Why does my prop keep spinning? - Page 3 - SailNet Community
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post #21 of 56 Old 05-22-2006
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Comparing a propeller in air to one in water is like the apples/oranges deal. How many times more dense is water, than air? Air compresses, water doesn't. Dragging a stagnant prop through the water takes more energy than a free-wheeling prop. More energy is spen deflecting all the water, where if free-wheeling, some of the energy is stored in the prop as energy. This energy helps keep the prop at speed, thus deflecting less water......less drag.
As for lube, no bearing surface will tolerate a lack of lube for long, so it boils down to the design. Now, under power vs free-wheeling, a bearing will tolerate a great deal of free-wheeling since the load on the bearings is negligible, as opposed to the load when motoring. Still, there are limits. This is analagous to towing a rear-wheel-drive vehicle. Typically, if towing over 25 miles, give-or-take, the manufacturer states that the driveshaft should be removed to prevent transmission damage since the tranny isn't pumping lube when being towed.
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post #22 of 56 Old 05-24-2006
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Probably best to change to a folding prop? The comment earlier about a Velvet Drive requiring lubrication from the front with the engine running may have validity, it is certainly the case with auto trans in your car and the structures of the gearboxes are very similar. Borg Warner would be able to clarify that but as the earlier comment also says, if it has been running like this for 20 years, it must be OK.
The reason I recommend a folding prop? If you stop your prop from spinning, you'll knock as much as 1/2 a knot off your boat speed depending on the size of the prop and it's pitch. If you're into voyaging, that represents 12 miles a day!
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post #23 of 56 Old 05-26-2006
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Originally Posted by bradleyjdonaldson
Actually there is NO FRICTION in a free spinning propeller on a turbine powered aircraft. There is no mechanical linkage between the propeller and the power turbine. If you watch a pre-flight on a turbo prop aircraft, you can free spin the prop like a windmill with no resistance what so ever. It is still detrimental to fully feather the prop as soon as you loose power to keep the dead windmilling engine from dragging the aircraft down. In fact if it is an air transport category aircraft it must have a AUTOFEATHER system to make the climb requirement after an engine failure on takeoff. ANYWAY..... Try this...Get a small prop from the toy store. Hold it out the window of your car while somebody else drives. Look at the difference in force aginst you with a spinning prop and a stopped one. You will be amazed how much drag a spinning prop makes!
well, not too sure how people will interpret this, had one of those toy pinwheels, stuck it out the window cruising into work, it spun happily along with a fair bend on the stick for a 1/4 mile or so, pulled it in and tied the blade so it wouldn't spin, stuck it back out the window, and the blades plastered back for a second or two, then the stick snapped off.

My view, stick was strong enough with the blade spinning, but not strong enough with the blade stopped, stick never changed, only the blade spinning or not, so I'd have to say drag increased when the blade stopped.

is a pinwheel identical to a prop? No, but many of the same 'laws' are in effect, and yes the entire surface area of a pinwheel is much greater than a prop.

had thought about sticking my trolling moter out the window and measuring the stress with a fishing scale, but figured that may raise questions.

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post #24 of 56 Old 05-27-2006
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i guess the problem is that a prop that has a good efficiency under power hasn't neccessarily low drag without power, either stopped or windmilling.

some designs might compromise to one degree or another, trading efficiency under power against lower drag unpowered.

as omatako said, best might be a folding prop, second an adjustable prop.

question as always is how much money has to be spent on it, if the speed increase is worth spending it, or if the money is available in the first place and not needed for other items.
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post #25 of 56 Old 05-27-2006
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Wink Pirates answer!!

Ahoy, dis ere Pirate has shouted me challange before, why don't ye just take de blooming thing off and try sailing for real? I ain't never had a prop on me vessels and never will. What's the point? Iffin I'm half way to home and dwe wind dies I anchor and wiat for god to deliver me home. Iffin he don't,den i figger I wasn't meant to go home yet. On the other hand iffin ye know your going to be sailing for extended periods why not just take it off and put it pack on when ye needs to navigate a tricky channel or git home to yer overbearing wife?. What ye be afrain of a little underwater exercise? Training might come in handy some day, like when your mast goes in de drink .
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post #26 of 56 Old 05-31-2006
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shaft brakes

Does anyone know who supplies a propeller shaft brake for a 1" diameter shaft?
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post #27 of 56 Old 05-31-2006
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There actually are two separate studies on boat and ship propellors that were done roughly 10 years ago now. The original study was looking at ship propellors with the idea of saving fuel by shutting down one engine at sea. The second looked at a wider range of applications including small yacht propellors.

The findings as they were published suggested that fixed blade propellers with more than two blades produced less drag if they are allowed to free wheel. There was a caviat in the study on yacht props which basically said, that as friction was added the drag on the free wheeling prop rapidly increased so that free spinning props with even small amounts of rotational drag actually produced more drag than fixed props, which is generally seen as bad news for the those considering adding a propshaft driven generator. For what it was worth, three bladed props on locked shafts were found to produce the least amount of drag in the 'Mickey Mouse' position, (two blades up and one down).

Two blade propellers produced less drag when locked. Further benefits were found to be gained with the prop locked in the vertical position.
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post #28 of 56 Old 07-27-2006
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Free spinning does not slow the boat. It was well proven that cross channel ferries after 1st & 2nd oil shock (To reduce fuel consumption) stopped one of the engines and allowed the prop to free wheel. Different strategies were tried. (Transaction published IME about 1982)
It will however cause more wear in traditional Stern tubes such as a yacht. not to good in oil lubricated either if shaft rotation too slow due to non-hyrodynamic lub condition.
I've never argued with a skipper even during a race, my choice would be to free rotate if the noise was tolerable, desperate for a little extra speed, would accept more maintenance.
Yep I'm an Marine Engineer
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post #29 of 56 Old 07-27-2006 Thread Starter
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Thanks for all the input, folks. I finally located my Perkins manual and it clearly states that freewheeling will not damage the drive. So, I'll keep it in neutral and let her spin for now...
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post #30 of 56 Old 07-27-2006
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A spinning prop is more drag than one that's fixed. Maybe put the transmission in gear.

(I know from airplanes, it's called windmilling)
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