25. Choosing a Propeller

Choosing a Propeller:
There are a number of factors that need to be taken into account when choosing a propeller for your model boat. The main ones are discussed in the following - in no particular order. We do not presume to provide a single answer for any situation.
Size: This is chosen to meet a number of factors:
- how big is the boat? - both the length and beam can be important factors
- how powerful is the engine and associated boiler.?
- how fast do want it to go?
- how much vertical space is going to be available in the skeg or with the propeller support brackets to allow the water to flow freely past with minimum interference from turbulence at the tip of the propeller?
Right hand (RH) or Left hand(LH) screw:
There are at least  two accepted methods of specifying this. We use the convention that when viewed from outside the boat looking toward the bow a RH propeller rotating clockwise will drive the boat astern and a LH propeller with the same clockwise rotation will drive it forward. We will not discuss the alternative definitions here. RH or LH selection is only important when the attached engine is non reversing and when planning a twin screw boat.
In simple terms a reversing engine can be run in either direction of rotation and achieve "ahead" or "astern" movement of the boat so the hand of the propeller is not important.
With twin screw boats, the same diameter propeller, one each RH & LH, from the same manufacturer should be chosen. This is to neutralise the effect the torque forces that are generated on the hull when turning the propeller. Likewise if replacing a damaged prop buy the replacement for the same manufacturer to maximise your chances of getting a balanced drive system.
Propeller Pitch:
This is the theoretical distance the propeller will move a boat on water in one turn of the propeller. This is analogous to the pitch of thread on a bolt i.e. one turn of the nut will move it exactly the pitch of the tread it is threaded on, except the actual distance moved by a boat is less than the "pitch" of the propeller because of slippage of water past the propeller. Propeller pitch is treated with great reverence and much analysis by specialists. 
Our propellers do not have a specified pitch in that they were designed to accommodate pitch into the shape of the blades and being precision investment cast they do not need any subsequent tuning or balancing.Test tank evaluation during their design/development outperformed alternative products when used with our engines. 
Fabricated or Cast Propellers:
Fabricated propellers are much cheaper than their cast competitors and can be much less efficient and more easily damaged.
On the other hand cast propellers can be highly efficient and not easily damaged. We offer a range suitable for use with the engines we make. We are making no comparative comments other than those related to pitch above, just let the products speak for themselves!