How sailboats sail

[Neil Gordon]

Do you realize, that while the helicopters blades do spin in the same direction when going from power to autorotation mode, the pitch of the blades is REVERSED from left hand to right hand (or visa versa) in order to provide the necessary resistance?

That's because the helicopter is going DOWN instead of UP. By analogy, if our boats were going backwards, our unlocked props would spin in reverse, too.

(Does having a tiller vs. a wheel affect whether you should lock your prop or not?)

[darmoose]

Matt,

I am glad that you decided to continue to share your expertise on this subject. I have in fact reread you excellent explanation of how a helicopter works in our earlier discussions on this subject and i must say that regardless of what i said earlier, i find nothing of what you have said that i disagree with regarding how the heliopter functions (not that i am any kind of an expert on that).

While i know that i am somewhat dogmatic on this subject, i am not so dogmatic that i refuse to understand obviously expert testimony. Indeed, many contributors on this board have offered many thought provoking ideas on this matter, and i respect them all.

However, here is my dilemma...

I do believe that it should be possible to place the relevant coefficent of drag for any speed of propeller rotation (forward or reverse and even locked) on a linear graph where one end is minimum drag and the other end is maximum drag. and somewhere on this graph you must put the locked prop. Does the locked prop condition go in the forward spinning area, or does it go in the reverse spinning area, or does it go between them?

We all seem to agree that a fast spinning prop spinning in the forward direction represents the minimum or least drag, and that a fast spinning prop spinning in the reverse direction represents the maximum or most drag.

We all seem to agree that as the prop loses rpms in the forward direction the drag increases, and when the prop loses rpms in the reverse direction the drag decreases. This alone seems to dictate that the locked prop position must be placed in the middle of the graph between forward and reverse.

If that is not enough, it seems apparent that while all of the forces at play on the propeller that have been identified (turbulence, vortexes, lift, water flow and so on) do in fact have some effect on the end result, it remains that the propeller is firmly attached to the shaft on all of our boats. and so, if we can learn something from the rotation of the shaft, we are at the same time learning something about the rotation of the propeller.

This is what i think we can learn.

At any rpm, in a forward spinning freewheeling condition while under sail, one must apply pressure to the shaft to slow it down, all the way down to the stopped (or locked) position. Under no circumstances, can one slow the shafts rpms without applying additional pressure. The fact that one has to apply additional pressure to slow the shaft (propeller) even with all those forces in play undoutebly means that there is more force (drag) attempting to speed up the rotation in evey case, and the most pressure needs to be applied to stop
the propeller.

It seems to me that a freewheeling propeller is releasing energy and reducing drag as it takes the path of least resistance through the water.

Exactly why the helicopters workings dont correlate with a freewheeling propeller i cant say, but i continue to believe that it does not for the reasons stated above.

If you or anyone can refute what i have stated above, i really would like to hear it.

Merry Christmas and Good Wishes To All

Darrell

[Sea Owl]

At the risk of being deluged by the more experienced members of this august board, I would like to offer a brief vignette to provide a different view on the prop locking issue....

I like to ride a pedal bike. In fact, I ride what is often called a 'racing bike'. I often had friends who were also into this who often had ideas on how to take grams off of the weight of the bike so I could go faster/easier. I was, at the time, 6'4" tall and 220 lbs. The bike went around 22 lbs. As I used to tell them, if there was a weight problem on the bike, it wasn't the bike!

As great as our CD's sail, they are cruisers, not racers. Everyone that I have seen sails fine and fast for its weight. I would find it hard to believe that locking or unlocking the prop would make any significant difference as compared to say, having a clean or fouled bottom. Particularly as a cruiser; most (admittedly not all!) of us aren't going that far in any one day's cruise that .2 knots (or whatever) is going to hurt/help us much.

Accordingly, my two cents is, do whatever "rocks your boat". The bigger virtue of locking your prop in my under experienced opinion would be lessening the total number of revolutions on the shaft per cruise, reducing bearing wear, and extending the life of the bearings/lubrication/shaft installation (sorry, thats the engineer in me speaking! ). That to me would be a bigger reason than worrying about the relatively minor speed differences in a cruising boat.

Fast Winter to all!

[Neil Gordon]

If you lock the rotors, the helicopter falls a whole lot faster than if it's spinning. Isn't that what we want except that we "fall" forward using sails vs. gravity? It's not lift that the spinning rotors cause, it's drag.

[tartansailor]

Sea Owl'
Actually you are right.
When the NY-40 was being developed, tank tests were run
and the difference was in the low tenths of a knot, favoring
the fixed prop, with the variable being the "cup" at the aft
side of the prop during its optimization regimen.

Dick

[wingreen]

I didn't know they put engines on the NY 40s? Were they internal combustion, steam, or coal?

BTW, I saw one for sale a few months back at a cool $1M or $2M. SSSSWWWWWwweeeeeetttt...

[John Vigor]

If you lock the rotors, the helicopter falls a whole lot faster than if it's spinning. Isn't that what we want except that we "fall" forward using sails vs. gravity? It's not lift that the spinning rotors cause, it's drag.

Thank you Neil, that's it exactly. Nicely put. And in language I can understand.

Cheers,

John Vigor

[John Vigor]

Sea Owl'
Actually you are right.
When the NY-40 was being developed, tank tests were run
and the difference was in the low tenths of a knot, favoring
the fixed prop, with the variable being the "cup" at the aft
side of the prop during its optimization regimen.

Dick

I can hardly believe my eyes. Tank tests proved that a fixed prop caused less drag than a spinning prop? Who'd a thunk it?

John V.

[Joe CD MS 300]

If you lock the rotors, the helicopter falls a whole lot faster than if it's spinning. Isn't that what we want except that we "fall" forward using sails vs. gravity? It's not lift that the spinning rotors cause, it's drag.

Thank you Neil, that's it exactly. Nicely put. And in language I can understand.

Cheers,

John Vigor

I'm not really on one side or the other in this discussion but I'm not sure that the helicopter analogy is really a good one.

When a engine fails on a helicopter the blades will continue to rotate in the direction that causes lift. Could that be the reason it appears to at least initially fall slower? Once the blades stop and lose all lift are we sure that it falls slower or at the same rate as if it were turned on its side? Once it starts downward its actually moving in reverse if you look at it as a prop on a sail boat. Haven't thought this all out, need more coffee.

Joe

[darmoose]

You will simply be ignored and ridiculed by John and his ilk. They would actually like you to believe this is about trying to squeeze a quarter or a half a knot out of a cape dory. They know well it is not, but cant discuss it, because they cant answer the questions i pose.

Not worth pursuing(i am sure they will take glee from that)

Cheers

darrell

[Matt Cawthorne]

Darrell,
The error in your thinking is that the curve between the force that tends to speed the boat or slow it is linear. It is not. When lots of power is delivered to the prop it generates thrust. The zero thrust condition is one where the engine is generating a small amount of torque, just enough to counter the drag of the blades through the water at zero angle of attack. Any rpm that is slower will generate a force that will slow the boat down. At some point the angle of attack gets large enough that an airfoil stalls. The prop goes from a lifting blade rotating at some angle of attack to a body draging through the water. This process is not linear. The ratio of drag of a stopped prop to the effective resitance of a spinning prop depends on many things so whether the locked or spinning prop is a bigger drag will vary from boat to boat, from prop to prop, and perhaps even from one speed to another. At some reverse rpm the tip speed is sufficient to to modify the vector diagram to decrease the angle of attack below the stall angle and the prop can generate a good deal more reverse thrust.

I will state my test data again. On my Cape Dory 36 I have on several occasions tried affecting the boat speed by puting the transmission in neutral or 'lock' position. I have never noticed any effect on the knotmeter.

[Michael Heintz]

WOW,

I just put up my sails and go where the wind blows me.......

[jerryaxler]

I've hesitated to get into this thread, but I've had the same experience as Matt. I have locked and unlocked the prop at various speeds and have not seen any change on either my knotmeter or GPS. I realize this is anecdotal and not scientific, but I tend to sail in the "real" world as opposed to the lab.

[darmoose]

Interesting that when offering up anecdotel evidence regarding thier own boats performance, people can easily recognize that all the forces (lift, vortexes, vertices, cups, blade tips, water flow, etc.) and anything else that might apply, are transferred thru the propeller, then thru the shaft, then thru the engine, and then to the boat, and then are readable in their instrumentation. Indeed,there are no applicable forces that arent present anywhere in the drivetrain.

But, the very same people, have a complete blind spot when it comes to measuring those same combined forces at a convenient point in the drive train, like the propeller shaft, and addressing or recognizing the additional force it takes to slow a freewheeling propeller (shaft) or stop it, and what that fact obviously means.

Truly remarkable

Darrell

[Matt Cawthorne]

Jerry,
Your approach is very scientific. You make observations, propose a hypothesis, test your theory by doing experiment, draw conclusions and communicate your results. Thanks for sharing.

Darrell,
Measuring the loads on a prop shaft is actually a pretty good way to find the answer. It leaves out the impact of the disturbed flow on the hull and rudder in the area of the prop, but other than that it would pretty much get the answer that you want. What you need to measure is the drag on the prop (that is the axial load in the prop shaft plus the friction in the cutlass bearing). Ignore the cutlass bearing for a moment. You need to buy a force transducer or try and strain gage the shaft. Next you need to resolve the issue of the the fact that the shaft is rotating sometimes. You can solve this by either buying a telemetry system or a pair of low-noise slip rings. This is a bad environment for slip rings. Next you need a data acquisition system. A number of companies make fine data acquisition systems that attach to PCs through either the com port or a usb cable. The last item will be the cheapest by far.

You would be better off just anchoring your boat in a fast current and measuring the loads on the anchor with the prop fixed and then rotating.


What you say? You want to measure the torque on the shaft? That would be great if you knew the drag versus torque relationship. No, it is not linear, particularly when the blades are stalled.

So lock your prop, or don't. Whatever feels right for you.

Matt