Do you put in gear or not when sailing?

[Juggler7823]

Stu,
We must have both contacted VP about the same time. Yes, according to the paper, leave out of gear for less drag. Admittedly this is the result of a practical experiment and could be affected by the experimental procedure and paramaters choosen.

 

[Adrian]

Not correct for boat propellors. This myth comes from helicopters and aeroplanes, and is called autorotation. There is a lot of gumph on it here...

http://en.wikipedia.org/wiki/Autorotation_(helicopter)

If the engine fails in flight in a helicopter, autorotation is used to enable a safe landing. Upon engine failure the pitch of the main blades must be set (rapidly!) to a point that allows this. The blades are kept spinning by the upflow of air through them enacting on a section of the blades where the pitch is such that that an upflow drives them. Then another section of the rotating blade has its pitch such that it pushes air down when rotating, hence keeping the descent rate to a level that is survivable on landing.

All helicopters must be able to perform a safe autorotation landing on engine failure to be licensed to fly. The blade and pitch system is carefully designed to enable this. This ability to do this leads to a lower efficiency of the blade system in powered flight, but is a price worth paying. So helicopter blades are designed to give lift when powered, and still give some lift when unpowered and autorotating.

Now to propellors on boats. These are designed to give thrust when powered. They are also designed to give some thrust, although less and less efficiently, when powered in reverse. There is no design consideration at all to give thrust when autorotating - in fact the opposite, you don't want it. So the thrust that marine propellors will give when autorotating is tiny, and much less than the drag of a locked propellor.

If I can remember, I'll get a video of this effect and post it here. My main engine is outboard so I can demonstrate this by releasing the hold-down catch and holding the engine down by hand. At 5 knots I can hold the engine down by hand with a freewheeling prop. Knock it in gear to lock the prop, and the kick up force nearly flips me off the boat. Once you've seen this, you'll never lock a prop again to reduce drag!

That's a great explanation, not sure if its right or not but very clear all the same!!

 

[Deleted member 36384]

No it requires energy to turn the prop so letting the prop will use more energy that allowing it to freewheel. Consequently, locking the prop will give less drag.

And drag consumes no energy? The energy caused by parasitic losses due to drag in the locked state is greater than the energy used to spin the free wheeling prop.

 

[Chris_Robb]

Roy
it was me that phoned up VP and talked my way past the receptionist to a chap called Lyn, he told me that on the VP2030 MS2 they used to tell peeps to put in reverse BUT that the box would lock in reverse, they advised to flick the starter to unlock it. Allegedly peeps were confused by this so at that point in time, 2003 ish they changed the advice to leave in neutral.
The box itself is just a bog std box, gears etc in a bath of oil so no probs with a pump to worry about.
So if we go back to the paper, leave spinning and it causes less drag?
Stu

Stu - (volvo 2003T) I put it is reverse gear, which of course locks. The VP advice to blip the starter motor is spot on, as starting the engine in reverse at 7 knots would impose an undue strain on the starter motor. I used to leave it free wheeling, until I heard the shaft seal making a buzzing noise some 200 after its last grease, so if covering huge distances I would recommend that it is put into reverse irrespective of the reduction in speed (if true).

 

[Pagetslady]

VP 2003 likes it in reverse

Some where during the past 20 years that I have been motoring with my VP2003 I have heard or read that it is not wise to let the prop rotate while sailing because unless the engine is running there is no cooling water passing through the gearbox allowing the gearbox oil to heat up, My engine has done over 6000 hours and is always locked when sailing as far as I am aware my gearbox is ok and as yet it has never stuck in reverse,(no doubt it will prove otherwise now)
Mike

 

[Spyro]

"My main engine is outboard so I can demonstrate this by releasing the hold-down catch and holding the engine down by hand. At 5 knots I can hold the engine down by hand with a freewheeling prop. Knock it in gear to lock the prop, and the kick up force nearly flips me off the boat. Once you've seen this, you'll never lock a prop again to reduce drag! "

That seems clear and simple to me. Is it a 3 blade prop?

 

[AngusMcDoon]

"My main engine is outboard so I can demonstrate this by releasing the hold-down catch and holding the engine down by hand. At 5 knots I can hold the engine down by hand with a freewheeling prop. Knock it in gear to lock the prop, and the kick up force nearly flips me off the boat. Once you've seen this, you'll never lock a prop again to reduce drag! "

That seems clear and simple to me. Is it a 3 blade prop?

Yes.

 

[cliffordpope]

Sail-assisted paddle steamer.
Lock the paddles when sailing, or allow to rotate?
Rotating the paddles uses energy, therefore better to lock them?
Therefore sticking an oar over the side makes you sail faster?

 

[lw395]

Sail-assisted paddle steamer.
Lock the paddles when sailing, or allow to rotate?
Rotating the paddles uses energy, therefore better to lock them?
Therefore sticking an oar over the side makes you sail faster?

Perhaps this is why paddle steamers are less efficient than screws?

To look at it another way, the drag of a freewheeling prop is gross, and the drag of a locked prop is hideous. Whether gross is slightly more or less than hideous doesn't really matter very much. A proper sailing boat needs a folding or feathering prop, or no prop at all!

 

[Ariadne]

We have a biggish (17x12) fixed 3 blade prop on our 110 Saildrive unit. If I lock it in reverse as suggested in the book it will lock up and need a flick of the starter motor to free it and will take nearly a complete knot of the boat speed.

On the other hand if I let it spin freely I have a slight whining noise, but I gain up to half a knot & I can start the engine without any worry when I need to.

So practical experience tells me to leave it free spin. I have only changed the oil seals once in 4 years constant use, and that was because I had the engine in bits for 3 months while I was working various bit of it and I couldn't run it up as normal for an hour a week.