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Mercury 9.9 Big Foot
Outboard Sail Motor

A High Thrust Outboard Sail Motor

Mercury Marine's 9.9 horsepower four stroke Big Foot sail motor.

This review is based upon a long term evaluation of a Mercury Marine 9.9 horsepower Big Foot outboard motor. Hundreds of hours were logged with this four-stroke rigged on the back of a 30-foot twin-keel sailboat. The 9.9 Bigfoot was an auxiliary engine used to motor through inlets, to dock and make way on flat calm days when the sails flogged and slatted.

Mercury BigFoot is a hybridization. Let me explain. Standard outboard motors are geared and their propeller is pitched to put a boat on plane. Conversely a BigFoot is intended to make more thrust in order to propel a heavy load at displacement speeds. Accordingly the 9.9-horsepower BigFoot is fitted with a 25-horsepower lower unit that spins a larger diameter gear set and larger diameter propeller. The larger lower unit allows attaching a bigger, slower turning propeller. Mercury BigFoot's gear ratio is lower: 2.2 to one as opposed to a conventional 2.10 to one. So with a Mercury 9.9 Bigfoot hanging off the transom a displacement hull feels and performs like the outboard is about a 20 horse. Just don't expect to go fast.

Mercury BigFoot can be operated from its tiller arm throttle and separate shift lever. Or it can be rigged with a throttle/shift box. Making the conversion required by a and TFT Extreme control cables All totaled the components cost me about $200.

Garelick Hydra mount provides the foundation for the Mercury BigFoot 9.9 horsepower sail motor

Tilting the Bigfoot outboard manually was difficult through no fault of the engine. To do so you must lean over the transom and hang over the cowling and grasp down behind the engine to release it. Then yank the back of the cowling to raise it up. Repeat the process to lower it. I mounted the 9.9 BigFoot outboard motor on a Garelick Hydra powered assisted outboard motor transom bracket. With it an electric motor powers a hydraulic pump that feeds a ram that lifts or lowers the engine. All the way up or down took about 20 seconds. I read the mounting instructions carefully, then read them again. I measured twice. and drilled the four holes just once. Instead of using big washers for backing plates, I cut and drilled 1/4 inch by four-inch aluminum plate. You should also know Garelick also makes significantly less expensive manual tilting brackets.

Doel Fin top view

Important to note, the 9.9 BigFoot is a Sail Motor which simply means it has an extra long propeller shaft (25" versus 20" or 15")  to better bury the propeller in undisturbed water. Even so in anything more than a flat calm, sailboats hobbyhorse. In rough water the up and down motion brings the tips of the propeller blades close enough to surface so that they suck air down around the wheel. With the propeller spinning in more air than water the engine races until the over-rev limited reduces rpm. I improved the situation somewhat by installing a Doel Fin hydrofoil that made it harder for the propeller to draw in air. Racor's fuel water separator filter for small outboard motors 025-rac-05

Early on in the test cycle I installed a Racor filter/water separator specifically designed for small outboard motors. Its ten micron-sized filter element keeps junk out of the carburetor and the fuel water separator took water out. It paid for itself in carburetor gaskets I did not have to buy and downtime I did not have to endure. The carbureted engine is slightly cold-blooded. Even in 90-degree weather it needed a couple of pumps on the engine-cowling-mounted primer bulb and full choke. Once warmed up, by playing with spark plug gap and the idle setting, I was able to get idle speed so slow the 9.9 tick-ticked like my grandfathers old watch.

oil filters extend engine life on mercury bigfoot outboardsAfter the first 20 hours, and about every 50 hours after that (the owner's manuals says 100 hours), I changed crankcase oil in a clean-hands, environmentally safe manner. With the engine warmed and all the nasty stuff in suspension, I poked the wand from a Moeller vacuum pump into the dipstick hole and sucked it all out. I also used the Moeller pump with an oil-fill pump fitting spliced onto the end to change gear oil. I was glad to see this engine had an oil filter because four-stroke outboard motors with their modest crankcase sump are particularly sensitive to dirty oil. The oil filter, and regular oil and filter changes, extend the lifespan.

I priced common repair parts, things like impellers and carburetor-to-intake-manifold gaskets and they are reasonably priced.