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Old 24-01-2016, 03:38   #61
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Re: Transmission nuetral or locked while sailing

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Originally Posted by Vipe6 View Post
First of all, I'm not trying to start an argument, I just want to have a better understanding on the subject.
At the beginning of the text in the first link provided i find this: "Think of those seed pods that spin as they fall. As they fall, the air makes them spin. (Because they're spinning, it's as if the mini wing was moving faster, so it creates more lift, and the seed pod doesn't fall as fast as it would if it didn't spin.)" Does this not mean that a spinning propeller creates more drag than a locked one? In adittion, the seed looks very much the same as one blade of a ships propeller……....
No argument, just a discussion and an exploration / understanding of the subject

OK lets have a closer look of the falling seed pod and then how that compares to a boat prop.

Imagine two seed pods, both the same mass and both the same cross sectional area. One has a "wing" shape that causes it to rotate as it falls and the other has a shape that doesn't allow it to rotate. They are both powered by gravity and as they have the same cross sectional area, they "should" fall at the same rate but of course, the spinning one falls slower as it creates lift while spinning. So yes, you could say it has more drag than the one that isn't spinning.

Now look at a very small model boat and imagine that you are towing the same
seed pods. This time, instead of gravity, the boat is powered by wind. The forces act the same however and the spinning seed pod will have the most drag compared to the non spinning one; so it might seem the spinning prop would have more drag…..

Now here are the big buts….

The reason the non-spinning seed pod isn't spinning is due to it's shape, not because it was "locked" against spinning.

To be useful to the boat analogy, we would have to compare a spinning seed pod that was free to spin and a spinning seed pod (of exactly the same dimensions) that was locked or prevented from spinning.

The force of the wind remains constant and once the model boat is travelling at a constant speed (i.e. thrust and drag is in equilibrium), the seed pod that is spinning will have a constant rpm. If you were to add a small motor and spin the seed pod faster (i.e. adding energy into the system), the boat speed will increase. Conversely if you were to slow the rpm of the seed pod down by using a brake (i.e. removing energy from the system), the boat speed will decrease. The extreme brake is to stop the seed pod from rotating altogether and this will slow the boat the most.

Another way to consider the boat prop question is to consider this.

Sailing at 1 kt with gearbox in neutral and motor running. The prop is unlikely to be turning as the forces on it aren't great enough to overcome the frictional losses in the shaft etc. Now the wind picks up and boat speed is say 6 kts, gearbox still in neutral (and engine running) but now the prop is rotating at say 200 rpm due to the water forces acting on it.

If you were to pop it into gear (ahead), the prop would now be spinning at say 1000 rpm due to the energy added to it by the engine. Of course we know the boat speed would increase. Now pop it back into neutral and after a short time, the prop would return to 200 rpm and the boat would slow down to the previous 6 kts (the wind remains constant).

Now if you were apply a brake to the shaft and slow the prop down to say 50 rpm, you are removing energy from the system (by the use of the brake) and it follows the boat would slow down somewhat. The most you can slow the prop is 0 rpm (by locking the prop) and this is where the boat slows the most.

Finally, there are many reasons why you can't compare airplane props to boat props but perhaps the most important one is that airplanes don't "lock" a prop blade to stop it rotating, they either alter it's pitch (if that system available) or alter their airspeed to ensure it doesn't rotate. Completely different horses for completely different reasons!
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Old 24-01-2016, 06:32   #62
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Re: Transmission nuetral or locked while sailing

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Originally Posted by crazyoldboatguy View Post
Where is the downside?
I can think of a few.

1) It will slow the boat in medium to light winds. A typical number is 1/4-1/2 knot out of 6 which adds 1-2 hours per day to a long passage. If winds are high then maybe not much speed lost but if sailing on the wind will cause more heel and leeward drift than usual reducing ability to point higher.

2) It adds a bunch of "stuff" to the prop shaft. Will all that stuff be properly balanced so as to not cause premature bearing wear? Can the running gear handle the side load created by the belt?

3) It has to be disengaged when motoring else the engine will be overloaded (assuming ideal prop and transmission beforehand). Also, because the shaft is going to be slow under sail the pulley ratio will be high and the alternator may fly apart when running under engine power. How to do the mechanical disconnect and interlocking so cannot start/run engine when alternator is engaged?

There is no free lunch...
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Old 24-01-2016, 06:56   #63
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Re: Transmission nuetral or locked while sailing

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Originally Posted by transmitterdan View Post
I can think of a few.

1) It will slow the boat in medium to light winds. A typical number is 1/4-1/2 knot out of 6 which adds 1-2 hours per day to a long passage. If winds are high then maybe not much speed lost but if sailing on the wind will cause more heel and leeward drift than usual reducing ability to point higher.

2) It adds a bunch of "stuff" to the prop shaft. Will all that stuff be properly balanced so as to not cause premature bearing wear? Can the running gear handle the side load created by the belt?

3) It has to be disengaged when motoring else the engine will be overloaded (assuming ideal prop and transmission beforehand). Also, because the shaft is going to be slow under sail the pulley ratio will be high and the alternator may fly apart when running under engine power. How to do the mechanical disconnect and interlocking so cannot start/run engine when alternator is engaged?

There is no free lunch...
I guess you could eliminate a lot of side load using a toothed belt or a bicycle type of chain. 3) could be solved by an electromagnetic clutch like the ones being used in autopilot drives.
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Old 24-01-2016, 08:06   #64
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Re: Transmission nuetral or locked while sailing

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Originally Posted by Wotname View Post
No argument, just a discussion and an exploration / understanding of the subject

OK lets have a closer look of the falling seed pod and then how that compares to a boat prop.

SNIP -- SNIP -- SNIP

Finally, there are many reasons why you can't compare airplane props to boat props but perhaps the most important one is that airplanes don't "lock" a prop blade to stop it rotating, they either alter it's pitch (if that system available) or alter their airspeed to ensure it doesn't rotate. Completely different horses for completely different reasons!
I'm sorry Wotname, I'm sure that you answered Vipes6 question somewhere in your response, but I kept falling asleep trying to find it .

Here's a Readers Digest Condensed Version.

The reason that a Maple seed pod falls slower than another object of the same mass is the very same reason why a unpowered, free spinning airplane propeller causes more drag than a lock one.

It's not the physical surface area of the "blade" that air is seeing, but the entire prop disk area (sorry, there is a proper term for this, but at the moment I cant think of it). It's the total circular area of the spinning "blade" that the air is reacting against. Which is many times larger.

Now before we go off on a "this verses that" tangent, this only applies to a low density medium. Such as air.
A high density media, such as water, the opposite is true.

Kinda think of it this way, water being heaver and thicker than air, doesn't want to "get out of the way", so it is easier to turn the prop than to "get out of the way".

Air will just sit and compress in front of the prop.

It's hard for most folks to comprehend this, this is when they have to just push the "I Believe" button.
Heck, even with all of my aerospace experience, I'm always awed by the fact that a 600,000 pound airplane is only held aloft by nothing more than air pressure differences.

And just to get off on yet another tangent, when "close hauled", air pressure differences is what is PULLING our sailboats.
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Old 24-01-2016, 08:11   #65
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Re: Transmission nuetral or locked while sailing

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Originally Posted by missourisailor View Post
I should clarify a bit on my last response.
We are of course discussing single engine aircraft here.
Mostly, although i watched a de havilland Caribou land without either engine working. BTW, passenger jets have been known to lose all power and glide into an airport many miles away. Airbus did that for 65 miles. Of course you dont have to worry about props with a jet.
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Old 24-01-2016, 08:23   #66
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Re: Transmission nuetral or locked while sailing

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Originally Posted by reed1v View Post
Mostly, although i watched a de havilland Caribou land without either engine working. BTW, passenger jets have been known to lose all power and glide into an airport many miles away. Airbus did that for 65 miles. Of course you dont have to worry about props with a jet.
Yeah, I did say "singles", and yes multi's do lose power. But with multi's you start having Yaw issues, with single engine failures. Which is another can of worms.

I was trying to keep it simple.
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Old 25-01-2016, 19:42   #67
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Re: Transmission nuetral or locked while sailing

Quote:
Originally Posted by missourisailor View Post
I'm sorry Wotname, I'm sure that you answered Vipes6 question somewhere in your response, but I kept falling asleep trying to find it .

Here's a Readers Digest Condensed Version.

The reason that a Maple seed pod falls slower than another object of the same mass is the very same reason why a unpowered, free spinning airplane propeller causes more drag than a lock one.

It's not the physical surface area of the "blade" that air is seeing, but the entire prop disk area (sorry, there is a proper term for this, but at the moment I cant think of it). It's the total circular area of the spinning "blade" that the air is reacting against. Which is many times larger.

Now before we go off on a "this verses that" tangent, this only applies to a low density medium. Such as air.
A high density media, such as water, the opposite is true.

Kinda think of it this way, water being heaver and thicker than air, doesn't want to "get out of the way", so it is easier to turn the prop than to "get out of the way".

Air will just sit and compress in front of the prop.

It's hard for most folks to comprehend this, this is when they have to just push the "I Believe" button.
Heck, even with all of my aerospace experience, I'm always awed by the fact that a 600,000 pound airplane is only held aloft by nothing more than air pressure differences.

And just to get off on yet another tangent, when "close hauled", air pressure differences is what is PULLING our sailboats.
Yep, you are right and the compressibility is something I failed to mention - my only defence is that I was trying (perhaps too hard) to keep it simple; sorry to have sent you asleep .

Leaving aside the airplane issues, the simplest yet most accurate way to consider the boat prop / neutral / locked issue is as follows:

Energy is never created or destroyed, merely converted from one form to another (leaving aside nuclear physics).

A sailboat travelling under sail with the gearbox in neutral (prop freely rotating) and a constant wind with be in equilibrium. That is, all the forces are balanced, in particular thrust and drag and the boat speed remains constant. Introduce a change of energy into the system and either the thrust or drag will increase thus changing the boat speed.

If one decides to apply a brake to the freely rotating prop, it will slow down. That energy that is a absorbed in the brake (mainly heat) has to come from somewhere and that is from the decrease in boat speed. Put another way, the drag has increased.

So what happens if the prop is slowed down to a complete stop, the brake is no longer getting hot so is it still absorbing energy from the system? Yes it is and this is what some often forget. The locking mechanism (whatever it might be) is absorbing energy thus increasing the drag. Where is the energy being absorbed - simple, it is heeling the boat over ever so slightly. The lock is operating counter to the rotation of the prop and is applied as a torque to the hull so wanting to heel the boat opposite to the free prop rotational direction.

Of course, the degree of heel is so small, one will not notice it but that is exactly what is happening to the force vectors.

Asleep yet :big grin:

Of course, adding energy into the system is far easier to understand - start the engine and put it in gear and boat speed increases - if this is true (and we know it is), then the opposite is also true!
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Old 25-01-2016, 20:57   #68
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Re: Transmission nuetral or locked while sailing

The reason a locked boat propeller causes more drag than a freely rotating propeller is simple. A locked propeller creates more turbulence than a freely rotating prop. Creating turbulence generates heat so requires energy from somewhere. The energy comes from the boat and this reduces the energy available for forward motion so the boat slows down. The turbulence (and thus heat) created by the freely rotating propeller is much less than when locked thus consuming less energy and slowing the boat less.

In the case of an airplane propeller the situation is reversed. A locked propeller also creates turbulence but in air it produces less heat (air being 800-1000 times less dense than water). If the airplane propeller is allowed to rotate then the energy consumed (heat generated) by the rotating propeller is more than the heat generated from the turbulence of a locked propeller. So the rotating airplane propeller generates more heat and slows down the plane more.
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Old 26-01-2016, 05:52   #69
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Re: Transmission nuetral or locked while sailing

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The reason a locked boat propeller causes more drag than a freely rotating propeller is simple. A locked propeller creates more turbulence than a freely rotating prop. Creating turbulence generates heat so requires energy from somewhere. The energy comes from the boat and this reduces the energy available for forward motion so the boat slows down. The turbulence (and thus heat) created by the freely rotating propeller is much less than when locked thus consuming less energy and slowing the boat less.

In the case of an airplane propeller the situation is reversed. A locked propeller also creates turbulence but in air it produces less heat (air being 800-1000 times less dense than water). If the airplane propeller is allowed to rotate then the energy consumed (heat generated) by the rotating propeller is more than the heat generated from the turbulence of a locked propeller. So the rotating airplane propeller generates more heat and slows down the plane more.
Yeah. What he said
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Old 26-01-2016, 05:59   #70
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Re: Transmission nuetral or locked while sailing

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Yep, you are right and the compressibility is something I failed to mention - my only defence is that I was trying (perhaps too hard) to keep it simple; sorry to have sent you asleep .

Leaving aside the airplane issues, the simplest yet most accurate way to consider the boat prop / neutral / locked issue is as follows:

Energy is never created or destroyed, merely converted from one form to another (leaving aside nuclear physics).

A sailboat travelling under sail with the gearbox in neutral (prop freely rotating) and a constant wind with be in equilibrium. That is, all the forces are balanced, in particular thrust and drag and the boat speed remains constant. Introduce a change of energy into the system and either the thrust or drag will increase thus changing the boat speed.

If one decides to apply a brake to the freely rotating prop, it will slow down. That energy that is a absorbed in the brake (mainly heat) has to come from somewhere and that is from the decrease in boat speed. Put another way, the drag has increased.

So what happens if the prop is slowed down to a complete stop, the brake is no longer getting hot so is it still absorbing energy from the system? Yes it is and this is what some often forget. The locking mechanism (whatever it might be) is absorbing energy thus increasing the drag. Where is the energy being absorbed - simple, it is heeling the boat over ever so slightly. The lock is operating counter to the rotation of the prop and is applied as a torque to the hull so wanting to heel the boat opposite to the free prop rotational direction.

Of course, the degree of heel is so small, one will not notice it but that is exactly what is happening to the force vectors.

Asleep yet :big grin:

Of course, adding energy into the system is far easier to understand - start the engine and put it in gear and boat speed increases - if this is true (and we know it is), then the opposite is also true!
Of course what you are saying about the heeling due to counter rotational forces is true, vector graphics prove that. But the larger force is the drag due to the cavitation that the stationary prop is producing.

Cavitation is what you keep omitting.
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Old 26-01-2016, 06:59   #71
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Re: Transmission nuetral or locked while sailing

We have beat this to death a few times but as this is how I have made my living for the last 30+ years or so I'll have a go so to speak.

For the record I was certain a rotating prop caused more drag as it does on an airplane, and this is very easily demonstrated, but it doesn't on a boat, and this has been demonstrated, I was proved wrong.

First a helicopter or seed pod for that matter does in fact fall much slower in autorotation, that is because of the length of the rotor has very different apparent airspeeds on the rotor, amounting to a driven and a driving region, a portion of the disk is being driven by another portion, creating lift. You have stalled and unstalled regions and even regions of reverse flow, helicopter aerodynamics are complicated.
This has nothing at all to do with a boat prop, so forget it.

I think, the biggest difference is if you look at the common airplane prop, the prop comprises probably less than 10% of the disk, the blade profile is rather thin, so drag from it being stopped is rather low. Known as a high aspect ratio for those that care.

Look at a boat prop, a three blade for example is probably close to 100% of the total disk, the blade shape is very broad, so it's drag is relatively high when stopped.
Doesn't matter though, too many times its been actually measured and proved there is less drag from a turning prop.

I solved my conundrum with a feathering prop, engine off, shaft stops, simple.
And I am sure there is less drag in that feathered prop than there is in a fixed prop, turning or not.

On edit, I a am also pretty sure that with a fixed two blade, if I indexed the shaft so I could ensure the two blade was in the "deadwood" of my full keel, I bet drag would be pretty low, sort of a poor mans feathering prop drag wise?
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Old 26-01-2016, 08:33   #72
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Re: Transmission nuetral or locked while sailing

Air compression is not why a boat moves forward. Its the relative air density, or vacuum created by the sails. The boat gets "sucked" forwards, much as a bumblebee creates a vacuum in order to move. A jet creates compression that moves it. That is why if you take a 150 jib and, keeping the square footage the same, cut it into two jibs and raise them on inner and outer fore stays, the boat will move faster. The slot between the sails acts to create a bigger density difference(or bigger vacuum), thus driving the boat faster, all else being equal

Which is why a boat can sail faster than the wind( assuming a racing boat).
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Old 26-01-2016, 09:12   #73
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Re: Transmission nuetral or locked while sailing

I had a Brunton autoprop self pitching /feathering on my last boat back in the UK and always shut the Yanmar 4JHE engine down at tickover speed rpm in forward gear. I would dearly love a Brunton on My current USA boat, also with a Yanmar, but this time a 3GM3, recommended to freewheel under sail. I cannot afford a Brunton on a pension but hate the nose of a rotating shaft and the potential for wear on the 'Volvo' style shaft seal and shaft itself from a constantly rottng transmission. I cannot imagine a simple effective and cheap shaft brake that is convenient to use without diving into the enginespace to engage it. I guess unless someone has a bright idea that earplugs are the only solution other than turning on the cockpit stereo speakers

If I submit to temptation, do I really risk gearbox damage by locking the shaft in gear and if not, is forward or reverse best?

I used to be just a little confused now I really am befuddled!
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Old 26-01-2016, 09:41   #74
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Re: Transmission nuetral or locked while sailing

Hey, even Universal and Hurth people can't make up their minds. These are the two sentences in their manual, one in the beginning and the other at the end.

CAUTION:
DO NOT LEAVE GEAR IN FORWARD WHEN SAILING. GEAR MUST BE IN NEUTRAL FOR FREE WHEELING OR SHIFTED INTO REVERSE TO LOCK PROPELLER WHILE SAILING.
.......

CAUTION: TRANSMISSION MUST BE LEFT IN NEUTRAL WHEN SAILING, DO NOT LEAVE IN FORWARD GEAR WHEN SAILING WITHOUT POWER AS IT WILL DAMAGE GEAR.
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Old 26-01-2016, 09:47   #75
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Re: Transmission nuetral or locked while sailing

Also I can't really understand how or why the idea that locked prop has less resistance than free-spinning one ever started. That's like thinking that automobile wheels will have less resistance when locked than when turning.
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