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Old 30-06-2008, 22:28   #1
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Neutral or In Gear When Sailing

Is it easier on the engine, reduction gears and other components to have the transmission in gear or to leave in neutral while under sail? What is the effect on boat speed if in gear? Is fwd or rev better if in gear? I'm talking a non-feathering prop.
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Old 30-06-2008, 23:51   #2
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This is like religion around here. There are many threads about it, an old study done by some MIT grad students about it and those that freewheel and those that don't will never convince each other differently.

Your engine and transmission maker may have something to say about it in terms of lubrication and wear so we all might agree to follow the manufacturer recomendations.

After that I recommend you experiment with your boat and see what you think.

BTW - I am a strong believer in stopping the (fixed pitch) prop. I also run a feathering prop on my boat which I think is awesome.
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Old 30-06-2008, 23:56   #3
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The first half of our circumnavigation, I kept the transmission in neutral and let the prop freewheel. The second half of our circumnavigation, I put the transmission in reverse on our Yanmar 3GM30F engines.

Putting the transmission in reverse was much quieter in the aft cabins in our catamaran. I now prefer having the transmission in reverse.

After 4500 hours on each engine, I still have the original gearboxes, and I have never had to have work on either transmission. So from a grearbox point of view, in the first 4500 hours of running, it didn't really matter whether we allowed it to freewheel or kept it in reverse.

I don't let the prop freewheel now because I like the quiet, and I don't want the wear and tear on my stern gland/stuffing box.
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Old 01-07-2008, 01:56   #4
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At one of the charter boats I saw big red letters warning, next to handle switching. "Never should be included in the" reverse "while driving under sail, to avoid damage gearbox!!!"
Apparently, some recommendations from the manufacturer?
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Old 01-07-2008, 02:59   #5
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an old study done by some MIT grad students about it and those that freewheel and those that don't will never convince each other differently.
Firstly, Can you direct me to that ??
Second, I don't think convincing anyone is any issue. The first rule is to read the manual. The manufacturer will tell you if the box should be locked or if it can free wheel. If you can tell me which box you have, someone could tell you what the manufacturer recomends.
The reason is in how the box is designed and how oil is distributed. Some will happily free wheel all the way round the planet with no problems and others will wreck themselves in short order.
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Old 01-07-2008, 03:23   #6
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Alan - Here is a link to the last big religious discussion. The MIT Study link is in there somewhere. I have to rush off or I'd get it out for you...

http://www.cruisersforum.com/forums/...ling-9923.html
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Old 01-07-2008, 03:39   #7
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An aside topic on this discussion. What is the view of putting a device (there are different types that do pretty much the same thing) for allowing the prop to free wheel and generate power while doing so? Just asking as this seems like a rather simple means with which to help recharge the batteries.

Michael

PS no I have not researched for this item and if it has been gone over my apologies.
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Old 01-07-2008, 04:18   #8
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Short version from brand new zf manual....

No problems with box in neutral when,sailing tugged or anchor.

CAUTION ,
use the shift position OPPOSITE to the direction of travel for locking the propeller shaft, otherwise the transmission will be damaged.
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Old 01-07-2008, 04:19   #9
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Originally Posted by Ex-Calif View Post
Alan - Here is a link to the last big religious discussion. The MIT Study link is in there somewhere. I have to rush off or I'd get it out for you...

http://www.cruisersforum.com/forums/...ling-9923.html
As far as I can tell, the Practical Sailor/MIT study* isn't currently available (free online).
* “To freewheel prop or Not” ~ "Practical Sailor" magazine, July 15, 2000 - Number 14

There is a newer study:
"Sailboat propeller drag" ~ P.M. MacKenzie, M.A. Forrester

Ocean Engineering (2007),
doi:10.1016/j.oceaneng.2007.07.004
for which an "authors'" pre-publication manuscript is freely available at:

http://eprints.cdlr.strath.ac.uk/567...rag_Jan_08.pdf

In which the authors conclude: “... The experimental results confirm that a locked propeller produces greater drag than does a freewheeling screw (up to 100% more drag was observed, this being at higher speeds). Furthermore, for the freewheeling case, the magnitude of the hydrodynamic resistance is significantly affected by the amount of frictional torque on the shaft, low torque being accompanied by low drag ...”

Goto:
ScienceDirect - Ocean Engineering : Sailboat propeller drag
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Old 01-07-2008, 04:47   #10
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BTW:
Dave Gerr, author of: “The Propeller Handbook”, “The Elements of Boat Strength”, and, “The Nature of Boats”

Says, in “The Nature of Boats”, that:

“... a propeller creates less drag when free to rotate ... However, if the wheel can be locked vertically, hidden behind a skeg or keel, it will produce less drag than when freely rotating ...”

Goto pages 250-251:
The Nature of Boats: Insights and ... - Google Book Search
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Old 01-07-2008, 05:17   #11
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Forward..........


but that is for a Max Prop.
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Old 01-07-2008, 07:15   #12
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Originally Posted by GordMay View Post
BTW:


“... a propeller creates less drag when free to rotate ... However, if the wheel can be locked vertically, hidden behind a skeg or keel, it will produce less drag than when freely rotating ...”
Wont help with my 19" 4 bladers.

I plan on letting one freewheel on passage alternating between engines for 3 to 4 hours as per ZF's recomendation

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Old 01-07-2008, 07:16   #13
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Most of the arguments are about whether your boat will go slower or faster if you lock the prop, and the answer seems to be you need to experiment--it depends on your boat and your prop.

There are no arguments that locking the prop is easier on the transmission, cutlass bearing, and crew.
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Old 01-07-2008, 07:32   #14
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Since we are on props can anyone tell me about Boyens feathering props? I found 2 3blade props in my bilge, and wonder why the P.O didn't use them? They fit the shaft perfectly. I am now running 2blade fixed on my SD20's.
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Old 01-07-2008, 18:27   #15
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Usually in this topic I drag out the aircraft examples, of which there are many, that state a stopped prop creates less drag.

One thing I haven't been called on is that when an airplane prop is windmilling it is turning the engine and gear train as there is no "clutch" on a fixed prop airplane. The drag on a sailboat prop should be less as in all cases I am aware of the engine is disconnected and you don't have to fight the engine compression.

However, I am reading a book called "Gentlemen Never Sail to Weather" - It's an old book that I snagged from the club library. The author swears he got at least a knot after learning to lock the prop.

I am still a lock the prop guy at heart tough.

Drag From Windmilling Propeller
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