18’ Seagull Catamaran cavitation

[joncbro]

18’ Seagull Catamaran 50hp Yamaha 4 stroke. All my props are 12” and aluminum. 3 blade 9 pitch horribly hole shot lots of cavitation top speed 21mph. 4 blade 9 pitch a little bit better hole shot 19mph. I was told I need an 8 pitch 3 blade? The same as a pontoon boat? I was also told I need a whale tail? I have an se 300 hydrofoil on it way and a 3 blade 8 pitch prop coming. Am I on track or just buying a bridged from salesmen? Next topic! Lol motor hight for single outboard catamaran? I understand monohulls and the cavitation plate but a cat I don’t know where to start at? I have water coming up the sides of the motor shaft hitting the sides of the cal cover. I have to trim the motor as high as it goes. Outboard is atlowest hole setting. I’m guessing it’s to low? Alright cat guys give me information?????? I do plan to upgrade motor this winter. I was thinking 115hp or 90? Yes 4 stroke! Love the gas savers and a stainless prop once I figure out what pitch?

 

[ahicks]

Interesting project!

Not knowing any better, I'd be tempted to start with the cavitation plate on the same line you would get if connecting the bottom of the 2 hulls. Then go from there.

Is there a pod or something hanging down just forward of the motor? A pic of your install from the right view might help.

When it comes to cavitation, I can see where a cat might struggle, and for that reason I'd be tempted to try a 4 blade prop - for it's increased surface area/bite.

Seems to me like all else being equal, going with less pitch is going to make your cavitation problem worse. You would want to focus on as large a diameter prop as possible, and prop blade design. Cupped for instance, often comes out of the hole with reduced cavitation.

I hate "hydrofoils".

 

[joncbro]

Thank you