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You may find some of the words used on this website unfamiliar. So we provide below some explanations to help you.
The full distance across blades, or the disc they occupy. Two or four blades can be measured straight across from tip to tip; three blade scan be measured from centre of hub to blade tip (radius) which when doubled = dia.
The theoretical distance the prop would move forward in one revolution. Two things it’s not, are the length of the prop or the angle of the blades (although this angle does indicate pitch). To work it out, measure the angle of the blade at a specific, measured radius from the centre. Use the formula 2Pi x R to work out the circumference at this point. Stretch this out as a straight line on the ‘x’ axis of a graph. Draw a line from this point at the angle you have measured, up to the ‘y’ axis. The height where the two intersect is the pitch.
No prop ever goes as fast as its pitch would suggest. Blades need some angle of attack to maintain some thrust. There are losses, including friction and turbulence, all of which can be loosely termed as Slip. Slip is the difference between theoretical pitch and actual distance covered. If tug-towing a heavy load, slip could be 50 - 60%. On a displacement hull it may drop to 30%, decreasing further for very light hulls such as hydroplanes. It is a useful exercise to work out your theoretical speed with a simple formula as follows: Prop shaft r.p.m. x pitch in inches / 1057 = speed in m.p.h before slip. Compare the figure you get with the actual performance to find out slip. Also before you fit a propeller, work out the speed it would give with the motor you plan to use. This may indicate a hideous mis-match which would cook the motor and turn your tramp steamer into a torpedo. It also happens that modellers choose low geared, or very low r.p.m., motors which don’t turn the prop fast enough to maintain scale speed. Both disappointments can be avoided by a little advanced knowledge. All Prop Shop props have a measured pitch (stated in their ref. number) which lets you work things out beforehand.
PITCH/DIAMETER RATIO (PDR)
A useful short cut for matching high-speed props to motors and hulls. A PDR of 1.4:1 is a medium pitch which works on all sorts of planing hulls. 1.7:1 is medium/high and works best as a surface prop on light monos, tunnels or hydros. Higher pitch ratios 1.8, 1.9 etc . are for progressively lighter and faster boats. All our high speed props except Multi/FSR have reference numbers of diameter in mm followed by PDR. For displacement hulls maximum efficiency is achieved with pitch/diameter ratios of around 0.8 to 1.1. These are the ratios typically used on our scale props. For planing hulls, especially surface drive, maximum efficiency is achieved at around 1.8.