Folding vs Feathering Prop
Date: November 29, 2001
From: Paul Uhl endvr32@endeavourowners.com
Selecting a prop is a bit of a science for which I don't feel entirely qualified to do. However I have read enough about them to offer the following advice:
Which is the best prop depends entirely on you're needs as each type has its advantages and disadvantages. There is a great article in SAILING, October 2001 that reviews the different types of props as well as the trade offs. The article lists 11 companies 'non-fixed' props breaking down into 3 categories: folding, feathering, and variable pitch.
Folding Props
(Martec, Gori, Tristream) do just that. When not in use water flowing across the blades causes them to fold into a tapered shape reducing drag. When the shaft is engaged, centrifugal force makes the blades pop open. These props are traditionally used by the 'go-fast' crowd. Folding props deliver good thrust in forward, little drag under sail, but have traditionally suffered from poor performance in reverse, at least compared to fixed and feathering props. A new generation of folding props however is rapidly changing that.
Feathering Props
(Max-Prop, Bowman, Autostream, Hydralign, Luke, Inc.) are different in that the blades shift or feather into a low drag position (basically the blades rotate to align fore and aft with the boat) when the engine is stopped. Once restarted the blades rotate about 45°. Feathering props are able to have more blade area than comparable fixed or folding props and are therefore more efficient under power. One of the greatest advantages is that the blades can rotate in either direction allowing for excellent thrust control in either direction. The difference when backing is dramatic. If you're concerned about prop walk this should be your choice.
Variable Pitch
(CDI, AutoProp, VPF) are really for BIG boats (+50ft) that are under power a lot more. I don't think they're suited to our needs. They're also very expensive.
Sizing
As for sizing a new feathering or folding prop, you really need to have a professional do that for you. I can tell you that with a folding or feathering prop you will have a 5-10% power loss over the fixed blade. You should also first determine if the size you have is correct for your engine. In simple terms you should be able to maintain hull speed at the continuous RPM rating for your engine. If you can't, your undersized, if you maintain speed at a lower RPM you oversized.
Example
Our E32 has a 16x12 2-balde prop--the Endeavour specified size. However we only have about a 1/2" hull clearance with it and can maintain hull speed just below our rated RPMs. That tells me that I should go down in size so I've been considering a 15" feathering prop. This gives me added hull to prop clearance and should bring the hull speed / engine RPM rating more into line. But before I go and spend the $1,300 on a prop, I'll speak to a professional. For instance a 15" blade reflects its length but you can also buy a wider or narrower blade to offset a loss or gain in power.
See why it's a science.
Prop walk is caused by the fact that a fixed blade prop is NOT symmetrical (forward vs reverse), that's why they have "right hand" and left hand" props--each of which will create prop walk in the opposite direction of their rotation. Feathering props are symmetrical and although they don't "grab" as quickly as a fixed blade, will virtually eliminate prop walk. A horizontal prop shaft will also reduce the effect but a feathering prop goes a lot further in eliminating the problem.
Correspondence with MaxProp Date: October 18, 2000 Thank you for your interest in the MAX-PROP Automatic feathering propeller. Based on the information provided, the following is our recommendation foryour Endeavour 32. Two-Blade 16x12 at $1420.00
Date: October 19, 2000 Thank you for the prop specification. I was also wondering if you might have another thought about sizing. Our current 16 x 12, which by the way is the recommended fixed blade prop size for our boat/engine, has only about 1/4" clearance from the hull--pretty tight. I know it's recommended that we have at least 1" of clearance between the blades and the boat. Would you have a sizing recommendation for a 15" prop suited to our boat and engine? My concerns about the smaller diameter prop would be that 1) it allow our engine to run at it's proper rpm, 2) that it doesn't over or under tax it, and 3) that we don't loose performance. Yanmar 3GMD I don't have the performance curves for the 3GMD, but we typically run it at 2,000 (5knts) to 2,500 (+6knts) in waves of about 1ft. Regarding your other comment: The Max-Prop Feathers automatically with the forward momentum of the If we stayed with the 16 x 12 feathering prop, what kind of forward performance loss could we expect compared tou our fixed blade? While I realize that it may take a few more rpm's to get the feathering prop to 'bite', will the feathering prop require us to operate our diesel at a different rpm for optimal performance, speed, etc.?
Date: October 19, 2000 Paul, Hope I have answered all your questions. Please contact me should you need additional assistance. Sincerely, Kelly S. Keeney PYI, INC. Date: October 29, 2003 I put a 3 blade Max Prop on a few years ago. The only modification was having to cut off part of the threaded portion of the shaft. The result? More speed under power, more speed under sail, noticeably better braking, no improvement in reverse handling, some reduction in prop walk but it is still there. I have learned to live with the prop walk, it can be a great advantage getting into tight places. Date: October 30, 2003 I have a '78 E-32, which was equipped with a 16" 3 bladed fixed prop. The prop walk to port, in reverse, was so bad as to make the boat almost unmanageable. Reversing to slow down was often a crap shoot, in part [I believe] because of the hull shape of the 32 and the rather steep angle of the prop shaft. I bought and installed a MaxProp 3-bladed feathering prop and the problem was solved. I can't speak highly enough of the MaxProp. I looked at several different varieties of folding/feathering/variable pitch props, and the MaxProp, for my money, was the best in terms of performance, price, etc. It also permits one to set the forward pitch independently from the reverse pitch; setting a "shallow" pitch for reverse helps eliminate the "paddlewheel" effect that causes propwalk and reduces the stress on the engine/transmission combination. On my boat, I had to cut a small amount of the propshaft off to fit the propeller (don't remember how much, but not much, less that 3/4"), but that really was a minor effort (especially as compared with replacing all the ports). Now, reverse actually works like it is supposed to, and I find I'm getting much more efficiency in forward, too (measured as knots/engine rpm). I'm a happy camper. ("My experience may not be typical, and results may vary for other owners.") By the way, I got a super deal because I bought the thing at the Pacific Sail Expo and got a "boat show" price. Hope this helps.
From: Kelly Keeney kkeeney@pyiinc.com
Three-Blade 16x11 at $2185.00
From: Paul Uhl endvr32@endeavourowners.com
22.5hp @3,700rpm
cone clutch
1" shaft
boatproviding the least possible drag. This on average will increase your
sailing speed 15%. Both the two and three blade propellers will power the
boat well in flat water, however, the three blade will maintain boat speed
up to 30% better than the two blade when going against heavy wind or seas.
The Max-Prop is approximately 80% more efficient in reverse over a fixed
blade, giving you better control, help reduce walking to port or starboard,
and allow the vessel to back down and maneuver more efficiently.
From: Kelly Keeney kkeeney@pyiinc.com
Typically the Yanmar engines are rated to 3600 rpm maximum which we find most people cruise around 2800-3000. One way to eliminate any fears of performance loss in forward would be to go to a 3-bladed propeller. The drag characteristics between the 2-blade and the 3-blade are nil. As for the performance loss on a feathering propeller, they are about 2-4% less efficient than a fixed blade of the same size so you may have to adjust your rpm a bit, however this being such a minimal amount, we find most people do not notice any loss what so ever. You may see a very slight decrease in power and thrust by going down to a 15" propeller, however, you would adjust the pitch higher to properly load the engine, for example, a 16x12 2-blade would be equivalent to a 15 x 13 2-blade, so as you drop in diameter, you increase in pitch.
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