does shaft length make a perform..diff?

[b.gagnon]

I have a 79 19ft manatee v-hull boat with a 88 merc 115 with a 20in shaft ..... the motor is no good ..I found a 88 merc black max 135hp for a real good price but it has a 25in shaft....the boat is rated for 145hp<br />the boat with the 115 has a good top speed but takes for ever to get out of the hole, even when new!!! I guess my question is, can i put a motor with a 25 in shaft on it ..<br />the boat is used in the ocean so debth is not much of a prob......

 

[JB]

Re: does shaft length make a perform..diff?

Howdy Gagnon. It is more likely that you need the proper prop for your boat, rather than a different engine with a too-long shaft.<br /><br />If the anti-cavitation plate on your present engine is about even with, and parallel to the boat bottom at zero trim it is the correct length. A 25" motor would turn your boat into a garbage barge.<br /><br />That Merc 115, properly tuned, should get from your boat about 90% of the performance you would get from a 135 (assuming both had optimum props).<br /><br />You need a prop that will allow your 115 to reach the high end of it's operating range with a light load and at least the low end of the operating range with a full load. I don't know the range of your engine, but most engines in that power range operate best at 4500 to 5500rpm, 6000rpm at most. What rpm are you getting at top speed?<br /><br />If you are unhappy with your hole shot you probably need to come down a couple of inches in your prop pitch. That might cost you a small bit on top end.<br /><br />Good luck.<br />JB

 

[b.gagnon]

Re: does shaft length make a perform..diff?

the 115 now has a rod hanging through a hole in the watter jacket!!! would a 25in shaft creat any probs.....

 

[JB]

Re: does shaft length make a perform..diff?

I guess I misunderstood what you meant by "no good".<br /><br />I think you would be very unhappy with the handling of your boat with a 25" shaft engine unless you jack-plated it up 5".<br /><br />Good luck.<br />JB

 

[b.gagnon]

Re: does shaft length make a perform..diff?

If i got a jack plate would the black max's extra hp make a noticeable performance diff.<br />or am i better off just getting another 115?<br />thank you for all your help!!!!!!

 

[savage]

Re: does shaft length make a perform..diff?

You might be able to put the 135 power head on your 115 center section/lower unit. This maybe worth your while depending on what you are paying for the 135. Check with a dealer or somebody that rebuilds power heads.

 

[12Footer]

Re: does shaft length make a perform..diff?

Yup...That's what I call "no good"!<br />sorry bout the bad luck.

<br />Got room for a jack plate?<br /><br /><br />------------------<br />Time to change my tagline

 

[Dhadley]

Re: does shaft length make a perform..diff?

Jack plates are like Jello -- theres always room. I've put a jack plate on a jack plate. Switch the p/h if you can. If not, the jack plate is a very simple answer.<br />Good luck!

 

[Dhadley]

Re: does shaft length make a perform..diff?

PS -- Take JB's advice about the prop. That's the key.