Shaft length on auxiliary engines

[jgrace]

Anyone have an opinion on this one?

The online Yamaha manual says shaft length is determined by where the cavitation plate is relative to the bottom of the boat. This is a screen capture from the manual.



OTOH my Yamaha dealer tells me an auxiliary should have the same shaft length as the main engine.

So which is best, option A with the cavitation plate in the correct position relative to the hull or option B with the prop at the same depth as the main prop?

[Jimbob]

B

[Jono]

C......















What was the question again?

[Lawrence]

Originally Posted by Jimbob

B

Why?

I am looking to get a larger auxiliary for my Ribcraft 5.3, probably a Suzuki/Johnson 6hp and will have to decide on standard or longshaft.
I am thinking of getting the longshaft but just based on the vague idea that the current Evinrude 3hp doesn't quite look deep enough in the water.

In position A the cavitation plate is below the hull where its mounted, does it need to be below the bottom of the boat?

[swginn]

Originally Posted by jgrace

OTOH my Yamaha dealer tells me an auxiliary should have the same shaft length as the main engine.

I have option A, for the simple reason that my main engine has an XL shaft and is mounted in a cut-out section of the transom, and there was no auxillary with a shaft long enough to be the same length as my main engine.

At displacement speeds I really can't see this making any difference TBH!! Ours pushes us along nicely at around 6-7kts. One advantage of having engine mounted as in option A is that it reduces your draft so you can use your aux to get you into shallow places your main engine can't.

Also you probably won't be able to use a long shaft on a tender, which can be quite handy if there's a gale blowing!!

[Pete7]

Originally Posted by Lawrence

Why?

Because if you have to use it in anger the waves will lift and roll the boat in a choppy seaway and an aux prop that is close to the surface will lift of out the water loosing grip. 4hp short shaft Yam just didn't do it in a chop, 5hp long shaft Merc, no problem.

Pete

[jgrace]

Originally Posted by Pete7

Because if you have to use it in anger the waves will lift and roll the boat in a choppy seaway and an aux prop that is close to the surface will lift of out the water loosing grip. 4hp short shaft Yam just didn't do it in a chop, 5hp long shaft Merc, no problem.

Pete

Well, I imagined that planing with the main engine and displacement with the aux would give different waterlines at the transom, so the auxiliary prop on a short shaft could be just as far below the surface when it's operating as the main prop is when IT'S operating. However, I won't argue with experience: Long shaft it's going to be!

One thing that worries me about the long shaft is whether the extra leverage it's going to exert on the bracket when it's in the up position (i.e. 99.9% of the time) is going to cause it to damage itself over time. And no I can't store the aux anywhere except on the transom, there's just no room on the 490.

[jgrace]

Originally Posted by swginn

I have option A, for the simple reason that my main engine has an XL shaft and is mounted in a cut-out section of the transom, and there was no auxillary with a shaft long enough to be the same length as my main engine.

But it's B really, because it's as long as shaft as you could get!

Nice neat transom arrangement btw. Mine's going to look a lot messier, I've got triangles of fabric going aft from the transom on either side of the main engine which act as splashguards. Not wanting to take 'em off, I'm currently designing a pivoted up'n'over bracket to swing the aux clear of them.

[Lawrence]

Originally Posted by Pete7

Because if you have to use it in anger the waves will lift and roll the boat in a choppy seaway and an aux prop that is close to the surface will lift of out the water loosing grip. 4hp short shaft Yam just didn't do it in a chop, 5hp long shaft Merc, no problem.

Pete

Thanks Pete, long shaft it will be then.