April 11, 2010

The Righteous Freewheelers

IF YOU WANT TO START an argument among sailors, all you have to do is ask whether a sailboat will go faster with its propeller locked or freewheeling.

A few Christmases ago I started one of the longest and most controversial threads on the Cape Dory bulletin board. I explained that Santa had just brought me a toy helicopter, and while I watched it fly around the room and crash into things it occurred to me that if the engine stopped, it fell to earth in two different ways – very quickly if the rotors stopped turning, and much slower if the rotors continued to spin of their own accord. I therefore concluded that a spinning, freewheeling propeller on a sailboat would cause more drag than one with its shaft locked.

I naturally cited several marine authorities who agreed with my theory but it didn’t silence the many Cape Dory critics who were surprisingly adamant that they, and they alone, were right. The Righteous Freewheelers, I call them.

Now, however, I have stumbled across more support for proplockers. I found it on a website at http://www.pelaginox.com/, which I believe is a British-based data base for a whole host of useful facts and figures for amateur sailors. I quote:
“Incidentally, some feel that drag with a spinning prop is less than that from a locked one; but consider the case of the helicopter. If the engine fails, the pilot performs an ‘autorotation’: the gearbox is taken out of gear and as the helicopter falls, air resistance causes the rotor to spin, and the machine achieves a stable descent attitude and a much-reduced rate of fall — the high drag from the spinning rotor allows it to land with a parachutic descent, albeit heavily, whereas with the rotor locked it would simply fall out of the sky. You can check this if you have a mechanical-shift gearbox (or a shaft brake) — test your speed with or without your prop locked; you’ll sail faster with it locked.”

I don’t know who to thank for those extremely wise words, but thank you anyway. I knew I was right all along. Proplockers rule!

Today’s Thought
You will find that the truth is often unpopular and the contest between agreeable fancy and disagreeable fact is unequal.
— Adlai E. Stevenson, NY Times 9 Jun 58

Boaters’ Rules of Thumb, #38
Compass cards. Small boats don’t need compass cards with markings of less than 5 degrees. It’s very easy to estimate the positions of single degrees between two markers 5 degrees apart. It’s not often that you can steer accurately to within less than 5 degrees on a sailboat in any case.

“How’s your wife getting on with her cookery lessons?”
“Man, her fame is spreading. Last night a pygmy from the Congo called and asked if he could dip his poison arrows in her stew.”


Aaron Headly said...

OK. I've let this one sit there all day, but I have to throw in my $0.02. And, no, I have no opinion on whether any captain should or should not lock his prop while under sail.

I just want to comment on people making an analogy between a fixed-pitch, low-aspect-ratio asymmetrical boat propeller and variable-pitch high-aspect-ratio symmetrical helicopter blades.

The analogy just doesn't work. A helicopter can 'pinwheel' down to the ground without power because part of the rotor blade (which has no 'twist' to it, unlike a boat prop) is at an angle that causes the relative wind (caused by the fact that the helicopter is falling through the air) to spin the whole rotor, and the spin of the rotor allows the rest of the blade to provide lift.

Ask any helicopter pilot and they will tell you that there is considerable art to autorotation (similar to trimming a sail): you have to control the blade angle (with a handle called the collective, which is their shorthand for 'collective pitch control') very carefully all the way down; pitch up too much and the rotor slows down or even stops spinning (and then the helicopter stalls and, usually, crashes), pitch down too far and too much of the blade ends up spinning the rotor instead of providing lift (which causes the rotor to spin too fast and allows the 'copter to fall too fast).

Even if your boat has a variable-pitch prop, your prop blades will have a considerable curve to them (unlike a helicopter blade), and the analogy still fails. (Plus: variable-pitch props usually have a 'feather' position that is calculated by the manufacturer to provide the least drag. And, of course, variable-pitch props add way too much complexity for the saltier among us).

Sorry for this rant, but weak analogies really get on my nerves.


PS - maybe it is time for the TV Show 'Mythbusters' to get involved in this; they did craft a pretty handy looking dinghy from Duck Tape© once, and might come up with a reasonably scientifically researched answer to this whole thing.

Jeff said...

I totally agree with what Aaron said. There are very few similarities between helicopter blades and a boat prop. Aaron touched on this, but it is important to understand that during an autorotation there are three segments of blade rotation- the driving segment (where the air is coming up through the rotor disk and actually turning the blades, a stalled segment (where you aren't creating and turning force or lift), and the lift segment (where you actually are creating lift).

The locations of the segments are dependent on the direction of rotation of the blades (counterclockwise for most helicopters), the speed of rotation, the pitch of the blades, and forward airspeed.

Airspeed is pretty important to consider as well- There isn't really a good equivalent in your analogy since no helicopter will autorotate straight down from any higher than about 10 or 15 feet. They require a forward (x-component). Getting too slow along the x axis and only moving vertically down along the y axis will result in a crash- not a nice auto. Since the boat prop is only moving along the axis of rotation (the equivalent of the helicopter falling straight down), it's not an even comparison.

I also think proper balance is important to consider. How many boat props out there have never nicked a lobster pot, had an anode corrode non-symmetrically, or have an even dispersion of barnacles. Any imbalance will effect the way the prop spins.

I don't know what the answer is to your original question, but I've always done what feels faster on whatever boat I'm sailing. My general rule is if I can feel the prop spinning out of balance at all, I lock it. I don't worry about it otherwise. I doubt there is a 'golden answer' that is right for all boats since there is an infinite variety of hull shapes and prop choices out there.

Dave G. said...

I'm going to also have to comment on the autorotation thing being a poor analogy. The forward component mentioned by Jeff is to maintain rotor rpms. This stored energy is released right near the ground when the flare occurs. You trade all that rotational mass for lift at just the right moment. That's the art part as you only get one chance... too high and you crash, too low and you smack the ground pretty hard. You are not using the main rotor as a "parachute" so much as a wing to keep forward airspeed. Think of it more like a 45-60 degree (sloped) fall rather than straight down.

As for the argument, this gentleman built a rig and tested it: http://www.sailnet.com/forums/gear-maintenance/53628-fixed-vs-free-wheeling-prop-test-data.html He found almost a 50% increase in drag on the fixed prop.

Now, if you think about it, it makes sense. When you apply power to your prop, it spins and forces the water away from the prop, to the rear. The resulting reaction is the boat moves forward. If you let the prop freewheel as you drag it through the water, it will tend to spin due to that shape of the blades (the same shape providing thrust under power). The rotation, if allowed, results in less drag as the blades yield to the passing water. Now, if you fix it in place, it will not yield and you will increase the drag as now the water cannot flow past freely. The water must now part around the non-moving blades.

In an airplane, I learned the same thing as a pilot. When you simulate an engine failure (routine practice), you don't actually shut the motor off. With the blade turning even a few hundred rpms, there is a significant effect on your glide speed and the resulting distance you can cover while descending towards the ground. Any pilot who has suffered a true engine failure will tell you that propeller being fixed results in a huge increase in drag and, as a result, a much shorter gliding distance as the angle of descent must be increased to keep up your airspeed.

Wouldn't you expect there to be some sort of locking mechanism in place if it provided a benefit? The marine engineers who design the systems are all aware of these forces and the effects of drag... much more so than any of us. Why would they have designed a free-wheeling propeller system for all boats if it were that inferior?