Special report Sam Fortescue reports on propeller choices
SAM FORTESCUE on propeller choices
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
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.
Feathered pros and cons
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
‘overdrive’ feature, where a coarser pitch can be engaged by jockeying the gear lever when motorsailing.
The Autoprop is a clever design that adjusts constantly. Because each blade’s weight is offset, they adopt varying pitches mechanically, pitching up or down to account for the driving force of the sails, reducing fuel consumption and engine rpm. This feature should also reduce prop walk astern.
Material options
The material of choice for propellers is an alloy of nickel, aluminium and copper, so-called aluminium bronze, which offers better strength and corrosion resistance than other bronzes.
However, there are a few notable exceptions. Australia’s Seahawk manufactures its Slipstream folding prop in low-carbon 316L stainless steel, sold in the UK by Steel Developments. “This being the same metal as most prop shafts and saildrive internal parts, it is less likely to promote electrolysis – the most common killer of props,” says Steel Developments’ Julie Hale.
The feathering Kiwi Prop’s stubby blades are made of special nylon-glass composite called Zytel. Glass may seem an odd choice for a propeller, but the material is incredibly tough, lighter than metal and corrosion free. It is also much cheaper than bronze.
Finally, Brunton has just announced that it will manufacture its two-, three- and four-blade Varifolds in titanium, after a successful project with a Baltic 115 cut 50kg out of the standard bronze prop. Titanium is much more expensive, but it offers excellent strength-to-weight characteristics and good corrosion resistance.
Most modern props are pretty much maintenance free. Some, like the Kiwi prop, require an annual squirt of grease into the boss, but many are water lubricated. Otherwise all props just need to be kept reasonably clean of marine life.
In the end, the choice of propeller is a compromise of performance under sail and under power – it all depends where on that spectrum the boat owner’s priorities lie.