Propeller pitch is all important, irrespective of the other propeller characteristics, the pitch of the propeller must be chosen carefully to match the desired top speed. Failure to do this results in a multitude of problems not limited to poor fuel economy, poor handling under power, poor response under power, etceteras.

Our example vessel has a speed requirement of 7 knots, and a maximum propeller RPM of 2000. This is very similar to stating the gearing and wheel size of a car. The first thing we need to do is convert our 7 knots into units closer to the units used to describe the propeller pitch, so we will convert 7 knots to feet per minute.

We do this by multiplying by 101.3 and see that 7 knots = 709 feet per minute. Note that the maximum propeller rotation speed is also counted over a minute.

If we divide the 709 feet per minute by 2000 RPM we get 0.35 Propeller Feet Per Minute.

Multiply 0.35 Propeller Feet Per Minute by 12 to get inches, 0.35 x 12 = 4.25.

This is our Theoretical Propeller Pitch in inches. If we were dealing with car tyres on tarmac it would be good enough, but since the propeller turns in liquid water we have to allow for Slip.

Slip varies, stick your hand in a bath full of water, hold your hand flat and move it slowly through the water (like a slap, not like a karate chop) and you will find it very easy, move your hand faster and it gets a lot harder. Use something with a greater area than your hand, for example a tin lid or a steel disk the size of a long playing record (or propeller) and you soon realise that while a baby can pull it slowly, even Arnold Schwarzenegger cannot pull it very fast.

Slip can be calculated, it is 1.4 / (required speed raised to the power (Exponent) 0.57) so for our required speed of 7 knots slip is 46.18%

So, we also have to include Wake Factor which is explained elsewhere onsite, which is 0.93 for our example hull.

The calculation is (Theoretical Prop Pitch x ( 1 + Slip))/Wake Factor which in our example boat gives a Required Pitch of 7 inches.

This is, in car parlance, the “correct gearing” to achieve our desired top speed.