Cavitation when trim is not fully down

[Rico23]

I have a 1999 bayliner with 3.0L engine which I have owned for a month or so. The prior owner told me he could get it up to around 45-50 MPH. I have not been above 35 MPH at WOT. This is with the trim all the way down. When I start to put the trim up, the boat starts to cavitate and the prop loses its grip on the water and I have hear then engine rev up higher and lose power. The water is not usually glass, but not real high waves either. What would cause this issue and why would it cavitate with the trip up just a little? I think if I can get the trip up and keep the prop's grip on the water, I might be able to get around 45MPH, but not close yet.

 

[Don S]

Re: Cavitation when trim is not fully down

You aren't going by what the trim gauge says are you?

 

[Oshkosh1]

Re: Cavitation when trim is not fully down

What size/pitch prop? What size boat? Is the prop in good condition?

Is the drive truly "all the way down"?...as in to the mechanical limit AND you can hear the pump change pitch...NOT relying on the trim gauge?

It COULD be that you think is full down is actually only at the trim limit...

 

[Bondo]

Re: Cavitation when trim is not fully down

What size/pitch prop? What size boat? Is the prop in good condition?

Is the drive truly "all the way down"?...as in to the mechanical limit AND you can hear the pump change pitch...NOT relying on the trim gauge?

It COULD be that you think is full down is actually only at the trim limit...

Ayuh,.... Sounds to me to be a Prop problem,...

 

[steelespike]

Re: Cavitation when trim is not fully down

Unless his speedometer is way off he out and out lied to you. Depending on the size of the boat and the load
3.0 speeds are about 40.
Is your prop in excellent condition? It is very likely the culprit in the ventilation but be sure about your trim setting and don't trust the gauge. Make some lightly loaded test runs after you read the stickys at the top of the prop forum.
Your prop size should be near the base of the blades or under the prop nut.

 

[emoney]

Re: Cavitation when trim is not fully down

Steelespike there has more than likely nailed it. Odds are, though, he wasn't intentionally lying to just you, but to himself too. I always hear these "I'm getting 50mph with 2 people" stories, but then you hit the water side by side with them and 35mph is about all she's got. May have been a situation where ONE TIME, going with a fast current, and the wind on his stern and NO people or gear on board, he hit over 40 and just "rounded up". Not sure why everyone is so concerned with top-end speed on a boat. What's the hurry anyway?

Having said all of that, make sure you've not got water-logged foam & wood in your boat and it's transom too. Little rot here and there in the original factory setup and the next thing you know you've added several hundred pounds to your boat. There's a "sweet-spot" in every boat's trim set-up and RPM range. I'd find that, memorize the feel and sound, and throw the speedometer overboard.

 

[Rico23]

Re: Cavitation when trim is not fully down

OK, I did a bit more research. First, I checked over the boat and did not see any water issues with the foam or wood. I thought I have had the trim all the way down, but I guess I will have to double-check this weekend. Given the feedback, I will make sure that I am not trusting the trim guage. The propellar is in good condition; not perfect, but no big dings.....The serial number on the prop is 48-832832-c1-21. From my reasearch the prop is 14-1/4 x 21P. I am not concerned of being able to do 45MPH, but just want to make sure the boat is running good.

 

[BonairII]

Re: Cavitation when trim is not fully down

Put your prop, gear ratio, and rpm, and prop slip(use .10 as general rule) in the prop calc and you'll get theoretical speed for your rig. You'll know what ballpark speed you should be getting.
http://www.rbbi.com/folders/prop/propcalc.htm

 

[Rico23]

Re: Cavitation when trim is not fully down

Well, I got a chance to run the boat last weekend. You guys were right in that the trim was not all the way down. I previously put the prop well below the bottom line on the trim, but it wasn't until I got to hear it go down did I realize that it had some more room. This did improve my top speed to around 44 MPH. Much better.

I guess I now know not to trust the guage, so hoepfully my post will help others with the same issue.