Prop ? for my 1978 Merc 115

[trihuller]

I have a 16ft. Larson 1600 Shark. its a tri hull. Currently I have a 12.75 X 21 Aluminum prop. The engine is a 1978 Mercury 115 outboard. 2:1 gears. At WOT I am just barely getting to 5000 RPM's. This is a rebuilt motor with about only 2 hours on it and I am running 25:1 fuel right now. Just wondering if I should make any changes right now or if I should wait til the engine is broken in. What changes do you guys recommend. I was thinking of dropping all the way down to a 13 X 17 stainless prop or a 13 X 19 aluminum. Thanks!

 

[Silvertip]

Re: Prop ? for my 1978 Merc 115

You would be wise to wait until the engine is broken in. Once it loosens up a little it may rev freer. Before you start buying props make sure the AV plate is at least even with or one inch above the keel. If its below the keel you have too much engine in the water. Less engine creates less drag, increases RPM and speed. Engine height is the first adjustment to make. Then, if the engine is performing properly you can begin looking at props.

 

[Dhadley]

Re: Prop ? for my 1978 Merc 115

A 21 may not be all that far off. Lets look at a couple of things. This should be the in-line 6 cyl, right?

 

[trihuller]

Re: Prop ? for my 1978 Merc 115

The AV plate is just the slightest above the keel. It is an inline 6 1978 Merc 115.

 

[trihuller]

Re: Prop ? for my 1978 Merc 115

Just a quick note. at about 4900 RPM's I was showing 40 mph on the GPS at WOT.

 

[trihuller]

Re: Prop ? for my 1978 Merc 115

 

[trihuller]

Re: Prop ? for my 1978 Merc 115

http://i38.photobucket.com/albums/e109/trihuller/1150Merc217.jpg <br /><br />Here is a link to a pic

 

[trihuller]

Re: Prop ? for my 1978 Merc 115

http://i38.photobucket.com/albums/e109/trihuller/1150Merc451.jpg <br /><br />Additional picture of the whole boat.

 

[Dhadley]

Re: Prop ? for my 1978 Merc 115

OK, that motor was produced with high compression (high dome) pistons. Do you know if it was rebuilt with the high dome or the later low dome pistons?<br /><br />Has the timing been set back for break-in?

 

[trihuller]

Re: Prop ? for my 1978 Merc 115

Yes the same pistons were used. The high dome. Advance timing has been set for 21 BTDC degrees and also at 4 BTDC. It was rebuilt by Clams Canino.

 

[trihuller]

Re: Prop ? for my 1978 Merc 115

Anyone have a prop suggestion after all the data provided?

 

[Dhadley]

Re: Prop ? for my 1978 Merc 115

What's his recommendation on rpm? Did the motor ever turn more rpm than 5000?

 

[trihuller]

Re: Prop ? for my 1978 Merc 115

? The maximum RPM rating is 5500 as stated in the manual...

 

[Dhadley]

Re: Prop ? for my 1978 Merc 115

Did it ever turn 5500?

 

[trihuller]

Re: Prop ? for my 1978 Merc 115

I do not know. Never had a tach on the boat before rebuild and bought it used.