catamarans & nacelle pod propulsion

I recently launched a new 'wharram type' cat. It's powered by an outboard mounted in a central nacelle pod, about 2/3's of the way back from the bows. Even though the outboard cavitation plate is 20 cm below the static waterline, the propeller still sucks air and cavitates when running at speed.

I'm considering a prop guard/kort nozzle kind of attachment around the prop to contain the thrust, but questioning whether it'll do any good.

Any insight you may have would be greatly appreciated

 

I've seen those nozzles on pod mounted outboards, as well as the hydrofoil fins you see bolted to the cavitation plates. Open the pdf to see nozzles on a cat.

The hydrofoils are cheap, and the thought is that the keep the water down on the prop. The nozzles improve thrust, which a cat needs more than those tiny little high speed propellers usually provide.

Do you have a structure of some kind the motor is attached to and that hinges? I have seen outboards mounted on vertical sliders where the motor just sticks down in the water have aeration problems that might improve with a structure in front to provide a kind of false transom.

 

nacelle pod propulsion

Keysdisease,

Thank you for your reply. My pod is a solid bow shaped structure with only 2 - 3" of static draft w/ non-movable engine. It's looking more and more that it's mounted too far forward and running in a trough,,, ugh!

A couple of people have said that the nozzle idea worked for them; certainly would be the easiest thing to try

 

I saw this on a Wharram forum of all places

http://wharrambuilders.ning.com/forum/topics/kort-nozzle

Here in the States the hydrofoils are dirt cheap, can't say I've seen many nozzles. Some of the flats boat guys claim they can jack their engines higher running in shallow water without cavitating with the hydrofoil.

Can't you just drop the sled a little deeper? Should not be an issue with a long shaft o/b

 

If it was me, I'd first swap to a stainless prop. They hold on better than alloy when starved for water. Better still, a motor that swings a bigger diameter prop with high gear reduction, for a further increment of improvement. If all that still leaves you with an issue, you will have to lower the engine height, preferable with a fairing/nacelle ahead of it.