Designers on Design: Displacement Power Cats
Article Page : 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.

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