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Propeller test - Yachting Monthly
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Propeller test - Yachting Monthly - 1

BOATS & EQUIPMENT GROUP GEAR TEST ULTIMATE PROPELLER TEST Which is the best prop for your boat? In the most comprehensive test ever published, Emrhys Barrell and the YM team compare 15 propellers for speed, thrust, drag, stopping distance, prop walk and price hat’s the difference between a bucket and a fixed, threebladed propeller? Not much, if you want to slow your boat down by at least half a knot when you are sailing. That’s the conclusion of our test on fixed, folding and feathering props – the most detailed ever conducted in the UK, and as far as we are aware, in the world. A fixed three-bladed prop, free to spin, exerts as much drag as hanging a bucket over the stern of our test boat. And if you lock the shaft, as many gearbox manufacturers require, then you might as well chuck another bucket over the stern and lose a whole knot of boatspeed. Put another way, prop drag will add about four hours to a typical cross-Channel passage. So what is the solution if you don’t want to sacrifice precious speed? The answer is, fit one of the many folding or feathering propellers on the market. But which one, and what are the drawbacks? We tested all the props we could get our hands on, measuring top speed under power, thrust (bollard pull) ahead and astern, stopping distance, and – for the first time in a sailing magazine – we measured the side-force (prop walk) generated when you put the engine into reverse. It is this vital, unwanted component that sends you swinging off to one side when you try to stop suddenly, or back-up in a marina. We then towed three typical props behind a test boat to measure their drag, and how it contributes to the overall drag of a yacht under sail. Prop drag has been an issue for sailing vessels since the propeller was invented. Early trials were carried out on English warships, using jointed shafts that could be lifted into a trunk in the hull when sailing. In the 1890s, Danish pilot boats used similar lifting shafts. For many years, long keels reduced the drag of two-bladed props on yachts, but the advent of the fin keel and exposed shafts brought the problem back again, with the added factor of more powerful engines making three-bladed props a necessity. The need to manoeuvre precisely in tightly packed marinas and overcrowded harbours has convinced even the owners of many long-keeled yachts to fit three-bladed props. Most cruising skippers simply ignored the unwanted drag and loss of speed, but in racing circles it became increasingly significant. This led to the development in the 1960s and ’70s of propellers whose blades folded backwards when sailing, reducing the drag markedly. The blades flew out under centrifugal force when the engine was put into ahead or astern. The earliest folding props had blades that moved independently, but this could lead to the lower blade dropping down when sailing, so the roots of the blades were linked with geared teeth, ensuring that they opened and closed together. At the same time, an alternative approach was developed: the feathering propeller. Here, the blades are set at right-angles to the boss, as in

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Propeller test - Yachting Monthly - 2

Folding, FeAthering And Fixed propellers Athering PROP FACTS The four main figures used to describe propellers are diameter, pitch, number of blades and rotation. Diameter is double the radius (distance from centre of the boss to tips of blades. As a rule of thumb, the more powerful your engine, the bigger the diameter you will need. Pitch is the measure of how far the propeller will advance in one revolution and thus how fast it will push your boat through the water for a given engine rpm (revolutions per minute). To understand pitch, imagine screwing a screw into a block of wood. The...

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Propeller test - Yachting Monthly - 3

BOATS & EQUIPMENT GROUP GEAR TEST THE TEST We used a Bénéteau Océanis 323, kindly lent to us by Sailtime in Lymington. She has a typical fin keel form, but atypically she has a built-in skeg carrying the shaft, rather than the P-bracket or saildrive of most other modern yachts. The skeg protects the shaft and prop from underwater damage, but a downside is increased vibration as the blades of the prop pass through the disturbed waterflow behind the skeg. This is resolved in normal use by fitting a three-bladed prop as standard rather than two, but otherwise did not alter the propulsive...

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Propeller test - Yachting Monthly - 4

Folding, FeAthering And Fixed propellers A Athering PROP TIPS FITTINg Some of the props on test are very simple to install, others are very complicated. However, while they all have instructions for DIY fitting, unless you’re very confident in your own skills, a piece of equipment as vital as a propeller ought to be professionally installed, both for safety and peace of mind. For our test, we had every prop fitted by the manufacturer’s representative, so there was no question about the installation, and they also observed all our tests and measurements. MATERIALS Propellers have been bronze...

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Propeller test - Yachting Monthly - 5

BOATS & EQUIPMENT GROUP GEAR TEST AUTOPROP Brunton’s Autoprop marked a completely different approach to feathering props when it was brought out in 1987. The three blades rotate independently, going from fully feathered while sailing, or in neutral, to fully twisted under power. The difference is that when you go into gear, the amount they rotate depends on engine speed and loading. Thus the ahead AUTOSTREAM THREE-BLADE From Australia comes the Autostream, a three-bladed feathering unit that has been in production for 26 years. Construction is all-stainless steel with a zinc anode at the...

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Propeller test - Yachting Monthly - 6

Folding, FeAthering And Fixed propellers A Athering FEATHERSTREAM THREE-BLADE Made in England for the past 18 months, this is a three-blade feathering unit with a bronze boss and stainless steel blades. The pitch of the blades can be adjusted externally, and can be different for ahead and astern. The blades swivel 180º to present the same leading edge in ahead FLExOFOLD THREE-BLADE Made in Denmark for the past 12 years, the Flexofold is an all-bronze folding unit, with blades fixed by stainless steel pins, and the anode, which covers the gearing, cleverly enclosed by the folded blades. On...

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