90hp 2 stroke SHORT shaft conversion??

[mitch184]

I know some of the racing boats have done it in the past, but has anyone here ever converted a 90hp 2 stroke into a 15" short shaft?

Shortening the driveshaft, water tube, shift linkage seems obvious. I'm not exactly sure how or if the tuner will need to be modified. The big kicker is trying to find a mid section that will bolt up. I've been search the gasket part number but haven't had any luck. Finding some interchangeability between older C75 motors, but nothing directly a short shaft.

Any advice?

[boscoe99]

Use a jack plate to raise the motor 5 inches instead of trying to make it 5 inches shorter would be my advice.

[pstephens46]

Use a jack plate to raise the motor 5 inches instead of trying to make it 5 inches shorter would be my advice.

Probably less expensive in the long run too.

[mitch184]

Agreed, but jack plate won't work for my situation.

I've read that some of the 80's Yamaha and Mariner 50-70hp motors were made in the short shaft AND bolt up, but haven't been able to find part numbers.

[boscoe99]

The biggest Yamaha that I can find which uses a short shaft is a 1994 50ESRS. There may be a snow balls chance in hades of a 90 HP power plant fitting on a 50 HP mid-section.

[mitch184]

Awesome thanks. I can purchase both gaskets and see how close they are. I don't think they share the same part number... but might be close.

[panasonic]

I have to ask....why?

[mitch184]

I have to ask....why?

This is going to seem petty, but....... I have small outboard jet sled i use for fishing. I'm building a Yami 90 to put on it along with adding a tunnel. The boat is a tiller. For a lot of the fishing I do, it's really nice to have a small motor so you can backbounce, side drift and just see over the motor. I'm 5'7", I am taking the mid section apart anyways and was just looking into shortening it. This also makes it so I wouldn't have to extend the transom height with the tunnel going in. I might even shave another 15lbs off transom too, which would be an added bonus.

If I wasn't already going to have the motor apart, I wouldn't even consider it. But with putting a new tuner and an outboard jet on, I'm most of the way there already. With the jet, I don't have to worry about shortening the driveshaft, water tube, linkage or anything like that. Literally just finding a mid housing that will bolt up.

[pstephens46]

Any chance you could post a picture of this boat? Sounds interesting.

[99yam40]

does not make sense to me to try and find parts that are not made for your outboard motor.
does not the jet already shorten the length of the motor when installed?

[mitch184]

does not make sense to me to try and find parts that are not made for your outboard motor.
does not the jet already shorten the length of the motor when installed?

I thought it would be an easier swap since I was told there were short shaft housing that would bolt up.

Jet does shorten the motor, but a 3" tunnel raises it back up. Being a tiller, and where I run this boat, I need that tiller in a good spot, heightwise, for steering. Not a lot of leeway on some of the creeks out here. If that tiller is up around your chest, its real hard to steer.

Other option is a drop tiller, which I haven't seen before, but could probably fabricate.