15 ' vs. 20"

[paulpost]

I can get an 15HP Yamaha brand new for a good price for my 12' inflatable but the shaft is 20". How much of a diference will make if any.
Thank you
Paul

 

[Hank49]

It shouldn't make any difference as long as you line the anticavitation plate on the motor up to be about even with the bottom of the boat. It's it's below, it'll drag... if it's above, it'll cavitate on turns.

 

[paulpost]

It shouldn't make any difference as long as you line the anti cavitation plate on the motor up to be about even with the bottom of the boat. It's it's below, it'll drag... if it's above, it'll gravitate on turns.

Excuse my lack of knowledge , this is new stuff for me, from what I've figured out you say to lift the motor up to the level that you say .
Thank you very much

 

[Hank49]

Yes, your 20 inch shaft power head will have to sit 5 inches higher than the same motor with a 15" shaft, assuming your 15 inch shaft was mounted correctly. It may not be possible if your transom isn't high enough to do this.

 

[paulpost]

OK If I dont do any change how bad it ill be?
Thank you

 

[Underwater Ogre]

It wont be bad...it will handle a bit different. A long leg is nice in big water much like a keel on a sailboat. Also in large swells it will be less likely to cavitate while you are holding in one place.
Good luck
O

 

[Hank49]

OK If I dont do any change how bad it ill be?
Thank you

My only experience on having on too low was with a 17 foot flats boat with a 115 Merc on it. It was a few inches too low and it actually lifted the boat too high out of the water to where it was dangerously out of control. I was cruising along and it started jerking back and forth after hitting the wake of another boat. I raised it up one hole and it worked fine.
Since your dealing with a 15, that won't happen but it will be pretty slow relative to if it's mounted correctly, I would bet. If you're going to go ahead with it, mount it as close as you can, which will probably mean as high as you can. Or, just shell out the extra bucks and get a 15 inch shaft. You may spend the difference in gas in a year or so if it's a real dog running too deep.

 

[paulpost]

Thanks to everybody who answered to this thread.
Regardless of the problems that I might encounter I cannot pass this deal, a brand new Yamaha 15HP for $1000 or maybe less. It is a 2000 model that has never been used.
Thanks again
Paul

 

[Hank49]

Let us know how it works.

 

[Nemrod]

A 20 inch shaft on an inflatable made for a 15 will result in a ton of drag, increased bow rise in planning, tons of spray and unstable porposing. You might be able to raise the transom by adding an extension to the transom, building it up, I have seen this done and am cosidering it for mine. I too am looking for a new motor for my Novurania Cancuro 420. I will look first for a 15 inch but if I find a 20 I will mod my transom. You do NOT want the motor buried in the water as would be the case with a 20 inch on a 15 inch transom, five inches is a bunch.

N