SPE­CIAL RE­PORT

Yachting World - - Front Page -

If you have a fixed three-blade pro­pel­ler fit­ted to your boat, you might as well sail along trail­ing a bucket off the stern. The up­shot is about the same: you lose be­tween 0.5 knots and 1.5 knots of boat speed, de­pend­ing on your hull shape. And yet an es­ti­mated 70% of Bri­tish boats still sail with fixed props.

How­ever, once a boat owner de­cides to up­grade their sterngear to some­thing a bit more stream­lined, there are a lot of de­ci­sions to be made. Al­though pro­pel­lers have been mov­ing wa­ter­craft for more than 250 years, their de­sign is still evolv­ing in re­sponse to new en­gines and new ma­te­ri­als.

There are two main op­tions: the fold­ing and the feath­er­ing prop. The ma­jor­ity of race boats favour the first, be­cause a two-blade fold­ing prop cre­ates al­most no drag at all in the closed po­si­tion, when it re­sem­bles a smooth clamshell. Lars Oster­gaard of Den­mark’s Gori says that Gori’s three-blade folder has a minis­cule drag of just 1.4N (c140g) at 6 knots; and the Gori ap­peared on 90% of the most re­cent IMOCA 60s, as well as be­ing fit­ted to the one-de­sign Volvo Ocean Race 65s.

“From a per­for­mance point of view, a fold­ing pro­pel­ler, no mat­ter the brand, is al­ways more ef­fi­cient than a feath­er­ing pro­pel­ler, due solely to the fact that the blades on a feath­er­ing pro­pel­ler are flat like a pad­dle, and the fold­ing pro­pel­ler blades are twisted and curved,” Oster­gaard ex­plains.

Den­mark’s Flex­o­fold has gone one step fur­ther and de­signed its two-blade fold­ing prop with an op­tional com­pos­ite boss, elim­i­nat­ing 2.8kg of weight from the unit, as well as the dan­ger of elec­trol­y­sis. There are also fold­ing three- and four-blade ver­sions from the likes of Gori, Flex­o­fold, Brun­tons and Sea­hawk, which have only slightly higher wa­ter re­sis­tance than a two-blader. Un­der sail, a fold­ing prop is less apt to tan­gle with fish­ing pots and wa­ter­borne de­bris. That ad­van­tage is much re­duced un­der power, how­ever.

Fold­ing props used to be nois­ier and less ef­fec­tive astern. In the early days of th­ese de­signs, the blades could be re­luc­tant to fully de­ploy when go­ing astern, al­though man­u­fac­tur­ers claim this ten­dency has now been elim­i­nated by gearing them so they open in uni­son. Tests con­ducted by our sis­ter ti­tle Yacht­ing Monthly showed that they still tend to be more

SAM FORTES­CUE

Feath­er­ing props are of­ten fit­ted to yachts where a skeg is too close to al­low a fold­ing model to be fit­ted

A Sea­hawk Slip­stream fold­ing prop in stain­less steel

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