Meet Malcolm Tennant - one of today's foremost power catamaran designers. Learn about his fuss free, excellent accommodation and economical displacement Powercat design.

A Challenge or A Designers Pipe Dream? Is it possible to get a craft that combines the best features of both of these types? A boat that has reasonable, even good performance with excellent accommodation, but is still economical to build and run, and has good sea keeping capabilities; or is this just one of those designer's pipe dreams ? A Look Back at Successful Attempts 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. Tom Fexas "Midnight Lace" 44 As Malcolm writes "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 waterline beam]. This was certainly narrow by contemporary planing boat standards but was not exceptional when compared with earlier boats, or with the types of hulls that Malcolm proposes should be used. Long and Fast Slim Jims of the 20th Century Before the improvement in the power to weight ratio of the internal combustion engine and the development of the hard chine low deadrise hull allowed boats to plane, there was only one way to go fast. You had to go long and slim, and in the first decade of the 20th Century we find boats such as "Slim Jim" that in 1905 were achieving speeds of 15 knots from a 15hp engine. Typical of these early boats was "Defender"; 16.2m [53'] long and with a maximum hull beam of 2.28m [7'6"]. Headroom in the flush decked hull was only 1.45m [4'9"] and she slept 6 in berths only 500mm wide [18"]. In anything of a seaway, it would have been incredibly wet and uncomfortable. More comfort, More Interior Space and More Speed?  The boat had a great deal of grace and elegance to her lines, but her rolling at sea and lack of accommodation would be totally unacceptable today except for one small detail; a forty eight horsepower 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 that the answer is yes -  if we use the catamaran configuration. 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. 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 much higher hull speeds than Froude ever envisioned from his observations, in the order of 30knts plus 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, then how much easier it should be 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 power boats exhibit, have shown no catastrophic increase in wave drag at speed/length ratios above approx 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. Is it that simple ...

The potential is there, but is it really that simple ? The answer, of course, is no, not quite. "Escape 13.5" - A Family Cruiser designed for extensive coastal cruises. If we compare a sailing catamaran with a keel boat, 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 for its stability. In the case of the power boat there is no such advantage. The catamaran may, in fact, be heavier than the monohull because of its increased skin 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 sometimes limited planing surface, but it does not mean that our dream is impossible. The Displacement Concept: What's Behind the Design  The displacement catamaran is not so 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 approach. To work successfully the L:B ratio of the hulls should probably 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 hulls 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 32ft [10m] unless they can be built light, or a very modest performance is required. This 25m Tennant Designed powercat was launched recently at PMY in Tauranga. Currently, sitting in Auckland’s Viaduct Harbor, the vessel makes 25 knots from her twin 800 HP engines. Her fine bows and careful distribution of reserve buoyancy have translated into a very smooth ride, a characteristic of Tennant Powercats. However, I have designed smaller displacement boats that achieve quite creditable 15 knot cruising speeds from very small horsepower (43hp per side) engines. To have performance on a par with that of planing vessels, the displacement boat must be able to have long slim hulls preferably without the planing boats low deadrise submerged chine sections. This increases the drag substantially, and even more if the chines break the surface. This is the approach that 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. Are These Boats Really Displacement Crafts? 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 then they must be planing. In fact, if we accept the usual definition of a 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. It is clear from the photographs that catamarans are not showing the traits normally associated with planing . The waters are muddied a little by the fact that there is no sudden jump from displacement to planning. It is a continuum and somewhere in the speed/length ratio range of from 1.5 to 2 the craft would be considered to be in "semi displacement" mode. However, it is clear from their behavior that these particular designs of catamaran are neither planing nor operating in a semi displacement mode. They are in fact true displacement boats. Let's take a closer look ... Malcolm Tennant 72' Power Catamaran

Zenith-47 with long and thin hulls. Displacement Catamarans, the Best of Both Possible Worlds  We have now designed a large number of displacement power cats exemplifying the "long and thin" approach to powerboat design . The Zenith - 47 displaces some 13 tonnes fully loaded and motors at 20knts max, and much more economically at 16knts, with only two 122kw [160hp] engines pushing hulls with a 24.5knt hull speed. A monohulled displacement boat of this length would have a hull speed of around 8.5 knots. The smaller "Nomad" and "Cortez" power boats also have a similar hull speed but are optimized more for economy with slower speeds from rather small engines. The "Icarus-46" has a top speed of 25 knots from two 150kw [200hp] 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 37.5knts which would yield a quite easy 30knts with the right horsepower of around 500 per side. Again in the interests of economy this boat is intended to cruise at 16knts with a maximum of 20knts using two by 150kw [200hp] engines. These performances are very much faster than those of the traditional displacement boat of comparable size. They are on a par with that of a planing boat of a similar displacement but with a lesser power requirement and subsequently greater economy. "Jaybee" 47'6" Power Displacement Cat Fuss Free, Excellent Accommodations and Economical Performance 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 lean" principle should not be applied to lighter weight boats with less superstructure and even finer hulls to produce 30 or perhaps even 40 knots of fuss free performance from quite modest horsepower. In fact, this belief has been partially tested with two offshore designs, "Red Diamond II" and the "Awesome 2000 ."  The 57" [ 17.5m] Red Diamond II, designed for a Japanese client, is capable of a top speed of 33 knots and a cruising speed of 24 knots from two 320kw [430hp] Yanmar diesel engines while the 65" [20m] Awesome 2000 has a top speed of 28 knots with a cruising range of 3000 nautical miles at a speed of 15 knots in open ocean conditions. This latter craft has made a trip from Longbeach California to the Hawaiian Islands using only its 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 types of craft regardless of the 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 early recognized the economic advantages of this concept. It has often been pointed out that many people with displacement boats try to push them too fast and consequently would probably 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.

St. John 44 The 44' Power Catamaran St. John 44 Malcolm Tennant is often asked "how small" a power catamaran can be and still be used for serious ocean going. This is really a "how long is a piece of string" type of question, but the St. John 44 is a serious attempt to actually answer it. Size is not Length The biggest problem when talking about the size of any catamaran is that people tend to confuse size with length. In the case of a monohull vessel, a longer boat is usually a bigger boat. In the case of the catamaran this is not necessarily so. You can have catamaran with, say, 18m hulls but with the superstructure of a 12m boat. This will be perceived by most people as being a big boat, when in fact, it is just a long boat that is still quite small in volume. It is also perceived that this longer boat will also be a much more expensive boat. But, again, this is not necessarily so. Given that the cost of the hulls of a catamaran is somewhere in the order of 10% of the total cost of the vessel, making the hulls a couple of meters longer has very little effect on the cost of the vessel. It may actually decrease the capital cost, as the boat may now be able to attain the same speed with smaller [cheaper] engines. St. John 44 Elevation Profile However, given that the general perception is that a long boat is a big boat and, of course, the usual marina berth problem. I have tried to restrict the overall length of the St. John 44 to what I consider is a workable minimum for an ocean-capable power cat. The effects of this restriction are twofold. Because people still want to carry pretty much the same equipment on the shorter boat that they would on the longer one, and are still looking for the same sort of range, we have had to use a wider hull than is our norm to get the necessary load carrying capability. This then has the effect of lowering the vessel's hull speed and reducing its efficiency slightly because we are now operating farther up the hulls resistance curve. But, we will still get around 25 knots out of 230 hp per side - perfectly satisfactory performance for a vessel of this type. Even if it is a little less than we could normally expect to get from one of our boats of this displacement. St. John 44 Plan Layout & Flybridge We often have all the cabins on the wing deck with walk-in access. However, this usually means that, if an enclosed wheelhouse is then placed on top of the main structure, and a good wing deck clearance is maintained, the overall height of the boat is substantial. In this design this was not an issue. The clients requirement of having an internal helm on the main deck meant that visibility out the front of the boat was a necessity. Consequently, the forward berths have sitting headroom only, so the helmsman can see forward over the top of them, and are placed in the wing with access from the hulls. This is the configuration that is common with most sailing catamarans. The owner's stateroom, however, is located on the wing deck with an en suite head and shower. The saloon, galley and dining areas are all located on the same wing deck level and use household appliances where possible. The stern cockpit features an aft-facing seat (great for fishing) and generous boarding platforms to port and starboard. The flybridge has a second helm, a davit and stowage for a rigid bottom inflatable dinghy. We have used our standard hull form that has become the shape of choice for most designers of displacement power catamarans since we first evolved it some fifteen years ago. This includes totally protected propellers, installed on a perfectly horizontal shaft in such a way as to completely eliminate the appendage drag normally associated with shafts and struts. Our usual hull beam, and buoyancy, increasing knuckle is there along with the underwing girder on the inboard hull side and the central wave breaking nacelle. All features that make our designs particularly good rough-water boats. Propulsion is by normal four-bladed propeller with the tail shaft attached to a thrust bearing so the boat is being pushed on a bulkhead rather than on the engine. The engine is connected to the thrust bearing by an intermediate shaft and can, therefore, be more softly mounted, considerably reducing noise and vibration. We have managed to get most of what the client wanted into this design, and we have still retained our generous wing-deck clearance and all the other features that, in our experience, make for a good offshore boat. The vessel also has a range of 2,500 nautical miles at 12 knots. So, I would have to say that, in my opinion, the St. John 44 is about the minimum size for an ocean-going power catamaran.

Side View of Malcolm Tennant New Yorker 51 The vessel is a Malcolm Tennant New Yorker 51 currently being constructed in Ilhabela, Brazil to American Boat and Yacht Council (ABYC) standards. Malcolm Tennant is widely considered the ‘authority’ on power catamaran designs in the marine industry and has published several papers on the subject, and this vessel is one of his flagship designs. The New Yorker is a serious offshore trawler style cruiser. It is an ocean cruiser with three or four ensuite double cabins and a 2.300 nautical mile cruising range at 9 knots. It has all Tennant’s rough water performance enhancing features such as the curved wing deck and anti-slamming nacelle. Boat Type: Length: Beam: Draft: Berths: Builder: Model: Year Built: Engines: Horse Power: Hull Material: Catamaran 51 feet / 15.54 meters 25 feet / 7.62 meters 5 feet / 1.52 meters 4 Brazil Boats Ltda Malcolm Tennant New Yorker 51 2004 2 (Diesel) 630 Composite What is especially interesting about this product is the fact it is still in the construction phase, and therefore can be completely customized to your taste and needs as it is being built. Brazil Boats have several interior designers available to aid with the process, or one is welcome to bring in their own concepts or people to tailor the yacht.

New Yorker 57 The New Yorker 57 - Long-Range Power Catamaran The first New Yorker 57 long-range power catamaran is currently under construction for an American client. This new Malcolm Tennant design will have its debut at the 2004 Annapolis Powerboat Show. Features Built of composite Airex foam and fiberglass, the power-cat will have a 5280 nautical mile cruising range at 8 knots using 2.6 liters of fuel per nautical mile [0.68 us gals per mile], 3190nm at 10 knots and 1980nm at 15 knots. The vessel's maximum speed is 23 knots and cruising speed is 15 knots. This type of performance would be impossible to achieve in a monohulled vessel of similar dimensions. The New Yorker 57 - Interior Layout The New Yorker 57 features Tennant Design's latest hull form, which is currently being evaluated by the Canadian Coast Guard to ascertain if it meets their seakeeping requirements. A 'softer' knuckle and more upsweep to the knuckle at the bow are some of the changes that have been made to improve the vessel's ocean going capabilities. The double arch wing deck, sweeping up to gunwale level well back from the bow and blending into an 'anti slam' nacelle up forward, is a now familiar feature on all of Tennant's ocean capable designs. Click on the picture to see a slide show of the most recent construction pictures.

Fuel Consumption Frequently in magazines and on web sites you will find claims of greater speeds, lower fuel consumption, longer range for a particular design. Here at Malcolm Tennant Design Ltd we pride ourselves at producing fuel efficient powerboats which give high speeds on displacement hulls with as low as possible horsepower requirements. Our efficient hulls allow for longer ranges with a given amount of fuel. To make sure we are delivering the best possible performance to our customers when possible, we like to compare our hull performance data with other boats as a validation of our design ethos. When fuel consumption tests are published it allows every body to look beyond the advertising blurb and estimated performance figures and get down to what a boat is really achieving. The fuel efficient 13.6m Escape We recently had the 13.6m Escape launched here in New Zealand and had a fuel test completed on her. We managed to find two other published fuel tests for recently launched (2003) vessels of comparable designs. These other two boats have been designed by reputable designers that are competent in designing power multihulls. The other results are for a round bottomed with chine semi-displacement powercat and a hard chine planing powercat. The fuel consumption was matched to the displacements of the vessels so that the slightly heavier planing cat would not be penalized. Our vessel the Escape is a full displacement hull that is Malcolm Tennant?s signature powerboat hull form. Let's Take a Look at the Fuel Graph The results are shown in the following graph. Right across the fuel range the Escape was using less fuel than either of her competitors. Her closest rival below 19knots was the semi-displacement powercat. However , as can be seen from the graph above 10 knots the Escape is using, on average, only 65% of the fuel of the semi-displacement vessel! Even below 10knots the Escape is only burning 60% of the semi's fuel. This means more than 40% more range for the Escape at a given speed. At all speeds compared the planing cat was using more fuel than the Escape. At the planing cat?s drag hump at around 11 knots, the Escape was using only 43% of the horsepower of the other boat! Above this speed the other boat gets onto the plane and her fuel consumption begins to drop until 18 knots where it begins to rise once more. At the top speed the Escape reached with her 200Hp motors of 23 knots she is burning only 90% of the fuel of the planning cat. The planning cat then uses an additional 480HP (total) for another 6.5 knots of speed! So, unless very high speeds are required in a boat of this length, the displacement cat is superior. Gain Three Days ... The Ideal Cruising Boat This figure also shows the second advantage of the displacement cat in that the speed can be reduced to a slower cruise speed for a greatly increased range. If we look at 15 knots, for example, the Escape is only using 50% of the fuel of the planing vessel. For a given amount of fuel this equates to twice the range! To increase her range, the planing cat could increase her speed to 18knots, at this point she would still only have 55% the range of the Escape. The other option for the planing cat would be to reduce her speed to 8 knots to achieve the same range the Escape achieves at 15 knots. At 15 knots the Escape has a range of 616 nm (with 10 % reserve), and she would cover this distance in 41 hours. The planing cat would take 77hours to complete this trip. This means she would arrive 1.5 days later! On a return trip cruise you would lose three days out of your holiday just to passagemaking! This has confirmed for us that our displacement powercat hull form is the ideal cruising power boat, capable of both high top speeds and extended cruising ranges that cannot be matched by the other conventional hull forms compared here. To the owner this equates to lower fuel costs and more time spent on holiday and less time passagemaking which should keep everybody happy!