Feathered pros and cons
effective astern at high revs rather than low, delivering around 25% less power astern. They can also make a loud, clunking noise when they deploy, and some are fitted with flexible stoppers for this reason. A feathering propeller, on the other hand, typically has a tiny bit more water resistance, but is nearly as efficient as a folding design for forward propulsion and usually rather better in reverse. It all depends on how well the propeller is engineered.
Feathering blades have flatter faces, so they set better in sailing mode, but that can cost 15-20% of its power. David Sheppard of Brunton’s explains: “Using a flat blade, the load is increased linearly towards the blade tips, which creates a poor radial load distribution and hence lower efficiency; it will also make the propeller blades noisier as they pass the hull.”
Bruntons’ feathering Autoprop tackles this issue with some clever design that builds in a curve to the blade, so that the pitch (and therefore effort) remains constant along its whole length. It is unique in this respect. Most folding blades, on the other hand, feature this technology – such as the Varifold’s so-called ‘helical pitch’.
Another advantage of a feathering prop, and the reason it performs so much better astern, is that the blades are able to swivel through nearly 180° to present their leading edge. On older boats and long-keelers there is not enough room between the prop and the rudder to fit a folding propeller, so owners have to choose a feathering unit instead.
Once you’ve got your head round all that, there are still a few quirks that distinguish the main brands of feathering and folding propeller. In folding props, the pitch is always set at the point of manufacture – literally cast in to the blades.
A feathering propeller, however, usually allows you to adjust the pitch with a few turns of an Allen key. So if you feel your engine is reaching full power before it gets to full revs
(over-pitched propeller) or vice versa, you can adjust accordingly. Some, like the Maxprop and the Autostream, allow you to set different pitch for forward and reverse, so you can tune the propeller to give more thrust at lower speeds.
“Sailboats very seldom run at maximum power and so the efficiency at half load/power is substantially lower,” says David Sheppard of Bruntons.
“This is the reason so many owners over-propped the engine to obtain higher boat speed at lower engine rpm, which will also reduce noise and vibration. This in effect overloads the engine, produces black smoke, reduces the engine service life and clogs up the jets.”
The Gori and the Autoprop from Bruntons both offer variable pitch, allowing them to adjust to the pace of the boat and revs of the engine. In the Gori, this is called the
From left: Bruntons Variprofile three-blade feathering prop in open position; the same in feathered position; two-blade Variprofile, feathered
Kiwi Prop’s feathering blades are made of a nylon-glass composite