Short shaft motor on a long transom? Risks / Problems?

[craigvalk]

I have a 16 foot tri-hull with a 20 inch transom, and currently am running a long shaft 71 Johnson 60 horse ( 3 cyl). Has a tired gear case and a lot of hours. I've had the incredible fortune of stumbling across a nearly brand new 71 Johnson short shaft. It's been sitting in a basement for 32 years.. and it runs! Identical to the present motor except for the shaft length... In my present set up, the cav plate is about 1/2 inch below the keel.<br />Clearly a short shaft is going to put that cav plate higher up. I understand the basic rules of shaft height, but what are the implications of sticking that short motor on my boat? Danger, bad performance? <br />Also, I know I could use my long shaft parts from the old motor, but I really don't want to tear in to a nearly new lower unit to replace the driveshaft.. plus the old motor does run and I could keep it for a spare.....but I might have no other choice...I do have the manual and am competent to do it... and I really don't want to pay 3 or 4 hours of shop time to have it done either.<br />Any advice appreciated....<br /><br />Craig

 

[richg99]

Re: Short shaft motor on a long transom? Risks / Problems?

I'd think that the short shaft motor would not have enough room to tilt forward, unless you mounted it 5 inches too high? Then it would not have enough prop in the water to properly drive the boat. <br /><br />Do you have a jack-plate or any sort of setback that would give you more clearance? Of course, any sort of setback would excaberate this " too high motor" problem ??? RichG

 

[mellowyellow]

Re: Short shaft motor on a long transom? Risks / Problems?

OMC has an extension kit to convert short to long.

 

[Chris1956]

Re: Short shaft motor on a long transom? Risks / Problems?

Craig, I would try the short shaft motor. If it is truly too short, you will ventilate and be unable to avhieve plane. If the boat planes without ventilation, and there is enough cooling water (very likely under this scenerio), you are all set.<br /><br />If you do have trouble, you can swap the driveshaft and gears from one gearcase to the other. Or if the motor has the 5" extension piece on the lower unit (my '59 johnson did), simply swap the driveshafts and add the extension pieces for the gearcase and water tube.<br /><br />Also, sometimes these old Johnsons had a drive shaft that was very easy to remove. You simply removed the water pump cover, and the driveshaft would pull out or the gearcase. Don't loose the waterpump impeller key.

 

[ED21]

Re: Short shaft motor on a long transom? Risks / Problems?

I put a short shaft motor on a boat meant for a long shaft once. The prop & water intakes weren't in the water. I didn't get to go out that day.

 

[JB]

Re: Short shaft motor on a long transom? Risks / Problems?

Moving to Outboards