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Old 31-01-2011, 18:16   #31
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You are lucky to not be restricted by space in your propeller choice; those of us with apertures are more limited.

I went through this last year, and things I think I learned are:

Folding props are not such a great idea for cruising boats: a small, strategically located barnacle can prevent it from opening. For boats cruising in the tropics or the Med this is a significant risk. And they are known to not always open reliably in reverse.

Autoprops work very well if correctly sized to the boat; if not they are a problem. Unfortunately you spend your money and take your chances - if the factory recommendation doesn't work they are not going to take it back and send a different size to you at no charge. Too risky for me - but then again I don't have room for it.

Feathering props are flat to appeal to racing sailors: flat minimizes drag under sail. Basically the trade-off is to try to perform under sail as best as possible compared to folding props while having the reliability advantages of feathering. The cost is efficiency under power, as has been noted earlier.

The only feathering prop I have located that has proper curved blades is the Luke (formerly Hyde, available since at least the 1920's). The curved blades and large hub do mean a higher drag under sail than more modern feathering props, but that is compensated by the higher efficiency under power. I have run into a number of cruisers with Lukes who swear by them, and I decided to go that way. The design is simple and stout. The disadvantage is that the pitch stops are machined into the hub, and would need to either go back to the factory or a machine shop to change.

I am in the midst of installing a new engine and stern gear so I can't provide any personal experience at this time. Good luck with the decision.
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Old 31-01-2011, 18:46   #32
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We have a Hundested controled pitch 24 inch 3-blade on our Camper & Nicholson 58. The engine is 115 HP Westerbeke that runs at 1100 with a 2.5:1 reduction & reversing hydrostatic transmission. Blades can be manually pitched from the cockpit from feathered to 90 degrees. This gives tremendous thrust at zero speed and pretty good efficiency steaming. Stuck on sand, I went to reverse and low pitch. The boat shot off the bar with water flying off the bow. We get about 4 mpg at 9 knots steaming. I think this is pretty good for 55,000 disp.

Note that the French report linked above looks like they did thrust comparisons with a restrained vessel. I could not read the French so I likely don't know the whole test, however, a "tractor-pull" is not a good indication of the things sailors care about in a prop. We really need to know power at constant speed and thrust and a copmarison of various props on the same vessel. The problem is that the proper test would not be using a conventional motor but rather an electric dyanamometer with an accurate speed and torque transducer as well as boat speed. With this data you can map the performace of the prop alone over a range of conditons. With this data, you can match the prop performance to an engine performance curve.
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Old 31-01-2011, 20:27   #33
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Quote by CatrinaPDX

"Folding props are not such a great idea for cruising boats: a small, strategically located barnacle can prevent it from opening. For boats cruising in the tropics or the Med this is a significant risk. And they are known to not always open reliably in reverse."

If you have a diver on board you are always checking/maintaining under the water and this is not an issue.
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Old 31-01-2011, 20:47   #34
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[QUOTE=CarinaPDX;609107]
Folding props are not such a great idea for cruising boats: a small, strategically located barnacle can prevent it from opening. For boats cruising in the tropics or the Med this is a significant risk. And they are known to not always open reliably in reverse.


Dissenting opinion, yet again:

In 35 years of sailing with a folding prop, and the last 24 years cruising in the South Pacific, I have never experienced the "barnacle preventing opening" problem, nor have any of my many acquaintances with folders.

The issue of failing to open in reverse was occasionally true of the old, non-geared props but the more modern designs, and the Flex-O-Fold in particular have eliminated that worry.

Barnacles can spoil the performance of any sort of prop, and should be discouraged, removed and cursed.

One may choose to not use a folding prop for various reasons, but the above ones are imaginary IMO.

Cheers,

Jim
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Old 31-01-2011, 21:16   #35
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You could see the problem arising with a boat that had sat on a mooring for a year or so. I could say that people who leave their boats on a mooring for that long deserve problems.
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Old 31-01-2011, 21:38   #36
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After using a fixed blade, then an Autoprop, I purchased a Gori 3-blade about two years and 600 hours ago. I've been very happy with it especially the 'overdrive' feature. In calm conditions, with the prop in 'overdrive', we can make 6 knots over ground at 1,300 RPM providing us with significant fuel savings. Furthermore, there is no maintenance to do whatsoever, other than monitoring the status of the zincs. When we're sailing, the prop folds completely and not only is the drag imperceptible but the shaft doesn't turn at all.

Calculating the correct diameter and pitch for a prop isn't rocket science and having a prop where the pitch can be changed seems unnecessary to me, unless you frequently change out your engine or transmission.

Fair winds and calm seas.
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Old 01-02-2011, 16:20   #37
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Infinite prop pitch control has increasing return based on displacement. With a very heavy boat, control of pitch is very useful. I can have very huge thrust at low boat speed if I can sett h4e pitch low. Cruising, I can pitch the prop steep based on matching rpm, manifold temperature and boat speed. This ability has been used by aircraft for many years to maximize fuel economy. There is no difference. If your boat is relatively light and you don't motor far or you tend to motor at constant speed or if the water is always relatively flat then a well-matched prop and motor are pretty good.

I know that with good pitch/rpm/speed matching, I can motor from Nova Scotia to Key West on one fill.
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Old 01-02-2011, 16:46   #38
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In light of what was said by Nicholson58, I would need to agree with him. Having infinite pitch control while underway is an ideal situation, however, is really only practical for larger yachts. On the other hand, for smaller yachts where the option of a Hundestad pitch control wasn't available, it remains a relatively simple formula to calculate the correct diameter and pitch for a prop.
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Old 02-02-2011, 01:45   #39
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There is another option for controllable pitch props: Sabb Motor, of Bergen, Norway. Their smallest controller is rated up to 150Nm torque: about a 50hp engine. At 43kg it is not too heavy as a replacement for a Borg Warner transmission.

I had an 18hp Sabb diesel installed until recently and did like the controllable pitch solution.
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Old 05-02-2011, 14:59   #40
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One note of caution about Brunton's Autoprop which I've learned through experience: under certain circumstances the blades can be thrown out of sync with each other (as they are free to move independently). With a flat aft section and if heeling considerably whilst motoring, air can reach the prop and one or more blades can dramatically change in pitch - to the point where the engine can violently stall. I've had this happen twice and it was frightening on both occasions. I've had similar problems motoring very close to rocks in a strong tide where the eddy vortices can similarly upset the equilibrium of the blades.

With a uniform flow of water over the blades it works well, but when the water flow becomes irregular and disturbed you may suddenly become aware that all three blades are doing their own thing independently of each other!
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Old 13-02-2011, 06:22   #41
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Just a general question - to those who have changed out fixed props for either folding or feathering - I realize that there will be less drag, and therefore more boat speed when sailing with folding or feathering props. However, is this difference really a big deal in areas where there 15-10knt winds are normal, as compared to light wind areas?

I could see reduce drag being significant if the true wind was only 8 - 10kts. But if there is normally enough wind for my 30'er to do hull speed or near, how much would I really be gaining in going to a folding or feathering prop? Currently have a reliable 3 blade fixed.
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Old 13-02-2011, 06:50   #42
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Quote:
Originally Posted by Northeaster View Post
Just a general question - to those who have changed out fixed props for either folding or feathering - I realize that there will be less drag, and therefore more boat speed when sailing with folding or feathering props. However, is this difference really a big deal in areas where there 15-10knt winds are normal, as compared to light wind areas?

I could see reduce drag being significant if the true wind was only 8 - 10kts. But if there is normally enough wind for my 30'er to do hull speed or near, how much would I really be gaining in going to a folding or feathering prop? Currently have a reliable 3 blade fixed.
You can answer that question yourself by dragging a bucket. Sail with and without the bucket in the water and make your own judgment.
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Old 13-02-2011, 07:12   #43
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Originally Posted by Northeaster View Post
Just a general question - to those who have changed out fixed props for either folding or feathering - I realize that there will be less drag, and therefore more boat speed when sailing with folding or feathering props. However, is this difference really a big deal in areas where there 15-10knt winds are normal, as compared to light wind areas?

I could see reduce drag being significant if the true wind was only 8 - 10kts. But if there is normally enough wind for my 30'er to do hull speed or near, how much would I really be gaining in going to a folding or feathering prop? Currently have a reliable 3 blade fixed.
I guess it's more a case you're more likely to get up to hull speed more often.

I agree with SailFastTri but have always likened it to driving your car with the handbrake partially on. Once you've experienced the benefits, you won't want to go back - and I'm saying this with a heavy cruising boat, not a lightweight flyer :-)
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Old 13-02-2011, 07:20   #44
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However, is this difference really a big deal in areas where there 15-10knt winds are normal, as compared to light wind areas?
I have a Max Prop on the boat for 25 years now and while I have no evidence to prove this but I think the benefit is as you state is in light air. I know that I can reach hull speed easily in 15 knots towing a dinghy and I would think that would present pretty good drag. Light air performance is enhanced by the feathering prop however two other benefits...1. shaft does not rotate(no need for brake) and 2. much better reverse. The one disadvantage is that there is some efficiency lost in forward since the blades are flat compared to the curved fix blade. Not sure if applicable but upon thinking about that last statement more, I've been in a small speed boat powered with a corvette engine with flat blades.
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Old 13-02-2011, 09:34   #45
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A significant part of my decision to go with a folding prop has to due with the fact that Rutea is a center cockpit boat with an aft cabin. The Beta Marine engine I recently installed came coupled to a ZF hydraulic transmission. With a fixed prop there would be no way to keep the shaft from turning while under sail. Short of installing a shaft brake, which I didn't want to do, it meant listening to a spinning prop while in our cabin.

A turning or locked prop while under sail does add substantial drag, however, there are many factors to calculate as to how much this would affect speed over ground.

Fair winds and calm seas.
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