A well-known truism of boat ownership is that sailing with a fixed propeller is akin to towing a bucket astern. We all know that a feathering or folding propeller has huge advantages under sail, but what are the costs of motoring with a sub-optimal propeller? Even purists will use the engine to manoeuvre in and out of busy harbours, and most of us are willing to fill in with horsepower when the wind drops.
You won’t be surprised to hear that propeller manufacturers believe that sailors often have the wrong screw fitted. And if that delicate balance between the propeller size, engine power and the boat’s speed potential don’t align, you’ll be losing money to inefficient performance under power.
‘It’s pretty common that props aren’t the right size,’ explains David Sheppard, managing director of Bruntons, maker of the Autoprop. ‘The usual first sign is that the engine is being overloaded or underloaded. If it doesn’t reach its full rpm, then it’s overloaded. And if it’s underpropped, you’ll get revs above the rated rpm. It’s more difficult to tell with modern diesels, but in the old days, you’d get black smoke coming out of the back because there’d be too much diesel going in and the engine wouldn’t be able to turn fast enough to burn it.’
There are some reasons why you might want to be overpropped, but engine manufacturers will
