1989 300 Sedan Bridge won't plane

[DosEquis]

I would like some feedback regarding my Sedan Bridge. I have rebuilt the heads and carburetors (quadrajets), new plugs, on both engines (mercruiser 5.7 260hp) and installed a closed cooling system. Timed and adjusted to spec, the engines sound and run great and you can feel them pull hard.
The boat has half fuel and two persons onboard, with no crazy overloads anywhere (dozen beer in the fridge). The trim tabs are working and they were trimmed bow down. The engines wouldn't go over 3500 rpm as she was plowing trying to get on plane.
I took pictures of the props when on the hard, but can't find them so I am unsure if the previous owner installed something other than recommended by Sea Ray (16x15p). I'll get the diver to go and read what they are, as I'm in Mexico and I'm not going in to this marina water. This was the second time I have taken the boat out since buying it, as I have been working on it (waiting for parts), and busy with business.

My question is; what props do other same boat operators have installed, for their best performance?
What rpm should I expect at 10 knots attempting to plane with this boat?

 

[SloBurn]

1st ?, how old is the gasoline? I've had bad gas (water) and had the same symptoms. I assume this problem is from first splash of this year?

 

[DosEquis]

My boat is always in the water. I put in $500 worth of Mexican fuel last October and some conditioner for preserving the fuel. There was about a quarter tank (each) before I added the fuel. This was American fuel from when, I don't know. You have a point and each tank is half full now. I can either burn it off at 6 knots or pump it out and burn it in my Ford.
I'll check the props this week and then all that is left if they are correct is the fuel.

 

[SCORPIO]

Is your bottom clean?

 

[DosEquis]

It was cleaned about a month ago. I expect the diver this afternoon to check the props and the bottom. I also know that the last time the zincs were changed on the trim tabs, they installed a puck on each side of the trim tab instead of just the top of the tab. I will get him to change that also.

 

[DosEquis]

The diver is down and he says both props are 16x15 which is according to Sea Ray the correct props for this boat. He is cleaning the bottom and replacing the trim tab zincs. I'll take it out tomorrow and if it doesn't perform all that is left is fuel.

 

[SCORPIO]

Has the boat ever planed properly since you've had it? I think I'd be doing compression checks and full tune ups with timing checks.

 

[DosEquis]

Mega (diver) said there was barnacles on the props and thru-hulls, and this was just two months since it was cleaned last. I'm thinking my bottom paint which is less than a year old wasn't such great bottom paint.
Both props are 16x15 which is according Sea Ray recommendations. I also had new zincs installed on the top of the trim tabs only. He also found the stainless bracket for my Lowrance sonar has corroded away after just 8 months, and the transducer was just hanging there.
Tomorrow I'll see if I have time to take her for a spin and check performance.

 

[DosEquis]

Has the boat ever planed properly since you've had it? I think I'd be doing compression checks and full tune ups with timing checks.

I've only had the boat out three times since I've owned it, and always working out an issue of some kind or another. I have never got it on plane since I have owned it, but last October I took it for a slow cruise and after I discovered the port engine had two frozen valves, and the stb engine a head gasket leak. So all the heads came off for overhaul and it just got back together a week ago.

 

[DosEquis]

The diver said it two months ago that he cleaned the bottom, not a month ago. I guess I have lost track of time here in Mexico, I should buy a calendar and maybe start wearing a watch again.
A full tuneup and timing was done when the heads were rebuilt. Haven't done a compression check.

 

[sandydlc]

I have a friend with a gasoline engine Carver who is having the same problem and they can't figure it out. The boat went to the mechanic for maintenance/tune-up and since they got it back - it won't plane. I'm very interested to hear the cause of your planing issue.


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[DosEquis]

Took the boat out on the bay this morning. Temps about 80 air and 75 sea at this time. The boat got on to plane and pulled 20-21 knots max. So I figure the last negative could be still some old fuel in the tank.
When I return from Canada in July I'll burn up what's in the tanks and then refill with fresh. She puttered along at 7-8 knots at 1500rpm which is decent

 

[DosEquis]

So, the dirty bottom and props were responsible for 10 knots. I don't know if removing the zinc from the bottom of the trim tab made much difference, but it all added up. The stb engine still pulls about 200rpm higher than the port engine, but they are on different tanks. I'm thinking I might install a cross flow fuel system. A friend was checking the exhaust while under power and he said they were both clean.