Possible cavitation issues?

[Drewfus]

Hello all,
Posting this question for a bud of mine in this section but I'm sure it could pertain to any year model.
He has a '78 24 ft Sedan Cruiser.
New factory built 260 Merc (350 Chevy).
15 pitch Alum prop (book recommended I believe).
3400 RPM @ 22 MPH @ WOT. Impeller replaced at rebuild.
Fresh water lake, in water slip kept.
Outdrive trimmed all down, trim tabs "level ", full of gas, 4 adults. Plane attitude is good.
Alright, so at WOT, the engine gets pretty darn hot after just a few minutes, and he has to back down to either high idle or slow speed to get the temp gage to come back down. At first he thought it might be algae growing in the outdrive cooling water intake, but it would seem counter-intuitive to have to slow down to cool down. I would think more pressure would mean better cooling. I did stick an ear over the transom and it sounds to me like the roaring of cavitation is what I'm hearing...
Personally, I think he should get more MPH and RPM's, so maybe a different prop, but that sounds like a different issue. Main thing is I don't want to see a overheated new engine. Any thoughts on what would cause excessive cavitation, or would a fix be one of the "whale tail" plates? Different prop? Thanks for any input! :thumbsup:
Drewfus

 

[ghuskin]

i assume you have an Alpha1 outdrive?

the onlythings i can think of are: 1) water not washing over the outdrive properly while on plane, or 2) blockage in your cooling system somewhere (maybe the manifolds?).

I'm not sure about the cause of cavitation, or if "whale tail" plates would help your situation.

Good luck.

Gregg

 

[Lazy Daze]

What temp is "overheating"? Bad T-stat? Bad circ pump? When the impeller was changed, was the condition of the wear plate/housing checked?

Can he run it on muffs (on land) and check for good exhaust water flow exiting?

WOT should be done with a very light load in the boat. The tabs should be all the way up. Drive trim needs to be UP - to the point just before you start catching some air. With a 15" pitch on there now, I doubt you should have any problems getting to 4400-4800 rpms.

 

[Sea Gull]

I'm no gas engine expert but 3400 RPM WOT seems very low for that engine. We usually see 4600-5000 as the top range. If so, the boat is severly overpropped which could lead to the overheating. I am sure those with more experience with gas i/o will be more helpful.

 

[Tail Chaser 661]

Numbers look good,
Except the WOT being only 3400rpm with a 15"pitch prop. Typo Error?
I'm overproped with a 17" P. My WOT was 4000 rpm at 28 kts.

Running the engine WOT will generate more heat, Then when you slow down.
Check all your water hoses from the transom plate up thru the power steering cooler. Water Pump, exhaust hoses etc.
Maybe there's a partially clogged hose or cooler somewhere.

If your still using the original riser/exhaust manifold.
Check them for carbon blockage and water jackets being clogged also
There also a flapper in the exhaust Y pipe.?
Check that its not stuck closed.
You can normally hear them clacking while at idle.

 

[Drewfus]

Thanks for all the input.
I'll pass all this along to my bud and see what he says. Might be a day or so but I'll definitely post back with his answers. Thanks again.
~Drewfus

 

[brains06]

Numbers look good,
Except the WOT being only 3400rpm with a 15"pitch prop. Typo Error?
I'm overproped with a 17" P. My WOT was 4000 rpm at 28 kts.

Running the engine WOT will generate more heat, Then when you slow down.
Check all your water hoses from the transom plate up thru the power steering cooler. Water Pump, exhaust hoses etc.
Maybe there's a partially clogged hose or cooler somewhere.

If your still using the original riser/exhaust manifold.
Check them for carbon blockage and water jackets being clogged also
There also a flapper in the exhaust Y pipe.?
Check that its not stuck closed.
You can normally hear them clacking while at idle.

I second that. That flapper can be a b%&*! Also check to make sure your circ pump isnt weeping.