Buyer’s Guide

Propellerheads

Nobody goes sailing for the motoring but the reality is that both engine and propeller are vital. Sam Fortescue explores which prop works best for you

Fixed props are still the most efficient when you’re under power alone, but as sailors, we’re more interested in using wind power. With the sails up in a good breeze, a fixed prop can generate 30-40kg of drag on a 40-footer – costing you up to a knot in boat speed. But a folding or feathering prop can eliminate 95 per cent of that unwanted friction.

If you are repowering your boat, it is a golden opportunity to upgrade a propeller that may well need to be changed in any case. But it’s worthwhile on its own merits. Club racing without a folding prop is no fun at all, while the prospect of 10 per cent faster passage times has obvious appeal to cruisers.

What you want in a prop

A good propeller must always be designed to suit your boat’s engine and gearbox, otherwise it cannot perform efficiently. Too small and the prop won’t get the boat up to hull speed or perform in heavier conditions (when you might need it most); too large and it can stop the engine from reaching optimum power.

“The main misunderstandings are the big differences between engines, even when their power ratings may be similar,” explains Glen Print, MD of Sillette. “Yes, I say to the boat owner, but your new engine rpm is 3200 and your old engine was 3800 and therefore the propeller requirement is vastly different. Same applies to different gearbox reduction ratios.”

A classic mistake is to install a more powerful engine, then discover that the larger prop needed to transmit that power to the water won’t fit under the boat.

“When repowering, the science must be fully understood,” Print adds. “It’s always best to start with the propeller and what limitations the boat has to swing the propeller that matches the new power unit.”

How many blades?

Racers with slippery hulls will often prefer a two-blade prop, because it offers minimum resistance and the least vibration. But the more powerful the engine and the slower the hull, the more blades you’ll want. It’s similar to car tyres, where you need enough surface area to transmit the power without slipping.

There’s also a relationship between propeller diameter and blade number. In principle, the most efficient set-up would be a really big two-blader, but since boats generally have limited clearance between the end of the prop shaft and the hull, the solution is three or four smaller blades.

Pitch is the other key characteristic, and refers to the angle of attack of the blades. Measured in inches, it refers to the forward distance the propeller

would travel in one complete revolution if it gripped the water perfectly. You can over-pitch a propeller to compensate for a restricted diameter, but it quickly becomes inefficient and wearing on the engine. Excessive pitch can also exaggerate prop walk astern.

Feathering vs folding

Racers will almost always prefer a folding propeller, whose blades shut like a clamshell. That’s because they have the lowest drag of all – less than a kilogram according to some tests. This is particularly the case when they are tucked behind the leg of a saildrive. On the other hand, they can be noisier when deployed, opening with a loud clunk as the blades spin out and hit their stops. They also tend to be less efficient in reverse – in part because they are trying to close themselves, and partly because they present the trailing edge. Several brands have geared the blades so they open in unison, and installed rubber or plastic ‘bumpers’ to reduce the noise. The folding Gori even allows its blades to swivel for greater efficiency aft.

Feathering propellers generally appeal more to cruisers, because they offer much better performance astern (thanks to swivelling blades) and in close quarters, albeit for marginally greater drag. Feathering units usually feature flatter blades, which are less efficient drivers of the boat than the curved blades of a fixed or folding propeller.

Most feathering propellers can be used in hybrid systems that generate power from the prop while sailing, and because of its unique design, the folding Gori can too. Every propeller mentioned here is available in shaft or saildrive variants, but expect to pay a little more for the saildrive version. Finally, when it comes to pricing, bear in mind that buying direct from European manufacturers may now be tax free, but incurs import VAT of 20 per cent and handling fees.

Gori

There has been little change recently in the design of the award-winning Gori – since 1994 it has offered a unique folding action that claims to have the thrust of a fixed prop. Engineered in Denmark from nickel aluminium bronze, the blades are geared so that they open in concert. They pivot through 180 degrees to give the same pitch in forward and reverse, which means optimum speeds in both directions. Goris also have an ‘overdrive’ function for motorsailing which deepens the pitch of the prop to give the same power from lower engine rpm, like the fifth gear of a car. “Still without any doubt the most efficient sailboat propeller!” according to senior VP of sales Lars Østergaard.

With the lowest drag of any folding prop, according to some German tests, Gori’s special two-bladed race variant is used by most of the Imoca 60s, Melges 37, ClubSwan 50, as well as many of the big racing trimarans, including François Gabart’s new ultim 101. Blades: 2, 3 or 4 Diameter: 11.5in to 30in Engines: 10hp to 300hp

Gori-propeller.com

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Gori's three blade folding propeller

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Bruntons Autoprop

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Bruntons Varifold

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A featherstream fitted to a Rustler 36 when you are motorsailing, they pitch up to account for the driving force of the sails, reducing fuel consumption. This feature should also reduce prop walk astern. With the prop shaft locked in sail mode, the blades balance in between, presenting a fine profile to the water.

Bruntons also manufacturers the Eco*star specially for hybrid or electrically powered boats. Based closely on the Autoprop, it has all the same features, but slightly different blade geometry that is optimised for spinning backwards in sail mode and generating electricity to recharge the batteries. This has seen it specced for Saildrone’s autonomous sailing craft. All are manufactured in Essex. Blades: 2 or 3 Diameter: 13in-35in Engines: up to 350hp

bruntonspropellers.com

Varifold

Autoprop

Bruntons has used lockdown to start work on a new range of controllable pitch propellers. But its existing feathering propeller already uses smart engineering to allow the pitch of each blade to change according to the need. So, Also manufactured by Bruntons in

Essex using high quality nickel aluminium bronze, the Varifold is designed to appeal to racers and fast cruisers. As a folder, it offers less resistance in sail mode than its Autoprop sister.

The company claims a unique design for the blades of the prop that cuts noise and vibration. A helical pitch, including reduced pitch towards the blades’ tips, reduces pressure and noisy pulses against the yacht’s hull. Blades: 2, 3 or 4 Diameter: 12in to 45in Engines: up to 1000hp

Bruntonspropellers.com

Featherstream

This UK-built propeller is manufactured and designed by Darglow, near Poole, using aluminium bronze for the hub and stainless for the blades. Recent improvements include a new lightweight hub suitable for small boats with saildrives. “It is approximately £4-500 less expensive than the large hub version which we used to supply,” says head engineer Chris Hares.

The pitch on a Featherstream is determined by the pitch cassette

installed with it. is can’t be altered when the boat is in the water, but it is always possible to send o for a di erently pitched cassette from Darglow and install it via a 10-minute operation when the boat is hauled. If you send the old cassette back, this is cost-free. e blades are completely at for frictionless sailing, and rotate through 180 degrees for optimum power ahead and astern. All models are now compatible with the Ambassador rope stripper, and up to 50mm sha diameter can be accommodated. Blades: 3, 4 or 5 Diameter: 12-24in

darglow.co.uk

Maxprop

Engineered in Italy since 1975 by Maximilian Bianchi, Maxprop owns its own foundry as well. e blades are at and pivot through 180 degrees, so that the leading edge is presented in both forward and reverse – making for excellent thrust either way.

Pitch can be changed in 2-degree increments at the turn of a screw, which can even be done in the water with the Easy and the Whisper models. It is not quite the variable pitch that the manufacturer claims, but it does o er some exibility. e blades are linked by gears so they open and close at the same time. And the Boomerang variant for larger boats with hydraulic transmission has patented springloaded blades that return to the feathered position without any need for a sha brake. Maxprop says its products are particularly good for catamarans. Blades: 2, 3, 4 or 5 Diameter: 12in to 44in

maxprop.it seaviewprogress.com

Kiwi Prop

Essentially unchanged over more than 20 years since its launch, the blades of the Kiwi Prop are unique in being made of a special nylon-glass composite called Zytel.

It is tough, lighter than metal and corrosion free, plus it is much cheaper than bronze - a fact re ected in the lower retail prices. e manufacturer also points out that the blades are sacri cial in the event of a collision, and much cheaper to replace than potential repairs to the stern gear.

Recent tweaks to this successful 20-year-old design include a new more e cient shape to the foil of the outer part of the blades, triangular rollers and titanium blade retention pins. e blades are fatter than some but free to rotate around their axis for excellent feathering. e unit o ers adjustable pitch ahead and maximum pitch astern for more power at low speeds.

Kiwi has developed a two-blade folding propeller for race boats in the same lightweight Zytel composite It will soon be available in diameters up to 17in, roughly suitable for a 40hp engine. Blades: 2, 3 and 4 Diameter: 14.5in to 21in

vectamarine.com

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A Maxprop tted and ready to go

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e Slipstream prop

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e Autosteam feathering prop

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Flexofold's twin blade folding prop adjusted - as can direction of rotation, in case a change of engine necessitates it. Blades: 3 Diameter: 15in to 20in Engines: up to 75hp

Seahawk.com.au

steeldevelopments.net

Slipstream

e folding prop from Seahawk is also in stainless steel – but this time low carbon 316 for long life. “ is being the same metal as most prop sha s and saildrive internal parts, it is less likely to promote electrolysis, the most common killer of props,” says Julie Hale of Steel Developments. e blades are geared, so they are synchronised, and the bushes can be replaced easily when worn. Polyurethane bump stops prevent the prop from clunking when it swings open, and the same plastic forms side plates to stop metal-on-metal wear. Seahawk provides a ve-year warranty. Blades: 2 or 3 Diameter: 14.5in to 18.5in Engines: up to 75hp

steeldevelopments.net

Autostream

is feathering prop is manufactured in Australia by Seahawk, but sold in the UK by Steel Developments in London. It is entirely made of 2507 duplex stainless steel for exceptional strength and resistance to corrosion, and features narrow blades that swing through 180 degrees in reverse. “Reverse acceleration is faster leading to less ‘prop walk’ and giving you full control over which direction you head,” says Julie Hale at Steel Developments.

Pitch is independently set for ahead and astern, and can be easily

Flexofold

Also designed and manufactured in Denmark, the Flexofold uses high quality nickel aluminium bronze in its construction. Being water lubricated, the gears on its folding blades need no grease. Its simplicity and competitive price have made it the go-to prop for boatbuilders including Beneteau, Hanse, Dehler, X-Yachts and more. And new this year, there is a larger hub available to suit ZF’s new SD15 saildrive, which is making headway in the 110-150hp sector where previously a sha drive

was the only option.

The Flexofold has large shock absorbers for quiet opening and in tests, it performs well both ahead and astern, although its blades don’t swivel to offer the leading edge. Pitch changes require a new set of blades which can be fitted to the existing hub.

Flexofold is also available in an award-winning race variant with a composite boss that saves 2.8kg of weight. It is only available for saildrives, but the use of composite makes the hub immune to electrolysis. Blades: 2, 3 or 4 Diameter: 13in to 27in

Flexofold.com

Variprop GP

Manufactured in Germany, near the North Sea coast, the Variprop is one of two offerings from SPW. It is a robust prop, built to order in ice-class nickel aluminium bronze (Nibral) for long life. The blades were recently redesigned with a Gawn profile, which SPW says make them 10 per cent more efficient than the old design. At the same time, the internal gears were switched from cast to machined, reducing vibration.

Flat, thin blades rotate 180 degrees in reverse to offer the leading edge in both ahead and astern. A simple turn of a screw changes the pitch, which can be independently set in both directions – ahead it is all about optimising thrust, while in reverse it is about reducing prop walk.

A patented multidisc brake acts as a shock absorber to eliminate the clunk when you engage the prop. SPW reckons it has the shortest hub in the business, making the prop really suitable for long-keeled boats with a small prop aperture. Blades: 2, 3 or 4 Diameter: 12 to 32in Engines: up to 750hp

Spw-gmbh.de

ABOVE LEFT The Variprop

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The Variprofile feathering prop

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EWOL's Orion SD prop

Variprofile

Similar to its Variprop, Variprofile is SPW’s cheaper, small-boat feathering prop. It is mass-produced for modern serial yachts with engines up to 140hp, but otherwise has many of the same features as its custom-built sister, including micro adjustment for the forward and aft pitch and Gawn-profile blades. Diameter: 12in to 18in Engines: up to 140hp

spw-gmbh.de

EWOL

This Italian company’s range of beautiful stainless steel propellers have undergone something of a revamp this year. Chief among the changes is a new design of ‘Sword’ blades which are said to be more efficient and quieter in rotation. The Andromeda unit is designed for sailing boats from 6-11m LOA, and engines up to 40hp. It is a threeblade feathering prop.

Other lines include the newly engineered Orion SD-15 – another three-blader with a stronger body designed for ZF’s new saildrives up to 150hp. And for 70ft-plus boats, the Pegasus four-blader should do the trick. An external pitch adjuster allows for fine tuning, while parts that wear over time can be easily replaced. There’s even a titanium bladed option. Popular in Italy, the prop has found its way onto many of the country’s performance cruising boats, as well as an 80ft Sunreef catamaran. Blades: 3-4

Diameter:

Engines: 30-150hp

ewoltech.com

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