Propellers are not quite a simple as they look, with varying types, pitches etc. Callum Smedley takes a look at how boat propellers work
A propeller is fitted to a vessel so that the power from the engine can be transmitted, via the gearbox and shaft, into the water at the aft end of the hull. Think of it as winding a nut along the threaded section on the shaft of a bolt. When the nut is turned it has to move forwards or backwards, and this is just the same with a boat.
However, in real life there will always be a bit of slip, because the water is a fluid rather than a stiff straight length of steel. And as the hull of a boat becomes more and more fouled there will be more and more slip, which means that the boat will start to move less fast for the same amount of power input.
Look at your propeller carefully and you will find numbers and letters stamped on the hub. Take 17 x 11 LH for instance. The first number (17) is the diameter of the propeller in inches; the second number (11) is the pitch of the prop, again in inches; and the letters LH tell us that it has left-hand rotation.
What is propeller rotation?
Propeller rotation is defined when you look at the propeller facing forward (towards the front of the boat from the back of the boat), with a clockwise action described as right-handed and an anti-clockwise action described as left-handed. So, in this case, our LH prop will rotate anti-clockwise when viewed from the rear with the gearbox set ‘ahead’.
It’s also important to note that, conversely, an engine’s direction of rotation is defined from the front looking aft – the opposite to propeller rotation. And never assume that the propeller’s rotation will be in the same direction as the engine’s rotation, because the gearbox may rotate the propeller shaft in the opposite direction when set ahead. The same engine, when fitted with a different gearbox, could have opposite propeller shaft rotation – so assume nothing when specifying a propeller.
What is propeller pitch
The term ‘propeller pitch’ is very important. In the above example, the pitch is described as 11 inches, which means that if there was no slip for one full turn of the propeller shaft the propeller would travel, either forwards or backwards, 11 inches. If the pitch was 12 inches the prop would travel 12 inches, and so forth.
A rule of thumb is that more pitch means more speed but with more power required.
There are three main types of propeller used on sailing yachts:
Fixed propellers are powerful and reliable for boats that rely heavily on their engines, but create a lot of drag.
Folding propellers have little drag while under sail, but they can be jerky when opening up and can cause poor performance astern.
Feathering propellers are more efficient than a folding propeller, while still hugely reducing drag.
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