Technical

Turning to propellers

Published on 01 april 2015 at 0h00

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First of all, to choose the right propeller, a few bits of information need to be collected. The boat’s displacement, hull shape, windage, all more important than on a monohull , are of course the first things to establish. Closely followed by the motor characteristics: graphs showing torque and power developed for a given level of rpm, type of gearbox and reduction ratio are all essential so that a real propeller specialist can work out your optimum prop, an appendage which is all too often overlooked. Be careful not to fall into bad habits inherited from years of monohull sailing. Twice the power installed in each hull does not equal the same power installed in the same-sized monohull. Also, if you really want to get to the heart of the matter, your expert should be able to provide further essential information: maximum torque on the propeller, and maximum diameter. And now you’re getting into a really in-depth debate, beyond the comprehension of mere mortals, but which can have big repercussions on the performance of your multihull. A good propeller is always going to be a very good investment. It is the only thing which can compensate if your boat feels under-powered. And in the case of increased power, it will always have a positive effect on fuel consumption, potentially resulting in a significant reduction.

What makes matters get even more complicated is that we like a propeller which offers maximum effect going astern, not just ahead, for maneuvering in harbor. We also want efficiency in the form of low fuel consumption if we have to motor long distances in light airs. And finally we want as little drag as possible when we’ve got the wind in our sails. One further asset would be in they could manage to avoid getting snagged on any of the debris which is sadly drifting around in our oceans. In theory what we need is a propeller shaped like a duckbill when we’re under sail, and a three or four bladed fixed prop for long distance motoring or maneuvering in tight spaces. Simple eh? In years gone by we have been happy with two-bladed fixed or folding propeller, as that seemed to be the best possible compromise. Thankfully a few brilliant minds developed controllable pitch propellers. Eureka! They found a solution to the question of working as well going astern (better than a fixed prop) as going ahead, dramatically reducing the braking effect they cause when they are not in use, and can even be optimized for the speed at which they are turning. Since then, three-bladed folding propellers have become the norm on the majority of our sailboats.

All that’s needed now is the choice of material. Bronze takes the lion’s share, but that’s undoubtedly because stainless steel versions are considered to be prohibitively expensive. As for propellers with composite-built blades, they still have a certain fragility, meaning that they are not necessarily recommended for the fastest multihulls. But watch out, the future is just beginning, not only ...

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