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
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