Conflicting Answers to Shaft Length Questions.

[AKCoastie]

Thanks ahead of time to any help you can provide. I am looking to upgrade to a higher HP motor for my boat, 70 to 115. I have found an engine for a good price (hard to do in Juneau, AK), the only problem is that it is a 25" shaft and my transom is 21". I have been looking all over the internet looking for an answer on whether or not this engine would still be ok, with no luck. I have heard anything from "It will work fine", to "it will work but will add a bit of drag and reduce overall performance", to "DEAR GOD! Don't do that, it's a death trap to add a few inchedto your shaft". So what's everyones experience with this? Will I be ok switching to a longer shaft or am I going to blow my boat up?

 

[tmcalavy]

Re: Conflicting Answers to Shaft Length Questions.

The rule of thumb is the cavitation plate should be even with, or within 1-2 inches of the bottom of the boat, up or down...down is better. What size shaft did your last/old motor have? Did it perform okay? If so, find one that length. If the deal is a really good deal, get it and try it on your boat. If you don't like the performance you can raise the motor with a ready-made-store-bought or homemade jack plate.

 

[tashasdaddy]

Re: Conflicting Answers to Shaft Length Questions.

you will need a jack plate to raise the motor to the correct height.

http://www.iboats.com/Marine_Store/...00623--submit.Search--submit.x.33--submit.y.5

 

[JB]

Re: Conflicting Answers to Shaft Length Questions.

It would work, but not well. Boat handling would be difficult and excess drag would use up a lot of fuel.

Use a jackplate to get the AV plate 1-2", PLUS 1" for each inch of setback the jackplate produces, above the keel. For a jackplate with 6" of setback that would be roughly 7" above the keel. Adjust the jackplate lift so that no prop ventilation (blowout) occurs when on plane and turning.

I am assuming that your boat is rated for at least 115HP. If not, forget it and get another 20" shaft engine within the boats rating.

 

[AKCoastie]

Re: Conflicting Answers to Shaft Length Questions.

I have actually found a 100hp with the right sized shaft that I think I am going to go with. I know this is very vague question with alot of variables, but, as a rough estimate, how much of a speed increase should I be expecting in switching from 70hp to 100hp? It is a 17ft Fiberform power boat.

 

[tashasdaddy]

Re: Conflicting Answers to Shaft Length Questions.

you are going up 42 % in hp, but don't expect a 42 % increase in speed.