Boatbuilding Tips and Tricks Blog

Tuesday, June 12, 2018

Transom Construction for Motor Size

I get this question periodically, so thought I'd clear this all up. People ask short shaft, long shaft, and so on, so I thought I'd clear up some of the confusion. Many people call 15: shaft, a "Short Shaft," and 20 inch a "Long Shaft" outboard. That may have been the case, back 50 years ago, but these days it's different. Normally, at least with the Japanese outboard builders, now pretty much duplicated by the American designs, a short shaft is 17-1/2", a standard shaft is 20-1/4", a Long Shaft is 23", and occasionally you'll see an extra long saft at 25", but the 25 inchers are generally specialized smaller motors intended as sailboat auxiliaries.

If you have a motor, new or old, your best bet is to measure it from the clamp area to the cavitation plate, as shown below:

Outboard Shaft Size

Build your transom such that the bottom of the cavitation plate is flush with the bottom of the boat. Be sure to allow for the bottom planking in this calculation. This means the framing from the bottom to the motor-mount board will be the shaft size less the bottom planking thickness.

Outboard transom flat

If you're building a V-Bottom boat, here's how you should calculate this design.

Outboard transom v

If you wish, you can lower it a bit after you test out the performance. It will never be higher, but some feel like the boats run better if they are 1/2" to 1" lower. The motor boards on my designs are amply robust to handle a bit of lowering if needed later.

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Jeff Spira, Naval Architect, marine engineer and boat designer

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