Hi all

The new boat i am going to crew in the local races has not arrived from France as yet so we are going to use mine in the Cock of the Huon race in the Pipe Opener series dssinc.org.au/yacht-racing/pipe-opener-series/

Normally when i sail i leave the box in neutral and let the 2 bladed prop rotate but when she was on the slip last week i marked the shaft with the prop in a vertical position lined up behind the skeg and i thought i might lock it in that position for the race to try and get her to be as quick as possible

I believe the second owner who used to race her up north replaced the original 3 blade prop with the 2 blade so he could do this and seemed to think it was worthwhile

Looking at info on the net Yanmar seem to advise leaving it in neutral when sailing but i am wondering if anyone has any thoughts on locking it in reverse and any potential problems it might cause

Thanks in advance

Regards Don

It is not a matter of the engine manufacturer's recommendation. It is a matter of the gearbox manufacturer's recommendation.

What is the clutch drive? Cone, disc, multi disc, torque converter or dog.

More information needed before a qualified answer can be made.

Select to expand quote

cisco said..

It is not a matter of the engine manufacturer's recommendation. It is a matter of the gearbox manufacturer's recommendation.

What is the clutch drive? Cone, disc, multi disc, torque converter or dog.

More information needed before a qualified answer can be made.

Hi Cisco

Sorry about that i should have been more precise

As far as i know it is a Kanzaki cone type box

Regards Don

When flying model aircraft, it's well known that a freely rotating propellor has more drag than one that is locked still. It seems counterintuitive to me, but it's been proven many times. I would imagine that boat propellors would behave the same way.

I think this would be a worthwhile endeavour, even if you can't lock it straight up and down.

Hi Nebbian

I think should be able to line it up behind the skeg by locking it in place while still on the mooring and just sailing it off the mooring to the race start

Regards Don

Donk my old Halvo yacht we would always lock the prop by shifting into reverse the old BMW gave up the ghost way before the gearbox had any hinot of trouble after 20 years of service. The new yanmar I fitted we also put it into reverse while sailing.

My new Adams strangely the prop doesn't spin while sailing in neutral.

My last Dunc 37 I would put a ring spanner onto the coupling nut to lock during long passages.

Not so sure about locking the prop in close quarters especially around the start of a race or moored yachts?

Hi Southace

The mooring area is on the outer edge of the other moorings in the bay so once we are off my mooring it is a clear run to the start line and if i lock it in reverse and and worse case have to start the motor for any reason it is only a matter of just knocking it in to neutral and starting it

I considered locking the shaft by other means with the box in neutral but it is probably a bit risky if a quick engine start is needed

Regards Don

My volvo says to lock in place

Select to expand quote

nebbian said..
When flying model aircraft, it's well known that a freely rotating propellor has more drag than one that is locked still. It seems counterintuitive to me, but it's been proven many times. I would imagine that boat propellors would behave the same way.

I think this would be a worthwhile endeavour, even if you can't lock it straight up and down.

I believe that the myth that a locked yacht prop causes less drag than a freely rotating one has been debunked many times, there are even video offerings from reputable people and organisations demonstrating this.
Air and water are very different.

I think the locked prop would be the better solution. Freely rotating props will have more drag and noise, just not enough to generate electricity!

Select to expand quote

UncleBob said..

nebbian said..
When flying model aircraft, it's well known that a freely rotating propellor has more drag than one that is locked still. It seems counterintuitive to me, but it's been proven many times. I would imagine that boat propellors would behave the same way.

I think this would be a worthwhile endeavour, even if you can't lock it straight up and down.

I believe that the myth that a locked yacht prop causes less drag than a freely rotating one has been debunked many times, there are even video offerings from reputable people and organisations demonstrating this.
Air and water are very different.

It's really not that simple. Factors are - how much friction in the shaft bearings and how much surface friction and form friction from the prop, a bit of friction at high revs creates significant wasted horse power and therefore drag, in a perfect world no friction would mean no drag. Also, what is the frontal area of the prop because that determines the drag with the prop stalled. A skinny free wheeling prop might not create much drag running free. A fat prop might create more drag when fixed and stalled. Best to suck it and see.

If you look in detail at the innards of a Kanzaki cone-clutch tranny, you willl see that the torque from the prop tries to force the cones together if it is locked in reverse, which is a good thing as it minimises any chance of the clutch cones slipping. Conversely if it is locked it in forward, the prop torque will try to force the cones apart which can lead to slipping. Slipping very quickly causes glazing of the cones, which requires disassembly to fix and is the biggest single problem with these trannies. It's not hard to do, but it is a hassle and will cost you a grand or so if you can't do it yourself.

Various studies I have seen quoted in forums support the locked prop from a measured drag point of view. Aircraft props might not be the best analogy since water, being non-compressible, behaves quite differently to air.

I find the noise of a rotating prop pretty annoying too.

Cheers, Graeme

From what I can understand from my 3ym30 Yanmah /KM2P-1 drive,,, when the engine is stopped, left in forward gear and the boat is sailing forward, my prop shaft can still (and does) rotate because the forward gear cone becomes unloaded from the forward gear cup. This happens because of the helix spline that the cone is driven by and slides on, is now been driven by the prop and not the engine. Force transmitted through the now driving propeller shaft act on the helix trying to push the cup and cone apart. This allows the cone to slip causing possible wear of the cone/cup

If you shift to reverse to stop the rotation of the prop when moving forward and the engine is stopped you are using the prop rotation to wind the reverse drive cone tight into the reverse gear cup. This can cause a tightness or loading up of the shift mechanism and if you try to shift back to neutral forcing it,,, without the motor running, you could do damage to the shifting mechanism.

If you must use reverse to stop the prop, starting the motor first before shifting back to neutral will unload the reverse drive cone from the reverse gear cup.

The principle of the helix spline is similar to that of a rotating nut and bolt One will end will tighten the other will loosen, The same reason for some applications having a left hand thread eg: Bench grinder

Like wongaga said:( I didn't see your post first before I posted)

Select to expand quote

wongaga said..
Aircraft props might not be the best analogy since water, being non-compressible, behaves quite differently to air.

At normal (non-supersonic) speeds, both air and water behave as a non-compressible fluid. The cutoff is a bit arbitrary, but anywhere below mach 0.1 (100 m/s in air) is considered incompressible. The equations are the same. Many airfoil tests for airplane wings and props have been done in water, to make it easier to see what's happening.

I know it doesn't make sense, but there you go.

I have a Yanma 50hp, not sure what gearbox though. I tried last week to slip it into Reverse and it went in no problem. The boat slowed immediately by over a knot and I had to start the engine in reverse to get her back into neutral. Not doing that again. I have a big 4 blade prop so maybe some difference there.

Select to expand quote

PhoenixStar said..

UncleBob said..

nebbian said..
When flying model aircraft, it's well known that a freely rotating propellor has more drag than one that is locked still. It seems counterintuitive to me, but it's been proven many times. I would imagine that boat propellors would behave the same way.

I think this would be a worthwhile endeavour, even if you can't lock it straight up and down.

I believe that the myth that a locked yacht prop causes less drag than a freely rotating one has been debunked many times, there are even video offerings from reputable people and organisations demonstrating this.
Air and water are very different.

It's really not that simple. Factors are - how much friction in the shaft bearings and how much surface friction and form friction from the prop, a bit of friction at high revs creates significant wasted horse power and therefore drag, in a perfect world no friction would mean no drag. Also, what is the frontal area of the prop because that determines the drag with the prop stalled. A skinny free wheeling prop might not create much drag running free. A fat prop might create more drag when fixed and stalled. Best to suck it and see.

Hi there are a couple of video's of fixed versus free wheeling prop tests to be found, one by an english sailing mag and another by an american gent using the name mainesail that come to the same conclusion, fixed props create a measurable increase in drag. I believe that a university somewhere came to the same definite conclusion.
Both the video,s show a simple test rig on the water that clearly demonstrate this.

Select to expand quote

UncleBob said..

PhoenixStar said..

UncleBob said..

nebbian said..
When flying model aircraft, it's well known that a freely rotating propellor has more drag than one that is locked still. It seems counterintuitive to me, but it's been proven many times. I would imagine that boat propellors would behave the same way.

I think this would be a worthwhile endeavour, even if you can't lock it straight up and down.

I believe that the myth that a locked yacht prop causes less drag than a freely rotating one has been debunked many times, there are even video offerings from reputable people and organisations demonstrating this.
Air and water are very different.

It's really not that simple. Factors are - how much friction in the shaft bearings and how much surface friction and form friction from the prop, a bit of friction at high revs creates significant wasted horse power and therefore drag, in a perfect world no friction would mean no drag. Also, what is the frontal area of the prop because that determines the drag with the prop stalled. A skinny free wheeling prop might not create much drag running free. A fat prop might create more drag when fixed and stalled. Best to suck it and see.

Hi there are a couple of video's of fixed versus free wheeling prop tests to be found, one by an english sailing mag and another by an american gent using the name mainesail that come to the same conclusion, fixed props create a measurable increase in drag. I believe that a university somewhere came to the same definite conclusion.
Both the video,s show a simple test rig on the water that clearly demonstrate this.

Hi Uncle Bob

What about if the fixed 2 blade prop is lined up behind a skeg as in my case ?

Regards Don

Select to expand quote

Donk107 said..

UncleBob said..

PhoenixStar said..

UncleBob said..

nebbian said..
When flying model aircraft, it's well known that a freely rotating propellor has more drag than one that is locked still. It seems counterintuitive to me, but it's been proven many times. I would imagine that boat propellors would behave the same way.

I think this would be a worthwhile endeavour, even if you can't lock it straight up and down.

I believe that the myth that a locked yacht prop causes less drag than a freely rotating one has been debunked many times, there are even video offerings from reputable people and organisations demonstrating this.
Air and water are very different.

It's really not that simple. Factors are - how much friction in the shaft bearings and how much surface friction and form friction from the prop, a bit of friction at high revs creates significant wasted horse power and therefore drag, in a perfect world no friction would mean no drag. Also, what is the frontal area of the prop because that determines the drag with the prop stalled. A skinny free wheeling prop might not create much drag running free. A fat prop might create more drag when fixed and stalled. Best to suck it and see.

Hi there are a couple of video's of fixed versus free wheeling prop tests to be found, one by an english sailing mag and another by an american gent using the name mainesail that come to the same conclusion, fixed props create a measurable increase in drag. I believe that a university somewhere came to the same definite conclusion.
Both the video,s show a simple test rig on the water that clearly demonstrate this.

Hi Uncle Bob

What about if the fixed 2 blade prop is lined up behind a skeg as in my case ?

Regards Don

Personally I reckon that would be a winner . The drag would have to be minimal as the blades are in the lee of the keel.
I guess that's how it was done by many over a long period of time.
The only thing that would be better would probably be a flexofold or similar but who can afford that solution.

I sailed on a boat for many years and we always lined up the 2 blade prop behind a skeg and locked it in reverse before starting each race, more times than not the prop would already be lined up and we just had to put it in gear, if your racing every little bit helps.

Select to expand quote

UncleBob said..

Donk107 said..

UncleBob said..

PhoenixStar said..

UncleBob said..

nebbian said..
When flying model aircraft, it's well known that a freely rotating propellor has more drag than one that is locked still. It seems counterintuitive to me, but it's been proven many times. I would imagine that boat propellors would behave the same way.

I think this would be a worthwhile endeavour, even if you can't lock it straight up and down.

I believe that the myth that a locked yacht prop causes less drag than a freely rotating one has been debunked many times, there are even video offerings from reputable people and organisations demonstrating this.
Air and water are very different.

It's really not that simple. Factors are - how much friction in the shaft bearings and how much surface friction and form friction from the prop, a bit of friction at high revs creates significant wasted horse power and therefore drag, in a perfect world no friction would mean no drag. Also, what is the frontal area of the prop because that determines the drag with the prop stalled. A skinny free wheeling prop might not create much drag running free. A fat prop might create more drag when fixed and stalled. Best to suck it and see.

Hi there are a couple of video's of fixed versus free wheeling prop tests to be found, one by an english sailing mag and another by an american gent using the name mainesail that come to the same conclusion, fixed props create a measurable increase in drag. I believe that a university somewhere came to the same definite conclusion.
Both the video,s show a simple test rig on the water that clearly demonstrate this.

Hi Uncle Bob

What about if the fixed 2 blade prop is lined up behind a skeg as in my case ?

Regards Don

Personally I reckon that would be a winner . The drag would have to be minimal as the blades are in the lee of the keel.
I guess that's how it was done by many over a long period of time.
The only thing that would be better would probably be a flexofold or similar but who can afford that solution.

Hi Uncle Bob

I think if i was to change the prop it would need to be a feathering type not folding as the rudder sits fairly close to the prop

Possibly a 3 bladed something like this http://www.hydralignprop.com/product-range.html

Regards Don

Select to expand quote

riverider said..
I sailed on a boat for many years and we always lined up the 2 blade prop behind a skeg and locked it in reverse before starting each race, more times than not the prop would already be lined up and we just had to put it in gear, if your racing every little bit helps.

Hi Riverider

The previous owner of mine thought it was worth the effort of replacing the original 3 blade with a two blade to enable him to do this

Regards Don

Interesting topic. My Yamaha 33 originally came with a 12 horsepower Yanmar & fixed two blade prop. The Yanmar manual recomended that the gear box be left in neutral when sailing. My flying experience told me that I should lock the prop in reverse to reduce drag.Speed tests showed the boat was faster with the prop freewheeling. Much of the published scientific literature on the subject is inconclusive, some suggest a cross over point, sighting factors such as prop blade length, pitch, keel type, p bracket location, shaft bearing friction etc as factors affecting drag.While others straight out argue you that fixed is fastest & freewheeling is slow or visa versa. I have since fitted a new engine & two bladed folding prop to the boat, so engaging reverse gear when looking for optimum performance is the norm. Ultimately you are going to be governed by the gear box manufactures recommendations if you don't what to void your equipment warranty. If your not bothered by that, let the speedo do the talking.

I don't get the locked/neutral propeller argument, ie why there is still a discussion.

A fixed propeller dragging through the water naturally wants to release energy by spinning.

If the transmission is locked, the only way that energy can be released is in the form of drag. If the propeller is left to spin, the energy can be partly dissipated through the transmission thus reducing drag.

I think this Yachting Monthly article largely confirms this:
www.yachtingmonthly.com/gear/lock-prop-let-spin-29526

Hmmm it appears to be related to pitch and diameter.



www.rcgroups.com/forums/showthread.php?730371-Spinning-Prop-vs-Stationary-Prop

It's certainly not a clear cut "one size fits all" solution of whether fixed or freewheeling has more drag.

The only explanation I can think of is that if the prop is stopped, then the flow stalls, and you end up with drag based purely on area.

When the blade is spinning, the blade might not be stalled, so might be providing 'lift' on the rear of the blade, pulling you backwards more 'efficiently' than just a flat out stalled blade. You can think of this like a helicopter doing an autorotation. Clearly for a helicopter, a spinning blade has far more drag than a stopped blade.

For boat propellors, which are probably on the other side of the Pitch/Diameter cutoff, it may well end up with less drag if the thing is left to freewheel. The yachting monthly article used a prop with a P/D ratio of 1, which ends up on the side of freewheeling being better.

I think a lot of the responses here are talking about a 3 blade also one without the sheltering Donk's 2blade is getting . Locked in a vertical position behind the keel there would be minimal if any drag.

Select to expand quote

nebbian said..
Hmmm it appears to be related to pitch and diameter.



www.rcgroups.com/forums/showthread.php?730371-Spinning-Prop-vs-Stationary-Prop

It's certainly not a clear cut "one size fits all" solution of whether fixed or freewheeling has more drag.

The only explanation I can think of is that if the prop is stopped, then the flow stalls, and you end up with drag based purely on area.

When the blade is spinning, the blade might not be stalled, so might be providing 'lift' on the rear of the blade, pulling you backwards more 'efficiently' than just a flat out stalled blade. You can think of this like a helicopter doing an autorotation. Clearly for a helicopter, a spinning blade has far more drag than a stopped blade.

For boat propellors, which are probably on the other side of the Pitch/Diameter cutoff, it may well end up with less drag if the thing is left to freewheel. The yachting monthly article used a prop with a P/D ratio of 1, which ends up on the side of freewheeling being better.

I agree with Nebbian. In the aircraft I flew the single engine performance with the dead prop not feathered was so poor the aircraft was unflyable. Feather the prop and she would fly on the other engine.
This was because the freewheeling prop created drag over the entire surface area of the spinning prop, pretty obvious from the drag created as the prop spins with the air flowing through it. The helicopter analogy is an excellent one.
Someone with more expertise in hydrodynamics and air/water properties could explain further but for me a stopped prop has to be better than a prop being spun by the water flow.

The answer to why an aero prop windmilling causes more drag than if it was stopped is complicated in one way but simple in another,, the answer is it dose and it doesn't.

A powered up spinning prop produces thrust from the pitch but it also produces lift opposing the thrust produced by the pitch because its blade/ foil with forward velocity. The thrust produced by pitch is far greater than the lift so the lift is negligible. However, if power is cut, the propeller stops producing thrust to a level that is below that of the opposing lift and it is now that the propeller is only producing what is like excessive drag/ reverse thrust (lift) whilst it is spinning.
The propeller needs to maintain rotating at this velocity and forward speed to continue producing the lift but as the drag slows the forward air speed the rotational speed of the prop will decay and to a point where the lift will be lost but the prop will continue windmilling. The windmilling blade will still have a greater angle of attack to the on coming air ( because it is spinning) than a locked blade so therefore it will still have less drag.
How does this compare to a boat prop?
It seems that a boat prop that is free wheeling in normal boating scenarios doesn't produce enough opposing lift to equal the drag of a locked blade therefore the free wheeling prop, having a greater angle of attack will always have less drag.

The angle of attck of airflow over a propeller is due to both the forward velocity of the aircraft and the rotational velocity of the propeller driven by the engine. When the engine fails, the rotational velocity will slow and eventually, the AoA will become negative and produce negative lift or in other words, lots of drag, more drag than if the propeller was stationary.

This diagram explains it nicely




www.pilotfriend.com/training/flight_training/fxd_wing/props.htm

This next diagram shows the drag crossover between a windmilling and a stationary propeller.





However, this is all pretty much contrary to many of those boat tests, that show the rotating propeller creates less drag. I think the answer to this is a combination of where the boat propeller would fit in the above graph due it's design, but also the effect of the other means of propulsion, being the sails. The propulsion from the sails may help keep the propeller spinning enough to maintain an AoA (drag level) that is better than that of a stationary propeller.

Select to expand quote

FreeRadical said..

However, this is all pretty much contrary to many of those boat tests, that show the rotating propeller creates less drag.

I don't think so. Your diagram and mine both show a crossover point, where the most efficient way to go is to change from freewheeling to fixed, or vice versa. They both show that a high speed prop with a high pitch will be better if left to freewheel. They also show that a low speed prop with a low pitch is better left locked. Note that the boat tests above used a high pitch prop. I bet that if they used a low pitch prop then they would have a different story to tell.

The crossover point on mine is 2/3 Pitch/Diameter. The crossover point on yours is at about 20 degrees. I wouldn't be surprised if they mean the same thing when measuring near the tip of the prop.

Really I doubt it makes very little difference.

The only reason I lock it is I like the silence and just to hear the water running past my hull.

Try this.

Sail one nautical mile with your prop locked in 10 knots of breeze and time yourself.

Then resail the mile with prop unlocked in 10 knots and compare times?