Here's a solution that offers zero cavitation using an outboard
to power a catamaran
- though it has obvious drawbacks, some might find it worth considering.
Put the outboard
on the tender
, and use the tender
to push the cat.
First, my reason for choosing this arrangement. I am preparing a cat ( a Woods Sagitta ) to use as a commercial fishing
boat. Here in the UK, you don't need a commercial fishing
licence if your boat is under 10m and it doesn't have an engine
. After extensive talks with the Marine
and Coastguard Agency, they have accepted that if I use a powered tender to push the cat, they will accept that the cat itself is sail-only (this saves me several thousand pounds and avoids many administrative hoops - I considered renaming the boat 'Loophole', but not for long!).
The boat came with 2 x 9.9 HT Yamahas. I have removed them, and I am rebuilding the cockpit
to get rid of all the lockers and make room for insulated fish
boxes. (I need to save all the weight I can to allow for carrying the weight of the catch.) I am building a fishing platform (this is for line fishing, at anchor) aft of the aft beam. Under sail, I plan to carry a small rib
there, which will have a 10 hp engine
I plan to use the engine only to get in and out of harbour. Using it will require pushing the rib
off the platform and attaching it to the middle of the beam that supports the new platform and use the rib to push the cat. The rib will have a loop of rope
going from one side of its stern through blocks on the stern of the cat and back to the other side of the stern of the rib. Adjusting this rope
will cantilever the rib and help steer the cat at low speeds. Otherwise, it just holds the rib straight onto the cat. The throttle will be controlled by an extension from the outboard handle made of that dense blue water
pipe, which can reach to the cockpit
of the cat (it is light and a bit flexible).
Obviously, the engine is a bit exposed and the whole arrangement susceptible in large waves, but I hope that in such conditions I will also have wind
, and power the cat under sail. There is also the drawback of having to get the rib into the water
and start the engine which obviously takes a little longer than just pressing 'Start'. However, I have experimented a little before I had the boat lifted into the yard for the alterations with a borrowed tender, and the cat was very easy to control being pushed from the middle of the stern. The outboard was 30hp, way bigger than I plan to use, but we used very little throttle to push the boat at 5-6 knots.
If I can come up with quick and simple arrangements for deploying the rib and starting the engine, I think this arrangement will suit my situation. Anyway, as I read about cavitation problems with outboards and cats, I realised that having an articulated floating outboard bracket (the rib) sorts this problem out (obviously creating other problems!).