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Designers on Design: Displacement Power Cats, A Visit with Malcolm Tennant

52ft Malcolm Tennant designed Power Catamaran For some fifteen years now, Malcolm Tennant has been designing power boats that combine something of the old and something of the very new. To make a leap forward in comfort and economy, Malcolm looked back to the close of the 19th century and the early years of the 20th. Malcolm Tennant accepted the powerboat wisdom of that time and used it in the designing of very modern power catamarans that can have very much more living space than their monohull cousins, and that easily surpass them in comfort and economy. "The Power of Multihulls" magazine calls Malcolm Tennant one of today's foremost power catamaran designers. Let's Get Technical...Froudes Law Revisited  When we take a closer look at the science behind the design, we start first with Froudes Law. Current thinking has it that to go fast in smaller craft, it is necessary to plane. This is because the usual monohull displacement craft is restricted to a speed of approximately 1.34 times the square root of their waterline length [Froudes Law]. To drive a normal displacement vessel faster than this requires an inordinate amount of horsepower and may even lead to the vessels foundering in their bow and stern waves, or rolling their gunwales under from the enormous torque produced. Planing is a way of circumventing Froudes Law by getting the vessel to plane on top of the water where the wave making drag is no longer a restriction on their performance. However, planing craft do need to be relatively light (ie: have good power to weight ratios, and planing surface area to weight ratios) and can prove to be very inefficient when they are not planing. In fact, they are not as economical to run at some speeds as is a displacement craft. So... it seems we have two distinct types of boat: One that is fast but not economical at slower speeds and that can have a bone jarring ride in a seaway; the other economical and comfortable in a seaway but is slow. Let's take a closer look at the design challenge: Can we combine the best features in both? Froudes Law *WATER LINE LENGTH determines maximum speed in displacement mode. Generally called Froudes Law, for non-planing, non hydrofoil boats there is a relationship between maximum speed and the square root of the water line length. Not an absolute limit, Froudes law is usually used to define a "hull speed" beyond which each, say, 10% increase in speed will require the propulsive power to double. For long narrow hulls, those with length/width ratio of 15 or more, the Froude barrier becomes softer, and it is possible to push thru to the so called "forced displacement" mode, but at significant drag price. A typical example of the use of Froude’s law would be to calculate, for the same power, how much faster 7m hulls are than 4m hulls. The answer is 32%. (disregarding skin friction which for small boats isn’t usually a big factor by the speeds at which the Froudes limit is being pushed anyway).