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Old 15-08-2004, 15:08   #1
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Sailing with tansmission in N or R

Had a question for all you seasoned deisel mechanic cruisers out there. I seem to get a lot of conflicting info on this topic

When under sail power only, should you stick the transmission of your engine in reverse so the prop does not spin to save wear and tear on the transmission or should you leave the trani in neutral and just let the prop spin creating less drag on the boat?

I am the owner of a 27 year old Volvo Penta MD2 (marine deisel two cyclinder) with a mechanical transmission which is comprised of two plates that are pushed against each other and pulled apart for forward and reverse (oversimplified but good enough)

If anyone knows the definitive answer to the Volvo Penta MD2 engine I would be darn gald to hear it. Volvo Penta is not all that interested in chatting with me. As soon as they hear I have an MD2 they promply tell me that they discontinued service on those engines years ago and to go somewhere else.

So here I am.
Andy
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Old 15-08-2004, 20:32   #2
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I have perkins 18s with Hurth tranmissions. Mine are enternally lubricated so leave them in gear does not hurt them. Which I think is the determining factor. If the tranny is lubed from engine then letting them spin will hurt them due to no lubrication if engine is not running
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Old 15-08-2004, 21:20   #3
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In gear

The question of less drag whether it is in gear or not will get many responses for and against. Both sides have a lot of good evidence. If it is a folding prop put the gearbox in reverse, you should also know where the prop is when you do this so you have to mark the shaft. If it is a fixed prop try it in gear and in neutral so see if there is any speed change. Letting the prop spin will wear the aft bearing which may be of not much concern but I would definately find out if the gearbox can be left spinning. Personaly I think you should put it in reverse. BC Mike C.
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Old 16-08-2004, 03:20   #4
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put it in gear definitely for the following reasons:

Reduces wear on the bearings.

Trials have proven that there is less drag from the prop, but you are still better off having a folding prop.

There is considerably less risk of fouling a stationary prop, than one wizzing around.
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Old 16-08-2004, 05:49   #5
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I use reverse

A little knowledge is a dangerous thing. I have (only) a little knowledge - so caveat emptor applies.

Propellers create drag under sail, whether locked or spinning free, unless they are locked in an aperture. Simply stated, whether the prop freewheels or is locked, you are creating drag.

Iíve read opinions that the locked prop actually produces less drag than the free-wheeling prop. On the other hand, Practical Sailor published a summary of a study they sponsored at MIT a few years ago - Relative to freewheeling, they concluded, though test lab limitations prevented full testing of freewheeling props, that less drag would be created for any given fixed prop if it freewheeled rather than held fixed. Motor Boating & Sailing has published similar results

I donít think itís quite that simple. Itís likely that he relationship of the propís characteristics (pitch, diameter, blade count & shape/area), the boatspeed, and other factors will determine the actual drag ratios (between fixed & freewheeling) in any given application.

Your log will be the best judge of whether leaving the prop to free wheel has more or less drag.

Your shop manual will be the best judge of whether leaving the prop to free wheel might cause damage to your gear box (etc).

When the propeller is allowed to freewheel some of the force propelling the boat is transferred into the rotational motion of the propeller (and shaft), thus creating drag at the propeller, and unless you can disengage the driveshaft at the aftermost shaft seal, at all the bearings between the propeller and the transmission (and for that matter, at the transmission itself).

On the other hand, if you lock down the propeller, you are creating a vortex behind the propeller caused by the water being forced to follow the blades and continuing in a helical pattern after leaving the blades. Also, because the water will not be flowing past the propeller at an angle of minimum slip, you will be creating a low pressure area aft of the blades, effectively pulling back against the forward motion of the vessel.

Dependent upon the shape of the hull below the waterline, the effects of drag can be reduced by up to 50%, if the propeller is in a vertical position when sailing. With long-keeled yachts where the propeller is located in a propeller well between the keel and the rudder, a vertical twin-bladed propeller causes very little or no drag. This is not the case with three bladed propellers. These can not be hidden behind any projections and can cause a decrease in top speed of 10% or more. If one does not want to compromise, one must dig deep into one's pocket and invest in a folding or feathering propeller.

The transmission on the Yanmar engine using the cone clutch device should always be left in REVERSE. Apparently leaving it in forward still allows the propeller to spin the shaft against the transmission. They went on to say that when you start the engine while sailing (remember your trans is in reverse) that you should also START it in reverse and then shift into forward (the water pressure on the prop makes it difficult to shift into neutral or forward while under sail).

A number of major gearbox manufacturers have dispelled an earlier unfounded fear of gearbox damage due to improper lubrication while freewheeling. However some gear box manufacturers do not advise leaving the prop to free wheel as this can cause mechanical problems within the box (overheating due to lack of lubrication).

Ie - From ďVelvet-DriveĒ:
It has been determined by tests and practical experience that all Velvet Drive marine transmissions call be free-wheeled without risking damage in sailing or trolling applications. Caution should be taken to be sure that proper oil level is maintained prior to freewheeling as well as normal running. Freewheeling one propeller of a twin engine boat at trolling speeds will not cause damage to the transmission connected to the freewheeling propeller.
Extended periods of free-wheeling at high speeds may cause the transmission to overheat; therefore, it is recommended that transmission sump temperature be monitored and free-wheeling discontinued whenever 230 degrees F or 111 degrees C is reached.

Since the hydraulic transmission needs hydraulic pressure to shift the transmission, moving the shift lever will have no effect. Even if the transmission was in gear before the engine was shut off, it will not be once the engine is stopped because there is no longer any hydraulic pressure. The pump also lubricates the gears and bearings. With no pressure, there is no lubrication. The only way to stop the prop from turning is to put a brake on the output shaft.

I lock my propeller (Reverse Gear) when sailing. Iím not certain whether this results in more or less drag on my boat - but I do feel that it places less strain on my drive train.

FWIW,
Gord
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Old 16-08-2004, 06:30   #6
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A quick follow up to Gordon's post, the MIT and US Naval Academy studies which were widely quoted were initially focused on ship propellors with the idea that an engine could be shut down to save fuel at sea. It was later expanded to look at smaller propellers. The conclusion was somewhat missreported. The findings of the study indicated that a prop with more than two blades that was allowed to freewheel produced less drag but only if the shaft was allowed to spin with minimal friction. When friction was added to the shaft, drag increased dramatically and quickly produced more drag than a prop that was locked. This has implication for people who are considering a water driven generator that would be powered off of the prop shaft. Locked three blade props produce the least drag when placed in the 'Mickey Mouse' position (one blade down and two up).

Two bladed props were found to produce less drag locked than when allowed to freewheel and produce the least drag in the vertical position.

As to your transmission, most small transmissions use a small 'splasher' of some kind to provide lubrication. I am not sure that is true of small Volvos as they have always marched to their own drummer. In any event, allowing the prop to free wheel takes a toll on the cutlass bearing, as well as the transmission seals, output bearing and internal output side bushings. If you care about transmission longevity then locking the transmission in reverse makes the most sense.

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Old 26-08-2004, 22:49   #7
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Plenty of Info

Thanks a bunch for the info. I think I will probably put the engine in reverse as the transmission is quite old and I would rather not use up the few spare parts I have been able to find. So I will stick it in reverse and make sure the drive shaft is not spinning. I suppose if I do let it spin it also wears on the drive shaft and the stuffing box too. It is nice to know that the amount of drag will not be all that much and I will try to lock it into the "micky mouse" position since my prop has three blades.

I have the shop manual and have inspected it a few times for such advice but found none. I also know that the transmission is lubricated from the engine so it would probably produce a lot more wear than I had previously expected.

I think I will still work on Volvo Penta for a definitive answer to my specific engine but thank you all for the good advice.
Andy
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Old 26-08-2004, 23:57   #8
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I just read an article writen by a guy called Nigel Warren of the UK on this very subject. His comment about a fixed prop, that the drag should be un-noticable to a crusing yacht and the drag effect may only make a difference of 0.1 of a knot, with the actuall drag effect being similar to towing a dinghy.
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Old 27-08-2004, 08:47   #9
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Props

The drag effect of a three bladed prop on a small boat is huge in my opinion. If the prop is near the rudder the turbulence can be sufficient to make steering over long distances impossible. The North Sails rep wisely will not sail with you to look at your sails, if the boat has a fixed prop. Towing our rubber ducky has little effect on speed but the three bladed prop is a killer. On a heavy cruising boat it may not be as noticable. Nigel wrote an excellent book on metal corrosion. Even the shaft has drag, more than a saildrive. BC Mike C
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Old 04-05-2006, 13:35   #10
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I'll throw my situation out there for your opinions. I have an old paragon transmission ( still functioning fine) that, by design, does not stay in reverse unless you hold it there. I have a three blade prop. Question, freewheel, in gear or add a shaft lock of some sort while sailing?
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Old 04-05-2006, 13:54   #11
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It is no good canvassing for opinions about this. Lots of people have conflicting opinions, but the only thing that matters is the advice from the gearbox manufacturer. Some gearboxes are designed to be turned only when the engione is running, as this provides the impetus for the oil within the gearbox. Some are designed that prefer for the shaft to be turning (avoids some of the stress on the thrust bearings.

If you have a folding prop, it avoids any problems!
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Old 04-05-2006, 16:44   #12
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Our prop is a 20" 2 blade fix and when sailing it goes "vertical" and we lock it by putting the tranny in reverse. Free wheeling seems to have an imperceptible impact on speed...if any at all. Who needs the tranny wearing out and there is a bit of "noise" of it rotating? I believe the Volvo manual recommends this.

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Old 04-05-2006, 17:43   #13
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Every transmission is different, and it is important to follow the manufactueres guidlines on which is best practice.
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Old 04-05-2006, 18:26   #14
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For my Hurth transmission they are clear that it should be placed in in gear in reverse while under sail. Drag or no drag I don't need to be replacing transmissions. I have no clue if the three blade is Mickey up or Mickey down. Sometimes you do get lucky though.
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Old 04-05-2006, 18:32   #15
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Excellent thread. I had always been a "freewheeling" man myself. I think I may change this opinion, or at least experiment a little bit with locking in the "Mickey Mouse" position. Luckily, I'm high and dry this week so I can mark off the shaft to be sure of obtaining the "Mickey Mouse" position.

But as Wheels says... probably should find some info on my transmission first.
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