[PCW] Drive options for displacement powercats

[Grahame Shannon]

The Volvo IPS drive units are bronze not aluminum, so corrosion should not
be an issue. They do not trim, only rotate, but the clever control system
means they can drive the boat in any direction, no thrusters are needed.
They only suit fast boats at the moment as the smallest units are in the 300
HP range, and they only come as twins.

The propulsive efficiency is about the best you can get, due to the forward
facing props, as in most prop driven aircraft. But they are very vulnerable
to damage from grounding or debris. Volvo says they are designed to break
away in such an event, without leaving a hole in the boat. All very well,
but how costly it must be to replace that beautifully machined lump of
bronze!

My own choice is a true inboard with an enclosed shaft in front of a rudder
which is hung top and bottom. In other an arrangement similar to what most
single screw trawlers have. It can be applied to any type of boat if
considered at the design stage. It should be able to take the ground without
damage.

Kind Regards,
Grahame Shannon
2101 Philip Avenue
North Vancouver, BC V7P 2W5