Feath­ered pros and cons

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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.

Pitch per­fect

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

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