View Full Version : Modifying for long shaft

04-04-2009, 02:37 PM
Yesterday I bought a 1979 Evinrude 35 at a steal. The motor needs an impeller and tune up and I will have about $100. The problem, it is long shaft (22" long) and my boat transom is 18" in the center and 16 on the outsides. I have a 1984 Kennedy Kraft 14' V-hull.

What modifying will I have to do to my boat to run the 35 long shaft? Do I need to bring the transom up 2 inches or 4 inches?

04-05-2009, 03:24 PM
First thing is to search the internet for the manufacturers installation notes. This will show you what the outboard manufacturer considers the optimum transom height for this motor.

You don't really have to modify the transom at all. Your new rig being a little too long will mean that the propeller will run deeper in the water. This will create some additional drag, but the propeller might benefit to some degree of being deeper in undisturbed water.

Better to test first and observe how deep the cavitation plate is. It's really annoying to have to lower a transom you just raised. I know.

Someone else on the forum might know how much extra drag you will induce if you do nothing at all and just let the propeller go deeper. I'm sorry I can't help you with that.


04-05-2009, 10:05 PM
If you try to run a long shaft engine on a transom built for a short shaft engine you run the risk of potentially dangerous handling. Older outboard engines were built with a spacer to extend the leg, I'm not sure if they were still doing it when your engine was built.

the standard transom heights were 15" for short shaft engines and 20" for long shaft. For racing we used to raise the engine up until the prop shaft was just below the bottom at the center line, however, for general use you'll want the anticavatation plate even with the bottom.

If you chose to raise the transom be sure to add stiffeners so the engine doesn't rip the top of the transom off.

Good luck. Bruce an old racer.

04-05-2009, 10:40 PM
mine is a solid exhaust housing and it doesn't have the 5" spacer. i found a short exhaust housing on ebay that i am battling for. from the bottom of the head to the lower unit actually measures 22" on the long shaft leg that i have. the short shaft 25 that i have got measures 17" and my dad's 35 measures 17".

i put the longshaft on my boat today and measured and it is 7" to the cavitation plate on the lower unit and that is extreme. I don't want to either sink my boat by ripping the transom off or the huge tidal wave that would accompany the long shaft. plus the lower would be way too deep to handle right. i have been told the kennedy's weren't known for using great material and have a tendency to weaken very rapidly.

if i get the short exhaust housing, i have the short drive shaft, water tubing, and can shorten the shift linkage to make it work. if not, i am either selling the motor or getting another boat....

04-07-2009, 07:27 PM
I have seen a transome built up in the middle for a long shaft engine. Do it right and it wont look bad at all. Its better than having the prop so far down below the boat that it might drive your stern sideways and flip you.

04-11-2009, 02:39 AM
I've personally seen a long shaft 40 horsepower on a boat, the cav plate was about 6" below the planing surface. Anyway, the boat handled quite well, no dramas and no long term structural damage.
In fact, it was great. It was a small boat and handled rough water very well, never cavitating or blowing out, even in teh roughest weather.
If you are feeling unsure, I have also seen large boards bolted onto transoms to raise an engine 6", think a board that extends from planing surface to enw height, thoroughly through bolted and about 18" wide, out of some very thick solid hardwood. You only want to support the part of the engine above the existing transom, at least a few bolts for the outboard clamp will extend through the existing transom.
Another bolt on option that may work well for you is an aluminium jackplate.

Regards, Andrew.

04-11-2009, 11:07 AM
Don't take 15" for short rig and 20" for long rig for granted. I bought a 1976 Mercury 50 Hp, and downloaded the user's manual which covered several models. "Short rig" turned out to be for transom height between 15 1/2" and 16 1/2" depending on model. "Long rig" could be between 20" and 22 1/2" depending on model. Measure or read the manual.


04-11-2009, 12:16 PM
Yeah, I need to get the manual for this motor as well. I am putting a mini jacker jackplate on it to raise the motor 3". 3" should put the cavitation plate about 2.5 inches under the stern and I think that should still be okay?

I may even need to find a 4" jackplate. That is the way that I am going to go though.

View Full Version : Modifying for long shaft