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Can I replace my short shaft with a long shaft?

J

Jhanratt

Regular Contributor
I have about a 1980 mirro-craft 14 deep fisherman which came with a factory matched 1980 Johnson 20hp short shaft tiller. Can I replace it with a modern 30 hp mercury long shaft tiller?

I have asked the seller to measure the distance from the transom to the anti-cavitation plate and the transom to the center of the prop shaft but I do not have the numbers yet.

I can see some potential problems navigating in shallow water which I can live with but will it lose efficiency and spray water all over the place?
How do you know how long of a shaft you can get away with??

The reason for the swap is that the old johnson is just not quite enough power to pull a tube now that my kids are bigger. Also I have overheated the Johnson and warped the head (fixed) and the prop shaft has some pits so I can't get the leg to seal anymore. (I change the oil regularly to compensate).

Any advice would be appreciated!
The boat is rated for a 35 hp.

Thank you!

Jeff
 
Thank you.

I have the measurements now.

Transom to cavitation plate. My Johnson is 18" the Mercury is 21"
Transom to prop shaft. My Johnson is 23" the Mercury is 26.75"

What you said about bow rise makes perfect sense to me as the torque is down low creating more upward thrust. The torque steer thing also makes some sense to me as there is more "rudder" in the water.

I will take your advice for sure. What can I use as a replacement? Short shaft only? how far off of my own measurements can I go" A short shaft 30 tiller seems to be hard to find in my area. anything over 20 seems to be on steering controls and this boat is set up for tiller.

Thanks for your advice!

Jeff
 
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Have you considered bolting on a motor board to raise the new motor higher on your transom?

I've made a bunch of them over the decades out of several layers of plywood glued together.

The other Jeff
 
No I haven't really considered using a motor board. Sadly I replaced the transom on this boat about 4 years ago and I could easily have raised it 2 inches or so. I think for now I will just keep my eye out for the correct motor. Mine is still working well so I'm not under pressure to change it yet.

I would like to know how much difference I could get away with though. Also what is the correct way to measure?

Thanks.
 
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