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Old 08-09-2010, 23:12   #31
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Not licensed but took some training including one autorotation that the instructor saved before I killed us. The reason I don't play drums or fly helicopters is I can't make my feet and hands do different uncoordinated things at the same time...

Thanks for the discussion!
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Old 08-09-2010, 23:24   #32
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Loved the discussion, and thank you too. Helicopters are not my thing either--tools of the powers of darkness comes to mind.
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Old 09-09-2010, 00:50   #33
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Originally Posted by Ex-Calif View Post
The (unpowered) helicopter trades speed (kinetic energy) & altitude (potential energy) into rotation of the rotor (kinetic energy) during descent by using the drag of the blades to create rotation.

The blade angle is actually very fine during descent. The pilot is not trying to achieve lift he is trying to achieve a target speed on rotor rpm. He is also spending parts of that energy in directional control by adjusting cyclic and tail rotor to fly to a suitable landing spot.

He arrives close to the ground with lots of rotor rpm and little lift. By applying the collective he increases the pitch of the rotor extracting all the kinetic energy he can and converts it into lift and hopefully when all the energy is expended he is only a couple of feet above the ground with a zero descent rate.

This is pretty much how the pilots I've talked to explained it to me. One offered to demonstrate (I declined). Apparently they do this often, in practice and certification trials so not only does it work, but works well enough they can do it repeatedly without damaging the aircraft (or themselves).

But, even a small helicopter has blades 20 feet long that weigh an awful lot more than most our sailboats propellers. And they have a variable pitch. So, I'm thinking in general, what works for one doesn't necessarily mean much to the other.

But if it's freewheeling down there with the water passing by, do you think it creates any prop thrust?
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Old 09-09-2010, 01:58   #34
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Isn't it interesting that these threads/discussions never lead to a clear answer/ consensus on the issue of prop drag free-wheeling or locked?

I sent an e-mail to Yanmar Sweden once asking what to do on my configuration at the time. (3HM30 with the red kind of oil in the tranny)

The answer I got- that took several months(!) first of all- was:

'put it in reverse'

And that wa it. No motivation as to WHY, no name under the message...could as well have been the person working at the counter at the entrance...

Makes me wonder if anyone really knows...I don't for sure... but as someone said, In light air, boat speed <4 knots, letting the prop free-wheel does increase boat speed. Period. At least on the boats I've been sailing.
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Old 09-09-2010, 02:06   #35
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The studies I have seen are on props in water tanks with no installation, hull, gearbox or mechanical effects.

Each installation is different. The true test would require putting your boat in a controlled environment and testing it. And mine. And his. etc...

It would require creating a drag curve for the prop stopped and the prop freewheeling.

We all know what it feels like to stick our hand in the water at 5 knots. Stick 3 hands in (like a prop) and the drag is palpable. The freewheeling theory has great merit - just no reliable data that I have seen.

The 10,000 miles of freewheeling wear on the system trumps the speed issue anyway.
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Old 09-09-2010, 02:15   #36
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Quote:
Originally Posted by Ex-Calif View Post
The studies I have seen are on props in water tanks with no installation, hull, gearbox or mechanical effects.

Each installation is different. The true test would require putting your boat in a controlled environment and testing it. And mine. And his. etc...

It would require creating a drag curve for the prop stopped and the prop freewheeling.

We all know what it feels like to stick our hand in the water at 5 knots. Stick 3 hands in (like a prop) and the drag is palpable. The freewheeling theory has great merit - just no reliable data that I have seen.

The 10,000 miles of freewheeling wear on the system trumps the speed issue anyway.

I agree. Goes with my (not very scientific perhaps) take on it.
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Old 09-09-2010, 02:28   #37
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knot related...

Could the drag be related more to the velocity of the water flowing past the blades in a non linear fashion?

For instance - one could imagine that at very low speeds water would flow easily past a stationary propeller, then as the water speed increases there may come a point where the turning propeller offers less drag. At higher speeds again the spinning propeller would present resistance generated by the disk area plus tip vorticies that may be greater than that presented by the stationary propeller.
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Old 09-09-2010, 02:35   #38
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Yes - I think you got it.

The drag of the gearbox does not change (much) with speed - i.e. it takes .10 hp to turn the drivetrain. Now the stuffing box may take more drag at higher speed but let's ignore that for now.

The drag of the stopped propeller is zero when the boat is not moving. The drag of the stopped propeller is likely exponential. I.e. the faster the boat goes the more drag produced.

At some point the curves cross.
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Old 09-09-2010, 02:46   #39
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I find that when my prop is in 'free wheel' mode, I can feel vibrations through the vessel as the prop turns. Im not sure that can be fantastic for the wear on mounting rubbers? Instead I now lock my props and it 'feels' better.....
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Old 09-09-2010, 06:00   #40
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FWIW, our boat's manual says to shut down the engine in gear. And leave it in gear while sailing (in forward). The prop is a Brunton self-pitching feathering Autoprop.

As an aside, I had problems getting the transmission into neutral to start the engine again after sailing with the prop feathered. It made me very nervous at the time. Evidentally the feathered prop puts some torque on the shaft which binds the gears. I eventually figured out that the way to deal with this is to just forget about shifting into neutral, and to just start the engine in gear. With a little torque on the shaft the engine is very happy to kick over that way.

Obviously this would not work if we had an interlock switch, which some boats have, preventing the starter from working if the transmission is not in neutral.
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Old 16-09-2010, 20:56   #41
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Yanmar now states that the trans should be in neutral since the metal cone clutches could be damaged and have difficulty in getting into neutral to start the engine.
I have a Kiwiprop which feathers.
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Old 17-09-2010, 07:02   #42
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Yanmar now states that the trans should be in neutral since the metal cone clutches could be damaged and have difficulty in getting into neutral to start the engine.
I have a Kiwiprop which feathers.
Just so people don't get confused, not all Yanmar products have cone clutches. The SD20 sail drive has a dog clutch which doesn't exhibit the problem.
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Old 17-09-2010, 13:12   #43
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According to Ted

In "Ted Brewer Explains Sailboat Design" he states that tank testing proved a free wheeling propeller produced more drag than a stationary one. The difference was slight but should not be ignored. Another test, while driving your car, stick a pinwheel out the window and feel the drag increase as it begines to spin. There is physical science behind this but I would be very poor at explaining it and I must get back to work.

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Old 17-09-2010, 13:57   #44
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Just so people don't get confused, not all Yanmar products have cone clutches. The SD20 sail drive has a dog clutch which doesn't exhibit the problem.
Maybe not but Yanmar still says put it in neutral not locked in reverse:


Yanmar TSB (LINK)


Advisory Number: MSA08-003:

DATE February 8, 2008 Dealers and OEMs
TO: All Marine Distributors
SUBJECT: Gear in Neutral While Sailing All MODELS:

All Sailboat Engines

We continue to get questions regarding the correct gear position while sailing with the engine OFF. This advisory is issued as a reminder; Yanmar requires that if sailing with the engine OFF (not running) the transmission shifter must be in the neutral position or internal damage to the gear or sail-drive will result. This damage will not be covered by Yanmar’s Limited Warranty. Please instruct customers and dealers who deliver the sailboat to the customer, of the correct (Neutral) position for the marine gear while sailing.

If the customer desires that the propeller shaft not spin while sailing, either a folding propeller, shaft break, or other suitable device may be used. However, Yanmar accepts no responsibility for the selection, installation, or operation of such devices. Please also refer to Marine service advisory “MSA07-001_Yanmar Sail Drive Propeller Selection” for additional information.

If you have any questions regarding this advisory please contact a Customer Support representative.
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Old 17-09-2010, 13:58   #45
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You can also ask yourself which foil creates more force and how the force is directed. Think a stalled foil versus a non-stalled (we know it is not stalled because the prop is spinning). How much force there is, and how is it directed?

If you do not like the stalled foil analogy, think about how fast a screw can be forced into a soft material - think a non-turning screw vs. one that has some turning moment.

Finally, one can make a small model of a prop and get themselves a fishing rod with a reel used for spinning. Then go do some tests - let the prop free wheel and then have it stop (e.g. by placing one of the blades in the reverse position). See which situation creates more drag.

If these examples do not ring the bell then let's go sailing identical boats - yours with the prop free wheeling. The last one at the bar pays for the beer.

b.
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