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Old 12-04-2008, 03:39 PM   #15
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Originally Posted by MMNETSEA View Post
To Freewheel or not to Freewheel - surely it depends?, as always - when discussing things pertaining to sailboats. Surely there is no one rule that fits all.

Which gearbox? How many blades on the prop? What pitch ? Is prop enclosed ? What width of keel ahead of the prop ? Does the Prop Shaft have pillar block bearings? etc... not to exclude ' what combination of any or all of the above'

Finally what does the the gearbox op-manual say on the matter, since it is common cause that the practice of freewheeling in certain types of transmissions has the greatest negative potential on the gearbox itself rather than on the rest of the drive train. Nigel Calder's contributions to cruising over the last thirty odd years have extraordinary merit.
Thanks for clearing up the "many circumstances" question I had.
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Old 12-05-2008, 08:03 AM   #16
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Another quick thought.

A spinning prop is far more likely to get fouled than a static one.

I am definitely an 'Engage Reverse' man!
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Old 01-03-2009, 02:52 AM   #17
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"A freewheeling propeller will, in many instances create more drag than a locked one."

I know very little about boats... but I know my car tires create more drag with the brakes are applied.

I am trying to think of an instance that a stopped propeller creates less drag than one being spun. I wish that guy would have given an example of the "many instances".

I thought the laws of physics were constant.
I work with an old(er) Naval architect and he has more old rules and anecdotes such as spinning wheels and drag and he always cites several instances from past tests that he can't give exact details about he just knows that what he's saying is the last word!

I own a substantial marine library as I suspect many of you here do also and there are more authors making statements of fact with no corroborating facts on all subjects relating to boats.

My 12 year old was at the helm and I told him to shift the transmission into neutral when he shut down the engine.

His asked why.

I said because it would hurt the gears to turn while the engine was off.

He asked why.

I said because there wouldn't be sufficient lubrication.

He asked why and how do I know.

I looked in the manual and no place does it say that.

As someone who has to answer questions relating to warranty of our products I know how important it is to have all questions answered by the O&M manual.

If the gears could be hurt from turning under sail without the engine running you would think it would be in bold print someplace to lock the shaft.

My manual doesn't say it.

I still shift into reverse because.....I heard from someone or read that you should do that.

That doesn't make it true.
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Old 04-20-2009, 05:11 PM   #18
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Another great job by "Maine Sail":

<object width="425" height="355"><param name="movie" value="http://www.youtube.com/v/jI-UG9RSlJo&hl=en"></param><param name="wmode" value="transparant"></param><embed src="http://www.youtube.com/v/jI-UG9RSlJo&hl=en" type="application/x-shockwave-flash" wmode="transparant" width="425" height="355"></embed></object>
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Old 04-21-2009, 12:34 AM   #19
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I love it Great little movie. I'm an advocate for the theory that in general free wheeling props create less drag than a locked prop so glad to see the video shows an example where this is true. The opposite would likely be true in a few special cases...for example...if one has a full keel sailboat with a two blade prop that can be locked with the blades hidden behind the keel from the free flow of water.

Great "myth buster" type movie.
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Old 04-21-2009, 02:27 AM   #20
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True the video clip does illustrate the theory very clearly that freewheeling (windmilling) produces less drag when using a RIB.

What is important to a Sailboat is the effect on the boat's transmission, coupling, prop shaft and bearings.

TMC and ZF gearboxes amongst others, where the clutches are hydraulically operated, require the transmission oil to be cooled - this process is enabled by utilizing the engine's oil cooling system via the Engines heat exchanger. When the engine is shut down and the propeller allowed to freewheel (to windmill, to backdrive), the components in the transmission will turn and heat up.

Extracts from Twindisk Technodrive - PDF manual #1016313 entitled "Marine Transmissions Owners Manual; relevant to this discussion :-

Windmilling1.jpg

Windmilling2.jpg

Windmilling3.jpg

What is necessary on all Hydraulically operated transmissions is knowing the transmission's oil pressure and temperature. Most modern ones have the facility to fit appropriate gauges and warning devices.
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Old 07-17-2009, 04:15 PM   #21
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If the difference in drag is significant, the result can be determined experimentally. If the difference in drag cannot be determined experimentally, then it isn't worth worrying about.

Collect enough data to have confidence, do the math, and see what you come up with on your boat. This will give you something to do other than drinking beer on long reaches.
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