags : Marine engineering

What is a Catamaran?

Before going into the advantages of catamarans, I’d just like to define the term. Catamarans are a class of boat that has two hulls, generally equal in size. The hulls are connected by a structure of some sort. Catamaran hull speeds are very high, making them a good choice for recreational uses. Catamaran sailboats are common, but catamaran ferry designs are quickly growing in popularity due to their unique advantages.

The catamaran is a ship design with an interesting history. The twin-hull design was first observed by Europeans in India in the late 17th century, but it was actually invented by South Pacific islanders. The capabilities of catamarans were more or less ignored in the West until 1877, when an American named Nathanael Herreshoff began developing and racing catamarans. His designs were so much faster than traditional monohull designs that sailing authorities banned them from competition for nearly a century. Today, catamarans are a popular design for recreational and racing sailboats, and are being used in efficient, low-wake ferries .

Image: Flickr , Catamaran in Cozumel.

Advantages of Catamarans

Catamarans are, in general, faster than single hull boats. This is due to several factors, the most important of which is the hull shape. Displacement hulls are the most common shapes for single hull craft. A displacement hull is supported on the water entirely by buoyancy effects. This creates a hydrodynamic drag barrier, which slows the craft. Catamaran hulls can be designed as planing or displacement hulls, but they are not slowed nearly as much by the drag barrier. Catamarans reduce drag by making use of a very thin and pointed hull design.

Catamarans are also very light, which further reduces drag and displacement. Catamarans save on weight because the multihull design eliminates the need for a keel counterweight, as the same purpose (righting the ship) is served by the hull spacing. This can make them difficult to turn, however.

Catamarans are also more stable than monohull ships, enabling designers to use more sail per foot of the boat. Stability comes from the wide beam legth; beam length is the distance from one side of the boat to the other. This wide beam and stability also allows catamarans to gain more power from heavy gusts, because they do not tend to heel over like monohulls do.

Catamaran Hull Speed

More efficient single hull boats are designed to travel with a low Froude number, which reduces but does not eliminate the drag barrier. This drag limits hull speed based on a relation involving length at waterline, given by the formula V=(gL/(2pi))1/2, where g is the gravitational constant and L is the waterline length. This is the theoretical limit for single hull displacement craft. Note that ships can sometimes exceed this theoretical limit, but to do so requires a very large power source.

The calculations for a catamaran are more complicated. The formula for catamaran hull speed is 1.34*(wetted length)1/2; however, this drag formula is generally not the limiting factor for catamaran hull speed. This is because boats with waterline length to beam ratios greater than 8:1 are not limited by hydrodynamic drag factors, whereas smaller boats need to plane to do so (planing requires enormous amounts of power for displacement hulls). A more important factor to consider is the prismatic coefficient, Cp. Cp = V/(LBP*Am), where V is the volume of water displaced by the hull, LBP is the length between perpendiculars, and Am is the area at midship.

Very fast boats actually require a high prismatic coefficient, which in turn requires a less-narrow boat. However, narrower hulls can get away with a lower prismatic coefficient. The ideal range of Cp for a catamaran is between 0.61 and 0.65. There are a few ways of increasing the prismatic coefficients: sailors can use bulb bows, a wide planing aft segment, or a flat hull rocker in conjunction with a bustle aft. Though high prismatic coefficients increase drag at low speed, at high speeds they can reduce drag by as much as ten percent.

Catamaran Hulls and Low-Wake craft.

As explained here , catamarans can be used to create low-wake ferries. Because of their wide bows and low weights, catamarans can be made to travel quickly with minimal wake. Catamaran ferries are also more fuel-efficient, because of the previously-mentioned factors.

Image Credit: Wikimedia Commons , Catamaran Ferry

Catamaran Boat Design

Hull Speed Calculations

Catamaran Hull Comparisons

• Featured Cats:
• Woodbridge 50′ Charter Cat
• Bloomfield 86′ MotorSailer Cat
• Shuttleworth 52 AeroRig

A Case for Displacement Power Catamarans

Article by Malcolm Tennant as published in Multihulls Magazine

In our Premier Issue, Malcolm Tennant, one of today’s foremost power catamaran designers, discusses the principles of planing vs. displacement catamarans. In this article he makes clear his choice of the displacement cat.

For some fifteen years now our office has been designing powerboats that combine something of the old and something of the very new. To make a leap forward in comfort and economy we looked back to the close of the 19th Century and the early years of the 20th. We have taken the powerboat wisdom of that time and used it in the designing of very modern power catamarans that can have much more living space than their monohull cousins, and that easily surpass them in comfort and economy. Current thinking has it that to go fast in smaller craft it is necessary to plane. This is because the usual monohull displacement craft are restricted to a speed of approximately 1.34 times the square root of their waterline length (Froudes Law).

To drive a normal displacement vessel faster than this requires an inordinate amount of horsepower and may even lead to foundering in their own bow and stern waves, or by rolling the gunwales under from the enormous torque produced. Planing is a way to circumventing Froudes Law by getting the vessel to plane on top of the water where the wave making drag is no longer a restriction on their performance. However, planing craft do need to be relatively light, i.e.: have good power-to-weight ratios, and planing surface-area-to-weight ratios; are very inefficient when they are not planing, and are not as economical to run at some speeds as the displacement craft.

So we seem to have two distinct type of boats: a. One that is fast, but uneconomical at slower speeds and can have a bone-jarring ride in a seaway; b. The other, that is economical and comfortable in a seaway, but is slow. Is it then even possible to get a craft that combines the best features of both these types? A boat that has reasonable, ­ even good ­ performance with excellent accommodations and is still economical to build and run and has good seakeeping capabilities: ­ or is this just one of those designers’ pipe dreams?

One quite successful attempt to achieve this dream was made by Tom Fexas with his Midnight Lace series of monohull designs, in which he used long, light, semi-displacement hulls to improve economy without compromising performance too much. These boats were, in fact, a compromise (aren’t all boats?) and, to me, only partially successful by reason of his definition of a slim hull which was, of course, forced on him by the need for stability, accommodation and sea keeping. To Tom Fexas a slim hull was one that had a length-to-beam ratio of four (the waterline length was four times the waterline beam). This was certainly narrow by contemporary planing boat standards, but was unexceptional when compared with earlier boats, or with types of hulls that I am proposing should be used.

Before the improvement of the power-to-weight ratio of the internal combustion engine, and the development of the hard-chine, low-deadrise hull that allowed boats to plane, there was only one way to go fast: building long-and-slim, and in the first decade of the 20th Century we find boats such as Slim Jim, that were achieving speeds of 15 knots from a 15 HP engine driving just such long hulls in 1905. Typical of the early boats was Defender: 16.2m (53′) long, having a maximum hull beam of 2.28m (7’6″). Headroom under the flush deck was only 1.45m (4’9″) and she slept six in berths only 500 mm (18″) wide. In anything of a seaway it would have been incredibly wet and uncomfortable.

The boat had a great deal of grace and elegance to her lines, but her rolling at sea, and lack of accommodations, would be totally unacceptable today except for one small detail: a 48 HP motor propelled this 16.2m boat at 16.5 knots! Is it possible, then, to reconcile these old, easily driven, but incredibly uncomfortable hull forms with the current, ever increasing demands for more interior space and more home comforts that can be the downfall of many a well-designed planing craft? I believe the answer is: catamarans! By joining two of these long, slim hulls together and surmounting them with an extensive superstructure, we are able to provide even more than the currently desirable amount of accommodation and at the same time stabilize the hulls so that rolling is no longer a problem.

So it would seem that all we have to do is to make power catamarans with long, slim hulls, and then we will have speed, economy and accommodation. The potential is there, but is it really that simple? The answer, of course, is “no” ­ not quite!

Even a very cursory look at sailing catamarans will show that they are not restricted by Froudes Law. Their very fine hulls place them on a very different part of Froudes’ wave making continuum, and results in their having a very much higher hull speed than he ever envisioned from his observations ­ in the order of 30+ knots is not unusual for these boats. Certainly the boats with this sort of performance are very lightly loaded racing craft, but even the more heavily laden cruising boats do not have much trouble breaking the 1.34 barrier. If these sorts of speeds can be achieved under sail, than it should be much easier under power.

Towing tank tests of long, slim hulls with high prismatic coefficients (fine hulls with a fairly even spread of displacement from bow to stern), such as our displacement powerboats exhibit, have shown no catastrophic increase in wave drag at speed/length ratios above approximately 1.4 ­ such as occurs with “normal” displacement hulls. These high prismatic hulls have a higher displacement hull speed than is “normal.” This test data is further supported by the precisely measured performance tests of such boats as the Zenith-47 Antaeus, the Awesome 2000, the Mako-61, the Jaybee and the Icarus 46 in the full-sized ocean test tank. All these boats have prismatic coefficients greater than 0.66 and all easily exceed their theoretical hull speeds, while returning exceptional fuel economy.

So it would seem that all we have to do is to make power catamarans with long, slim hulls, and then we will have speed, economy and accommodation. The potential is there, but is it really that simple? The answer, of course, is “no” ­ not quite! If we compare a sailing catamaran with a keelboat, we will see that the catamaran has one immediately obvious advantage. It is lighter because it is able to eliminate the lead keel upon which the keelboat depends on for its stability. In the case of the powerboat, there is no such advantage. The catamaran may, in fact, be heavier than the monohull because of its increased area. All is not lost, however, because while the skin area is increasing by the square, the interior volume is increasing by the cube! This possible increase in weight may be a problem with planing catamarans because of their limited planing surface, but it does not mean that our dream is impossible.

The displacement catamaran is not as susceptible to overloading as is the planing craft. The hull speed of the displacement boat is largely dependent on the L:B ratio of the hulls and this does not change very much with modest overloading. This does, however, bring up one of the limitations of the displacement boat. To work successfully, the L:B ratio of the hulls should be in excess of 10, and preferably higher. Consequently, if high displacements and length restrictions force short, fat hulls on the designer, then the displacement approach will not be successful. In this situation the only recourse is to lengthen the hull until the requisite L:B ratio is obtained, or to use a planing hull form.

It will be apparent from this, that the displacement concept would seem to have little place in boats shorter than 10m (32′), unless they can be built light ­ or a very modest performance is required. I have designed smaller displacement boats that achieve quite credible 15-knot cruising speeds from very small horsepower (43 HP per side) engines. But if performance on par with planing vessels is required, then the displacement boat must be able to have long, slim hulls, preferably without the planing boats’ low deadrise, submerged chine sections, as this increases the drag substantially, and even more if the chines break the surface. This, then, is the approach we have taken with a lot of our power catamaran designs: long, slim, easily driven round-bilge, minimum wetted surface hulls that give performance on a par with planing craft, but with considerably better sea-keeping capability and better fuel economy.

It is, of course, possible to question whether these boats really are displacement craft. Current theory says that for vessels of this length, to go this fast, they must be planing. In fact, if we accept the usual definition of planing vessel, namely: that it has a speed/length ratio of more than 2, then these boats are clearly planing. However, a boat is said to be planing when most of its mass is supported dynamically by the downward directed thrust of the water. A vessel that is planing will typically have a bow out trim and will have bodily risen out of the water. The waters are muddied a little by the fact that there is no sudden jump from displacement to planing. It is a continuum and somewhere in the speed/length ratio range from 1.5 to 2 the craft would be considered to be in a “semi-displacement” mode. We have now designed a large number of displacement power cats exemplifying the “long and slim” approach of powerboat design.

The Zenith-47 displaces 13 tons fully loaded, and motors at 20 knots maximum ­ much more economically at 16 knots ­ with only two 122 kw (160 HP) pushing hulls with a 24.5 knot hull speed. A monohulled displacement boat of this length would have a hull speed of about 8.5 knots. The smaller Nomad and Cortez powerboats also have a similar hull speed but are optimized more for economy with slower speeds with small engines. The Icarus-46 has a top speed of 25 knots from two 150 kw (200 HP) turbo-charged diesels. At the upper end of the scale is the Mako-61, an 18.6m (61′) game fishing boat with a hull speed of 37.5 knots which would yield an easy 30 knots with around 500 HP per side. In the interest of economy, this boat is intended to cruise at 16 knots ­ with a maximum of 20 knots ­ using twin 150 kw

These performances are very much faster than those of the traditional displacement boats of comparable size and are on a par with that of a planing boat of similar displacement, but with lesser power requirement and subsequently greater economy. I believe the performance of these designs demonstrates the potential of the displacement power catamaran to be that very elusive and ephemeral animal; the best of all possible worlds: combining excellent accommodation, comfort, and economical performance with good old-fashioned seaworthiness. It seems to me that there is no reason why this old “long and slim” principle should not be applied to lightweight boats with less superstructure and even finer hulls, to produce 30 or even perhaps 40 knots of fuss-free performance from quite modest horsepower.

In fact, this belief has been partially tested with two offshore designs: the 17.5m (57′) Red Diamond II, designed for a Japanese client, capable of a top speed of 33 knots (cruising at 24) from twin 320 kw (430 HP) Yanmar diesels; and the 20m (65′) Awesome 2000, which has a top speed of 28 knots, and an open ocean cruising range of 3,000 miles at 15-knot speed. This craft has made the trip from Long Beach, California to Hawaii using only her internal tanks. Although these displacement cats may not be the fastest things around in flat water, they have demonstrated an ability to maintain much higher average speeds than most other craft regardless of sea conditions. In situations where the high-speed planing monohull is forced to drastically reduce its speed, the displacement catamaran is able to continue on with very little reduction in performance.

This ability is displayed day in and day out by the rapidly expanding commercial catamaran ferry fleets whose operators recognized the economic advantages of this concept early on. It has often been pointed out that many people with displacement boats try to push them too fast and, consequently, would be better off with a planing boat. For these people there is now another alternative: displacement boats with the performance of planing craft and the frugal thirst and smooth comfort of the traditional displacement boat.

Did You Know That We Offer Contract to Closing Services? Click Here to Find Out More.

Need Marine Financing? Apply Here With Our Partner, First Approval Source

• Catamaran Interviews
• Catamaran Reviews
• Buying Advice
• Selling Advice
• Woods Design Advice
•     Admiral 38
•     Admiral 40
•     Admiral 50
•     Americat 3014
•     Antares 44
•     Aquila 44
•     Aquila 48 Power Catamaran
•     Aventura 37
•     Balance 442
•     Balance 482
•     Balance 526
•     Bali 4.0
•     Bali 4.1
•     Bali 4.2
•     Bali 4.3
•     Bali 4.4
•     Bali 4.5
•     Bali 4.6
•     Bali 4.8
•     Bali 40 Catspace
•     Bali 5.4
•     Bali Catsmart
•     Beneteau Blue II
•     Broadblue 346
•     Broadblue 38 Prestige
•     Broadblue 385
•     Broadblue 435
•     Broadblue 46
•     Rapier 400
•     Rapier 550
•     Catalac 10M
•     Catalac 11M
•     Catalac 12M
•     Catalac 8M
•     Catalac 900
•     Catalac 9M
•     Catana 381
•     Catana 39
•     Catana 401
•     Catana 40S
•     Catana 411
•     Catana 42
•     Catana 42 S
•     Catana 431
•     Catana 44
•     Catana 471
•     Catana 50
•     Catana 521
•     Catana 531
•     Catana 55
•     Catana 581
•     Catana 65
•     Catathai 44
• Chris White
•     Chris White 48 Voyager
•     Chris White 55
•     Condor 40
•     Contour 34
•     Corsair F28 R
• De Villiers
•     Dean 365
•     Dean 400
•     Dean 440
•     Dean 500
•     Dix DH550
•     Dolphin 380
•     Dolphin 460
•     Edel 35
•     Endeavour 30
•     Endeavour 35 Victory
•     Endeavour 36
•     Endeavour 44
•     Endeavour 44 TrawlerCat
•     Endeavour 50 Pilothouse Trawler
•     Excess 11
•     Excess 15
•     F-41
•     Fastback 43
•     Fastcat 445
•     Fisher 28
•     Fisher 32
•     Fortuna 36 Island Spirit
•     Fortuna 401 Island Spirit
• Fountaine Pajot
•     FP 32 Maldives
•     FP 35 Tobago
•     FP 36 Mahe
•     FP 37 Antigua
•     FP 37 MY
•     FP 38 Athena
•     FP 39 Fidji
•     FP 40 Isla
•     FP 40 Lavezzi
•     FP 40 Lucia
•     FP 40 MY
•     FP 40 Summerland MY
•     FP 41 Lipari
•     FP 42 Astrea
•     FP 42 Venezia
•     FP 43 Belize
•     FP 44 Helia
•     FP 44 Orana
•     FP 45 Elba
•     FP 46 Bahia
•     FP 46 Casamance
•     FP 48 Salina
•     FP 50 Saba
•     FP 56 Marquises
•     FP 57 Sanya
•     FP 58 Ipanema
•     FP 60 Eleuthera
•     FP Saona 47
•     Freydis 43
•     Fusion 40
•     Gemini 105
•     Gemini 3000
•     Gemini 3200
•     Gemini 3400
•     Gemini Freestyle 37
•     Gemini Freestyle 399 Power
•     Gemini Legacy 35
•     Grainger 420 Mystery Cove
•     Gunboat 55
•     Hirondelle 7M
•     HopYacht 30
• Island Packet
•     Island Packet Cat 35
•     Kennex 420
•     Knysna 440
•     Knysna 480
•     Knysna 500
•     Knysna 550
•     Lagoon 35
•     Lagoon 37 TPI
•     Lagoon 380
•     Lagoon 39
•     Lagoon 40
•     Lagoon 400
•     Lagoon 410
•     Lagoon 42
•     Lagoon 42 TPI
•     Lagoon 420
•     Lagoon 421
•     Lagoon 43 PC
•     Lagoon 44 Power Cat
•     Lagoon 440
•     Lagoon 450
•     Lagoon 46
•     Lagoon 470
•     Lagoon 50
•     Lagoon 500
•     Lagoon 52F
•     Lagoon 55
•     Lagoon 560
•     Lagoon 570
•     Lagoon 620
•     Lagoon Seventy 8
•     Lagoon Sixty 7
•     Leeuwin 42
•     Leopard 38
•     Leopard 39
•     Leopard 39 PowerCat
•     Leopard 40
•     Leopard 42
•     Leopard 43
•     Leopard 44
•     Leopard 45
•     Leopard 45 Classic
•     Leopard 46
•     Leopard 46 Lion PowerCat
•     Leopard 47
•     Leopard 47 PowerCat
•     Leopard 48
•     Leopard 50
•     Leopard 51 PowerCat
•     Leopard 53 PowerCat
•     Leopard 58
•     Lidgard 73 Executive
•     Looping 50
•     Maine Cat 30
•     Maine Cat 38
•     Maine Cat 41
•     Manta 40
•     Manta 42
•     Matrix 450 Vision
•     Matrix 760 Silhouette
•     Maverick 400
•     Maverick 420
•     Maverick 440
•     Moxie 61
•     Nautitech 40
•     Nautitech 40 Open
•     Nautitech 44 Open
•     Nautitech 442
•     Nautitech 46 Open
•     Nautitech 47
•     Nautitech 47 Power
•     Nautitech 475
•     Nautitech 65
•     Neel 45
•     Neel 47
•     Outremer 40
•     Outremer 45
•     Outremer 50 Standard
•     Outremer 51
•     Outremer 55
•     Outremer 5X
•     PDQ 32
•     PDQ 36
•     PDQ 42 Antares
•     Privilege 37
•     Privilege 39
•     Privilege 42
•     Privilege 43
•     Privilege 435
•     Privilege 445
•     Privilege 45
•     Privilege 465
•     Privilege 48 Transcat
•     Privilege 482
•     Privilege 495
•     Privilege 510
•     Privilege 65
•     Privilege Serie 5
•     Prout 31 Quest
•     Prout 33 Quest
•     Prout 34 Event
•     Prout 35 Snowgoose
•     Prout 37 Snowgoose
•     Prout 37 Snowgoose Elite
•     Prout 38
•     Prout 38 Manta
•     Prout 39 Escale
•     Prout 45
•     Prout 46
•     Royal Cape 45
•     Royal Cape 500 Majestic
•     Royal Cape 530 Majestic
•     Sailcraft 30 Iroquois
•     Sailcraft 32 Comanche
•     Sailcraft 35 Cherokee
•     Sailcraft 41 Apache
•     Sailcraft 44 Apache
•     Scape 39
•     Wildcat 350
•     Seacart 30
•     Seawind 1000
•     Seawind 1160
•     Seawind 1190
•     Seawind 1200
•     Seawind 1260
•     Seawind 1600
•     Simpson 48
•     Solaris 36 Sunrise
•     Solaris 36 Sunstar
•     Solaris 42
•     St Francis 44
•     St Francis 48
•     St Francis 50
•     Stealth 11.8
•     Sunreef 60
•     Sunreef 62
•     Sunreef 70
•     Sunreef 74C
•     Sunreef 82 DD
•     Sunreef 88 DD
•     Switch 51
•     Switch 55
•     TRT 1200
•     Heavenly Twins 26
•     Ocean Twins 38
•     Vaan R5
•     Vision 444
•     Voyage 380 Maxim
•     Voyage 400 Norseman
•     Voyage 430 Norseman
•     Voyage 440
•     Voyage 450 Cabriolet
•     Voyage 47 Mayotte
•     Voyage 480
•     Voyage 500
•     Voyage 580
•     Voyage 590
•     Kronos 45
•     Wharram 38 Tiki
•     AMI 320 Renaissance
•     Woods 22 Wizard
•     Woods 35 Banshee
•     Woods 35 Flica
•     Woods 36 Scylla
•     Woods 36 Vardo
•     Woods 38 Transit
•     Woods 40 Meander
•     Xquisite X5
•     Xquisite X5+

Catamaran Design Formulas

• Post author By Rick
• Post date June 29, 2010
• 10 Comments on Catamaran Design Formulas

Part 2: W ith permission from Terho Halme – Naval Architect

While Part 1 showcased design comments from Richard Woods , this second webpage on catamaran design is from a paper on “How to dimension a sailing catamaran”, written by the Finnish boat designer, Terho Halme. I found his paper easy to follow and all the Catamaran hull design equations were in one place.  Terho was kind enough to grant permission to reproduce his work here.

Below are basic equations and parameters of catamaran design, courtesy of Terho Halme. There are also a few references from ISO boat standards. The first step of catamaran design is to decide the length of the boat and her purpose. Then we’ll try to optimize other dimensions, to give her decent performance. All dimensions on this page are metric, linear dimensions are in meters (m), areas are in square meters (m2), displacement volumes in cubic meters (m3), masses (displacement, weight) are in kilograms (kg), forces in Newton’s (N), powers in kilowatts (kW) and speeds in knots. 

Please see our catamarans for sale by owner page if you are looking for great deals on affordable catamarans sold directly by their owners.

Length, Draft and Beam

There are two major dimensions of a boat hull: The length of the hull L H  and length of waterline L WL  . The following consist of arbitrary values to illustrate a calculated example. 

L H  = 12.20      L WL  = 12.00

After deciding how big a boat we want we next enter the length/beam ratio of each hull, L BR . Heavy boats have low value and light racers high value. L BR  below “8” leads to increased wave making and this should be avoided. Lower values increase loading capacity. Normal L BR  for a cruiser is somewhere between 9 and 12. L BR  has a definitive effect on boat displacement estimate.  

• Tags Buying Advice , Catamaran Designers

Owner of a Catalac 8M and Catamaransite webmaster.

10 replies on “Catamaran Design Formulas”

Im working though these formuals to help in the conversion of a cat from diesel to electric. Range, Speed, effect of extra weight on the boat….. Im having a bit of trouble with the B_TR. First off what is it? You don’t call it out as to what it is anywhere that i could find. Second its listed as B TR = B WL / T c but then directly after that you have T c = B WL / B TR. these two equasion are circular….

Yes, I noted the same thing. I guess that TR means resistance.

I am new here and very intetested to continue the discussion! I believe that TR had to be looked at as in Btr (small letter = underscore). B = beam, t= draft and r (I believe) = ratio! As in Lbr, here it is Btr = Beam to draft ratio! This goes along with the further elaboration on the subject! Let me know if I am wrong! Regards PETER

I posted the author’s contact info. You have to contact him as he’s not going to answer here. – Rick

Thank you these formulas as I am planning a catamaran hull/ house boat. The planned length will be about thirty six ft. In length. This will help me in this new venture.

You have to ask the author. His link was above. https://www.facebook.com/terho.halme

I understood everything, accept nothing makes sense from Cm=Am/Tc*Bwl. Almost all equations from here on after is basically the answer to the dividend being divided into itself, which gives a constant answer of “1”. What am I missing? I contacted the original author on Facebook, but due to Facebook regulations, he’s bound never to receive it.

Hi Brian, B WL is the maximum hull breadth at the waterline and Tc is the maximum draft.

The equation B TW = B WL/Tc can be rearranged by multiplying both sides of the equation by Tc:

B TW * Tc = Tc * B WL / Tc

On the right hand side the Tc on the top is divided by the Tc on the bottom so the equal 1 and can both be crossed out.

Then divide both sides by B TW:

Cross out that B TW when it is on the top and the bottom and you get the new equation:

Tc = B WL/ B TW

Thank you all for this very useful article

Parfait j aimerais participer à une formation en ligne (perfect I would like to participate in an online training)

Leave a Reply Cancel reply

Your email address will not be published. Required fields are marked *

Save my name, email, and website in this browser for the next time I comment.

A Complete Guide to Displacement Hulls (Illustrated)

The displacement hull is the classic go-to hull design for sailboats and one of the most recognizable ones out there. In this guide, I explain all there is to know about them.

What's a displacement hull? A displacement hull is a boat hull design that uses buoyancy to support its weight. It lies partially submerged and displaces water when moving, hence its name. The amount of water it displaces is equal to its weight. It's very stable in rough waters. That's why this design is widely used on cruisers and sailboats.

Displacement hulls are great and reliable. Below we'll talk all about that. But they all have one major setback. Read on to find out what.

On this page:

Displacement hull features, how a displacement hull actually works, why it's so fuel-efficient, setback: maximum hull speed, advantages & disadvantages of displacement hulls, who might like this type of hull, in conclusion.

Nearly all sailboats have displacement hulls. Displacement hulls are great for operating in rough waters. They are less affected by waves than planing hulls. Because they're so steady, they are to go-to design for many ocean-going boats. Examples of boats with displacement hulls are: sailboats, canoes, and fishing boats and trawlers.

The displacement hull is:

• the most reliable & efficient hull in rough water
• the most fuel-efficient hull
• the most buoyant hull
• the hull with the largest cargo capacity

I'll explain all these points later on, but first, I want to just describe the hull design for you.

Design Features

Displacement hulls are pretty bulky. They have round bilges. The bilge is where the boat's bottom curve meets its vertical sides. The hull itself is round. It's round because that creates less resistance when moving through the water. That roundness is what makes it such a comfortable ride, even in waves.

But that roundness also makes it easy to roll (think of canoes, for example). That's not a good feature in heavy weather. To offset it, sailboats have a heavy keel that runs deep into the water. This counterbalances any roll, making the boat very stable. Sailboats with a long keel are very difficult to capsize.

The hull is rounded throughout, running from bow (front) to aft (back).

The displacement hull is generally pretty heavy. That's okay, since it is supported by its buoyancy, so it doesn't need a lot of power to propel (more on this later). The weight actually helps it be more stable and unbothered by nature's pull. I think it's fair to call the displacement hull with the whale among boats . It uses the water's upforce to carry it, and gently peddles along.

How Fast Is It?

Since this hull needs to move a lot of water before going anywhere, displacement hulls are pretty slow. Actually, it very well may be the slowest hull type out there. On average, their cruising speed lies anywhere between 6 - 8 knots. They can go faster, but most boats with displacement hulls don't have the power to do so.

They are great at low speeds. Thanks to their shape, they are easy to move and don't require a lot of power. They're actually one of the most fuel-efficient designs out there.

Compared to other hull types:

• Displacement Hull - Partially submerged, buoyant, moves water
• Planing Hull - Glides over water surfaces, generates lift |
• Semi-Displacement Hull - Displaces at low speed, lifts partially at cruising speed

I've written an Illustrated Guide to Boat Hull Types , where I go over 11 different examples of the most common boat hulls . That article will give a great and quick overview to get you up to speed, so if you don't know anything about boat hulls yet, that article is a great place to start.

The shape of the hull creates a sort of air bubble that floats on top of the water. At the same time, the weight of the boat pushes down (or actually, gravity pulls it down). This submerges the boat a bit, anchoring it, in a way. This push-pull gives it its characteristic reliability, making it more stable and better at keeping course.

As with anything that is really good at floating, it doesn't require a lot of energy to propel it. Since it can use the water to carry it, it's great for carrying cargo. You can really load her up without drastically increasing fuel consumption.

A planing hull needs to get up to speed before it generates lift, and until it does, it's absolutely rubbish in terms of a smooth ride. That's why planing hulls can get so uncomfortable in waves. They can't get up to speed, and their hull isn't made for displacing - rather flying - so it becomes a terrible ride.

The one major setback for displacement hulls is the upper-speed limit. As I've noted before, they are pretty slow. But the thing is: they can't go beyond their upper-speed limit, even if you gear her up with massive outboard engines and so on. The reason for this is called the maximum hull speed .

To understand the maximum hull speed and how it works, I want you to think of yourself lying in the Mediterranean Sea. That's just arbitrary, but since I can pick any sea I like in these kinds of visualizations, I prefer the Mediterranean. So you're lying in the Meds and along comes a sailboat. The sailboat hauls a rope behind it (I know, a line). You grab on to the rope and hold tight. The sailboat gently drags you along. It accelerates. The pull increases, you have to grab on even tighter. It accelerates even more. You have to really clench now.

The reason you have to increase your grip when the sailboat accelerates is simple. Your body displaces water when you move. When the speed increases, it has to displace the same amount of water, but faster. The water resistance (drag) increases.

The power needed to displace water increases exponentially with speed.

So now you can probably imagine that there will be a point where you can no longer hold on and have to let go. You have to slow down. That's your maximum hull speed working.

In the same way, there's a point where the boat's drag becomes so large, that it becomes almost impossible to propel it, no matter the amount of power. That speed is called the maximum hull speed. Every displacement hull has one, and it is a direct correlation with the boat's length. If you want to check out the maximum hull speeds for different boat lengths and learn how to easily calculate it yourself, you can check out a previous article. In it, I go over average sailboat speeds and the formula for calculating maximum hull speed .

By the way, the reason planing hulls can go faster, is that they generate lift at a certain speed. In terms of our story just now, that's the same as if you got yourself a wakeboard. Then, when the boat accelerates, at one point you pull yourself out of the water, and glide over the surface, instead of lying in it.

As with anything, this design has both pros and cons. I'll go over each one briefly down below.

• handle well in rough waters
• very hard to sink
• smooth ride
• large cargo capacity
• requires little power: very efficient
• very dependable
• can be very heavy
• large range

Disadvantages

• has a maximum hull speed
• tends to roll
• can capsize if it has no keel
• if it does have a keel, it has a deep draft

If you don't care about speed and are all about range, safety or comfort, the displacement hull is the way to go. It's by far the most comfortable ride of all hull types and will get you anywhere. You can cross oceans, cruise inland - it doesn't really matter. It has the largest range of all the hull types, and the fuel-economy is really impressive. With cruising speeds averaging between 6 - 8 knots, this hull type is the slowest, but also the steadiest. The perfect boat for long-range cruisers and liveaboards.

Displacement hulls have been around for centuries, and they are the most well-known hull for a reason. They're reliable and efficient. Those are perhaps the two most important trades when you're at sea. Nearly all sailboats have displacement hulls, and for cruising, the benefits outweigh the drawbacks big time. If you like speed, however, you should consider getting something with a planing hull or semi-displacement hull. You can learn everything about semi-displacement hulls here .

Jacques Burgalat

Great tutorial ! Do you know of anyone (or company) who could help me with using a fully electric power train on a 19m/40 ton Tjalk? I (and the tjalk) are currently on the Saint Johns River (Florida) which is more akin to a lake or canal than an actual “river”, so fighting waves and currents is not an issue.

Thank you for your help.

Leave a comment

You may also like, the illustrated guide to boat hull types (11 examples).

I didn't understand anything about boat hull types. So I've researched what hulls I need for different conditions. Here's a complete list of the most common hulls.

Semi-Displacement Hulls Explained (Illustrated Guide)

CAT COMPARISONS

VooXdoo Yachts new XF60.

Power catamarans are becoming an increasingly prominent part of the boating scene. Despite this relative familiarity there is still a perception that all catamarans are the same. It is obviously not true that all monohulls are the same, so why should it be true for catamarans? Just as for monohulled vessels, there are numerous sorts of power catamaran.

Which one is used should depend on the purpose for which it is intended. And, just like monohulls, you need to have the right boat for the job. However, unlike the monohulled vessel, there is a much greater overlap in performance and behaviour between the two basic types, the planning craft and the displacement vessel. This is largely because a displacement catamaran can, unlike its monohulled cousin, often go just as fast as a planning one, and in some sea state conditions, faster. However, it is not quite that simple. There are many more variables than just the hull form. So, let’s look at some of the parameters that differentiate the different types of catamaran and see if we can determine how they may affect your choice.

Although the hull form is not the only variable to a lot of people it is certainly one of the most obvious. If we ignore the more exotic examples of the breed, such as the SES [surface effect ships/side wall hovercraft] craft and the SWATH [small water plane area – twin hull] which is really a very extreme displacement vessel, then there are two basic types; the planning hull and the displacement hull.

The hull of a planning catamaran is similar in shape to that of a planning monohull. It is generally a relatively low deadrise hard chine hull with no rocker. Just like a monohull, it may have planning strakes and the currently very fashionable steps. There are planning catamaran hulls in which the deadrise angle is constant for at least the aft half of the hull (monohedron hulls) and others where it will vary along the length (warped plane hulls). Others may be similar in section to the deep vee monohull. There are also a number of variations on the single chine/multiple chine/longitudinal step (strakes), theme.

However, there is one planning catamaran hull form that you will be very unlikely to see on a monohull.

This is the asymmetrical planning hull as epitomised by many of the designs of Scott Robson, as far back as in the 1980. Today Robson continues to develop new hybrid designs utilising the best concepts from both asymmetrical and symmetrical hull forms. This means that the inside of the hulls is flat with all the shape on the outboard side.

This configuration is considered to have superior performance to the symmetrical shape in some conditions and is particularly suitable for fitting hydrofoils. It can also be designed to lean into a turn just like a monohull, whereas the symmetrical-hulled planning cat will tend to lean outward in a turn. Displacement cats also lean outward in a turn but because the angle of heel is around half a degree it is usually not even noticed. Planning power cats, are essentially two narrow planing monohulls placed side by side with a bridge deck in between. Nic De Waal of Teknicraft uses planning hulls that are symmetrical forward but asymmetrical down aft.

Robson amd Hill throw an extra wrinkle into the mix by fitting foils to their boats that are going to exceed 30 knots. Foils allow the boats to plane sooner and also carry heavier loads when on the plane. The retrofitting of foils is sometimes used to enable a planning cat that is showing a disinclination to plane, to do so.

 The fitting of foils is an approach that is difficult to use on a monohull but the catamaran configuration is perfectly suited to them as the main load-carrying foil is generally fitted between the hulls. Foils, particularly the active systems, are also sometimes fitted to the larger displacement hull forms, such as those found on high speed ferries, to improve the ride quality. The new generation of Voodoo high speed power cats are all fitted with foils.

Robson says the rule of thumb if you want to go over 45 knots is you need a foil. Under that speed you can get away without one. Hill agrees, if you want to achieve high speeds, in excess of 35-40 knots in a big cat you need to have foils and Hill has certainly proven to be a master at marrying foils to big cats. His latest project, the 20m Carbon Cat being built by Scott Lane Boatbuilders is designed to achieve around 45 knots, a speed that would not be attainable without the use of foils.

However, Robson also says he often puts foils on boats achieving 25-30 knots as it enhances the ride and performance of the boat.

PLANING HULL

Generally, the planing cat is designed for relatively flat water, but it will still handle rougher conditions better than a monohull, provided there is sufficient wing deck clearance. It is also usually a relatively short-range vessel and is also relatively small. Just like a monohull, it becomes increasingly difficult to make the vessel plane as it gets larger. This is largely because while the planning area is increasing by the square the displacement is increasing by the cube.

It just requires more and more horsepower to plane and at somewhere around 18 to 20m it makes more sense to go to a displacement hull form which at this particular size will go just as fast, if not faster, with considerably less horsepower. Works well if a boat has a top speed up to around 26 knots.

Just as with monohulls, it is expensive to run a large, fast, planning vessel. However, the converse size wise is also true. It becomes increasingly difficult to justify the use of the displacement hull forms below about 10 m in length unless you are willing to go quite slow or have hulls that can only be used for stowage. Essentially, if you want to go fast in a “short” catamaran you use a planning hull form.

DISPLACEMENT HULL FORM

So, what characterises the displacement hull form? It is usually a round bilge form of minimum wetted surface that depends on its length, plus the hull water line length to hull water line beam ratio, to achieve high speeds. Unlike the monohulled displacement craft, the hull speed (in knots) of the displacement catamaran is not restricted by the familiar 1.34 times the square root of the waterline length (in feet), of Froude’s Law.

Like the planing hull cats, there are a number of different design approaches to the shape of the displacement hulls. Many of the French catamaran companies just fit larger engines into their sailing hulls, probably because they have the moulds handy. This is fine as long as you are not going to exceed around 15 to 18 knots. Beyond this speed the hulls start squatting and assuming a bow-up attitude. They are essentially trying to go up-hill and require increasingly large amounts of horsepower to move.

Alan Wright, and a number of other designers, takes a somewhat different approach. He took the traditional trawler displacement hull, made it much narrower so it had a higher hull speed and then joined two of them together with a wing deck.

When in 1983 the writer’s design office staff started looking at power catamaran design, they used the sailing cat hull shape because they knew how fast they could make them go from many years’ experience of designing sailing catamarans. However, to prevent the squatting normally associated with this hull form, they increased the buoyancy down aft by fitting a bustle with a vertical trailing edge. This proved to be a difficult hull shape to construct as a one-off and so the buttock lines were straightened and, instead of kicking the hull lines up toward the surface down aft, as with the traditional displacement hull shape, they drew them in to form a canoe stern. They then placed a large amount of buoyancy above this in the form of a flat section to prevent squatting.

Since then this form has been refined by the addition of a concave surface above the propeller complete with some kick down toward the aft end. The distribution of the buoyancy has changed slightly, and the entry has been fined up even further. Now that this particular form that they developed has proven to be so successful it has become the preferred shape for an increasing number of displacement catamaran designers world-wide.

BULBOUS BOWS

It is not unusual to see a displacement hull with a bulbous bow and there are several reasons why a designer might employ such a device. One of the parameters that determine the resistance of a displacement hull is the hull’s half-entry angle. The finer the entry, the lower the resistance. By its very nature, any catamaran hull is finer in the forward sections than an equivalent monohull and this tendency is more exaggerated in the low-resistance displacement hull. As a consequence of this, the longitudinal centre of buoyancy (LCB) of such a hull tends to be located well aft. If the superstructure of such a vessel is then located well forward, it will be necessary to fit a bulb to move the LCB forward to where the LCG is located, if a bow-down trim is to be avoided.

This characteristic is clearly illustrated by the fine entry and bulbs that are typical of a Craig Loomes wave-piercing displacement catamaran, or some of the high-speed displacement ferries. A bulb may also be fitted after launching, to correct a bow-down trim problem.

THE WEIGHT ISSUE

The displacement hull form is particularly suited for long range, and particularly long range at speeds that owners of monohulls can only dream of. However, this weight issue does raise another point of differentiation. The height, shape and span, both transverse and longitudinal, of the wing deck, will also vary from design to design and from designer to designer.

Although weight does not have much effect on the performance of the displacement hull form, increasing the weight for a given design will lower the height of the wing deck off the water. If this lowering is extreme then it will impact quite literally on the vessel’s performance. Consequently, if the vessel is to operate in rough water conditions, the wing deck and the various structures associated with it should be designed to suit those conditions. The most important requirement is to keep the wing deck as high off the water as is feasible.

However, the striving for a stylishly low profile often forces the wing deck closer to the water than is desirable from a performance point of view. The writer has seen it said that the fine bows and deep forefoot that are typical of the displacement hull form, and that allow it to slice so smoothly through the seas, can lead to wing deck slamming and also bow steering, or even broaching, in large following seas. There is some truth to this assertion if unheeded, but of course the designers of this type of vessel are aware of these potential problems and take the appropriate design action.

So, in fact the perceived problem is just that. It is only someone’s perception, and in reality, does not exist in boats where the appropriate design action has been taken. In a similar way the “wave piercing” displacement cat uses the larger central “hull” to prevent burying the low freeboard hulls and minimise slamming of the wing deck. All the above features also mitigate against bow steering, wave stuffing and broaching. But, when you are in the middle of a very wide expanse of ocean there can be no room for error.

The span of the wing deck is a direct reflection of the hull spacing. On a planing cat the hull spacing appears to have little effect on the performance. Conversely the spacing can have quite significant effects on the performance of a displacement power cat. If the space between the hulls is too small then there will be hull interaction and an increase in resistance.

So how close is too close? It depends on the length-to-beam ratio of the hulls and how fast the boat is going. The Malcolm Tennant design office designed two cats with identical hulls. One had an overall beam of 7m and the other, 5m. With the same horsepower the boat with the 5m beam was 2 knots slower. It should be obvious from all this that there is a much larger size/speed overlap between the planning and displacement hull form catamarans than occurs in the equivalent monohulled vessels.

With monohulls the displacement boat is on the low end of the speed scale and the planning boat on the high end. Admittedly there is an amorphous hybrid called a semi-displacement craft but generally speaking the planning and displacement vessels are quite distinct. Because it is not possible to separate the planning and displacement catamarans in the same way on the basis of top speed, it is just not possible to say which type is superior in a lot of conditions unless you define your requirements very carefully. If one of your requirements is to minimise fuel consumption, and for many owners this is the case, then there is ample evidence that a well-designed round-bilged displacement cat is hard to beat.

ACCOMMODATION SPACE

It is generally considered that a catamaran will always have more accommodation than a monohulled vessel but this is not necessarily so. If the beam of the catamaran is restricted to the same as that of a similar monohull, then the monohull may in fact have more interior space. Hard to believe? Then consider that essentially what has been done with the catamaran is cut a piece out of the centre line of the monohull. If the catamaran then has the same beam and length of the monohull then it does in fact now have less interior volume because we have cut that piece out and not replaced it by making the vessel wider.

So, restricting the overall beam of the catamaran to that of a monohull can impact quite severely on the accommodation.

YOUR CHOICE

So, is it possible to generalise in any way concerning which type of vessel should be your choice? It is very difficult to do this because often the choice of boat is influenced by emotional factors as much as rational ones. It probably depends mostly on where your priorities lie. If the styling is the dominant factor, and if you are looking for a vessel that has a profile of no greater height than a monohulled craft of the same size, then you are probably going to have to go with a planning cat with a very low tunnel height. You will have to accept that the wing will hit sooner in rough water and you will generally require more horsepower to drive it.

Conversely, if you want a high tunnel height and good rough water capability combined with economy and extended range, then you are going to have to live with the higher profile that accompanies these characteristics. If you are looking for lots of accommodation in a short boat then you will need a boat whose beam is going to make the use of a marina berth problematical.

If easy engine access is your major requirement then perhaps you should be looking at monohulled craft because no catamaran (particularly displacement hull cats) has the wide-open engine rooms that can be found on some monohulled vessels. However, planning vessels will generally have wider hulls and will often have the engines located further aft than in a displacement boat, particularly if surface drives are being used. If fuel economy is the number one priority then a high-speed displacement cat (up to around 26 knots) can give this to you over a wide speed range, with a significant increase in cruising range for any given fuel capacity.

In addition, well-designed displacement cats have outstanding seakeeping, but they will usually have less accommodation space in the hulls than do planing cats. However, this will usually be offset to some extent by the fact that displacement cats tend to have greater overall beam than do planning ones. It should be mentioned that beamy displacement cats can be too stable with a very uncomfortable ‘snap’ roll in beam sea conditions.

An there is the question of power. The majority of power cats over 10m will have a pair of diesels, either through shafts or sterndrives. Smaller lighter power cats favour twin petrol outboards, but with the advent of high horsepower and high torque diesel outboard, there is a whole new power source available for larger power cats.

This is a very brief summary of what is a very complex topic. The possible permutations and combinations of the hull type, style, drive system and accommodations of a power catamaran are considerable – much greater than for monohulled vessels. But hopefully the article shows that there are some generalisations that can be made about the large number of different types and styles of catamaran, each of which will have a particular advantage or disadvantage depending on its intended purpose. Ultimately, it always comes down to horses for courses and, of course, your personal preference. Along with this must go the recognition that all boats are compromises and that you just cannot have a boat that will excel at everything.

THE RIGHT BATTERY

Silent power, related articles, looking back on 40 years of bridge to..., the history of kiwi trailer boats by the..., a-z guide on trailers, marine batteries, getting further on your fuel, dmw trailers celebrate 50 years, power cats – the past, present & future, horizon yachts open house 2024.

D&B Publishing is proud to have delivered the Pacific regions best power boat read for over 25 years. Our PowerBoat team brings you the news, views and reviews from around the world. A team second to none when it comes to power boating.

• Boat Reviews
• Boat Safety
• Destinations
• Buyer Guide
• General Interest
• Company Profile
• [email protected]
• +64 9 428 2328