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Go Back   SailNet Community > General Interest Forums > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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  #11  
Old 01-30-2001
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In Gear

On a two bladed prop if close to keel and lined up with it, will give less drag if locked cross ways...but another factor is the turbulance the locked prop creates. It now effects the rudder surface(it will also put a drag on the vessel) which will not allow you to point as high, plus much less control when heeled over...turbulance creates air bubbles and pockets of different pressures not needed by the rudder
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  #12  
Old 02-05-2001
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In Gear

I just spent two days last week attending the fifteenth Chesapeake Sailing Yacht Symposium (CSYS). There was a room full of some of the world''s best yacht designers and researchers. There was a lot of information flowing. I tried to get some answers to some of the common issues that have been discussed on various BB''s either from the lectures or in discussions with presenters and yacht design professionals.

One of the papers dealt with a simplified VPP (velocity prediction program) written on Excel and designed as tool for quick analysis of varying designs. In the follow up discussion members of the audience questioned the use of the particular coefficients of drag for propellers as presented in recent testing results. Practical Sailor had published one such study performed at MIT that suggested that a free wheeling propeller had less drag than a locked propeller. These results were roundly questioned by design professionals who had worked in the field of propeller design. After this discussion I had the opportunity to discuss this issue with a Professor of Mechanical Engineering teaching Naval Architecture and whose students had actually performed the same type of tests as the MIT students. His results are very different than those presented in the MIT study. Here''s what I came away with in this conversation. (I also had the chance to discuss this with a number of other people in the field this includes inf from the general trends of this discussion.)

4 or more bladed, steeply pitched, propellers such as used on big ships, typically have less drag when permitted to free wheel than when they are locked up. This occurs because of the small incident angle of the water flow on the blades (therefore they are not stalled out) and the comparatively small amount of rotational friction of the propeller shaft and bearing when compared to the large amount of drive generated by the blades.

In the case of Sailboat propellers, the pitch is quite flat and so the blades are generally in a partially stalled condition when they are allowed to freewheel. This is even more pronounced when friction is applied to the shaft and the blades are turning slower than the flow of water passing over them. In this circumstance the propeller produces a ball of turbulent water. This ball of water generally has a greater drag than a locked prop. This professor explained the MIT results by saying that the MIT study actually used a propeller that was powered at equal speed as the water to replicate a freewheeling state and so had a very low drag. In his study they allowed the propeller to actually free wheel and then applied increased friction to the shaft measuring the resultant drag. His study concluded that there was more drag in the freewheeling prop than the fixed one. He went on to add that the differences between locked and freewheeling were much smaller in a three-bladed prop than a two-bladed one.

I also had confirmed in later conversation that prop position was found to have a real effect. A two-bladed prop locked in the vertical position and a three-bladed locked with one blade vertical in the down position had substantially less drag than the same props in other points of rotation.

We discussed the idea of using the freewheeling prop to generate electricity. He indicted that in terms of drag this was the worst condition because the partially constrained shaft would produce the greatest drag. Apparently, the propellers designed for water driven generators are specifically designed to have a minimal drag using blades with a very large pitch.

Conclusion: You give up a fair amount of speed if you permit your prop to freewheel. You loose more speed free spinning a two-bladed prop than a three bladed prop but loose the most speed with a partially constrained three-bladed prop.
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Old 02-07-2001
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In Gear

Jim-
The drag dynamics aside, the transmission gets its cooling from the engines cooling system, no engine, no cooling and bad for your tranny. Put it in reverse and keep it there when sailing with the engine off. On caveat though is the possibility of it locking under extreme pressure, to avoid this potential when sailing at hull speed make sure that the transmission switch that usually requires that you start the engine in neutral is disabled, this way if you are unable to get it back into neutral you can always start the engine and save the day.
Bermuda
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  #14  
Old 02-08-2001
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In Gear

Actually, Jim, none of that is true for many if not most sailboat engines. By that I mean most sailboat transmissions are not "cooled" by anything related to the engine.(In fact all that I have ever owned or worked on were not cooled by the main engine cooling system) Some are lubricated by shaft driven pump that is driven by the engine input shaft. Others are simply "splash" lubricated. Splash lubrication is by far the most common form of lubrication on small sailboat transmissions. Most splash lubed trannys can be allowed to freewheel without burning up.

Also I have yet to run into boat with a marine engine that has a switch that requires you to start in nuetral. They may be out there, and you may in fact own one, but I have never seen one.

BUT is freewheeling a good idea? NO. even if the transmission doesn''t burn up it does increase wear and tear on the transmission parts and besides freewheeling creates more drag.

Respectfully
Jeff
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Old 02-08-2001
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In Gear

I send my first message on this board and I''m flamed. What arrogance and from someone with so little experience admits to having never seen a neutral safety switch. In case he (and you) are interested, neutral safety switches are as much a part of engine systems as fuel filters. In fact the entire fleet of Island Packets have one, mine included, they are an intregal part of the shift mechanism. Shirt, my previous, 1982 S-2 had one installed. Anyway, and this is my last word on the subject, your engine is no doubt a Yanmar 3GM, and if so, put it in reverse when the sails are up and not motorsailing, IMHO, and that of the entire Yanmar engineering team.
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  #16  
Old 02-08-2001
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In Gear

Jim:
None of what I posted was intended as a flame. While I disagree with genreal applicability of the points that you make I am not questioning that what you discribe may be true for some of the boats out there and certainly may be true for the boats that you are familiar with. I must admit that most of my diesel experience has been with European engines (Buhk, and Volvos), I have also worked on Graymarines, AT4''s and Universal Blue Jackets, Perkins and Westebekes. I have never seen a nuetral lock out sensor on any of these engines. My limited time onboard IP''s never exposed me to their engine bays.

I apologize if my comments hit you as being a personal attack. It was in no way intended to be that. I solely wanted to point out that transmission coolers and neutral sensors are a rarity in small sailboats rather than the norm.

Respectfully
Jeff
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  #17  
Old 02-08-2001
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In Gear

I also have not seen a neutral saftey on small sailboats....2 & 3 cyl...but I always let my prop freewheel...my last boat for 15 years...sailed to bermuda, bahamas, florida, carolinas...never had a problem with the trany...I went to the N.Y. boat show and Atlantic City boat show(2001)...The Yanmar Factory Reps and their dealers said it is OK to freewheel...won''t hurt tranny...I do feel when prop is locked it creates more drag and water disturbance pass the rudder(air bubbles and such) giving less control...In my racing I find when I lock it, my speed goes down...I lock it several times in different positions but lose speed...
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  #18  
Old 02-08-2001
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mack boring recccomends to put the engine in reverse when sailing.
eric
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  #19  
Old 05-22-2001
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I have a Yanmar 3Qm and when I talked to the yanmar tec rep about it he said to put it in reverse when sailing.
SV
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  #20  
Old 05-29-2001
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Why don''t you take the prop off and be a real sailor.
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