engine in gear when sailing or.......?

J

jay

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I have a Volvo MD2020 engine mated to a conventional shaft drive and a fixed 3 blade prop. The engine manual says this should be left in neutral when sailing. Obviously this means the prop spins at a considerable rate of knots. Surely this will cause undue wear in the shaft and seals – and creates an annoying rumble throughout the aft part of the boat. The only other option (that I know of anyway) is to put the engine in gear – My previous sailing has been as racing crew and so has always been on boats with folding props where this was the custom to ensure folding. What is the correct procedure and why?

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Check what the gearbox manual says!!

Some gearboxes must be left in neutral some can be put astern when sailing ahead and some can be put ahead when sailing ahead.

For a pure mechanical gear box it should not really matter but some have cone clutches and other little niceties that get "messed up" (polite description) when run backwards.

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There is no hard and fast rule, it depends on your priorities:

1) If minimising drag is your priority, let it spin while sailing at low speeds. As you get near hull speed under sail, the prop will begin to stall and you will hear a lot of rumble and feel the vibration. It is then best to stop the prop by selecting reverse gear as the drag of a stopped and stalled prop is less than a turning and stalled prop.

2) If you are sailing in areas with debris in water, best to stop the prop as a turning prop is more likely to get rope, nets, fishing line etc horribly entwined around it than a stopped prop.

3) If wear on your bearings and stern gland is your big concern then stop the prop - though I suspect that this is not really a factor in coastal cruising boats. It may become more significant on non-stop blue water boats though.

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I have recently acquired a boat with a Yanmar 3GM30F. On passage I put the gear shift into astern to stop the prop, but found it very difficult to shift into neutral for the next startup. The manual doesn't give any instructions - does anyone know the correct procedure for the Yanmar, I guess it can vary from one make of engine/gearbox to another. (In case you are wondering - the previous owner isn't available to ask). The gear shift linkage is otherwise quite free.

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Happens on my 1GM10's every time.

I just find waggling the stick a little free's it off to get into neutral. I do not think it causes any harm, much like some manual cars will not go into reverse every time, summit to do with synchro mesh or no clutch, never really looked into it as I have never been worried by it.

I bet this does not help at all /forums/images/icons/smile.gif

<hr width=100% size=1><A target="_blank" HREF=http://www.topcatsail.co.uk>Woof</A>
 
If ShipsWoofy's waggling of the stick does not do it then a quick jab on the starter while jambed in reverse will free it (move to neutral after the quick jab). That is what should be done with Volvo's as I understand it (and what we do if the stick wiggle doesn't work) and same for others it seems.

Not sure about Ric's comment about letting freewheel at slow sailing speeds when prop is not stalled - I know where he is coming from on that but I would have thought the prop will always be stalled due to the very high angle of attack and rotation being constrained by the gearbox and bearing frictions. Would be interested in any sound hydrodynamic views on that.

John

<hr width=100% size=1>I am the cat but I am only 6.
 
This is a bit of a technical reply from a cat isn't it?

Your hydrodynamics are, however, quite correct. I've always locked the prop in reverse, although it is possible to buy a shaft brake if worried about ignoring gearbox instructions.

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A stopped prop is less drag in all conditions than a spinning one.

But the main point that has to be taken into consideration is what type of gearbox you have. Hydraulic box will not care if stopped or not. Mechanical box may suffer over a period from running without adequate oil flow etc.



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Volvo gearboxes are different from most of the others (hurth, zf etc): the disks transmitting power from the engine shaft to the propeller shaft are 1. initially moved by a small lever, then 2.kept together adjacent to each other by the propeller thrust
The engine manual is correct (strange isn t it? -.<) in saying to keep the gear in neutral (afair it also says to let the engine run in neutral every 10 hours or so to properly lubricate parts, or else fit an external shaft brake): if put in forward, the small lever will try to keep the (rotating) prop shaft disk against the (fixed) engine shaft disk but will be unable to properly do so, causing wear between the two disks; on the contrary when in reverse, propeller rotation will try to push together the two disks, and the small lever would have a very difficult job in trying to set them apart, that s why it is often almost impossible to put the gear lever from reverse to neutral when underway

awful description but I hope it helps, if you have the occasion of looking at a volvo gearbox drawing it will be much clearer

r

[edit] should have said more properly conical surfaces instead of disks -.<)

<hr width=100% size=1><P ID="edit"><FONT SIZE=-1>Edited by Roberto on 21/09/2004 08:48 (server time).</FONT></P>
 
You are most definitely wrong that "a stopped prop is always less drag than a spinning one". That is a gross simplification. For example, if the prop is rotating at a speed such that the angle of attack of the blade is zero, the flow over the prop is laminar and the only drag is skin drag. If you then stop the prop, the flow is entirely stalled and the drag increases rapidly. However, if the prop is allowed to freewheel in the stalled state, then the drag can be even higher than the stopped and stalled condition.

The points where these changes occur depends on the speed of the boat through the water, the pitch of the prop, and the speed it which it is rotating.

Understanding this concept requires some understanding of vectors etc, and so people who don't understand such things mistakenly conclude that it is either "always best to let the prop freewheel" or equally incorrectly that it is "always best to stop it". Hence you often get long and misinformed arguments on the subject in sailing boards.

The correct answer, as I have explained above, is that it depends entirely on the angle of attack of the leading edge of the prop - and that is dictated by the vector combination of the boat speed and the speed of rotation of the prop, and the angle that this flow makes with the pitch of the blade.

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But is the rotational speed of the freewheeling prop ever sufficient at any boat speed given the bearing and gearbox frictions? I don't think I have seen a prop on a yacht freewheeling at anything remotely close to where I would expect laminar flow over it.

For example, to try and put some measure on it, if a 380mm pitch propeller was to freewheel with no slip at 1 knot then it would have to rotate at approx 1.5 rpm and at 2 knots 3 rpm. I believe that our prop hardly rotates at all at those boat speeds when the gearbox is in neutral so I would say that our prop (fixed 3 blade of design for yacht auxiliary) is essentially always stalled and I expect that most yachts would be the same.

John

<hr width=100% size=1>I am the cat but I am only 6.
 
I think that the description above of the Volvo 'box also applies to the Yanmar box. Put in reverse when sailing as this pulls the cone clutch parts into tight engagement; if you use forward the parts tend to separate under load, can slip, and cause excessive wear.

I use reverse with my 3GM just to stop the dreadful rumbling; with the wetted area on my long-keeler drag one way or the other is not much of an issue! However, it regularly jams in reverse and as already stated, a quick blip of the starter immediately frees it and you can then put it into neutral.

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If there is zero angle of attack there is no force causing the prop to rotate therefore the prop is stationary.

That is why people have feathering props.

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You're in a helicopter at 1,000 feet. The engine's just stopped. The pilot asks you whether you'd prefer him to leave the rotors spinning, or stopped.

Which do you choose?



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I have the Perkins perama M50 which is the same base engine as the VP 2020 and has a Hurth Gearbox with wet clutch plates. I seem to remember that if you want to stop the prop spinning you must put it in REVERSE.
Personally I cannot see the difference between wether in Forward & Reverse as the whole of the G/box internals are immersed in oil - surely everything is lubricated and cannot 'lockup'?
Not sure however if the VP2020 has it's own gearbox or is fitted with a Hurth.

<hr width=100% size=1>dickh
I'd rather be sailing... :-) /forums/images/icons/smile.gif
 
if it is a Hurth (and almost anything mechanical except volvo)as it has been said it should be blocked in reverse

if the clutch lever is put into forward, the helicoidal wheel will function very much like a bicycle gear: when you push the pedals it spins the wheel (as when the engine turns the propeller turns as well), if you stop the pedals the wheel will keep on turning (with engine stopped, the propeller would be kept still only by friction, a sufficient boat speed will be capable to cause enough force on the propeller to win the friction and let the propeller turn, with forward gear on)



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OK - so I simplified it to a general matter ......

So who is going to calculate the speed at which to stop it or let if freewheel ?

The simplest is to stop it as what I am informed by others in better knowledge of these things is that its actually quite a low speed that most props make the change. Let it freewheel at that point or greater speed of boat effectively presents a disc and not blades.

Sorry I crossed your learned borders - But I tend to look at these thing sin best interest and simplest general case.

BTW - I let mine freewheel as I don't care a toss about speed on a motor sailer that can't catch the proverbial grandma ...

OK ?

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