An outboard motor fairing to prevent propeller ventilation
Attaching an outboard motor to almost any type of double ended hull generally involves using a bracket mounted on the side of the hull. Some small catamarans also have their outboards mounted to a bracket between the hulls. Without a transom or deep fairing in front of the leg of the motor, severe ventilation of the propeller can occur and reduce the thrust of the motor. Ventilation (where air is sucked down into the propeller) occurs because of the turbulence behind the leg of the outboard motor.
I decided to fabricate a fairing around the leg of the motor to reduce the turbulence and hopefully increase the thrust. I considered many possible methods and materials, and I didn’t want something that would be too difficult to remove if it didn’t work as I expected. Flat panels of fiberglass bent around the leg in a foil shape seemed to be the easiest solution. Sheet metal could also do the job, but my workshop was better equipped to make it out of fiberglass.
The next day I tested the fit by bending the vee panel around the leg of the motor, with the bottom edge of the panels resting on the top of the anti-ventilation plate. I was pleased with the curved shape and decided to go ahead with bonding it to the leg. I applied packaging tape along one trailing edge of the panel with the tape overhanging to allow it to hold the two panels together when bent around the leg. I applied a bead of silicone sealant down each side of the leg, and a generous bead of resin/glue powder mixture along the inside of one trailing edge. I folded the panel around the leg, and wrapped the tape over the trailing edge. So far so good and it didn’t spring loose and fly across the shop.
It’s important to be able to rinse out any salt water from the inside of the foil to avoid any corrosion of the leg, so I drilled two holes in the top panel. The two bottom corners of the foil already had small openings for drainage where they overhang the anti-ventilation plate.
I choose the Honda engine because it’s a four stroke, it’s air cooled, and it has a centrifugal clutch, but the clutch can cause a small problem when approaching the shore or a dock. If you use the motor to steer (rather than a rudder), reducing the throttle to slow stops the propeller turning, and you lose steering control. An unexpected benefit of the large foil around the leg is that it functions as a rudder after the propeller has stopped.
Ideally this fairing could be made of some high density foam in two halves, so that it could all be removed with a couple of screws. There is exhaust flowing in the leg though, so maybe having cool water around it is best.