IMPROVING APERTURES

Apertures generally degrade propeller performance. If the deadwood in front of the propeller is anywhere near wide as a blade, it starves the blades of water, causing performance losses. Furthermore, apertures don't usually have enough room to accommodate even the minimum recommended size prop. Propellers need 20% of their diameter clearance at the tips and 30% fore and aft. Less than this causes noise and vibration.

Because of these factors, propellers in an aperture need to be larger in diameter by about 1-2 inches and have 1-2 inches more pitch than the same prop being driven on a strut in open water.

Two ways to improve performance: Have a qualified yard 1) open up the aperture and 2) fair (ie streamline) the deadwood so that it is like the trailing edge of a wing.

Doing both of these things will allow you to install a bigger prop and not starve it of water. You can expect 30% more thrust from your present motor if you enlarge and streamline. In rough conditions the blades won't stall as readily and your average speed in a chop will increase significantly.

There have been three C & C owners who modified their apertures and deadwoods in this way. They were able to increase their prop diameter from 11" to 12" for dramatic performance gains using their original Atomic 4 motors.

Keep in mind that it's probably not a good idea to do this if the area you need to enlarge and/or fair has metal bars for reinforcement in it. Absent the bars, it's a lot cheaper and less risky to open up the deadwood than it is to put in a bigger motor. Motoring performance is prop limited, not motor limited, so bigger motors won't help without a bigger aperture.

If you can't or don't want to alter your aperture or deadwood, your options are to put the largest PerfectPitch prop you can fit with 10% tip clearance. You will notice some performance gains in rough conditions because the blades tend to stall less and the Flex Pitch will help in both forward and reverse.

Or you can install a three or four bladed prop and live with the drag.