I recently purchased a small outboard motor for my Prindle 18 catamaran and had to figure out how to mount it on my boat. I wanted something that was easily removable and was lightweight...
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I had an old Hobie 16 boom on hand, so that is what I used from the crossbar.

I decided to bolt the mount to the lip of the hulls so I had two matching 1/4" aluminum riser brackets made at a local weld shop. He welded a piece of 1/2" aluminum rod to the part of the bracket that went inside the hull lip. This matched the contour of the inner lip perfectly. The brackets are approximately 10" wide x 8" high. The front is angled so there is clearance for the rudder tiller handles.

I drilled and tapped down through the center of this rod for a 1/4" SS bolt that would fasten the bracket to the lip. In order to make a flat surface for a bolt and washer to sit on the hull lip, I applied thickened epoxy to make a raised section on top of the lip where the hole would be drilled for the bolt.

After mounting the side brackets to the boat, I cut the boom to length and brought the boat to the weld shop to have the boom welded in place. I cut a piece of 1 x 4 x 5 mahogany to make the motor mount on the crossbar. I bolted this to a stainless steel rod (Hobie shroud mount) that was slid in the sail track and braced it with some aluminum bar that I riveted to the boom.

The motor doesn't have a friction lock to hold it in the forward position, so operating it was a pain, since you had to steer it and the rudders. I figured out that by using a bungee cord to hold the motor tiller to the rudder crossbar center it made operating much easier when cruising longer distances.

This motor doesn't have a lock to hold it in the stored position, so I made a bracket to hold the tiller handle securely to the crossbar.

Jack Hoying jmhoying(at)adelphia(dot)net Fort Loramie, Ohio