Prop and gear ratio help?

[RickR]

Just got back from Grouper fishing in Forrest's neck of the woods (FLA Panhandle) and I have some problems with "The Other Boat"

The "Other" is a Sportcraft 25' walkaround cuddy similar to the Donzi F-25 (5000lbs dry wo/engine)

Problem is we blew up the 200 Yamaha for the 3rd time so replaced it with a 1995 225 Saltwater rebuilt Yamaha.

Performance is similar to the 200Y but RPMS are down 700. I am assuming the 225 has a different gear ratio.

Our partner did some prop swapping and I've got a prop slipping problem.

We are running a 15" large diameter Yamaha Stainless which has a unique blade the is about half way between a round ear and a cleaver. It cavitates till 4200RPMs. When it hooks up RPMs drop 300 and speed increases 3MPH. After it hooks up it will hold down to 3700RPMs then start slipping again.

With the prop "hooked up" 33MPH@4500RPMs 36MPH@5000RPMs with 1/1.75 gear ratio that would be 17.5% slip, 1.86 12% slip

With prop cavitating 16MPH@3900RPMs (49% slip @ 1/1.86 :( )

Is the different gear ratio causing the slipping?
Can I remedy the problem with a round ear with a larger blade surface area and a smaller pitch?

Are large diameter 13" pitch props available for the Yamaha?

Anyone have one for sale?

Thanks!

[tmdog]

Rick ------- Are you sure the drive is deep enough in the water. I had a similar problem with a outboard . Ran fine in the everglades [ fresh ] and cavitated in the intercoastal [salt]. Lost a lot of speed. Bouyancy of the salt water made the stern rise, resulting in the prop running close to the surface ---- cavitation. Hope this helps. JIM "MISS THAT SALT AIR"

[BigGrizzly]

Talk to Poodle he uses a Turbo something says there great on his Y's for both his dad and him in their contenders

[Forrest]

Dad-gum-it! Rick, we got wrapped around the axle here in town doing concrete work and we missed you this time. You guys know how to catch and cook Grouper and anything else out of the Gulf for that matter, and I was sure lookin' to scarf a good seafood dinner off you and Ray! Next time it will me my turn to treat. Who knows, maybe by then, the stove will be working at my Blue Crab Lane house. I now have a good dock there now, but the SOB across the canal has petitioned the DEP tho keep me from putting in a boat lift. What a jerk! It ain't over yet.

Anyway, do you think that you might have a hook in that old Sportcraft's hull? That could do it. Also, keep the X-dimention low. Remember, it ain't a Donzi!

[RickR]

TMDOG
Same shaft length as the 200

Forrest
I'll check for hook but she did not have the cavitation problem with the last engine. Made some Etouffee with shrimp from My Way tonight YUM!! :D

Everything is the same except more HP, different prop and gear ratio.

Thanks!

[Fish boy]

Rick,
jsut a thought but this may help your cavitation problem. Click on the link and then on the "porta-brackets" loink and scroll down to the stabilizer plate. http://www.bobsmachine.com/index.htm

http://www.donzi.net/photos/jstaples44.jpg

Good luck,

Jody