Long to short interchange?

[shelby78]

I have a 1984 25hp merc that runs great but its a long or x-tra long shaft motor and i use it on a 14ft aluminum boat (not deephull). Does 29mph but i know the motor is way to deep in the water.

I just found a blown up (or close to it) 1986 20 hp merc short shaft and am wondering it i can swap out the botom end/shift lever/housings etc etc and have it bolt up to my motor?

I,m guessing my boat should be good for at least 3mph more but the big point is its a fishing boat and i don,t want the motor as deep as it runs with the long shaft.

Thanks for any help. I have looked a parts schematics but they don,t seem to help to tell me if the parts will bolt up or not.

 

[James R]

Re: Long to short interchange?

As far as I can see that motor came in two flavors, 15inch and 20inch. If yours is a 20inch,long shaft, then from the transom mount where it hooks onto the transom, to the centerline of the prop shaft it will measure in the order of 29inches. If this motor is a converted 15inch there will be a 5inch spacer bolted between the housing and the lower unit.
If there is no spacer then you are stuck with the 20inch. You can consider raising the motor on the transom with a jack plate.

 

[Laddies]

Re: Long to short interchange?

Yes you can put your upper end on the short shaft mid section and lower unit and it will work. Mercury don't have a extension kit like the J rudes do a Mercury long shaft has a different mid section and gear case housing than a short shaft not a spacer kit like the rudes.

 

[sschefer]

Re: Long to short interchange?

Keep the long shaft and raise the transom. You'll be able to stay out on the lake a little longer when it gets rough and you won't have to worry as much about getting caught on a rough lake. Raising the transom is pretty easy. It just takes a decent welder and and new transom board.
I have a 14' with a 15" transom and I love the boat but hate the low transom. I've got a 18hp 2004 Nissan short shaft that gets me up to 26 mph on the GPS and runs great. It's not my only boat so I can choose the right time to use it vs. the bigger boat.