WOT Thrust line problem?

[bdwernychuk]

By the time I get this boat/engine sorted out, the season will be over ...

At full throttle with a light load in the boat the bow of my 17.5 Campion Cuddy sits too low in the water, way lower than best trim angle for max speed, limiting my top speed to about 27 kts. and my max rpm to 4800. This with the engine trimmed full up.

(With a heavy load aft, I can get get 31+ kts, more like 5000-5100 rpm max.)

Is there any adjustment I can make to the way my '85 90hp is mounted to bring the bow up a bit at full throttle?

Thanks,
BDD

[Ken]

BDD,
Maybe drop the engine down one notch on the transom mount?
Maybe shift some weight to aft - say move starting battery as far aft as possible?
I will refer you to a prop expert on another forum - go to www.maverickboats.com and look up Marcus, aka "Funky Monkey" - he is the Power Tech prop rep in Louisiana. Email him about your problem - or call him. I have read that different props have better "bow lift" than others. Maybe a fix to your problem.
Good luck,
Ken K

[bdwernychuk]

Thanks, Ken.

I've got both batteries aft, plus a porta-potty with a full water tank. Not much else I can move. The boat's out of the water right now so I'll try moving the engine down.
BDD

[Hamfisted]

What prop are you using now? Sounds like the boat is just underpowered and can't raise the bow. You can try a lower pitch prop but I don't think you're gonna get much out of it. The boat oughta have a 150 on it.


Mike

[bdwernychuk]

Current prop is a 17", giving me about 31-32 kts at WOT, 5100 or so max RPM. This boat's got a deep V hull all the way aft so it's no rocket. Sure takes a rough sea well, though. I've been considering a 19" for lower fuel consumption but I'm starting to think that's not a good idea.

BDD

[Ken]

BDD,
You definitely do not want to go to a 19" prop if you currently have a 17" and are getting 5100 rpm at WOT.
The max rpm range for your Yammie is 4500-5500 and you are there already - 5100 is good.
I tend to agree with Mike in that you are probably a little under-powered.
Get in touch with Marcus like I suggested - he may be able to get a little more bow lift out of that 90 hp with a different style prop.
Good luck,
Ken K