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Mark Howe
Moderator
Username: unclemark

Post Number: 693
Registered: 08-2003
Posted on Tuesday, January 08, 2013 - 06:56 pm:   Edit Post Print Post

One of my students, a pilot, just asked for this link and that got me back here again. Based on the actual experiments in Boris' link I have ceased talking about reducing drag and merely mention kelp, wear, etc.

I had a chopper pilot say one time that it isn't the autorotation that cushions a crash, it is the spinning momentum of the blades with a 'flare' just before landing that does it. Not real sure of how this all works physically, but I think this has to do with being able to adjust the pitch of the blades to create last minute lift. [My mind is boggled by verbiage like 'pull back on the collectives and flare something'; they drive those things so instinctively it must be like an extension of their bodies.]
 

Eric Gritzmacher
Intermediate Member
Username: eneveaux

Post Number: 18
Registered: 08-2008
Posted on Tuesday, December 27, 2011 - 08:34 pm:   Edit Post Print Post

I have an AutoProp but the hub is not the right size for my shaft (i bought it for another boat). Given all i have learned about prop drag, i am considering having it re-fitted so i can use it on Cool Man. My experience with it was as pretty much as advertised: performance under power was noticeably better and improved fuel usage. Only downside was reverse; there is a delay in getting thrust. The real issue is the cost of retrofit. Too late for Santa?
 

Marc Hughston
Moderator
Username: hughston

Post Number: 799
Registered: 03-2003
Posted on Monday, December 26, 2011 - 10:42 pm:   Edit Post Print Post

I'm with Eric on continuing to sail with the transmission in reverse when the engine is off, despite the additional drag. I don't like the noise and would rather save the wear on the drive train. Mark Howe also points out the potential for wrapping a line or kelp when the prop is spinning. I wish the "feathering" prop on Betty would actually feather, but it doesn't. Did Santa bring one of those folding or feathering jobs for anyone?
 

Eric Gritzmacher
Intermediate Member
Username: eneveaux

Post Number: 17
Registered: 08-2008
Posted on Monday, December 26, 2011 - 05:01 pm:   Edit Post Print Post

Thanks for this. I thought I was done with the subject but this is worth reading. It pretty much puts the the issue to rest, at least as far as what is more efficient. So the question becomes: how much, if any, damage or wear is done to the transmission and prop shaft? By the way, the study done by MIT that the article provides the link too is also worth reading, even though it is very technical. It doesn't address the question of locked vs. free, but does provide some great insights on the performance of various props.
 

Boris Buzan
Junior Member
Username: grayeagle

Post Number: 5
Registered: 09-2007
Posted on Monday, December 26, 2011 - 12:45 pm:   Edit Post Print Post

Here is a link to a test done locked vs freewheeling. Freewheeling wins hands down, cavieat being can the tranny survive freewheeling with no engine load. Probably depends on which shaft the lubricating pump is on. Only your manufacturer knows for sure.

http://www.catamaransite.com/propeller_drag_test.html
 

Marc Hughston
Moderator
Username: hughston

Post Number: 797
Registered: 03-2003
Posted on Monday, December 26, 2011 - 11:40 am:   Edit Post Print Post

Happy Holidays! Just a couple of thoughts.

I have had the problem of occasionally not being able to get the transmission out of reverse. This is on the Catalina 42, Betty, which has a three blade max prop that is supposed to feather, but doesn't. Unlike some larger boats, Betty's engine will still start if the transmission is in reverse - so that's what I do, and then immediately shift into neutral.

On the question of drag, I think a propeller on a plane is analogous the real-world situation on the sailboat, but the helicopter is not. Especially if the propeller is at the rear of the airplane, as some are, pushing it along. When the propeller is being driven by the engine, it spins in the same direction it would if it were free-wheeling. Spinning in that direction, the propeller provides thrust for forward movement. If spinning provides thrust for forward propulsion, it follows that the plane will travel faster through the air with the prop spinning than with it fixed in place. Same thing for the boat I would say.

The problem with the helicopter analogy is that the rotors are trying to keep the aircraft up – this is like having the prop spin in reverse. So of course, the helicopter falls faster with the rotor blades fixed, and the boat sails faster than having the prop spin in reverse. That’s where the analogy breaks down – I say the prop on a boat won’t free-wheel in reverse, but will spin in the same direction it would have if it were being used for propulsion, therefore causing less drag than a fixed prop.

OK, so tear me apart now. :-)
 

Eric Gritzmacher
Intermediate Member
Username: eneveaux

Post Number: 16
Registered: 08-2008
Posted on Saturday, December 24, 2011 - 02:28 pm:   Edit Post Print Post

Well, i think we have exhausted the subject. A few good points David. Hard to refute. I think you observation about the gear box not getting properly lubricated makes some sense. My engine guy's comment was to check with the manufacturer of the transmission - it is not really so much of an issue with the engine itself. Why Yanmar stands being their service advisory saying keep it in neutral is not clear. Personally, i can't really tell the difference in boat speed between neutral and reverse anyway so will keep doing what i have always done - reverse
 

David Sheriff
Board Administrator
Username: admin

Post Number: 347
Registered: 01-2004
Posted on Friday, December 23, 2011 - 08:13 pm:   Edit Post Print Post

My understanding is that rotating the engine end of the transmission supplies oil pressure to the transmission bearings. Sail in neutral with the prop rotating long enough and your transmission will wear, perhaps fail, from un-lubricated bearing surfaces. The cutlass bearing is in the water. I don't see that a freewheeling prop would have any bad effect on it.

Reverse is almost always a higher reduction ratio than forward. So the engine gets less back force from the prop in reverse. Any wear vibration from turbulence around the prop causes is lower and on the least-used gear surfaces. That's about all the mechanical reason I can think of.

"sail in reverse" is such a hoary universal rule that it may not be as important to today's small sailboat engines compared to all engines in sailboats throughout history. It's easier to know at a glance the transmission is in gear under sail if it is always in gear the same way. Reverse, forward or with a shaft brake applied, do not sail with the reduction gear in neutral.
 

Mark Howe
Moderator
Username: unclemark

Post Number: 639
Registered: 08-2003
Posted on Thursday, December 22, 2011 - 07:42 am:   Edit Post Print Post

Eric Gritzmacher wrote:
In the seemingly never ending discussion about what gear to put your transmission in when sailing (engine off), I came across this a while back and thought I would share it. It came from Sailboatowners.com forum. The wording is a little ambiguous: does this only apply to sail-drives? The consensus from the group was that it applies to all Yanmar engines. Probably would be a good idea to go to Yanmar’s website and check out the Marine service advisory to be sure this is true.
Recently I spoke with the Yanmar dealer in Costa Mesa and they told me leaving it in reverse is best but didn’t seem to have strong feelings about it. It never made sense to me why you can’t leave it in neutral; I would think there would be less drag (unless you have a folding prop). How could the prop turning damage the transmission or any other part of the drive train? Some people said they get some vibration in neutral at higher speeds but they also had big three blade props. But otherwise, if you experience vibration, then perhaps something else is going on.
Some comments to this post suggested that locking the shaft can cause wear on the cone surfaces. Whatever. Wouldn’t the manufacturer of the gear box have a bearing (no pun) on this issue? So there you go. It seems everyone has a point of view on this and I didn’t intend to start a barrage of emails, but welcome everyone’s opinion. I am sending this to Ryan Lee who does my engine work. I will be curious what he has to say.

Ric Maxfield wrote:
Actually, the problem is with the cutlass bearing. If you leave the engine in neutral, the prop spins the shaft without providing lubrication to the bearing and it burns up the bearing. So, it's not just the type of engine, but the overall setup that has to be looked at. At least that is my understanding of it from what I was taught.

Mark Howe wrote:
The new engines from Universal used to come with a tag that said sailing in forward voided the warranty. I used to have one of those tags to show to the class.
Sailing in neutral, while apparently causing no damage, has several detractions. A spinning prop can catch 'seaweeds' or lines accidentally falling overboard. A spinning prop also causes drag.
This latter is counter-intuitive unless you are a pilot; particularly a chopper pilot. Autorotation [freewheeling] of a propeller causes a chopper to fall slower, making survival of a crash more likely. If the prop jams, you fall like a rock.
Someone explained it to me long ago "A spinning prop occupies the area of a disc".

Marc Hughston wrote:
Here is what Nigel Calder says in the discussion of transmissions in "Marine Diesel Engines:"
"When a boat is being towed or is under sail with the motor shut down, the flow of water over a fixed-blade propeller will spin the propeller unless the propeller shaft is locked in some way. This is of little concern with manual and two-shaft transmissions, except that it creates unnecessary wear on bearings, oil seals, and the stuffing box. But on some hydraulic transmissions (e.g., some Detroit Diesels), it will lead to a complete failure of the transmission since no oil is pumped to the bearings when the engine istn't running. Some output-shaft oil seals(particularly rawhide seals) also will fail."
So it looks to me like Calder is saying it creates unnecessary wear, but that's about it. I need to respond to Mark Howe's theory of drag though.

Eric Gritzmacher wrote:
For what it is worth, I have spoken to two pilots – one is a captain on a 777 and former Air Force pilot. While they don’t have any insights on the wear or stresses created by a prop turning, or not turning as the case may be, they both felt that less drag would be created by having the prop in neutral and spinning while under sail. Depending on the prop size, it may not be of much consequence one way or the other however.
I emailed Yanmar regarding the service advisory that started all of this so will see what they say; assuming they say anything.
Here is what I got back from Yanmar which is the same advisory as the one I attached to the first email. Some people on the sailboatowners.com site think this is a CYA position; it may not be optimal for efficiency but protects Yanmar from ???. I did see a few complaints on the board about people who had problems getting their gear out of reverse after long sails but no way of knowing if leaving it in gear had anything to do with it.
So, there you go. I have always put it in reverse but can’t say I really notice any difference in boat speed. Maybe it is too small to be detected. I welcome any further comments but unless I come across any “new” information, will put this issue to rest and stop cluttering up your inboxes.

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