Prop rotation when sailing

[Plevier]

Also in aeromodelling: free flight rubber powered contest planes have free wheeling props for best efficiency when the power runs out. The props are high pitch and high blade area with minimal shaft load and low airspeed.

Sorry I think that's a red herring. It's better freewheeling because the drive disengages and there is as near as dammit no mechanical resistance to its rotation (minimal shaft load as you say), not because of the pitch or blade area. If you think about it a high pitch prop will actually have less drag locked than a fine pitch one with similar blade size.

 

[lw395]

I think Maine Sail is to be congratualted on such a thorough (and practial!) investigation!

As has been said, my only criticism would be that the "freewheeling" prop probably did have ratehr less resistance to motion that a real one turning a shaft and some gearbox innards. Certainly mine aslo passes througha long stern tube full of grease and a couple of stuffing boxes.

Sadly, I'm not in the water at present, but has anyone considered trying the following:

1. Find a patch of the calmest water possible (harbour or lake, ideally) and with as little tidal flow as possible.

2. Get the boat up to a set speed, and then cut the power, kick it into neutral, and start a stopwatch. Time how long it takes to loose 2-3 knots.

3. Repeat in the opposite direction and average.

4. Do the above a few times to see if consistent results can be established.

5. Do the same again with the prop locked and see if it takes more or less time to get down to the same speed.

That would seem the purest way of checking on any particular boat, and doesn't need any equipment apart from a stopwatch and log. The log doesn't even have to be accurate as long as its reasonably linear and consistent.

(And apologies if that's already been suggested - I haven't read the whole thread)!

It needs a yacht with a fixed prop of course!
Which is some thing I wouldn't admit to.
However I like the idea of your test.
Far closer to real life, but of course specific to one boat.

 

[Plevier]

Put it another way: the induced losses of a freewheeling prop vs the profile and parasite drag of a stopped prop can be plotted on a curve of drag vs rpm for a particular combination of boat and prop and boatspeed. The position of minimum drag will vary depending on the factors I mentioned above. For most boats and most props the least drag condition will be letting it freewheel. But a few boats may fall outside the norm and have less drag with the prop stopped. It really does depend on the way the various factors add up and the effect that has on the drag curve. Anecdotally some posters here report this to be so for their boats.

If the freewheeling prop has no resistance to rotation there is no induced drag, only form and profile. Granted that there is in all real cases some resistance, the induced drag will be different for different blade profiles, but if the resistance is small so will be the induced drag.
I can't disprove mathematically what you suggest but I'm really struggling to imagine a transmission with so much resistance that you would be better with the prop locked.
In the aircraft situation you're talking about the prop turning over a piston engine against compression and doing a lot of work - I haven't heard of that happening on a yacht.

 

[lw395]

I...
I can't disprove mathematically what you suggest but I'm really struggling to imagine a transmission with so much resistance that you would be better with the prop locked.
I...

It's not really ever so much resistance.
The drag measured by Maine Sail is of the order of 30lb.
Say 150N. Take the speed to be 2.5 m/s.
The power loss is therefore around 375W or half a horse power.

I can well imagine the losses in a gearbox, cutless bearing and shaft seal being in that region, half a hp loss is less than you would expect from a gearbox for 40hp but it would be much better to measure it. It might well vary a lot with gearbox designs.
If freewheel was a design consideration, you might design the gearbox differently.

 

[Maine Sail]

The fact that folding props do not instantly fold on their own is an absolute red herring. The ones I have had are held open by centrifugal force at anything above half revs (probably less?). That is a key part of their design, to enable them to work in astern.

While I can observe for myself a free spinning prop is very low drag, having demonstrated this by towing a tender with the outboard down, the outboard will only kick up if it is in gear. But I can spin the prop of an outboard in neutral with my little finger.
I can't spin the prop of any yacht that easily, even when the cutless bearing is wet. I've observed this diving to clean yachts, as well as rotating the shaft from inside.
I'm curious as to what the value of that resistance to rotation is, in any engineering units, at any representative speed.
How much friction or rotating drag is in the Maine Sail rig, numerically?
I don't see any controlled drag on the shaft in the rig.
I'm guessing in a real gearbox, the drag is mostly viscous and increases significantly at higher rpm. Has anyone ever measured his properly and published the results?

My impression based on a few yacht is that the prop is rotating way slower when windmilling than if it was driving the boat at that speed. Is it possible to measure this and work out what the slip factor of the prop is in drive and windmill?

If you read it carefully you'd see that my boat begins to freewheel at about .8 - 1.2 knots speed through water and I tested this numerous times to ascertain when this begins.. Below .8 knots the friction keeps it in place.. I matched the test jig to when my boat began to free spin and it began freewheeling at about 1 knot. The jig has a friction bearing than can be compressed to increase the load/driveline drag on the jig. This was by design. It matched up perfectly with when my own boat begins to freewheel. My gear box is a Hurth 150 v-drive with 1" shaft and 4" cutlass bearing. It turns easily but with some resistance.

 

[lw395]

If you read it carefully you'd see that my boat begins to freewheel at about .8 - 1.2 knots speed through water and I tested this numerous times to ascertain when this begins.. Below .8 knots the friction keeps it in place.. I matched the test jig to when my boat began to free spin and it began freewheeling at about 1 knot. The jig has a friction bearing than can be compressed to increase the load/driveline drag on the jig. This was by design. It matched up perfectly with when my own boat begins to freewheel. My gear box is a Hurth 150 v-drive with 1" shaft and 4" cutlass bearing. It turns easily but with some resistance.

Yes, I read that bit.
It matches the static friction.
Which isn't the same as the resistance to turning at speed, which I would expect to have a viscous component from the gearbox oil, which would increase at higher speeds.
I'm not familiar with the innards of Hurth gearboxes. How much viscous drag is going to depend on the design of the gearbox.
Are your drag measurements corrected for the angle of the line between the strut and the spring balance?

It's good to have some real measurements of prop drag, people can take them and look at the drag/speed curve of a typical yacht hull, then justify buying a decent feathering prop.
It's a much cheaper way of buying boatspeed than getting a bigger boat!

 

[rob2]

Like so many things in life, it is unlikely that anyone will invest the time and money required to plot the effects of all the variables involved. I have noticed an increasing tendency for owners of sailboats to fit what (in my youth) were considered motor boat props. The prop on test certainly falls into that category! Having a liking for RORC style 70's designs of hull, I wouldn't want to live with a three bladed prop, let alone one with such a large blade area - received wisdom being that it will induce too much drag.

My own boat has a two-bladed, folding prop and its performance is quite adequate to manouvre the boat. A previous boat which was quite a performer when fitted with a slender, fixed two-bladed prop became the last boat in the fleet after an engine change which involved a three-bladed, motor boat prop. Mind you, it could tow a barge with the new prop.

It would be interesting to see the experiment repeated with a two-blade, sailboat prop with the friction held constant.

Rob.

 

[Tranona]

It is a pity that more people who are contributing to this thread have not read the report of the test. This means they tend to go off at a tangent and start introducing factors that are simply not relevant. The test is very clear about what it is trying to show. That is the difference in drag of a 3 bladed prop, similar to that fitted to many boats, when fixed or rotating, in water at speeds comparable with those achieved by small yachts, ie up to 6 knots. Comparisons are also made with a towed bucket and a folding prop. The results are consistent with all the similar empirical studies, some of which are already mentioned, but there are more, in both the UK and USA dating back nearly 100 years. All come to the same general conclusion.

Quite why anybody wants to make any comparison with aircraft props or helicopters is beyond me - different shapes operating in a different medium at different speeds, so completely irrelevant.

Many of the comments attempt to criticise the methodology, or alter the conditions to try and show that the results would be different - for example using a two bladed prop or locking it behind a keel etc. Of course changing a key variable will alter the results - the report discusses the possible impact of some of these changes. However, it is the absolute values that will change and possibly the level of difference between fixed and rotating. For example a feathering prop of similar size to a fixed would produce a similar amout of drag when locked, but less than a fixed when spinning - ie a wider differential - but the difference will be there.

What is less clear, or is less easy to predict is the difference in speed under sail as a result of reducing drag, however that is achieved. This particular test found an improvement in speed (or rather a reduction in power requirement for a given speed), but that is specific to that boat. The actual improvement will depend on how much of the overall drag of the boat is caused by the prop and the reduction in overall drag from either leaving the prop spinning or having folding/feathering blades. Many people report improvements in the .4 to .8 knot in the cruising range of 4-6knots, ie 10-15% from a folding prop over a fixed, again consistent with this test.

A folding prop will presumably have an influence on a boat's handicap when racing. It would be interesting to see some empirical evidence of the improvement in speed (reflected in time allowance) of a folding prop. Not my field but suspect someone here would have access to such data.

 

[lw395]

..

What is less clear, or is less easy to predict is the difference in speed under sail as a result of reducing drag, however that is achieved. ......

That information is fairly easy to find for typical yachts. Try Larsson and Elliason (sp?) or any other good book on yacht design.

I think it probably pays to realise that the speed increases will be most when sailing upwind in lighter weather. Being blown downwind in F5, the change in drag will make a trivial difference as the hull is on the steep bit of the 'hull speed' hump.

Rating allowances for fixed props are significant, but not enough to make anyone consider it viable for anything more serious than Round the Island.

 

[noelex]

For example a feathering prop of similar size to a fixed would produce a similar amout of drag when locked, but less than a fixed when spinning - ie a wider differential - but the difference will be there.

A feathering prop will produce very, little drag when sailing.
When feathered the prop will not rotate

 

[Tranona]

A feathering prop will produce very, little drag when sailing.
When feathered the prop will not rotate

I know that - whatever gave you the idea that I need your confirmation.

Again if you bothered to read the test report they actually got measurements for a folding prop, which coincidentally was the same as I have on my boat.

 

[Tranona]

I think it probably pays to realise that the speed increases will be most when sailing upwind in lighter weather. Being blown downwind in F5, the change in drag will make a trivial difference as the hull is on the steep bit of the 'hull speed' hump.

You are mixing two things up there - it is not the point of sailing that affects the improvement in speed, it is the proportion of drag from the prop which is greater at low speeds. The increased resistance as you approach "hull speed" (whether going upwind, downwind or on a reach) is nothing to do with drag from the prop but with wave resistance.

 

[noelex]

I know that - whatever gave you the idea that I need your confirmation.

Again if you bothered to read the test report they actually got measurements for a folding prop, which coincidentally was the same as I have on my boat.

I think you have already sent me to the naughty corner a couple of times for daring to comment. I could mention that a folding and feathering prop have different properties, but you know that and don't need my confirmation.
Lighten up, no one has even mentioned Bavaria in this this thread

 

[Plevier]

It's not really ever so much resistance.
The drag measured by Maine Sail is of the order of 30lb.
Say 150N. Take the speed to be 2.5 m/s.
The power loss is therefore around 375W or half a horse power.

I can well imagine the losses in a gearbox, cutless bearing and shaft seal being in that region, half a hp loss is less than you would expect from a gearbox for 40hp but it would be much better to measure it. It might well vary a lot with gearbox designs.
If freewheel was a design consideration, you might design the gearbox differently.

Good call - wish I'd thought to calculate that. However Maine Sail points out you need to subtract the drag of the rig without propeller from those figures.

Quote -

Strut drag removed:

Free Spinning = 8-13 Pounds Drag
Locked = 33-38 Pounds Drag

On the low end of both drag range numbers, 8 pounds free and 33 pounds locked, that is a 312.5% increase in DRAG by locking the prop!

On the high end of the drag range numbers 13 pounds to 38 pounds that is a 192% increase in drag cause by locking the prop!

Unquote

So that makes it about .125-.25HP free, .5-.75HP locked. He does say that this was set up to give the same prop friction as his gearbox but I haven't noticed anywhere that says what that gearbox is. I'm also not clear on whether the force measurement was compensated for any out of line pull on the gauge.

It needs someone to do towing tests linked to a real gearbox doesn't it! On my last boat with a VP2002 engine and standard 2 blade prop, I had to leave it in gear otherwise the current at my mooring made it spin so that was pretty free.

 

[snooks]

It needs someone to do towing tests linked to a real gearbox doesn't it!

Maybe someone could do a test using a gearbox that you would find attached to an engine, how about using an outboard engine gearbox so it would be like a sail drive?

Obviously whoever did such a test would have to find figures without the leg then with a leg, then with a fixed prop and a spinning prop.....now where could we find such a test?......I wonder

 

[Plevier]

Maybe someone could do a test using a gearbox that you would find attached to an engine, how about using an outboard engine gearbox so it would be like a sail drive?

Obviously whoever did such a test would have to find figures without the leg then with a leg, then with a fixed prop and a spinning prop.....now where could we find such a test?......I wonder

Ah. Yeeeesssss (with Paxmanic overtones.)
That not so subtle hint sent me off to look at my newly delivered YM.

However in the article I can't see any mention of measuring and offsetting the drag of the leg at any speed, only its "weight" - which I assume actually means the couple tending to hold it down. As far as I can tell from the article, only the total combined drag of prop and leg seems to have been measured. (Not sure what "figures without the leg" means?)
It seems to me the wording at the foot of p69 should be "which therefore equalled the combined drag of the prop and the leg." Have I misunderstood something?

Was there any testing to justify the assumption that the gearbox of this outboard provided the same level of resistance as a typical shaft or saildrive gearbox? I would suggest the outboard box is likely to be more compact and lightly built; I've no idea if it would be more or less lossy.

Unless the experimental procedure has been simplified for the article, I think it still leaves some points open which is a shame.

 

[lw395]

You are mixing two things up there - it is not the point of sailing that affects the improvement in speed, it is the proportion of drag from the prop which is greater at low speeds. The increased resistance as you approach "hull speed" (whether going upwind, downwind or on a reach) is nothing to do with drag from the prop but with wave resistance.

Drag makes a bigger difference upwind.
A lower drag boat can set the sails flatter and point higher.
Downwind, the higher drag slows the boat a little which increases the apparent wind, offseting the higher drag a bit, hence reducing the effect. Bethwaite explains that better.

 

[Little Rascal]

Sorry I think that's a red herring. It's better freewheeling because the drive disengages and there is as near as dammit no mechanical resistance to its rotation (minimal shaft load as you say), not because of the pitch or blade area....

It's not really a red herring just an example at the extreme end of a spectrum where the correct solution for minimum drag is reversed from the 'norm'. I grant you it's largely because the disengaged prop has low frictional loads but there is still induced drag present.

Induced drag is present as soon as there is lift and there is lift between the stall alpha and zero lift alpha (alpha = angle of attack) The frictional losses are still a big factor because they change the alpha and increase the lift coefficient of the prop - meaning more lift is neccesary to freewheel and therefore more drag is created.

Blade area makes a difference because it changes the value of the stopped drag. If this is high and induced drag is low because of low friction then there can be a set up where freewheeling is best, even in aircraft.

If you think about it a high pitch prop will actually have less drag locked than a fine pitch one with similar blade size.

Yes it would, but the higher pitch prop will also spin at lower revs for the same speed. So it's really a question of advance ratio (pitch speed vs air/boatspeed). Fine pitch at high speed will freewheel at very high rpm and be much draggier than a course pitch prop at low speed.

The comparison with aircraft props is completely valid (that's what Reynolds numbers are for ) Any propeller system that is operating in incompressible flow will obey the same principles. (The Helicopter analogy is a non-starter though: the rotor is variable pitch and during auto-rotation airflow through the rotor disk is reversed from normal flight. A different thing altogether.)

The best drag condition will change depending on pitch, blade area, boatspeed and shaft load. That's my only point. There is no black and white answer like 'Freewheeling is always less drag' as some have concluded. It is not fair to scorn anecdotal evidence just because a test of one set of conditions leads to a certain conclusion. My point is simply that a greater understanding of the actual physics shows that there is no convenient one size fits all answer.

It may well be a 'one size fits nearly all' thing.

Each system should be examined for it's own best drag condition. It just so happens to be that for most set ups on boats freewheeling is lower drag. If you have a boat and prop like the one tested then that's great but don't take it for granted - an exception could be possible given the right conditions.

 

[Tranona]

I think you have already sent me to the naughty corner a couple of times for daring to comment. I could mention that a folding and feathering prop have different properties, but you know that and don't need my confirmation.
Lighten up, no one has even mentioned Bavaria in this this thread

Just wondered why you bothered to make a specific response to me about something that was not relevant.

If you actually read the test, and what I (and others) have written, it is very clear that different props will both have different drag and a different level of reduction in drag when they are either locked or feathered/folded. In fact I made that specific point.

So, still unclear what the point is that you are trying to make.

 

[Little Rascal]

Drag makes a bigger difference upwind.
A lower drag boat can set the sails flatter and point higher.
...

Exactly. For pointing ability upwind, the lift/drag ratio of the keel/hull is just as important as the rig.