Noisy Sailing!

[upthesolent]

Being a newbie to having an inboard engine I wonder if the following problem is a common one.

When making good progress under sail, there is a noise which I assume is coming from the propeller which I guess is turning round with the force of the water against it. The first time I heard it we were goosewinged in a brisk 4 and I thought it was actually made by the wind going through the rigging, but soon realised that is was a whirring noise. I did try putting the cockpit lever into drive position but that made no difference.

I have an 11 h.p. Vetus twin turning a twin blade prop (looks like it has a rope cutter added) via a Volvo Saildrive unit on my Leisure 23.

I suppose folding props would be the answer but I feel that`s going to be expensive. Any thoughts, comments on this please. I find it a most unpleasant noise!

 

[firstascent2002]

In a boat with a prop sharf then you will hear the prop spinning whilst sailing if the shart is not locked. This is an annoying noise and actually the drag of a spinning prop is actually greater than a stationary prop. Most people fix the prop by putting the gear box into reverse.

I'm not sure what the form is with a saildrive but as the hustler we've just bought (and not yet sailed) has one I'd be intrigued to know what others here have to say.

j

 

[VicS]

We always put the gears into astern (Volvo MS gear box) That stops the prop from turning.

 

[EdWingfield]

Best go to the engine manufacturer's website to check 'cos my Volvo 2030 instructions are to leave in neutral.

 

[peteandthira]

Same here re the 2030, although NOT if you have a folding prop. Sometimes our folder doesn't quite fold itself away if it has been a while since lift-out, but a quick plop into reverse folds it nicely and then back into neutral, with silence.

 

[Inselaffe]

Hi,

I know its a done to death discussion but.... /forums/images/graemlins/laugh.gif

Reading Principles of Yacht Design (Larsson & Eliasson, Adlard Coles Nautical) at the moment again, I noticed that they say 'If the propellor is completely free to rotate, its resistance is reduced to about one fourth of that of a locked propellor. ...... In practice the clutch and friction will slow down the rotation.'
They say a folding prop has normally less than 5% of a fixed and locked one.

The drag coefficients they quote are:

Fixed, Locked: CD = 1.2
Fixed, free to rotate: CD = 0.3
Folding: CD = 0.06

There is also a graph of prop resistance (0.5 x water density x Speed^2 x CD x Prop frontal area) vs Speed.

They don't say where they got these figures from though.

They go on to say that for the 40ft example they use as an illustration, at 6.8 Knts prop resistance would be 29% total drag if locked, reducing speed by about 0.8 Knts, whereas if it was completely free to rotate there would only be a 0.2 Knt reduction in speed.

My own Noddy Experimental Study: I have an 9.9 horsepower outboard in a well on my little Etap 22 and my prop (High Thrust) also makes a noise, but when I put it in gear the leg is pushed back, quite severely, which must mean more drag. I don't have access to a towing tank anymore so can't do a proper test!

Since my outboard has little shaft friction and the noise is not very bad at all I leave it rotating, but if it was noisey then I would probably shut it up!

Does the view that a fixed prop reduces drag come from the times when two-bladed props could be kept in the wake of long keels?

Or, and very possibly, have I missed something bloody obvious?

 

[ColinHuggins]

Previous opinion has been divided on this: noise & wear v drag & manufacturer's recommendation.

IMHO there is much more chance of a stray rope wrapping round the prop when it's turning. Yes, I'll admit to becoming anchored by the stern in the Looe Channel without due cause but with much impediment!

CJH

 

[john_morris_uk]

It is a done to death discussion, but:

It depends on the prop. (Sailing boat props often have smalle blade areas than a prop on a power boat.)

It depends on lots of other factors. The drag in the drive train etc.

Ultimately, I can't stand the noise of the spinning prop and shaft and always lock it.

I admit to having a fancy feather prop now anyway so the debate has gone cold for us on our boat.

One final thought. If you were a helicopter with a failed engine, you wouldn't lock the blades in the hope of producing more drag as you descend!

 

[Richard10002]

We definitley lose about 0.5 knots at 5 or 6 knots with the prop locked, (3 blades, not folding).

Alternatively, the wind drops the moment we lock the propellor, and picks up as soon as we release it - every time! <g>

 

[Channel Ribs]

Our Galion 22 has a very similar noise, but we suspect it is not the prop but the rudder oscilating very slightly; might be worth checking?

 

[fluffc]

If your engine has a hydraulic gearbox, then putting it into 'gear' has no effect with the engine off, as the gearbox oil pump is not being driven, so there is no pressure in the system and the (hydraulic) clutch cannot engage the gears.

Note that the PRM manual for hydraulic gearbox specifically states that no damage can be done to the gearbox by leaving the propeller free to rotate. Makes sense as no gears are being meshed - it's just a sumb lump of metal spinning inside an oil bath! Of course, your stern gland is still going to wear out...

 

[thalassa]

Could also be too much "play" in the outer cutless bearing, or an unbalanced propellor (nicks etc.) . Two-bladed propellors are more sensitive to this than 3 bladed ones. A good propellor firm should fix this.

 

[upthesolent]

Should have guessed this was a hot chestnut! Thanks for the replies. And hello VicS, I will try selecting reverse when the boat is back in the water to see what happens. I did a quick Google on it and found a few people had problems in getting the lever back to neutral, the general consensus in overcoming this was to actually start the engine in reverse and then selecting neutral. (Never had this sort of problem with the Seawych lol.)

Nevertheless, would still be interested to hear from those who do not advocate this practice.

Thanks all.

 

[Avocet]

We have a conventional 3 baleded prop and an old fashioned shaft with stuffing boxes at either end. At any speed right up to hull speed, it is completely silent although when spinning, the prop sets up a faint vibration that we can feel through the tiller.

 

[Marsupial]

Hi
I cannt find anything on ship props but (in terms of drag) they work the same way as aeroplane props - look here for the science

http://selair.selkirk.bc.ca/aerodynamics1/Multi/Page2.html

 

[hebdena]

Prop. Drag.

If I remember correctly, if a ship is running at full ahead but the prop is turning at half ahead there is more drag than if the prop was stationary or running astern.
I believe the technique for an emergency stop was to run the prop at half the ship speed until down to a couple of knots and then go half astern. I'm sure this must be all about prop slip and cavitation etc.
This could be just a theory I picked up on but it does sound logical, a bit like ABS brakes for ships............must try it out one day.

Andy.

 

[Ric]

[ QUOTE ]

One final thought. If you were a helicopter with a failed engine, you wouldn't lock the blades in the hope of producing more drag as you descend!

[/ QUOTE ]

For gads sake not that one again. You can't even begin to compare the situation of a stalled helicopter rotor to a stalled prop - the Reynolds numbers are of orders of magnitude different.

The drag on a free-turning prop is about one tenth of the drag of a locked and stalled prop. If you don't want to believe it, you are free to bask in ignorance for the rest of your lifef, but I would suggest you get a book on fluid dynamics and propeller design and study it.

 

[temptress]

[ QUOTE ]
[ QUOTE ]

.....The drag on a free-turning prop is about one tenth of the drag of a locked and stalled prop. If you don't want to believe it, you are free to bask in ignorance for the rest of your lifef, but I would suggest you get a book on fluid dynamics and propeller design and study it.

[/ QUOTE ]

I hear what you are saying but when I have a fixed prop and I am sailing at 7 knots if I leave the prop to rotate I travel about 1 knot slower (6 knots) that if I lock it in place (7 knots). This is observed fact on my current boat (SO 47). As a former delivery skipper I got to play wiht all sorts of boats and I have also seen similar speed reductions on other boats I've sailed with fixed props.

I'm not disputing your theroy but actual observation shows otherwise. /forums/images/graemlins/confused.gif Anyone on here care to explain this? /forums/images/graemlins/smirk.gif

 

[Vara]

I think I've posted on this about 3 times since I've joined the forum.
Forget the theory,take your boat out on a calm day preferably in a tide-less situation motor up to 6 knots,cut engine and measure the time it takes your speed to drop back to (say) 3 knots prop unlocked and locked,repeat a few times,analyse your results,and go with the results.
Theory is great but nothing like a bit of empirical testing.
Remember the apocryphal Bumble Bee,its a well known scientific fact that they can't fly. /forums/images/graemlins/smile.gif
If cruising I always lock my prop regardless, as the noise is irritating,and probably wears things out..

 

[charles_reed]

If drive in neutral

it's probably the propeller and shaft turning.

Try leaving it in gear and seeing if that cures the problem.

If you have a cone-clutch gearbox, putting the gearbox into gear with the engine stopped will probably have no effect as the engine needs to be turning to energise the clutch.

Screws vary, some need to be put into reverse, others into forward gear to stop the rotation. Most fixed screws need to be braked in reverse.