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Old 01-07-2008, 19:00   #16
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On my past two boats I have had Hurth tansmissions and the lubrication is such that an unlocked transmission will wear poorly. You can disagree on which is less drag but you can'[t disagree on the specs for the tranmission. No transmission will be damaged locked in reverse but some will be damaged freewheeeling. It's not always a matter of opinion.
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Old 01-07-2008, 19:16   #17
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Dulcinea

We have a MaxProp on Dulcinea, so this isn't an issue for us. When underway I just drop it into reverse for a few seconds, the prop goes to feathering mode, and then it doesn't matter! I of course put things back into neutral, and when we fire up the iron jib it takes a second or 2 for things to "unfeather" and then life goes on.

A thing of beauty!

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Old 02-07-2008, 02:38   #18
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Originally Posted by Ex-Calif View Post
Usually in this topic I drag out the aircraft examples, of which there are many, that state a stopped prop creates less drag.

One thing I haven't been called on is that when an airplane prop is windmilling it is turning the engine and gear train as there is no "clutch" on a fixed prop airplane. The drag on a sailboat prop should be less as in all cases I am aware of the engine is disconnected and you don't have to fight the engine compression.

However, I am reading a book called "Gentlemen Never Sail to Weather" - It's an old book that I snagged from the club library. The author swears he got at least a knot after learning to lock the prop.

I am still a lock the prop guy at heart tough.

Drag From Windmilling Propeller
Ex,

I don't think we are comparing apples with apples in your airplane example.
The reason why the fixed (but constant speed) airplane prop stops spinning is because it is feathered, the airflow is no longer over the airfoil and therefore the drag is significantly reduced. This is not the case with a fixed boat prop as it can't be feathered in the same manner.

I don't know anything about damage to trannies but I am sure it depends on the type (as others have posted); however, it easy to work out which method (in gear or in netural) gives the most drag.

Place prop in netural, go sailing -if the prob starts spinning then the only way to stop it is to apply some force on the drive train somewhere (ie via gear box, brake etc). That force is now absorbed by the prop and shows up as drag. Put another way, if you have to apply a force to stop the prop. from turning, then the amount of force required to stop the prop. is the same as the increase in drag when it is forcibily stopped.

If the prob doesn't spin when in netural, then placing it in gear will have no effect anyway.

Therefore, being in netural is always the lowest (or worst case -same) drag UNLESS it is a 2 balde fixed pitch prob that can be aligned with the keel and locked in that position.

IMHO.

I am a locked prop guy myself, I just hate the sound of it turning when under sail
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Old 02-07-2008, 02:49   #19
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No transmission will be damaged locked in reverse but some will be damaged freewheeeling. It's not always a matter of opinion.
Absolutely and it is certainly not a case of Religion. It is pure and simple mechanical logic and the age old rule applies. Read the Manual!!
The MIT article I remember from way back. It actually offered no real information in the end.
A prop is a rotary wing. As it turns it actually sucks itself forward. 55% is suck forward and 45% is propulsion off the back. Stopping it's rotation creates a Stall effect over the wing. The wing now creates turbulence which is seen as drag. However, the drag from a stationary prop is not as great as if the prop was spinning. The reason is quite technical and long winded to explain. But it can be easily explained with this simple image. If a plane is flying through the air, the wing creates lift and that keeps the plane aloft. But if you stop the plane from moving forward, the wing stalls and can no longer support the weight of the plane in the air. So the plane falls. If the plane could maintain a horizontal attitude, it would fall straight down. This is like our stationary prop. The drag from the wing can not stop the plane from falling and it will fall at a very high speed. But if you managed to point the nose down and gain forward speed, eventually the wing has enough air flowing over it and the wing gains lift again. Maintaining the forward motion will create a lift capable of stopping the plane from free falling straight down. The result is that the lift from the wing moving forward is greater than the drag of the stopped wing going straight down.
Now that is only part of it. Because a wing on an aircraft doesn't have to impart energy to the engines. A prop on a boat is both a wing and the turning force of the Gearbox/engine. So the amount of force it takes to turn the shaft is then seen as effort against the motion of the prop. The effort against the prop is now seen as effort against forward motion. A generator turned by the prop is going to result in the boat slowing. The energy it takes to turn that prop is a direct relationship to the force it takes to push the boat forward. So if the force against the boat moving forward is great enough, it will be seen as a loss of speed.
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Old 02-07-2008, 04:07   #20
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Ex,

I don't think we are comparing apples with apples in your airplane example.
The reason why the fixed (but constant speed) airplane prop stops spinning is because it is feathered, the airflow is no longer over the airfoil and therefore the drag is significantly reduced. This is not the case with a fixed boat prop as it can't be feathered in the same manner.
We are definitely mixing apples and oranges - sorry I brought it up.

However - I am a pilot and aeronautical engineer so I can't let this one go.
By definition a fixed pitch prop on an airplane does not feather.

It will windmill. You actually have to slow the airplane down by pitching up, in order to stop the prop. Once stopped you can increase speed to a certain point, after which the prop may start windmilling again. This speed, however, is usually below a speed known as best glide speed, the most efficient speed at which you will cover distance over the ground. There is a different speed (slower) at which you will stay aloft the most amount of time.

There are of course feathering aircraft props as well.
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Old 02-07-2008, 04:59   #21
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Absolutely and it is certainly not a case of Religion. It is pure and simple mechanical logic and the age old rule applies. Read the Manual!!
I agree - Read the manual for your transmission and engine.

However it's not really mechanical logic, it's physics and while your analogy is interesting the fault lies in the fluids we are dealing with. It's been a while since I did fluid dynamics but air is compressible and for our purposes water is not.

An airfoil has a shape designed to compress the air and bring bernoulli into the game. As the wing moves through the air it compresses the air on the leading edge and top 1/4 or so of the wing. After that the air accelerates rapidly and creates a low pressure on the top of the wing. This is lift.

There is also a downwash created on the underside of the wing due to angle of attack and that brings newton into play. Aircraft props work the same way.

Putting air aside, there is very little airfoil shape to a boat propeller. The boat propeller creates thrust primarily due to Newton.

One way to visuallize this is to imagine the prop stopped and the water moving. Let's also assume for a moment that we are talking about slow speeds <8kts so the effect of angle of attack of the boat prop can be nulified for the moment. Let's also assume a CW rotating prop.

When the prop is powered the water strikes perpendicular to the the aft side of the prop and is deflected or thrust rearward by the pitch of the prop. A low pitch prop has to turn faster relative to a high pitch prop to push the same amount of water per second.

On the forward side of the prop the water wants to separate and so a balance of pitch and prop speed must be designed to minimize this separation, called cavitation.

If we add back in the forward motion of the boat, it becomes logical that a prop will start by cavitating and "lugging' the engine slightly as the boat starts moving. Eventually the angle of attack of the water moves forward and the cavitation effect is reduced and the prop "unloads" slightly. The speed of the hull must also be considered when designing a prop as must the weight of the boat as the weight of the boat will be a huge factor in getting the boat moving and the prop unloaded.

High speed boats generally will have a lot of pitch because the angle of attack moves forward so much.

There is more twist at the root of the blade than at the tip, because the speed of the prop at the tip is different (faster) than the speed at the root. So the diameter of the propeller is also considered when designing the shape of the propeller.

Chord length and shape is also factored in as a function of boat speed, engine speed and prop diameter. Low speed props are generally more paddle like with less twist than a high speed prop.

There is another thing happening - the water does not move straight across the prop. It moves outward as well due to the shape of the prop.

At the tip of the blade the water spills off and creates a vortex. This vortex is not usefull and creates a lot of drag on the prop as the "higher pressure water on the backside mixes with the lower pressure water on the front side. The same thing happens on aero wings which is why designers have tried all sorts of things from droop tips, to saw blades, to circular wings to winglets to control the vortices.

So lets freewheel the prop and run the water in reverse.

The water now strikes the front side of the prop - as it is now coming not from the perpendicular but from the front of the boat. The water pushes on the front of the prop and the prop starts to move CW. The faster the boat goes the harder the water pushes and the faster the prop goes.

The prop is changing the direction of the water hitting the front side (the same deflection angle as in forward) but importantly as it moves CW it also has to displace the water on the backside of the prop which is now hitting the rear of the prop perpendicular. The faster it goes the more water has to be both deflected and displaced and drag continues to go up. The vortices also exist as water spills off the tip.

If the prop is stopped the water must be deflected but the back side water does not have to be displaced. The vortices still exist to a lesser degree and it is possible that at a certain speed the back side cavitates and that creates (a lot of) drag. Also no matter how fast you go the angle of attack never changes on the stopped prop. It is always from the front of the prop so the drag curve will definitely look different than a freewheeling prop.

I will not state that a stopped prop is faster. All props are different.

I would surmise that a small prop could freewheel and not make be a big deal, but a large prop and a fast sailing speed freewheeling would be bad In my humble and unscientifically tested opinion.

However if I were going to do 10,000 miles under sail, I'd sure practice with my boat and try to figure it out.





Quote:
Originally Posted by Alan Wheeler View Post
The MIT article I remember from way back. It actually offered no real information in the end.
They did a great job comparing several 2 and 3 bladed fixed props and a few folding and feathering props under power.

What I had trouble with is that by their own admission they did not test any of the props in a freewheeling condition. Their conlcusions were calculated.
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Old 02-07-2008, 07:54   #22
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Originally Posted by Ex-Calif View Post
......One thing I haven't been called on is that when an airplane prop is windmilling it is turning the engine and gear train as there is no "clutch" on a fixed prop airplane.....
Drag From Windmilling Propeller
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....The reason why the fixed (but constant speed) airplane prop stops spinning is because it is feathered, .....
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Originally Posted by Ex-Calif View Post
....However - I am a pilot and aeronautical engineer so I can't let this one go.
By definition a fixed pitch prop on an airplane does not feather....

There are of course feathering aircraft props as well.
Ex,
I bow to greater wisdom and knowledge. I misunderstood you first reference to "fixed prop airplane" and the link that discussed feathering of a windmilling prop to mean a "airplane where the prop was "fixed to the engine crank" as opposed to say a PT6 (where it isn't) rather than mean "fixed pitch" because as you state "a fixed pitch prop on an airplane does not feather". That was the only way I could reconcile the link to "fixed prop airplane".

So I used the term "fixed" in a similar way but added "(constant speed)" to indicate the ability to feather so as not to confuse the term "fixed" with "fixed pitch" - didn't work did it.

I now see you actual meaning in you first post.
This is probably as clear as mud - sorry - but as a low time ex pilot and high time LAME, I couldn't let this one go.
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Old 02-07-2008, 08:33   #23
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Different views

After reading Wheels post (#19) and Ex's (post #21) which give differing accounts of what happens with boat props and drag, I would like to go back to first principles and then get a considered opinion from these gentlemen.

Consider a boat moving through the water with a fixed pitch prop that is not restrained (essentially the tranny is in netural) and the boat (driven by the sails) is moving fast enough to cause the prop to rotate.

With the boat at constant speed, the thrust of the sails must equal the drag (leaving aside some constants like heel and leeway).

In order to stop the prop rotating some external force must be applied to it. The energy required to stop the rotation must come from somewhere and as the only energy source in the system is the thrust from the sails, it must be taken from there thus decreasing the available thrust to move the boat.

However for the thrust and drag to remain equal, the drag now has to decrease to match this lowered thrust value. The only way to decrease the drag value is for the boat speed to decrease (essentially giving a lowered drag value).Therfore the boat slows down until the thrust and drag are equal again.

So the locking off the free wheeling spinning prop shows up as an increase in drag (because the effect is a slower boat speed) although it is really a case of the energy required to lock it off being subtracted from the available energy to move the boat. In effect, locking off a spinning prop is adding a form of parasite drag.

Please let me know if I am holding the wrong end of the cat here, I won't would to stay ignorant if I am missing something basic.
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Old 02-07-2008, 09:32   #24
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The problem is that you gotta believe a moving prop creates more drag than a stopped one. It's a tough pill to swallow.

And I would add that not all boat and prop combinations are going to act the same.

My hypothesis allows that some combinations will have more drag when locked...

Also don't forget that in your terms there is "energy" required to stop the prop. There is also energy required to move the prop. Otherwise the prop wouldn't move...
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Old 02-07-2008, 09:34   #25
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Ex,
I bow to greater wisdom and knowledge. I misunderstood you first reference to "fixed prop airplane" and the link that discussed feathering of a windmilling prop to mean a "airplane where the prop was "fixed to the engine crank" as opposed to say a PT6 (where it isn't) rather than mean "fixed pitch" because as you state "a fixed pitch prop on an airplane does not feather". That was the only way I could reconcile the link to "fixed prop airplane".
Sorry if I sounded cocky - It's not about wisdom (of which I have a little bit) it's more about we miscommunicated.

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Cheers...
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Old 02-07-2008, 10:50   #26
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Sorry if I sounded cocky - It's not about wisdom (of which I have a little bit) it's more about we miscommunicated.

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Cheers...
Absolutly no offence taken and I didn't read it as "cocky". I concur with the miscommunication and I trust I also have not sounded "cocky"; at least not in this instance. I might have had my tongue very slightly in my cheek while I was bowing

BTW, some of my best friends are pilots - lets have a cyber beer.
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Old 02-07-2008, 10:58   #27
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OK, can one of you two blokes please tell me the acronym definition of "LAME" in these posts

I did a search & ummm, "LAME"[1] & "pilot" don't give any good result examples

[1] in-fact, LAME on its own doesn't give a good result example
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Old 02-07-2008, 11:18   #28
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The problem is that you gotta believe a moving prop creates more drag than a stopped one. It's a tough pill to swallow.

And I would add that not all boat and prop combinations are going to act the same.

My hypothesis allows that some combinations will have more drag when locked...

Also don't forget that in your terms there is "energy" required to stop the prop. There is also energy required to move the prop. Otherwise the prop wouldn't move...
All this is true but taking you last point first (am I Irish?), if there is not enough energy acting on a "free" prop to make it move in the first place, locking it has no effect at all in the system. The prop wasn't moving before it was locked and isn't moving after it is locked, there isn't any difference.

One only "locks" a prob if it is already moving (or will move). The question then becomes "does the boat go faster or slower after locking it, all other conssiderations being equal?".

In my hypothesis, the boat must slow down after locking it (except it that rare instance whereby a 2 blade prop can be "hidden" behind the skeg or keel).

If the boat was to speed up after locking the prop, then the process of locking it must be adding a "thrust vector" into the situation (or decreasing the drag??). In my mind, that would require an enegry source rather than the energy lost in holding it locked.

That is the pill I can't swallow (yet) - but hey, I am no rocket scientist and would be happy if someone can point out any errors of my logic here.
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Old 02-07-2008, 11:23   #29
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OK, can one of you two blokes please tell me the acronym definition of "LAME" in these posts

I did a search & ummm, "LAME"[1] & "pilot" don't give any good result examples

[1] in-fact, LAME on its own doesn't give a good result example
Sorry exfish, we got off the boating theme of this forum - Licensed Aircraft Maintenance Engineer - really just glorified spanner head or in my case - radio nerd, but we use terms like LAME to make us sound far more important than we are. Also helps in getting paid more. Mind you, if we stuff up, it doesn't matter what we are called - 'cause it's all bad. One of the reasons you notice why I am a bit pedantic here and elsewhere, I don't like stuffing up, bad for employment prospects
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Old 02-07-2008, 13:31   #30
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Aircraft Maintenance Engineers are licensed by (insert jurisdiction) to inspect and certify that work done on an aircraft complies with written airworthiness standards.
To gain an aircraft maintenance engineering licence, certain basic criteria must be met.
You must:
Be a high school graduate, at least 21 years of age.
Be able to read, write and understand English.
Not suffer from any disability likely to affect your technical skill or judgement.
Have 2 years post-secondary education/training.
There may be other requirements, depending upon the specific jurisdiction.

Sounds easy; but it ain't.
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