15hp long versus short shaft

I wanted to see what you folks think. I have a long shaft 15hp on a 15" transom. When I apply full throttle, I can actually see and feel the transom bow. I have seen posts recommending a shim, but it seems the higher I raise the motor, the more leverage it will have over the transom and bow even more. I thought about making a new transom with more height, maybe not the full five inches, but in effect that would be the same as a shim. I don't like the idea of shims in fear that the motor will pull itself off having so little area to clamp to. I'd really like to utilize the motors full power/speed to cover more fishing. The motor seems to be reaching full RPM by sound (it roars). It is a standard 10X9 prop and runs well, but doesn't seem to move much faster above plane. The boat is a 14 ft aluminum at about 350 lbs without gear. I was just looking for your thoughts or experiences. Thanks in advance.

Re: 15hp long versus short shaft

First thing I'd check is that the transom is solid. It should not bow when using any motor within the horsepower range its rated for regardless of whether its a long or short shaft. Replace the plywood if necessary, and take a 12 x 12" or so piece of 1/4 inch aluminum, through bolt it to the original transom so there is 4-5 inches sticking up, and screw a block of wood where the motor now mounts

Re: 15hp long versus short shaft

I would consider one of the following,
1: either get or change your engine to a short shaft.
2: rebuild the transom to fit a long shaft.
You cant "shim" a long shaft to fit a short transom.
As for some of the speed issue, you just plain have to much leg in the water that is causing excess drag.

As opposed to our friend above, Dont! go slapping a aluminum plate on the transom. If your not going to do it right, dont waste the time or money and just do not do it at all.

Re: 15hp long versus short shaft

You already have a bad transom. Replace it and convert your outboard to a short shaft or make it a 20" transom.

I would prefer converting the outboard to a short shaft over altering the boat's structure with a taller transom unless the transom is already cut out to make it 15". Then just don't cut out the replacement transom.

Jerry rigging your transom as is would be an invitation to disaster.

Re: 15hp long versus short shaft

Your transom must be checked for defects.But there other things going on as well.
You have a lot of extra resistance in the water because of the extra five inches that your long shaft has.This does 2 things.First, it slows you down considerably and makes it quite hard to get onto plane,You probably have a very big bow wave. Second, the push emanating from your propeller is larger because of the greater distance( and thereby leverage) from your transom.Add to that a possible wrong setting of the tilt of the engine and your little 15 horse will be trying to push the bow under water.
As long as you don't change to a short shaft,which you should do regardless, try to shim the engine up a bit and set the trim so that the propeller sits farther away from the boat.This will allow the boat boat to cooperate with engine instead of fight against it.
Something I forgot to mention. If you have a 50s 0r 60s johnnyrude your long shaft is actually 23 inches and the short is 18.

Re: 15hp long versus short shaft

bifflefan said:

I would consider one of the following,
1: either get or change your engine to a short shaft.
2: rebuild the transom to fit a long shaft.
You cant "shim" a long shaft to fit a short transom.
As for some of the speed issue, you just plain have to much leg in the water that is causing excess drag.

As opposed to our friend above, Dont! go slapping a aluminum plate on the transom. If your not going to do it right, dont waste the time or money and just do not do it at all.

Click to expand...

Just curious why not? A friend of mine did it on a solid transom, properly bedding everything, and it works great, extremely solid! The transom rivets would rip out before you ever bend a 12" wide piece of 1/4 in aluminum. Think "Jack Plate that doesn't move" Of course, don't just "slap" it on or cobble it together either.