50HP Yamaha 15" Mid and Lower Strength?

[hellbents-10]

We have a 15" 50hp 1988 Yamaha that is a rare bird!

I would like to run a 90hp Yamaha powerhead on top with a spline adapter to the smaller 50hp vertical shaft.

Anyone abuse this 50hp Yamaha lower unit?

We may build another shorty out of the 90hp mid and lower and have 2 complete engines ready for battle in case the 50hp lower is too brittle.

This will be going on a sub 500lb carbon 16 Tuff.

[mattmak77]

First off, very cool combo - Tuff16/Yam90. Post pics, please!

Regarding the small lower - if you mean the 3cyl 40/Pro50 mid/lower (old style - push pull gear change, pre-1994), It won't bolt up to the 90 adapter plate. The Yamaha 2cyl 55hp short mid does bolt up. Regarding the lower itself, I kinda have the same concerns - the propshaft looks a bit small for that much power, but I do think some have run this combo. The drag guys down south seem to prefer the larger 90hp gearcase. If you want to combine the large 90 gearcase and the short 55hp mid, you will find the 90hp water pump too tall by about 3/4in. So that is another thing that needs a solution, besides the driveshaft length..

[Tomanytoys]

All the things he said x 2.
We went through all this a few years ago when sorting out a power upgrade for Jrs Allison 15R.
The gear case off the 90 accepts all the prop options for a V-6 so in our case it opened up a wide selection for prop testing & set up.
The 90 mid will stand up to a lot more abuse as well.
If you choose to go with a shortened 90 you can retain the factory self contained trim or put the trim pump in the boat and go with a single ram trim set up.
if you get up this way for a Sunday fun run with the guys on Lake Muskoka stop in and take a look at ours.

[hellbents-10]

Awesome thanks for all of this info!!!! We might make it for a Sunday run later this year as well.