Fast Electric Model Catamarans

By Paul Williams.
Article posted 17th January 2009.

Catamarans

A catamaran is essentially a monohull sawn in half lengthways and the two halves joined back together with a bridging section. The two hulls, or sponsons, and the bridge form a tunnel, hence cats are often refered to as "tunnel hulls" or "tunnel boats".

Section through a typical deep catamaran.
Section through a typical catamaran.

The purpose of the tunnel is twofold. Firstly, the tunnel spaces the planing surfaces apart creating a wide, stable platform that does not exhibit the multitude of handling quirks that plague monohulls. Secondly, and this is where it gets interesting, the tunnel traps air under the hull and supports part of the boat's weight on a cushion of air, unlike a monohull which must support the entire weight of the boat on its planing surfaces.

Wetted area of conventional (left) and stepped catamarans.

Catamarans, especially those moulded in fibreglass or epoxy-glass, usually have sponsons with stepped planing surfaces to reduce drag. Steps also act as trimming devices which force the boat to ride level, effectively turning the cat into a four-point hydroplane.

Scale catamaran running at speed.
Scale offshore catamaran running at speed.

Monohulls have varying degrees of deadrise angle, anything from none at all in the case of the "Crackerbox", to as much as 27 degrees for a deep vee offshore racer. Cats however, often have very little or no deadrise angle at all.

Tunnel Width

The width of the tunnel on a cat is a compromise. A wide tunnel increases stability, but it also increase the aerodynamic lift the boat generates. A full size offshore racing cat is often long and narrow, to reduce the lift created and make the boat less likely to blow over in rough water. An offshore racing course may be from point-to-point, with little or no cornering. If the course does have turns, an offshore racer must slow right down to corner - it cannot turn at high speed without risking an accident. A formula tunnel boat however will be much wider in comparison to an offshore boat as it will be turning every few seconds or so on an oval course and is required to turn at high speed.

Small catamaran rounding a turn bouy. This boat is powered by 2S lipos and a Kontronik motor.

Model racing cats are often a hybrid of formula tunnel boats and offshore cats. In effect, a model racer will often be nearer to a tunnel boat but with the outward appearance of an offshore racer. Truly scale offshore boats would be far too narrow to successfully race on an oval course. However, there are now a number of very large, high powered electric scale offshore cats running in the USA and Europe, with setups using ten-series (10S) lipos and hulls in excess of two metres in length. They are fantastically fast in a straight line, but must slow right down to corner.

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Last modified: 08th July 2010 @ 09:05