OUR SPECIFICATIONS AND ADVANCED INFORMATION
Continue below to see detailed information on our processes, boat specifications and everything else you need to know about our cats.
Power catamarans come in many shapes, and in different parts of the world preference is given to specific types. This page explains the 10 hull shapes of medium- and high-speed existing boats
BRIEF DESCRIPTION OF HULL TYPES
A. Australian type with symmetrical sponsons, fine entry, medium-square tunnel, low deadrise.
B. Sailing-boat type symmetrical, round-bilge and tunnel, deep forefoot, no strakes.
C. Asymmetrical sponsons with low deadrise bottoms and no-trip chine, medium height square tunnel.
D. Split monohull with narrow, low square tunnel with high attack angle at bows.
E. Super-slim sponsons with medium-to-high tunnel, fine entry, designed to be used on protected waters.
F. SES (solid side skirt) hovercraft with low tunnel and skirts at bow and stern.
G. Kenton cat type with low round tunnel and round bottoms, tunnel lifting at bow.
H. HySuCat with one main foil and two trim foils, high deadrise bottoms and medium-high tunnel.
I. Bobkat with round, asymmetrical sponsons, high tunnel with tunnel-chines and bow steps.
J. Bobkat with HySuCat foils.
A POINT SCORING TABLE OF HULL CONFIGURATIONS
Different Hull Configurations – Point Scoring Table
Aspect a b c d e f g h i j
1. Low vertical accel – sponsons 2 7 5 4 9 9 5 6 7 8
2. Low vertical accel – tunnel 3 3 3 1 5 3 1 6 7 9
3. Inward banking in turns 1 1 9 7 6 4 5 7 9 9
4. Non-broaching in following sea 2 3 6 7 4 6 7 5 8 8
5. Non-weaving in quartering sea 8 8 2 3 4 7 7 3 8 8
6. Resistance to barrel-rolling 1 5 9 3 7 7 7 5 9 9
7. Load carrying ability 5 5 6 7 2 3 8 5 6 7
8. Transverse stability 6 6 7 3 4 3 4 7 7 7
9. Pitch stability 4 5 6 7 4 3 6 7 7 8
10. Dry ride in small chop 6 6 6 3 7 2 2 7 7 7
11. Economy at planing speeds 8 4 7 4 2 9 7 9 6 8
12. Economy of construction 8 9 8 7 5 1 6 7 9 7
Please advance below to view detailed descriptions of each hull configuration
Australian type with symmetrical sponsons, fine entry, medium-square tunnel, low deadrise.
This shape became popular in Australia when it was found that two identical symmetrical sponsons cut construction costs and gave a beamy boat with lots of deck space. Lateral stability at rest is very good and a vast improvement on monohulls. With a fine, deep forefoot it slips through a small chop without fuss and gives comfort and economy, provided that the wavelet heights are less than half the tunnel height.
By twisting the bottom from almost vertical at the bows to almost horizontal at the stern – a severely warped bottom – the resistance in calm water is kept low with a small wetted area and low wave-making resistance. This type is still popular on rivers and in large harbors, but the problem starts when it ventures out to sea where conditions are not as favorable. Even on a balmy day there can be large rollers, flowing in unobstructed for thousands of miles – at 24 knots. If the water is deep enough they have a sinusoidal shape with the steepest gradient always less than 20° and mostly 10° or less.
When Type-A runs straight into these rollers, it will try to act like a wavepiercer with its fine, low-lift bows until buoyancy lifts the bows with the help of the tunnel roof, if necessary. In the process it will slow down a fair amount because of the increased resistance caused by the extra wetted area and, of course, the gradient. No vehicle or vessel will go faster up a hill than down. And talking about the downward run, after cresting a head sea it is usually an exhilarating feeling, the acceleration and the higher speed. If the wave length is long enough, say six times the boat’s length, then nothing strange should happen when you arrive at that trough and start up the next wave. Running beam-on to the big swells, either on top or in the bottom, is no problem. Quartering the head seas without wind and chop may make the passengers feel a small uneasiness when the cat leans away from the higher water, but there is no real chance of overturning.
It is on the way home and running with the swells that Type-A starts having some fun and games, and gets the adrenaline pumping. Running with the swell can be fun if you sit on the crest and hold the boat there with the right amount of poise and use quick steering corrections to cope with the small turbulence sometimes found when the bottom of the sea starts shelving.
But don’t overtake the roller and go screaming down into the trough at a great rate of knots! Our Type-A will bury its bows and take some green water on deck while stopping as if it has disc brakes. In severe cases it will pop back like a giant cork to the great discomfort and danger of the passengers. The Americans call this "stuffing it" – which is descriptive, if nothing else. The other trick habit that Type-A has is the severe lean when quartering large swells after overtaking them. The actual number of degrees may only be 10 or so, but it feels like much more because a good cat normally only rolls about 3° and so spoils the passengers who come to expect this lateral stability all the time. The grand debacle is still to come when Type-A arrives at speed at the trough, at an angle, and the leading bow digs in to cause a broach that no amount of steering correction can stop. Nobody on board enjoys this because turning turtle is a real possibility.
The reason for this bad behavior is in the fine bows which have no buoyancy lift, and the symmetrical shape that has no non-trip chines on the outside. Thus it banks to the outside of a turn like a car, not like a motorbike that banks into a turn for stability, safety, and comfort.
Other bad habits include severe tunnel-roof slamming in high, short seas and the choking of the tunnel to slow down when those symmetrical sponsons channel half of the water displaced into the tunnel. You would have understood by now that Type-A rates as a good smooth-water boat, but a very poor offshore craft. And this is before we start on the high vertical deceleration – bumping – that occurs at speed in confused and uneven seas when the boat leaves the water and lands on the flat-aft-bottoms. This can cause structural damage and change your idea that it is always most comfortable at the stern of a planing boat. They are also load-sensitive, and frequently a slight overload will prevent it from getting on the plane, resulting in high fuel consumption as it struggles along, bows up, at 15 knots instead of planing at 32 knots.
Sailing-boat type symmetrical, round-bilge and tunnel, deep forefoot, no strakes.
The sailing boat type. This has symmetrical round bilge sponsons and wide, lowish tunnel. It has a deep forefoot without spray rails or chines, and keels in miniature. After the French proved that a catamaran can outsail any monohull of the same size, powerboaters started to look at this hull configuration for medium speed cruiser’s without sails. Without lift rails and chines the amount of lift at speed is negligible, so there is no reduction off wetted area. But the long, slim sponsons with their fine entry have very little wave-making resistance which partly compensates.
Nevertheless, this type must be operated in the 15-25 knot speed range to give reasonable economy when loaded in the way powerboaters are inclined to do. This is a lot quicker than normal displacement speed for a hull of the same length, and nicely fills the gap left by the demise of the semi-planing hull.
The round-bilge shape gives a soft ride and can’t slam as such, but the short and white flat tunnel does that with a vengeance when you try to go directly into a head sea – a situation that is almost unknown to sailing man, so they never allowed for that in their design and development.
Because of the wide overall beam and the lack of vertical tunnel sides low down, the Type-B leans to the outside on corners but to a lesser extent than Type-A. The fine entry and deep forefoot slice through the chop nicely, but it lacks the buoyancy or lifting surfaces to save it from some stuffing into the back of the next wave.
Type-B has a relatively small water plane area so it can carry light loads relatively easily before slowing down drastically when overloaded to the extent where the tunnel roof stays in contact with the sea for a large increase in resistance. Type-B can benefit from some new patented idea such as the HySuCat to lift it at speed and improve the top speed.
Kenton cat type with low round tunnel and round bottoms, tunnel lifting at bow.
I apologize for jumping the queue but there is a certain number of common features with Type-B such as round-bilge sponsons and symmetrical bows sections. But Type-G has a lower, full-length, rounded tunnel and a lot less beam to change its sea-behavior completely. That soft entry and landing of the rounded bottom of the sponsons are completely overshadowed by the bang that occurs when solid water hits that low, round-tunnel roof and finds that it has nowhere to go.
As a matter of interest, it is our conviction that the well-known spitting (sneezing) of a bucketful of water forward, out of the tunnel mouth, at speed, is caused by the speed of sound in the two phase medium being exceeded.
This happens when aerated water is suddenly compressed. Many of the low-tunnel builders put in small steps in the tunnel roof to try and capture this foreward flow but we doubt if they were successful.
Type-G usually uses chines on the outside and, together with the tunnel which is submerged at rest, has considerable lift at speed. As a matter of fact it probably is the best load carrier of all catamarans, providing it can get over the hump – another big difference from Type-B which has no real hump in its resistance curve.
Type-G behaves reasonably well in following seas when that large bellmouth comes to the rescue, when diving is imminent, but it does slow down in the process.
Asymmetrical sponsons with low deadrise bottoms and no-trip chine, medium height square tunnel.
The fully asymmetric sponson- shape lowish deadrise bottoms and non-trip chines have a lot going for it. It will bank to the inside in turns and have a good directional stability in head seas and following seas.
However, in quartering seas it weaves as the seaward-curved bow causes it to "steer." A steering correction to the opposite side gets aggravated when the other bow enters the same wave and does the same, resulting in an uncomfortable corkscrew yawing motion which is highly conducive to producing motion sickness. The medium-height tunnel also has a limit on wave heights that it can handle at speed without severe slamming. The many flat panels, as opposed to compound curves, makes this shape a good candidate for construction in aluminum, steel or plywood, but the flat panels need more stiffness to prevent panting that can cause fatigue.
Depending on the deadrise angles of the bottoms it can have a good ride even in rough seas and will react safely when sliding sideways off large swells. In following seas it behaves well because of the full bows, but at an angle, a broaching action may be felt when the leading bow hits the bottom of the trough and then veers off. Lateral stability is excellent and it will need abnormal loads to make it roll too far. Ride wetness will depend on the detail shape of the forward chines and the amount of flare in the bows, but it should be much better than the Type-A. It can carry reasonable loads and its Center of Gravity (CG) position is not critical, within bounds, of course. Economy should be good with a well-balanced unit.
Split monohull with narrow, low square tunnel with high attack angle at bows.
Strictly speaking, this should not be called a catamaran because its parent was a monohull that got split down the middle, and the halves were moved apart by a small amount, and the gap covered over. The result is a hybrid which inherited the worst characteristics of both monohull and a bad catamaran. It slams and bangs in any kind of head sea, or even chop, and does it with a noise like a thunderclap. The transverse stability has been improved from that of its original monohull, but not to the extent that would match any decent catamaran. The point-scoring table tells you that it has pitch stability and can be constructed economically, and should bank to the inside on corners. Load capacity is good and the economy in smooth water is reasonable. Ride is dry, except for tunnel-spitting similar to the Type-G.
Super-slim sponsons with medium-to-high tunnel, fine entry, designed to be used on protected waters.
The idea of a super-slim sponsons was started by English designer Nigel Irens when he designed the 21m trimaran, Ilan Voyager, which broke many records previously held by planing craft. This type has high aspect-ratio sponsons that have very little wave-making resistance. You only have to look at one of Philip Hercus’ wavepiercers in action to see what we mean. At certain speeds there is an advantage for the super-slim. However, in doing this, a large area is running wet, and skin friction resistance has increased over that of a similar sized planing craft. Because of the minimal bow lift there is no dynamic lift and almost no buoyant lift. Therefore Type-E is fine in small chop and wave lengths of less than half the boat length, but it urgently needs a helping hand in the way of a third sponson or tunnel roof extension to prevent stuffing.
To counter this unacceptable quick stopping action, Hercus decided to take advantage of scale and, by all accounts, his 120m ferries are doing very well, but they are ships and not compatible with the boats we are discussing here. Vertical acceleration from the sponsons is very low, but the tunnel roof will slam if it is flat and not high enough for the sea state. Because of the low water plane area, Type-E is sensitive to load shifts and it becomes important to control people movement and other factors that can offset the critical center of gravity.
Because of its wave penetrating action, it cannot be used offshore or where large waves and rollers occur. In other words, this is a protected-water boat similar to Type-A and Type-G. The narrow sponsons pose problems in installing wide engines and long cardanshafts, such as those used in SWATHs and SES may be needed and that adds to the cost.
Lateral stability will depend on aspect ratio, but is less than on other, more normal types of catamarans. You should have a dry ride until the chop overrides the bows and runs up the sides. Construction is not difficult but speeds (for economy) are within a narrow range.
SES (solid side skirt) hovercraft with low tunnel and skirts at bow and stern.
The solid side skirt hovercraft is not considered a catamaran by many, but it does have two long, slim sponsons almost like Type-E, but with the addition of flexible skirts fore and aft. The skirts are there to contain the cushion – air that is pumped into the big empty space between the sponsons, skirts and tunnel roof to lift the craft up to where it has minimal draft and wetted area. The SES was developed by people who were unhappy with the normal hovercraft where air-propulsion is needed. They thought that these slim sponsons would allow populsion by waterjet or propeller and so make it more efficient. Another handicap of a pure hovercraft is its susceptibility to cross winds and its consequent need to wheathercock to counter them. So don’t be surprised if you see one traveling almost sideways to go along a certain course. Having slim hulls in the water helps offset this to a considerable extent.
But it is costly to build and maintain and its ride characteristics are not acceptable to many. It is load-sensitive and the CG has to be dead right. The ride is wet and becomes hard when the waves hit the relatively low tunnel. On the upside, it is capable of good speeds in calm conditions. As an afterthought, it is probably unfair to compare it with normal cats.
HySuCat with one main foil and two trim foils, high deadrise bottoms and medium-high tunnel.
Professor Günther Hoppe was testing one of the early Bobkat catamaran models in the circulating tank at Stellenbosch University when he decided that the resistance-to-weight ratio was too high and needed improvement.
Firstly, he changed the cross section of the sponsons by introducing a wide, and low-deadrise bottom with no non-trip chines. This immediately reduced their resistance, but not enough for Hoppe, and he continued experimenting until he hit on the novel idea of fitting a foil between the sponsons to carry part of the load and, in so doing, reduced the wetted area.
It is a proven fact that long and narrow wings of aeroplanes produce more lift at low and medium speeds than short, wide ones. The same goes for hydrofoils. The Russians developed hydrofoil craft for use on their rivers where a low wash was needed, together with economy at high speeds. Many configurations were tried out but all lifted the hull completely clear of the water which gave them the best speed, but introduced other problems. Among these were deep draft at rest and wide foils extending beyond the sides of the boat to make docking difficult, and sometimes downright dangerous. It was also very expensive to produce, and large shaft angles made propulsion inefficient.
The Hoppe solution, registered as HySuCat, is a low-cost compromise that has been developed to give excellent results within its effective speed range. The foils between the sponsons are positioned to not only lift the boat when planing speed is reached, but also to adjust the trim for optimal main foil and sponson attack angles. In the early HySuCat designs the main foil was placed just forward of the Center of Gravity and small trim foils were mounted near the transom, all of them above the bottoms of the sponsons. However, the world patent covers many other possible configurations.
Production models of the HySuCat had a higher deadrise to improve the ride in rough water and help the vertical tunnel sides for banking less in turns. Without the non-trips, the lateral stability – in extreme conditions – could lead to tripping and flipping if the Center of Gravity is too high.
The sweeping bow with the chine going right up to the gunwales has poor buoyancy and dynamic lift with all the problems previously mentioned for asymmetrical hulls. At low speeds the tunnel may slam a bit, but once the foils come into action at about 14-18 knots, and lift the whole boat a considerable amount, the tunnel clearance is also increased and very much larger waves are needed to create an uncomfortable slamming. We have found that the foils also dampen action such as heaving and pitching, which improves the ride even further. The main advantage of the foil system is the dramatic reduction in the resistance, resulting in a higher top speed and improved economy. Recent applications of the HySuCat system on other hull shapes such as Type-A improved the speed and lifted the tunnel a bit but it could not cure the other inherent bad habits in the basic design.
If, for instance, the bows have very little lift of their own, the existing HySuCat system can lift the bows and make it run well in a small chop, but in really rough water when speed needs to be reduced to where the foils don’t lift, they might even hold the hull down. In very small boats the position of the Center of Gravity becomes very important and the driver has to position the passengers correctly, or else he can expect some strange maneuvers. Expect to see improvements of the system in the future.
Bobkat with round, asymmetrical sponsons, high tunnel with tunnel-chines and bow steps.
Bobkat with HySuCat foils.
The registered trademark Bobkat covers a range of power catamarans from 2.5m-33m that have similar looking hull shapes, but with detailed changes for different sizes and speeds. The shape took 23 years to develop after which it underwent a series of refinements for the next 10 years. It is still not perfect, but when compromises were forced on the design, safety and comfort always took precedence over the outright speed.
The almost symmetrical bows prevent wandering when quartering seas and provide plenty of buoyancy when needed. It is almost unknown to take green water over the bows even in the Cape of Storms’ worst seas. Stepping the bows in the front section of the tunnel allows asymmetrical sponsons for the rest of the way to give that all important banking to the inside, which is considered a prerequisite for safe seakeeping – and keeping your passengers on board.
The convex shape incorporates the equally important non-trip below the wide chines to further improve safety in beam seas and quartering swells in a large following sea. The rounded section does not slam and gives a comfortable ride in rough water, even when jumping the large waves at high speed. Spray rails are used at the bows to keep spray down and to provide a dry ride, even with a southeaster off the bows.
The patented tunnel chines running fore and aft behind the bow-step are there to mix this solid water with air to form a two-phase medium that is compressible and therefore reduces the tunnel slamming experienced by other tunnel shapes. The tunnel is also the highest of the boats listed and when the foils are fitted on the Type-J the effective tunnel height allows high-speed travel in severe sea states. The 20m patrol boat, for instance, can take 3m high head seas at 26 kns without discomfort.
At other headings there is almost no limit to speed accept in following seas when the boat starts flying off the crest and loses propulsion while airborne. The tunnel chines lift up in a flattened S-shape near the stern as does the tunnel roof, to provide an increased tunnel area for waves to enter when traveling at speeds below 20 knots in following seas. This not only lifts the stern but prevents that sudden forward lurch you get with other transoms. For the sports fisherman this feature also allows for backing down at speed when fighting a large fish without any danger of swamping.
The overall aspect ratio of 3:1 with a sponson ratio of 10:1 reduces wave making resistance, especially on the foil, while giving excellent lateral stability. The progressive increase of sponsons width with draft gives a load ability which is further enhanced in the foil-assisted J-type. The metacentric height of most catamarans results in a restricted roll action of small amplitude that people soon accept and prefer to the motion of monohulls, where alarming roll angles can develop in beam seas and rough waters. Type-J is fast, economical, and most seaworthy with a built-in characteristic of looking after its occupants.