A variety of clamping and bolting devices is used to mount outboard motors such as sail-boat auxiliaries. The type of fitting will depend upon the stern shape, height of the transom, position of the rudder, etc. Mariner Optimax Outboards are designed to operate with the anti-cavitation plate or exhaust outlet about 2-3 inches (50-75 mm) below the surface of the water. At a depth less than this. the outboard will vibrate. race as the stern lifts to a wave, and overheat. If the mounting bracket is too low. the motor will operate below its optimum speed, power output will be reduced, and fuel consumption will increase. A selection of clamps, alternative lengths of propeller shaft 15-19 inches (380-480 mm) and a choice of propellers of different pitch and diameter enable motors to be tailor to the boat.
Mariner Optimax engines are designed to withstand the occasional splashing and the ubiquitous spray. They can, however, be seriously damaged if submerged. To prevent this, and as security against theft, it is sensible to chain the outboard motor to the boat.
Stern mounting Mariner Optimax engines
In many boats and in particular sailboat auxiliaries, the outboards are double as rudders. The motor is clamped at the center of the transom and the boat is steered by the tiller fitted to the outboard. This works very well, provided one remembers that steerage is lost once the motor is cut or slipped into neutral.
The propeller should turn in water that is as free from turbulence as possible. It is often impossible or undesirable to mount the motor so that the propeller is deep below the keel. In such a position it would be vulnerable to damage. The normal mounting point for the outboard is on the center-line immediately aft of the skeg. In this position, the motor may tend to vibrate, and the forward motion relative to the output of the motor may seem disappointing. The likeliest reason for this is that the turbulence created by the skeg causes the propeller to cavitate. Move the motor to one side, or trim the skeg to reduce turbulence.
As a propeller turns in the water, it produces forwards motion and slight sideways thrust. This is because the lower part of the propeller is operating in deeper (and there before denser) water than the upper half – and the stern is pushed sideways in the direction of rotation. A propeller turning clockwise will threaten the stern to starboard – altering the heading to port. The bias which is the result of the offset mounted engine position can be afore compensated for by mounting the motor on the appropriate side of the boat.