2 ENGINES AND PROPULSION MATCHING THE MACHINERY


It’s essential that the boat and the various components in the drive train are well matched to your intended usage. This means there has to be the right engine for the boat and the right prop for the engine. Judging this isn’t as easy as it sounds and almost always involves making compromises, since any combination will only operate at its peak efficiency over a narrow range of conditions. Yet we often expect too much. For instance, we might have a very powerful engine in a high speed boat and ask it to troll at just 3 knots for several hours when it would be much happier roaring along at 30. Not only will the engine be running inefficiently but it might actually be suffering damage. In those circumstances a slower boat with a smaller engine might have been the better choice.


Unfortunately, this is too big a subject to explore in depth here. Luckily, there’s plenty of information out there to refer to.


DIAMETER, PITCH AND SLIP


Propellers are classified by their diameter, pitch and direction of rotation. The first is, of course, the diameter of the circle swept by the blades, and rotation is the direction the upper blades move if viewed from astern. For example, if seen to turn clockwise, the upper blades will move to starboard and you have a ‘right handed’ prop. If they turn the other way – ie, anticlockwise – the prop is left handed.


Pitch is the least easy to understand. It is best thought of as the distance a propeller would screw itself forward if it was to make one revolution in an unyielding material. Sadly, water isn’t unyielding so this is a largely theoretical concept. In reality, the prop will never ‘advance’ 100% of its pitch, and that shortfall between advance and pitch is called ‘slip’ – and there can be a lot of it in headwinds or, perhaps, with a fouled or heavily laden boat. Expressed as a percentage of pitch, slip can easily be as much as 50%.


Advance Slip


A right handed prop. Viewed from the stern the blades when at the top turn to the right.


Pitch


14


RYA Seamanship for Sea Anglers


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