Prop rotation when sailing

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oldgitofthewest

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Convincing evidence in YM this month that allowing free prop spin when sailing reduces drag substantially. But do we know which gearboxes tolerate this and the implications for wear on bearings etc. I have a Beta 20 with TMC 40 gearbox. I have always put in gear (astern) mainly because of the noise the spinning shaft makes. What's the experts view on the gearbox implications?
 
Haven't seen this month's comic, but there is plenty of conventional wisdom that a shaft brake is a good idea. I'll look out for the mag in the club.
Personally I'd have a folding or feathering prop anyway.
 
+1 for a folding prop.

I have to say that when sailing with a fixed blade prop, I CANNOT stand the noise and vibration if it is left to rotate. Might as well motor.:)
 
The article only shows what has been known for a long time - a rotating prop produces marginally less drag than a fixed one. It also makes the point that many gearboxes are quite happy left in neutral, except some hydraulic boxes that have an engine driven pump for lubrication so should be locked. Most boxes also make noise when free wheeling and many people engage reverse to stop this. However, it can result in the box locking making it difficult to get neutral to start the engine.

Shaft brakes have limited value, except to lock hydraulic boxes, or, as the article suggests if you have a two bladed prop behind a long keel and the shaft can be locked with the blades vertically behind the keel.

The graphs are pretty clear - if you want to minimise drag then you need a folding or feathering prop. However as many threads have discussed, this is not a cheap optiuon and the benefits are not always as clearcut as they might seem.
 
Ex-SolentBoy said:
What's a little bit of drag compared to the depreciation cost of a new gearbox!
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Not sure there is any evidence it leads to higher wear on gearboxes. Both Yanmar and Volvo recommend you leave the gearbox in neutral so the prop is rotating, so one might conclude the opposite is the case.
 
Tranona said:
Not sure there is any evidence it leads to higher wear on gearboxes. Both Yanmar and Volvo recommend you leave the gearbox in neutral so the prop is rotating, so one might conclude the opposite is the case.
Click to expand...

On the other hand Kubota say you shouldn't.

Not sure there is any evidence that it doesn't lead to higher wear either.
 
snooks said:
Do Kubota make gearboxes?:confused:

Or are you talking about ZF, PRM or TMC gearboxes which attach to a kubota?
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Ive told this story many times. When I bought my first boat in 2001 I phoned VP up, blagged my way thru to a chap called Lynn in tech support. He said (talking about the MS2) that VP used to recommend putting in reverse to lock the prop shaft. They found that if the box locked up in reverse, that peeps were loth to give a burst on the starter motor (as recommended by them) to unlock it, so they then changed the advice to leave in neutral and spin!
Stu.
 
Categorically advised to put in reverse by marine engineer (Cornwall). Kanzaki gearbox (Yanmar 3GM).

That was after he'd had to re-glaze the cones as the gearbox was slipping considerably.

As usual, if you look long enough you'll get all possible opinions !!
 
I suspect the true answer varies according to the speed and the drag on the prop. Driving the propshaft and half the gearbox is not exactly spinning freely. You would have to be sure that the measurements are in realistic conditions.
It's not an exact analogy, but helicopters fall out of the sky a lot quicker if the rotors stop than if the rotors are freewheeling. You get less lift off the blades when they are fully stalled.

I see it as a 'which is the lesser of two evils?' debate, dragging a prop through the water on a sailing boat is gross.
 
I did not read the YM report, but I have seen some figures, from what looked like good experimental design, that showed the opposite.
I suspect as others have said that it varies with many factors, but there is probably not much in it.
Be careful with your gearbox though. Some are very particular.
 
When I stop motoring I always leave the gearbox lever in ahead. There must be a reason why everyone else seems to prefer astern but I can't think of one.
 
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