Designers on Design: Displacement Power Cats
Article Page : Proof of the Theory

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.