Diy: auxiliary outboard motor mount

Written by pauline gill
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Auxiliary outboard motor mounts enable the use of outboard motors to drive a vessel whenever the normal propulsion mode is either unavailable or undesired. The key factor for auxiliary motor mounts is the ease of tilting the motor down or up to get the propeller in and out of the water without undue back strain. This auxiliary outboard motor mount only has to fit one boat--yours--so implementation is easier than a one-size-fits-all. It is very helpful to know your boat's natural waterline at rest before starting this project.

Design Strategy

This fixed mount uses corrosion-resistant aluminium angle struts and stainless steel hardware to provide a long-lasting, trouble-free mounting system. The motor clamping plate is fabricated of light, stabilised, high-impact UHMW black polythene, reinforced with flat aluminium plate. The auxiliary outboard motor mount has a wide stance mounting on the boat's transom, providing extra strength when the motor is tightly turned to rotate the boat on its own axis. This greatly enhances maneuverability while mooring or departing the slip.

Component Preparation

Starting with the motor mount pad, you will need a 1-foot square piece of 1-1/4 inch-thick UHMW polythene sheet, as defined above. This is sandwiched top and bottom by four 1-foot long pieces of 1/4 inch thick by 2-inch wide aluminium strip plates that are attached horizontally to the UHMW sheet with 1/4-inch by 20 thread 1-inch long flat head Phillips screws. These strips provide clamping surfaces for the motor's own transom clamping system. Try the finished plate in the motor's clamp bracket to confirm plenty of clearance for the other brackets. Cut eight 1-foot long pieces of 2-inch wide 1/4-inch thick aluminium angle and drill a 1/2-inch hole centred at both ends of both of the faces. The hole centres should be 3/4-inch from either open side so they will line up when the pieces overlap. This gives you eight pieces with four holes each. Two of these pieces attach vertically to the motor plate, two vertically to the boat's transom, and the four remaining angles connect the top and bottom corners horizontally such that the finished structure will be a rigid box frame.

Final Assembly

Attach the vertical angles to the inboard right and left sides of the motor plate, top and bottom with 1/2-inch by 3-inch long SS carriage bolts and locking SS nuts over SS washers. Attach the four horizontal strut angles with 1/2-inch by 2-inch-long hex head SS bolts and locking SS nuts, with SS washers on both sides. Next, attach the two vertical transom mount angles to the front of the four horizontal struts the same way. If the holes have been drilled with precision, the fully tightened structure will be exceptionally rigid.

Installation and Test on the Boat

The auxiliary outboard motor mount should be attached to the transom with 1/2-inch diameter SS through-bolts so that the motor's cavitation plate on top of the propeller is at least four inches under the waterline, at rest. Try the motor under varying conditions to make sure it doesn't cavitate or lift from the water. If the motor is a little hard to tilt out of the water, attach a booster handle to the back of it, which can supply extra leverage for this task.

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