motor board for malahini

Designs for inboard or outboard power

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motor board for malahini

Post by ctiger » Mon Apr 05, 2010 2:42 pm

I'm putting a larger engine on my malahini and the instructions call for a 2.25" total thickness of motorboard. On the plans the motor board is 1.5" thick including transom. Is the extra thickness added all the way down into the transom knee? If so, the extra thickness must be cut from the knee. Is this right? Or do you put the extra motor board over the knee to keep from notching the knee?

Thanks in advance

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Re: motor board for malahini

Post by Iggy » Mon Apr 05, 2010 5:41 pm

I am still in the middle of my Malahini build, and I am also using a large motor (90Hp).

I built up my transom a fair amount.. Ontop of my 3/4" outboard mahongany plywood I epoxied a 1/2" douglas fir and then another 3/4" mahogany plywood layer for a total of 2". You can use 3/4" instead of the 1/2" core if you want a 2 1/4" overall thickness.

In my case, I made the 'motorboard' area as wide as my middle batten, where the motor well will attach to the side of the batten to keep the thinkess uniform. Ontop of my 7/8" transom frame laminations I added a 3/8" board of mahogany ply at the bottom up to match the two overall thicknesses.


The Knee goes ontop of everything.. requiring longer bolts.. so keep that in mind. I actually had to countersink mine a little into the knee to make my bolts fit.

I will likely add a 2" board accross the top of the transom cutout opening to add a bit of width for the motor mount to hang off.

I figure with design of the motor well, it should be plenty strong and transfer the load quite well. If it was just the transom with no motor well, I would probably have made it thicker, just to be safe.
Ian (aka Iggy)
My Malahini Build

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Re: motor board for malahini

Post by ctiger » Mon Apr 05, 2010 7:39 pm


Thanks a ton. That is a perfect solution. Awesome.

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Re: motor board for malahini

Post by Angelo » Sun Aug 07, 2016 1:00 pm

Are those bolts in the right position? I countersunk and through bolted the knee to the keel. There is never going to be any serious force vertically on that knee. So isn't bolting it to the keel the resist forces on the x plane more logical???

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Re: motor board for malahini

Post by PeterG » Mon Aug 08, 2016 8:05 am

EDIT: Aw shucks, just saw this was an old post/string from SIX years ago (!) Sorry for bloviating on this... but my comments may be helpful to others?

Just out of curiosity, what motor are you installing? I suspect your instructions specify a maximum transom thickness, not the minimum.

I am building a Malahini too, with a slightly modified transom to accept the 1965 Mercury 65hp long shaft motor that I will be using. I used the Mercury outboard owner's manual for 1966-1974 motors (also good for a 1965 65hp motor) which gives dimensions and minimum clearances for the installation. That manual specifies a maximum transom thickness of 2-3/8" for the 65hp, but not a necessarily a minimum. My transom will be at about 1.5" with two layers of Hydrotek meranti plywood (each layer 18 mm thick with 11 plies, very strong). In way of the motor clamps I will be installing a 1/2" thick white oak board that will serve more as protection for the transom plywood than extra thickness.

When you look at the design of the transom structure, extra thickness will not provide any improved strength. Some stiffening of the transom comes from the knee/keel connection, but much of the stiffening is provided by the combination of the 2x4 cross piece at the top of the transom knee and engine well bottom plywood. These take the push/load from the motor and transfer it to the two plywood pieces that are at each end of the 2x4. Those plywood pieces are connected to the battens at the hull bottom and the deck at the top and spread that load into the rest of the hull.

My point is: Don't bother with extra transom thickness, the 1.5" thickness should fit your motor just fine. You can add a wood clamping pad to the inside of the transom to build up the thickness in that area. The structure as designed will easily handle up to 85hp without redesign. Note that if you are using a long shaft motor which needs a 20" transom height, the 2x4 cross piece should be moved up the transom and the upper leg of the transom knee should be extended up to meet the 2x4. If you want to maximize the stiffness of the transom, use two 4" wide laminated layers of 3/4" plywood in place of the 2x4 solid wood piece.
Murphy's Law: Anything that can go wrong, will go wrong.
Griffin's Law: Murphy was an optimist.

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