Swing away motor

The F36 plans call for a traditional inboard diesel engine and propeller shaft thru the bottom of the boat. We can’t do a sail drive because we want the boat to be able to sit on its hulls in zero water, as in the extreme tides of Baja or England. So… Here we go “off plan”, switching to an outboard engine on a swing-up mount on the back of the starboard beam. I’ve been thinking up an approach that is both temporary and long term; if we don’t like the outboard the entire mount can be easily removed with only 1lb of hinges left behind and the trampoline put back in the engine’s place. Or when a cost-effective electric pod drive solution becomes available we can switch over. Or if an inboard diesel is really the answer, that can be retrofitted at any future time.

The measurements for a Yamaha 25 long shaft were found online and we made a faux motor of Luan door skins to practice with placement. The net lashings tube almost looks like a motor mount hinge, but it’s not parallel to the waterline and it wasn’t built to the shear forces and weight of the motor. So we need to work around that feature, and not cut it off the boat. The rudder gudgeons seem like a good model for the motor mount hinges and we have some spare G10 tube and a 16″ piece of 1/2″ stainless steel rod that will make a nice hinge pin with the right bushings in place. First up is fabricating the hinges.


The funny shapes of the two pieces are what fit over the net lashing tubes on the beam. The other three will run on the top tab of the motor mount (yellow board is a stand in to see how this is going to work). I’m thinking the motor should be mounted as close to the hull as possible, as the hull shape curves away considerably down by the waterline. The weight should be tucked in close there, and things like the freezer and galley on the port side should counter balance this engine placement.

We’ll pick this project back up next week once the bushings are in hand so we can align all the hinges and permanently affix the two big ones to the beam (instead of that blue tape :)
Thinking we’ll want to wrap them at least 18″ around the beam in both directions with heavy weight uni-directional cloth.
PS – the mounting box will be made to push up against the big diagonal brace under the beam, so the motor forces are spread out on a couple of surfaces, not just the hinge pin.

9 thoughts on “Swing away motor

  1. Greg,
    Seems like a well thought out plan. Working out side the box often ends up the norm. I’m wondering if it would be worth building a soft mock-up to confirm the strength and stresses the motor and it’s weight, will put on the mount and hull.

    Thanks for keeping those of us that can only dream of a project like this so very well informed. I for one, have certainly learned a great deal.



    • Bob, what do you have in mind for a soft mock up? My game plan is to build the swinging ‘sled’ to structural (but not cosmetic) finish and go find/borrow a dead 150lb outboard. There is a motor shop nearby where I bought an engine for the Whaler years ago and I’m guessing they will help.


  2. Hi Greg,

    Good to see you are not loading up with all the cost, smell,upkeep and weight of the inboard.We have a very substantial but light sled on Skateaway which has served us well for 22 years and currently mounts a really nice 4 stroke15 hp Tohatsu (2006 model). The previous motor was a 2 stroke 15 hp Johnson which lasted for 13 years so on a annual cost basis we are talking maybe $150 a year.The original 25 hp Johnson was too heavy, powerful and used double the gas, most of the motoring we do is in very light air motor-sailing and 15 hp is plenty. Our sled hinges on a fully laminated in,horizontal,leading edge shaft that protrudes out inboard and outboard 1 1/4″ long and 1 1/4″ diameter. The inboard end plugs into a thick wall delrin bush set in the hull, the outboard end fits into a delrin bushed alloy block which is bolted to the underside of the wing deck (which you don’t have) Because the almost 6′ long sled is hinged well ahead of the beam the motor stows nested right up against the beams aft face with all the protection that affords. The underside of the sled is flat foreword and develops down to a 90 degree V form aft which breaks the waves in front of the motor, forms a nice, stable trough to keep the prop biting. Skateaway is all stripped down getting a full refinish right now but I will see if I can dig out some photos.
    Because there is no wing deck foreword of your beam you might want to consider a large gusset bonded to the main hull right beneath the net to provide the outboard bearing support.
    The sled needs to be quite long to minimize the angular change when it is lowered that way you get an effective thrust angle and a good shield for motor, the shape of the sled will skim the surface and in short chop even lift and fall to keep the outboard driving when you need it most. Don’t bond anything on until you are confident with the mock up.

    Best regards, Keith.


    • Great info Keith. I had thought the sled should end just below the motor mount, but you’re saying to take it down to the waterline. Protect the motor leg and break waves – got it. I don’t understand the shape at the waterline so hopefully you can find a photo or I can call you. Thanks!


    • Kieth, I was about to offer some thoughts on a mock-up for Greg’s plan. You have far more knowledge of the boats structure then I.. Perhaps you can offer a suitable Mock up idea. My concern was vibration and the fatigue factor of both the metal used and Glassed area.


  3. Greg, found some 23 year old photos which I can copy, enlarge add some explanatory notes to and mail. Old tech but then we can compare notes on the finer details.
    What would be a good mailing address?


      • Hi Greg,
        It’s in the mail, talk to you next week (off to Jersey to work on the St Francis for a couple days) Best, Keith.


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