Engine RPM sharp increase, when at top speed

[all yeah boy]

I have restored an 1972 Crestliner with an outboard 1973 Merc 65hP. The boat has a hydraulic trim that works ok, but I am having trouble getting the motor to stay at top speed. When I hit top speed regardless if the motor is trimed up the prop seems to leave the water and the RPMs go through the roof. The motor is still sucking water for the cooling system through. I am not sure that prop is leaveing the water but that is my guess, it seems like when this happens the prop has zero resistance. The position of the motor is not changing when this happens thats why i guess the prop is leaving the water. Also the boat doesen't like to plane to well so it is usually at a steep angle

Any ideas?? Is my motor to low or high, when I repaired the transom i put it back in it's original position. Any help would be great!!!

 

[j_martin]

Re: Engine RPM sharp increase, when at top speed

Your prop hub could be slipping, or you could be cavitating if it's too high.

A prop in poor condition could cavitate even if submerged.

hope it helps
John

 

[jimmbo]

Re: Engine RPM sharp increase, when at top speed

Sounds like the prop is ventilating. Where, in relation the hull bottom is the anti-ventilation plate? Above. below, flush? As said earlier, a prop in poor condition can lose its bite. A cupped prop will benifical here.

 

[all yeah boy]

Re: Engine RPM sharp increase, when at top speed

Thanks for the help, after roaming around the forums and various websites I have a feeling that the prop is ventalating. I am not going to be around my boat until thrusday and the I will look to see where vertically the ventelation plate sits. What is the best case for the plate, even with bottom of hull, 1 in, 2 in, 3 in below?

Also will trimming the boat while under way help this problem? It seems like trimming the motor will make it worse.


I rebuilt the transom myself and know that the motor is as low as it will go. The motor sits at the same place vertically as it came from the factory. The prop is a bit dinged up, but the boat came with a second prop that looks in better shape so I will swap it out and try again. I operate the boat at around 5,300ft above sea level could this play into it?

 

[jimmbo]

Re: Engine RPM sharp increase, when at top speed

On a fast(over 45mph) boat, having the plate above bottom of the hull will reduce drag improve handling but will reduce the ability to hold the bow out of the water at speed. On boats boat that can't go that fast even with the bottom of the hull is usually the best. On very heavy or non-planing hulls, putting the plate an inch or two below the hull insures solid water.
Trimmiing of the boat or engine can make a difference. Tucking the engine in close to the transom usually reduces venting and allows the prop to regrip the water.
Does your boat have prominent keel on it? Older glass boat often did and many aluminum ones still do. If it does, does it extend right to the trnsom or has it tapered into the hull 2 to 3 feet ahead of the transom?
Make sure you prop your engine to run at 5300 rpm to 5500 rpm at WOT. At 5300 feet above sea level it will make a lot less hp than 65hp even when correctly proped.