Designers on Design: Displacement Power Cats
Article Page : The Challenge

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 ...