Freewheeling propellers: the chopper theory

[wingreen]

I'll throw in my $0.02...

Since it is necessary to apply a force to stop the propeller from turning, it seems to me that a free-wheeling propeller would produce less drag, since it is not acting against the flow of water like a locked-up propeller would.

[Loren]

Sorry but $.02 more.

If the answer were black and white, there would be results of experiments that would show that at a variety of rpms and prop sizes and shapes, one way or the other showed decrease in drag. Its possible that freewheeling has advantages at certain speeds while locking may be better with other speeds. Hull shape and prop shape and placement may change results also. The dynamics at 20 rpms may not be the same as at 1000 rpms. Even the Michigan prop source stated only his hunch.

I have not seen proof to this point. I believe that its likely that the answer may be different at 1 knot compared to 7 knots. For my sailing I'm more concerned with what is better at 1-3knots. (That 1/10 of a knot makes all the difference once a year at our marina fun race - always more fun if you win and we have had light winds many years)

I would guess that a number of CDers will try their own experiment next season at different speeds and see if they can detect any advantage. I know that I will do it and see if there is a measurable difference. I may have been so confident that freewheeling would be better at low speeds that I thought I observed it. I will try to be more objective next season. (-16F windchill today so I'll have to wait a while) Theory is nice but if I see a measurable difference the theory may go out the window.

I still find many of the posts interesting and if I get tired of it I'll quit reading this string. I still wonder if either way is right all of the time.

Loren

[Neil Gordon]

As we start to experiment on the water (which I'm really looking forward to), could we remember to observe and report the RPM's the prop makes. We tend to ignore the friction of the cutlass bearing and transmission, but that for sure is keeping the prop from freewheeling the way a pinwheel would. I suspect that even freewheeling, the prop is in large part dragging, anyway. (My theory remains that the placebo effect of locking or 'wheeling has more effect on sailing performance than the underwater dynamics.)

[darmoose]

John,

I am sooo sorryyy. nobody told me that we needed your permission to post. had i known that, i would never have been sooo brazen as to not let you and yours have the last word.

However, once again, the views keep coming, and others continue to post their $.02, as they should. i keep thinking that someone of the "proplocker ilk" will offer up some logic or evidence to support your position. just saying it over and over and over aint gonna get it for me. inasmuch as the helicopter analogy doesnt seem to hold up after all, there doesnt seem to be any reason for one to start off with the theory that a locked propeller creates less drag.

someone did offer that the water rushing past the prop is pushing on the blades to make them spin, thereby using energy. but, does anyone think that if you lock your propeller the water stops pushing against the blades? i dont think so.

to understand this problem, you need to "be the prop". try this, repeat after me...".Hmmmmmmmmmmmmmmmm, be the prop, be the prop" if you are the prop, and you are being pulled through the water, you must spin (unless you are restricted by friction) cause spinning is the path of least resistance.

if we are to get away from our hypothetical boat (which has no drive train friction) for a moment, what Loren says is no doubt true. each boat will exhibit different amounts of friction when freewheeling. heck, i think it was Warren, who said a long time ago, that when he has tried freewheeling his propeller still dont spin (now theres drive train friction) he wouldnt see any difference at all, would he?

but since the ultimate friction stops the prop completely, it only follows that this creates the greatest drag. there hasnt been a single shred of evidence to prove otherwise, in spite of the theories of vortex and lift or whatever else. no actual boat is going to be without some friction in the drive train. but the less friction present, the faster the prop will spin and the less drag it will create.

BTW John, i think misquoting someone is beneath anyone of your reputation. i am dissapointed. for all my faults (and i admittedly have them), when i thought i was wrong, i said so. (it just turned out that i was mistaken about being wrong) i hope you will correct yourself.

if we wernt talking about this, what would we be talkin about of any consequence? surely, the fact that after years of debate, this subject remains unresolved in so many minds, makes it worthy.

Cheers back at ya
darrell

[Dick Barthel]

Darmoose,

Have you had a chance to read my last post which contained the response I got from the engineer at Michigan Wheel? I think the first citation he provided is worthy of at least a comment from the free-wheelers. It's an past article from Cruising World:

http://www.shaftlok.com/FreewheelStoryShaftLok.htm

By the way, for what it's worth, I still haven't been convinced that either side has won. I lean towards the lockers though.

And remember, just because John's helicopter analogy was wrong does not mean that the free wheelers prevail. It just means the analogy has nothing to do with the solution.

Here's a quote from one of the smartest guys ever to help keep things in perspective: One thing only I know, and that is that I know nothing. Socrates

And a plea for continued civility as we plow ahead toward 10,000 hits:

“what you want , above all things, on a raft, is for everybody to be satisfied, and feel right and kind towards the others.” Huckleberry Finn

Let's keep the "fun" going. Maybe we'll end up with a Guinness Record for our beloved Message Board! I never thought we catch Fenix's Thread (which isn't finished either) but now I'm not so sure.

Dick

[darmoose]

Dick,

you have been fair, openminded, and thought provoking in your past posts. i appreciate that. yes, i have read your previous post and all three of the web sites you posted. the second two are just discussion boards like this one, but dont seem to be as active, and certainly dont add anything to our arguments.

the first website looked promising, and i thought at first that there was going to be something here. the author started by admitting he did not have the answer yet. he said that logic supported the freewheeling theory. then he went on to give us alot of information about which i can only honestly say i didnt understand a word of it. if you or others understood him, perhaps you will help me to understand him as well. he refered to charts and graphs which were not there. i am very open to hearing someone explain that article.

i also thought your engineer from Michigan Wheel was gonna help, but alas, he too did not know.

i mean no malice toward anyone. i only want to attempt to answer this question, if it is possible for this board to do so. i have listened while some have ridiculed those who did not agree with them (i guess they are trying to be cute)

Johns helicopter analogy had promise, and only through close scrutiny could the the flaw be identified. it is what it is, no more, no less. and yes, just because it doesnt apply doesnt mean i am right. i may still be proven wrong. i only take the position that i take because there is logic to support it. i truly dont see how one can come to the opposite conclusion from the get go, as i dont see any logic to even get you to the starting line.

i would like to anticipate that somewhere down the road, we will all together find the answer to this question. i can only promise you that if it turns out that i am wrong, i will say so. i have seen nothing from the "proplockers" yet that makes me think they will do likewise.

finally, if others wish to attack me personally, as above, rather than contribute to our collective knowledge, i reserve the right to respond. i think you might address your concerns for civility elsewhere. seems to me that alot of the good natured humor and entertaining wit that we all enjoyed in the earlier days of this thread began to dissappear as the worm began his turn

darrell

[Dick Barthel]

Darmoose,

My reminder for continued civility was meant as a general call and not meant to be directed at you. I apologize if I left you with that perception - that was not my intent. If we are to reach 10,000 hits it should be because the discussion is being advanced and not because of personal pyrotechnics that only serve to diminish our Board.

I don't think it is possible to solve this long standing debate by discussion. I think as Tod suggested it would take expensive and directed testing in controlled conditions. I've asked for and received the opinion of a propeller engineer and he's not sure but leans toward locking. I've pointed out how David Gerr, (The Propeller Handbook) a recognized expert on the subject has waffled and made contradictory conclusions on the matter. Gerr is cited as a propeller authority by both John V and Nigel Calder, both recognized authors of good sailing literature.

I think from a reasoning standpoint, the thread is dieing of its own weight. I have spent my entire adult life in dispute resolution. At this point, because of the number of posts, it would take many hours to sift through everyone's posts to extract the valid points that have been made on both sides. So any continued discussion would necessarily be myopic.

Its been fun. I do hope that as OJ hinted in another thread that he is going to read and respond to that link the propeller engineer provided. If he can't figure out the article, then it's a bad job. I hope others also weigh in. As for me I'm exhausted and not smart enough to bother with it.

As for John V. I took his post to you as slightly provocative but mostly funny. He wasn't "putting words in your mouth" because he used all of your words! Newspaper people do this all the time. Most of us know and love John's writing and I think he was just fun'in with yah.

You have established Darmoose on our Board with this thread as someone to be reckoned with. I for one have totally enjoyed your unbridled enthusiasm. Your apology post stands as a testament to your sincerity. I hope you keep posting and stirring things up for many years to come. And that I'm around to read them!

Dick

[Neil Gordon]

i only take the position that i take because there is logic to support it.

What I've learned from this thread, from sailing and from life in general is that logic requires an understanding of all the forces involved.

If we relied on logic:

We wouldn't be able to sail to windward.
There would be no such thing as hull speed.
We'd never mount a transducer inside the hull.
We'd always replace a tiller with a wheel.
Given the cost per nautical mile, we wouldn't sail at all.

[Cathy Monaghan]

Not to beat a dead horse, but why not just follow the instructions in the engine or transmission owner's manual? According to the Hurth transmission manual, "...Use the shift position opposite to the direction of travel for locking the propeller shaft, otherwise the transmission will be damaged...." So while sailing, with engine off, the transmission should be locked in Reverse gear if you've got a Hurth transmission.

Cathy
CD32 Realization, #3
Rahway, NJ
Raritan Bay

[Lew Gresham]

I lost a Hurth transmission on my 30 Catalina right after it was commissioned due to the transmissions dip stick coming apart and was ground up in the gears. Hurth replaced the transmission without cost. I was glad of that, it only cost $975 in 1985. I remember the dealer mentioned at the time of commissioning my boat it was very important to lock the transmission in reverse gear when sailing. As Cathy says it's in the book. I might add, which is kinda funny, I replaced the transmission myself which saved me a large labor cost. It was not a bad job, but before I started the installation I removed the dip stick and examined it closely, and low and behold, the dip stick came apart with a little twisting and pulling. I mentioned it to Hurth personally and they sent me another dip stick with a pin installed through the nut. Hurth told me the slightest vibration caused some to come apart and drop into the gear box. You would think they would have had a recall on a bad design. Lock em boys!

[Neil Gordon]

... why not just follow the instructions in the engine or transmission owner's manual?

That's just for protecting the transmission. Too bad there's no instruction that reads, "We suggest locking the transmission to protect it despite the loss of some boat speed," or "We suggest locking the transmission both to protect it and to increase boat speed."

Interesting that there's little consistency in the instructions, with some suggesting freewheeling and some locking. Either way, it's not worth a transmission job for the theoretical tenth or so of a knot of boat speed. For sure, it's not worth building and referring to a table and locking/unlocking as the wind speed rises and falls and boat speed reacts accordingly.

[John Vigor]

John,

I am sooo sorryyy. nobody told me that we needed your permission to post. had i known that, i would never have been sooo brazen as to not let you and yours have the last word.
darrell

Good boy, Darrell. Apology accepted. Just try to do better in future, there's a good fellow . . .

One other thing while I'm here. Too many posters have been saying my helicopter analogy was wrong. I have borne the pain of this unwarranted calumny with fortitude for many sleepness nights, but the time has come to reiterate my original post:

If you lock the rotors of a helicopter in flight, the airfoils will stall and the whole aircraft will fall like a brick.

But if you allow the rotors to freewheel, they will generate enough air resistance to stop the helicopter falling and allow a safe landing. That's the theory (and well-tested practice) of the autogiro.

There is no question of that being wrong. It's fact.

Now, if you just think of a boat as being a helicopter on its side . . .

Cheers,

John V.

[Oswego John]

Okay, that's it. That last post is just what I needed.

I'm baaaaack. Let's go for 10,000 posts. Damn the torpedoes. Full speed ahead, locked or unlocked.


O J

[Derek Matheson]

Dear Fellow Sufferers:

Having read most of the previous posts and it being a snowy evening in Maryland, etc., etc., I offer the following.

1. Our Volvo MD-7A manual says to lock the gearbox in reverse, so we do that. Although I am a mechanical engineer and have designed gearboxes and lubrication systems for same, I have neither the time nor the inclination to challenge the Volvo engineering and technical writing staff at this time on this subject.

2. This problem is solvable in 3 different ways. (All propellers have different foil shapes, aspect ratios, etc., so the solution will apply only to a particular propeller with particular shaft rotational drag.)

2.1 Empirical Solution: Attach the example propeller to a shaft that is free to rotate and place in a stream of water e.g. hang off the side of a boat. Measure the force produced along the axis of the shaft versus boat speed and graph results. The apparatus should also allow a variable braking torque to be applied to the shaft. To include hull effects, one could disconnect the shaft coupling and measure the real drag force from inside the boat. I suggest using a low friction thrust bearing and spring-scale device. A good candidate boat would be an inboard that has been converted to an outboard, so that the boat does not have to be towed.

2.2 Theoretical Solution: Use classical fluid dynamic theory as applied to turbomachinery to predict the behaviour. I'm sure the appropriate formulas could be entered into Mathcad and we could all enter our propeller data and get results without getting wet.

2.3 Computational Solution: Use computer modelling to create 3D models of propellers and apply computational fluid dynamics (CFD) to produce a solution. Many have done this before. We run this program at work. It's a lot of work to set up the problem and run it. See http://www.cfdesign.com/Applications/default.asp They show a propeller example under the 'Aerospace & Defense' link.

I vote for 2.1, as it is the low tech, high fun approach, but more so in summer. Volunteers?

[bottomscraper]

There seems to be conflicting information on Hurth gearboxes. The Hurth manual says:

2. Boat sailing, moving in tow or anchoring

When the engine is off, and the boat sails, moves in tow or is anchored, the propeller may turn with the water current.

CAUTION. Idling position of the propeller: gear shift lever must be in «O» position. Use the shift position opposite to the direction of travel for locking the propeller shaft, otherwise the transmission will be damaged.

My interpretation of this is that neutral is just fine or locking it in the opposite direction if you want to lock it. But I think locking it is optional. Never lock it in forward (unless you are sailing backwards!) Since this manual is in 4 languages maybe there is another interpretation of what "the propeller may turn with the water current" was intended to mean. Are they saying it is ok to let it turn or are they saying that it might turn but you shouldn't let it?

The Perkins 4.108 handbook says this:

The Hurth mechanically operated gearbox and the TMP12000 Series gearbox may be trailed indefinitely. When prolonged propeller shaft trailing is being carried out, the gearbox oil level should be maintained on the full mark. Other makes of gearboxes may be fitted to the Perkins 4.108 marine engine and in this case, the gearbox manufacturers recommendations for propeller trailing should be followed.

This one seemed a bit clearer to me. It says you can let it turn but make sure the gearbox is full (which is a good thing anyway). It doesn't say a thing about locking it!