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Thoughts on folding and feathering propellers for sailing yachts
All yachtsmen want their boats to be as fast as possible under sail. After all, an extra knot could cut a couple of hours off a cross channel Solent to Cherbourg. We also want our boats to have the power to punch into heavy seas and strong tides, and be manoeuvrable under power in our crowded waters. For motoring we need propeller blade area, but big blades mean increased drag and slower sailing. A conundrum!
Research has shown that a 3-blade fixed prop can account for up to 40% of a hull.s total resistance below hull speed. (Hull speed being the theoretical maximum speed of a displacement hull calculated at 1.34 x the square root of the waterline length). In tests even a small 15-inch 3-blade fixed prop increased the drag of the hull by around 50 lbs., the equivalent of dragging a large bucket behind the boat. The faster you go, the worse it gets because resistance increases with the square root of speed. Other lab tests showed a 16" propeller producing a drag of 70lbs at 8 knots.
By comparison, a 2-blade folder only increases drag by about 4 lbs. and a 3-blade feathering prop by only 7 lbs. So, great under sail but how do they perform under power? We know that 2-blade folders provide the least resistance under sail. Under power however, they rely on centrifugal force to open the blades, but to ensure full opening the blades are often broader, thicker and heavier at the tips and this reduces the overall hydrodynamic efficiency. In astern, the blades may not fully open and at best, thrust astern can be very disappointing, so it is often necessary to rev up the engine to increase centrifugal force and open the blades as much as possible. That is not ideal for relaxed control in crowded waters and marinas.
3-blade folders have a greater blade area and so when motoring they transfer more power to the water but when folded they produce more drag than 2-blades. They are heavier and so they create a bigger shock wave to the transmission as the blades are flung open. This will also create wear on the blade teeth, hub and pivot pins, resulting in jammed or loose blades and even more transmission shock to the gearbox etc.
Feathering propellers work on the principle that the blades do not fold but pivot on the hub. Most have blades connected internally to a bevel gear on the shaft so that when the shaft makes its first rotation, the bevel gears turn the blades from the low drag sailing position to the drive position. On some the pitch is pre-set, on others the pitch is adjustable internally after dismantling and on a few it is adjustable externally but at high initial cost. Under sail most line up the blades with the shaft/saildrive. They usually have no twist to the blades in order to create the least drag when feathered so they are about 4% - 5% less efficient under power ahead than an equivalent fixed blade prop. They are very efficient astern at all engine speeds because the blades immediately adopt a full pitch reverse position. Under sail they can increase speed in almost all conditions by up to 1 - 1 = knots.
Overall then we see that a 3-blade feathering propeller best solves the conundrum. It comes very close to the power performance of a fixed 3-blade yet virtually eliminates the very high drag problems under sail. Compared to a folding prop it has full power astern, none of the wear and transmission shock wave problems and it has sometimes the benefit of pitch adjustment. So what is the downside? Well until the introduction of the New Zealand designed Kiwiprop, it was the initial high cost and then expensive ongoing costs when factory repairs are necessary.
The Kiwiprop was launched in New Zealand six years ago with the clear objective of substantially reducing the initial purchase price and ongoing maintenance costs yet introducing patented new features for better performance and elimination of blade corrosion. This prop is a 3-blade feathering design for engines from 15-55 hp. For performance it is unique in allowing each blade to .windvane. individually to line up with its local water flow and not just with the shaft which is often at an angle to the buttock water flow. This reduces sailing drag even more than a geared design. The Kiwiprop hub components are made from either marine quality AB2 bronze or stainless steel. The blades are made from 40% glass reinforced Zytel which is an extremely strong Dupont material with especially high resistance to impact (Dupont also make Kevlar). They are so light compared to bronze or stainless steel that they create almost nil shockwave to the transmission and they eliminate the very costly blade corrosion that can be a serious problem. Individual blades can be replaced easily within a few minutes at low cost and without the necessity of balancing blade weight etc. Pitch is easily adjustable externally and lubrication is simple via external grease points.
So the Kiwiprop brings innovative design for advantages in performance and maintenance and it cracks the only serious drawback of feathering props, price! FOR ALL SIZES THE PRICE IS THE SAME AT £845 plus VAT. This is less than many folders, and in most sizes is about half the cost of most competing feathering props. Too good to be true? No, this innovative prop was designed to be efficient, tough and low cost. It is!
Do contact us for a full information pack.
New Zealand.s best-kept sailing secret is now engineered in the UK by Vecta Marine
Click here to read about the Kiwiprop Design Philosophy
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Hamble Point Marina|
Unit 6 Firefly Rd
Hamble, Southampton, Hampshire SO31 4NB, United Kingdom
Tel +44 (0)23 8045 7278
Fax +44 (0)23 8045 7308