Freewheeling propellers: the chopper theory

[tartansailor]

For every person that comes up with a new concept, even if it is solid and based on observable, measurable FACT!!! there are hoards of those crowd followers who will dispute.

So, before arguing with a fool, be sure that he is not similarly engaged.

Vigor's conclusions are correct.

Dick

Cedat Fortuna Peritas

[Neil Gordon]

... as good as the tiller vs. wheel debate.

[Maine Sail]

My boat sails faster with the prop spinning. Of course I sail a light displacement, fin keel very responsive boat. When measured against both paddle wheel and GPS 99.5% of the time I can gain at least 2 to 4 tenths of a knot by putting my boat in neutral. I know this is contrary to the popular belief but I've checked it literally 70 times and I always do it both ways. I measure speed increases when I unlock the shaft and speed decreases when I lock the shaft. I have a three blade prop though..

[Al Levesque]

I think one needs to understand the effect of stall to recognize the difference between freewheeling and locked. There is also another misunderstanding. Two bladed props are said to hide behind the deadwood. Most of my three bladed prop also hides behind the deadwood. I am convinced that the water driven by my prop comes mostly from the sides. If the prop is allowed to rotate I expect that more water sees the prop than if the prop is locked.

I believe that the combination of stall over the blades and some degree of stagnancy ahead of the prop result in least drag. However, the analysis is far from simple and would require tank testing to confirm. In the meantime, I am also minimizing the wear on my system by placing the transmission in reverse when sailing.

[Dean Abramson]

To me, the boat should go faster when the prop freewheels. This is about drag. What does the prop do if left unencumbered? It spins. So that is the path of least resistance; in other words, the least drag. The boat goes faster.

Having said that, however, I always put my tranny in reverse and stop the spinning. This is to save wear on the tranny and shaft. A tiny speed gain is not worth wearing out your transmission by letting it whirl around all day for no really good reason.

A freewheeling prop does not result in reverse drag; the prop spins in the SAME direction as it turns when notoring in forward, does it not? It just passively screws thru the water in its normal forward rotation as the wind pulls the boat (and it) along; the prop turns more slowly, of course, than it would be doing if it were pushing the boat. If you stick it in Forward and hit the throttle, it will then turn faster, and push the boat, but it will not reverse direction. It seems to me that the only time a prop would freewheel in reverse is if the boat was somehow sailing backwards, or was being towed backwards.

I have always assumed that the reason manufacturers say to put the tranny in Reverse is that that is even more counter to the prop's natural (forward-spinning) inclination, and therefore locks it more effectively than putting it in Forward. Throws a bigger wrench in the works, as it were. But I don't want to confuse you with technical terms.

I am not an engineer, but I did stay at a Holiday Inn Express last night.

Dean

[Dick Barthel]

A helicopter blade under power is perfectly analogous to a sailboat propeller under power but when not under power and falling to the earth, the auto rotation is caused by the wind flowing up through the blades from below. That would only be analogous to pulling the boat backwards through the water. That "drag" or "lift" is enough to save the copter from a crash but not enough to make it go back up.

When the hull is being pushed or pulled through the water in a forward direction by the sails I would think the spinning propeller would help a small amount but not enough to worry about or to risk the tranny over. A better solution would be to lock the prop in line with the keel if you have a two blade and are really concerned.

Dick

[John Vigor]

A freewheeling prop does not result in reverse drag; the prop spins in the SAME direction as it turns when notoring in forward, does it not? It just passively screws thru the water in its normal forward rotation

Well, finally someone realized that a freewheeling prop doesn't spin in reverse. Nice work, Dean.

But you're only halfway there, I'm afraid. The prop doesn't screw passively through the water. The fact is that it takes energy to spin a prop, as you'll know if you've ever ever tried to turn one when your boat's out of the water.

The whole point of the Helicopter Theory is that a stalled foil creates less resistance than a spinning foil. That is why a stalled helicopter falls from the sky like a stone, while one whose blades are allowed to rotate can glide to a landing.

At least one British engineering firm I know of (P. Newall Petticrow, Ltd.) has conducted tests and found that dragging a freewheeling propeller through the water creates more vortices than a propeller fixed in place. And the energy it takes to create those vortices is subtracted from the boat's forward motion. That firm also found that the drag of a spinning propeller is greatest at about 100 prm.

One of America's greatest naval architects, Francis S. Kinney, asserts in Skene's Elements of Yacht Design that "the shaft should be locked so that the propeller cannot revolve. It has been found that a revolving propeller creates more drag...."

But what brought it all home to me was watching my Christmas helicopter stall and crash. It seems so obvious now.

Cheers,

John V.

[marilou]

What is the benefit of the prop to be free-wheelin? Volvo, also, states that if you have a folding prop, engage in reverse. Why?

[Dick Barthel]

I think of trying to push a 1 foot square piece of plywood through the water - extremely difficult. A prop is not quite that bad because of its shape and size, but its still being pushed or pulled throught the water. Maybe a slow moving blade is like trying to push a bunch of propellers throught the water since the drag is being spread out in a circle and not just one line across the diameter? That might be what creating vortices is all about? In other words you'd be effectively spreading out the disturbance area to the full area of circle.

By the way, when a helicopter is in an autorotating mode, I think the blades are still rotating is the same direction as when under power. Otherwise, like putting you boat in reverse, the copter would fall like a rock.

See what playing with toys has stimulated. And they say Christmas is for kids!

Dick

[M. R. Bober]

all due respect to all contributors and posters, but the question at hand has to do with only drag and resistance, as offered up in john vigors original post...


if one is sailing along at, say, 4kts, under sail and turns on the engine and revs it up to,say, 3000rpms in forward gear, one will likely increase his speed to, say, 6kts. under sail and engine. everyone should readily agree with this premace, no? [/b]

with no change in the wind conditions, one can then lower his rpms to a point where one is again going 4kts, and if one continues to lower his rpms he will begin to slow the boat to below 4kts at some point. the slower rpms are increasing drag...


let the leisurely repenting commence

darrell randolph
s/v mystic rose

Au contraire, mon frere...

Running the engine in forward gear--while under sail--doesn't seem to slow RESPITE down. By actual measure, motorsailing at any rpm increases VMG (up to a point) over the VMG of sail alone.

OBTW: Universal M-30 engine manual states reverse or neutral when under sail (or tow, I'd guess).

Mitchell Bober
Sunny Annapolis (where sailors with street cred get their props), MD

[Oswego John]

Dick, John V., anyone,

This is the first time I have touched upon this thread, and this is for several reasons, valid or otherwise.

First off, there are several things that I am not sure of after reading several conflicting versions on the topic.

Secondly, the outcome isn't that crucial to me since I sail a Ty with an outboard which swings up when sailing.

With that being said, I really am intrigued with the physics and mechanics associated with the thread.

Speaking of toys, I have one question that if I get the correct answer to, it will clear up several doubts in my mind at this time.

My question of the day. Drumroll, maestro. Ta dah.

If a child has a toy pinwheel on a stick and waves it to and fro, sideways, in front of him without rolling his wrist, do the blades rotate in the same direction whether moving from right to left and left to right, or do the blades change rotational direction?

The correct answer to this will give me a jump start in understanding the dynamics of this thread.

O J

[darmoose]

FOLKS

this is a simple fluid dynamics question. it doesnt deserve all of the guessing and flopping around that is going on. the answer to this question is in fact based in science and math and not in anything else.

the question is "does a fixed(locked) propellar cause more or less drag(resistance) on a sailing vessel than a freewheeling propellar" the question is not limited to two bladed propellars. another way to phrase this question is "does it take more or less force to pull a boat thru the water with the propellar fixed(locked) than if the propellar were allowed to freewheel"

those who have postulated that it takes more energy to turn the propellar are simply wrong. the turning(freewheeling) propellar is a little like a relief valve, by spinning it is reducing the amount of energy it takes to pull the boat thru the water, and allowing the water to pass thru the propellar faster than if it were locked in position.

to answer Marilous question (cause it deserves an answer), the reason they recommend locking the folding propellar is so that the blades do not open up and cause more friction than they do in the closed position. so, yes the better solution would be to have a folding propellar with the blades folded, or have no propellar at all. unfortunately, that was not the original question.

i am very surprised that some of the otherwise intelligent and truth seeking contributors to this board, can get so bogged down in their dogma and so closed minded not to readily see the answer to this question. some even to the point of refering to others as "fools"

if you have ever pulled a nail out of a board, you know it requires a certain amount of force. if you try to pull a similar sized screw out of the same board, it will require alot more force because of the threads(which are like our propellars). but if i can cause the screw to spin or rotate in the proper direction while you are pulling it out, the amount of force required will drop considerably.

i dont know what else to offer you? you cant claim a theory that doesnt hold up in real life.

darrell

[darmoose]

O.J.

the toy will spin in a different direction each time the child waves it back and forth. and the child can wave it faster because it is spinning than if it were fixed and not allowed to spin.

hope you are feeling well

darrell

[tartansailor]

With all due respect to the thought and conceptualization that you have offered here, please let me suggest that the ONE critical issue that has been ignored, and that is: LIFT!

My friends, you are conceptualizing MECHANICS, while the reality is FLUID DYNAMICS.
If the prop were flat, you would all be correct!
In my former life as crew, I was privileged to take tank readings. I was there, I recorded the numbers, and my superiors the owner, the skipper, and other boats that I am familiar with all locked their shafts during the big boat races.

But far be it for me to even attempt to convince anyone to the contrary; just go on believing and practicing what you will.

Dick

[Steve Laume]

So Darrell, have you forgotten the energy it takes to turn the screw to make it easier to pull out? I would imagine it takes quite a few foot pounds to turn that transmission too. There just isn't any free ride in physics. Fool that I am, I just don't care weather the prop creates a bit of drag or not. This reminds me of my cycling days when guys would spend a whole mess of money on a titanium bolt or some such thing when all they had to do was lose a couple of pounds or train a bit more. It's not about the prop. It is a really interesting problem though. I think a tiller causes less drag too because it responds quicker than a wheel, Steve.