1988 mercury 70 ?'s

[ebtate]

I have just purchased a 1988 15' astroglass with a 70 hp mercury that has only about 50 hours on it. I am wondering how many rpms I should be taching at wide open throttle and approximatley what kind of speed I should be looking at? The prop is 12.75x21. What can I do to get optimum performance out of this set-up? I am also getting quite a bit of spray between the transom and motor, any reason why? I appreciate any comments or suggestions.

 

[S.T. Marine]

Re: 1988 mercury 70 ?'s

Get the right pitch prop for the boat. Also make sure its trimmed to the right setting or you will keep getting that spray

 

[ebtate]

Re: 1988 mercury 70 ?'s

Thanks for the quick response S.T.. Does that pitch seem to be wrong? Is it possible that when the motor is trimmed completely down that it is too much even for coming out of the hole?

 

[DJ]

Re: 1988 mercury 70 ?'s

A 21" pitch prop. seems a bit steep for that engine. It's only 70 hp. and a relatively small displacement also.<br /><br />I believe that engine should turn at least 5500 at WOT.<br /><br />Spray. Perhaps the engine is set a bit too low on the transom. The cavitation plate should be even or slightly (about 1/2-1") above the keel of the boat. You may want to move it up one notch and see if that helps with the spray issue.

 

[ebtate]

Re: 1988 mercury 70 ?'s

Thanks djohn, W.O.T. is about 4200 rpms with this prop. I have not tried another, that is the one that came with the boat and motor. Would a stainless steel prop help performance matters? I tried a Stingray hydrofoil but it killed top end speed. Though it did stabilize the boat and got it up on plane much faster.