engine in gear when sailing or.......?

In consideration of laminar flow etc

Fo efficiency of turning a shaft compare the shape of your propeller [driving tool] with that of the twisty thing [driven tool] on a trailed log. The shape of the foil is in a mirror image and the pitch is much curlier [with a finer point of attack??]. The latter is designed for efficientcy in being driven. The former is designed for efficiency of driving. So make your own theory and deductions. Suffice to say, under most conditions when sailing a fixed non turning prop presents less drag.

Problems occur with different gearbox requirements and design restrictions which may require gear box to be moving/not moving when engine is not running. Best option when not running the engine is to lock the prop shaft with a shaft brake. Warning brake must automatically disengage or be manually disengagesd if the engine is running and certainly if gearbox is enghaged. Many horror tales when shaft brake is a makeshift screw driveer of vice grip jamned in somewhere and forgotten when engine started and gearbox engaged ???????!!!!!!!!****+++++!!!!

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Nigel, Hi

Whew, I was sitting behind you and he didn't ask me - you've saved my life!

Imagine that your prop is the rotor. If it's 'autorotating' its applying more reverse thrust than if stopped.

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Re: In consideration of laminar flow etc

How do you arrive at "suffice to say". I am still waiting for some figures to back up all these assumptions. A comparison with a helicopter blade in air bears no relation to a boat prop in water. Ric is the only poster I have read on the right track, shame he has no experimental data to back it up. Has no-one any test results from towing boat props at varying speeds in a test tank to settle this one once and for all?

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Re: In consideration of laminar flow etc

I'm not sure why you say that there is no comparison. They're both fluids, and other than the fact that a foil in air stalls at about 20 degrees, about half that in water, there is no real difference.

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Re: In consideration of laminar flow etc

I think the huge difference in the ratio of swept area/blade area will give totally different results but what I would like to see is some real test data not unsubstantiated guesses. All I have seen were some tests on model aeroplane props which gave quite surprising results, as I think boat prop tests would do. As Ric said there is a changeover point but where is it? Anyone got any real data?

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Re: In consideration of laminar flow etc

I am actually digging some out, but it's going to take a while. Also can't be ar*ed to do it from old text books.

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I can only relate to :

I used to fly model helicopters many moons ago - when I had money to burn !!

Autorotate as you know is to get the rotor up to sufficient speed by use of pitch control that when near to the ground you whack on lots of pitch and the helicopter doesn't smack itself into the ground and demolish chopper and passengers .....

I thionk it a bit different to the prop bit ....... here we don't want the movement through medium to spin the prop up and give it momentum .....

Phew - I have to lie down - just remembered how much I spent flying model choppers !!!!


<hr width=100% size=1>http://groups.yahoo.com/group/gps-navigator/
 
Argh ..... time to engage shaft lock

So there we are leaving dock and manouevring around all those Jet-skis and kids in daddy's rib etc. Quick shufty at speed log .... making sure its speed through water as well !! ...... "Darling can you engage the 'shaft-lock' please ......." Answer would probably run to something akin to Sado-Masochistic tendencies ????

Yeh - I'm really worried about when the change-over occurs ..... like most - good technical point but naff all use on a boat !!

IMHO of course ....

Apologies if I offended anyone ...


<hr width=100% size=1>http://groups.yahoo.com/group/gps-navigator/
 
Re: I can only relate to :

Nigel - have you seen what they're doing with electric motors in model planes and 'copters, up to about 0.61" power and 10-15 mins duration, silently...

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Re: Argh ..... time to engage shaft lock

No offence taken but I am always happy to learn, I feel it is better than making statements such as "A stopped prop is less drag in all conditions than a spinning one" from ignorance. I feel it is better to not make sweeping statements without a good knowledge of the subject as one tends to look a bit of a wally.

IMHO of course.

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A helicopter has a variable pitch prop and so you cannot make a simple analogy likening it to a fixed pitch boat prop. You need to look at some prop vector diagrams. Very hard to explain in words.

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If you can see that, why cannot you grasp the simple concept that fixed prop will under normal circumstances provide less drag than freewheeling prop. I thought it was well documented??? Consider a helicopter with blades fixed, or an autogyro. They will provide more lift if rotating than if fixed

<hr width=100% size=1>Me transmitte sursum, caledoni
 
It's tested regularly! Every time someone forgets to lock the prop while they are racing....

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Easy if you have an outboard in a well. Sail along at about 5 knots and drop it into reverse. The motor kicks BACK on its rubber mounts showing that the drag has increased by a few KILOS. Knock it into free wheel and you can hear it sigh with relief!

Geoff

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Helicopter head-bashed-in..

I had the joy of going around the Westlands factory in Yeovil when at school.

One of our party asked why the helicopters weren't fitted with ejection seats.. The engineer taking us around never bothered to answer for some reason.

Regards

Richard.


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Re: Helicopter head-bashed-in..

Avocet's prop turns quite quickly when freewheeling. Even at low speeds. I guess at about 3 knots it will be doing a few dozen RPM. I might time it next time I'm out and post the results. At hull speed (a whole 6 knots!) it will spin fast enough for me to feel a slight vibration through the tiller (long keel, with full-length rudder and prop in a cut-out). The vibration stops when I put the gear lever astern. I've never been able to make up my mind about whether the boat goes faster though!

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I am amazed how quiet this thread has gone since this observation about outboard reaction to being in or out of gear was posted. From what I understand it doesn't seem to go along with the idea that less drag is created by a prop that is prevented from freewheeling. Perhaps I have misunderstood so will those who insist that a prop must be stopped from freewheeling to reduce drag please give some evidence to support that theory. So far all I have ever seen is statements about how well known/obvious it is that the prop should be stopped to reduce drag but nobody has refered to any actual proper hydrodynamic data to support this. I am trying to keep an open mind so any real evidence would be welcome.

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