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

 

[craigvalk]

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

Thanks all for the advice. <br />I know what's involved on the shaft change, unfortunately it is a lower unit teardown to do it. There is a pinion nut on the end of it that needs be accessed through the hub. <br />Since it's essentially the same motor, controls, cables and all don't have to be changed, so I figure an hour or two to bolt it up and try it out... why not?<br />If it works great, if not I can switch back and will now have a winter project to occupy me!!<br /><br />Thank you all for the advice!!<br /><br />Craig

 

[quantumleap]

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

Mellow Yellow already solved your problem.

 

[craigvalk]

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

Thanks for the tip on the shaft conversion kits.<br />Does anyone know where to get one? <br />I thought OMC was gone and dead.<br />I guess I can check with a dealer, but if there is a part number or something that someone can provide it might help.<br /><br />Craig

 

[JasonJ]

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

If it were me, I would start parts swapping. The shortshaft will probably not perform well. The prop will not be getting clean water, and the possibility of overheating is very high. It may look okay at rest, but once you go on plane (if it even gets on plane), half the prop and all of the water intake will be out of the water. Two minutes later you have destroyed your near new motor, not worth the risk to me. If you are competent, you'll have the parts swap done in a day, and be stress free. Good luck....

 

[DJ]

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

Check with these guys.<br /><br />www.twincityoutboard.com

 

[craigvalk]

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

Thanks all. I am going to look into the conversion kits. At worst I can parts swap from the old motor as suggested. Only concern there is if I do that I don't have 2 running motors.<br />That's a minor concern though. I think my winter project is set... I will get the season out of the present rig and then yank the motor and spend some winter nights working on it.<br />I started the short shaft on the stand tonight and ran it for a 1/2 hour, the thing is SWEET.<br />Thank you all again, this board is the BEST resource for old motor junkies like me.<br /><br />Craig

 

[craigvalk]

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

Jason by the way.... NICE work on that center console... that thing is quite nice!!<br /><br /><br />Craig

 

[JasonJ]

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

Thanks Craig. Glad you are not going to try the shortshaft. You would be able to putter around and thats it, definately no fun. I am curious, why do you feel your gearcase is tired? Is it shifting poorly, did some teeth break off, does the oil get contaminated or end up with metal shavings all the time?

 

[craigvalk]

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

Hi Jason.. yeah it's an Hydroelectric Shift and it's giving me grief. You can read the sorry tale on the J/E repair board. Good news is I think I have a working lower end after a couple rounds of parts swapping, and I will re-install it this weekend at the lake and see what happens. Looks hopeful but you never know.<br /><br />After Labor Day my boating time subsides, so about November or so I figure I will get that short shaft converted over in the basement, and will install it in the spring.

 

[JasonJ]

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

Sounds good. Ahhh, the joys of living in the north. Our two day boating season sucks, but it makes us appreciate it that much more (and gives us the rest of the year to tinker)....

 

[RickinNC]

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

What will the boat do if the motor is too low on the transom? The reason I ask is I have a longshaft evinrude 70 on an old bass boat the motor sounds likes it running ok but the boat just plows through the water and never gets moving fast enough to get up on plane