NOW THAT WINTER is nearly upon us and the water is getting
colder, Old Wotsisname is asking every passerby whether he should reduce the
size of his prop. This is typical OW. He's just showing off. He wants people to
know how clever he is. He read somewhere that because colder water is denser,
propeller diameter should be reduced about 1 percent for every 10°F drop in
water temperature, based on a
"normal" of 70°F.
I believe this is true. I also read it somewhere. In one of
my own reference books, actually. But I don't go around shouting about it. My
natural modesty forbids it. However, there's no doubt that many sailors show a
serious interest in propellers and many of them go to great lengths to match
their propellers to their engines for maximum efficiency and miles per gallon.
If you're one of them, you might be interested in some more
propeller bric-a-brac I've collected over a lifetime of frustrating engagements
with marine engines and propulsion systems:
Ø
Increased diameter absorbs more power than increased pitch.
Ø
A decrease of 1 inch in propeller diameter will increase propeller rpm by about
300.
Ø
A decrease of one inch in pitch is good for an increase of propeller rpm of
about 200.
Ø
On auxiliary sailboats, the minimum clearance round the tips of the propeller
blades is 10 percent of the propeller diameter.
Ø
Powerboats need 20 percent or more clearance to avoid vibration.
Ø
After installing a propeller on a shaft, the thinner lock nut should be put on
first. This allows the larger nut, with more thread area, to assume the load
when it is tightened up against the thin lock nut.
Ø
For the greatest efficiency, the pitch-to-diameter ratio should be less than
1.4 : 1, except on high-speed boats doing 35 knots plus.
Ø
A large-diameter propeller is always more efficient than a smaller one, except
on boats designed for continuous operation at 35 knots or more.
Ø
There is disagreement about whether to allow a sailboat's prop to spin when
you're under sail only. New York naval architect Dave Gerr says a prop creates
less drag when it's free to rotate. British author Eric Hiscock says the
opposite: "Experiments made by P. Newall Petticrow Ltd. have shown that a
2- or 3-bladed propeller offers less drag when it is locked than when it is
free to spin, and that the drag of a spinning propeller is greatest at about
100 rpm." Another renowned American naval architect, Francis S. Kinney,
agrees with Hiscock.
I myself think it varies from boat to boat, and all you have
to do is anchor in a current, attach a fishing scale to your rode, and put the
(non-running) engine in and out of gear. (I also have a theory concerning rotor
blades on a falling helicopter, which definitely encounter more drag when
rotating than when locked, but it proved highly controversial and caused much
ill feeling and backbiting on one bulletin board; so I won't dwell on it here.)
Today's Thought
It is the darling delusion of mankind that the world is
progressive in religion, toleration, freedom, as it is progressive in machinery.
— Moncure D. Conway, Dogma and Science.
Tailpiece
Some sporting definitions:
Greyhound racing: The curs of the working classes.Table tennis: The sport of pings.
Polo: Jockey hockey.
Weightlifting: Careless rupture.
Bull-fighting: He who hesitates is tossed.
(Drop by every Monday, Wednesday, Friday for a new Mainly
about Boats column.)
6 comments:
Old Wotsisname... isn't he the one with a seized injector, a gearbox full of sawdust, and four inches of barnacles on a lumpy ferro?
Yes, I'm sure fine-tuning his prop according to water density will make a noticeable difference ;)
Re. locking the shaft: If the gearbox manufacturer says not to let it freewheel, the choice is made for you (burned out clutches are not fun). Otherwise, I agree with you, testing it on your particular boat is the only way to be sure. Not all boats swing the same way.
Re. helicopters: Unfortunately, autorotation doesn't give any insight into the boat prop problem. Autorotation isn't about spinning vs. fixed; it's about adjusting the blade pitch as the helicopter falls so that they're stalled near the hub, driven by the airflow in the middle, and lifting near the tips. Get that collective pitch even a tiny bit wrong and she'll fall like a stone, spinning or not.
I'm sure you've seen Maine Sail's experiment on fixed/rotating props, and confirming results from other studies he refers to in his article. Do you disagree with those? And if so, why?
No ill-feeling or backbiting, I promise. Just trying to make up my own mind. Thanks.
Dear Anon: I have seen the experiment you refer to. I don't subscribe to the theory that it is correct for all boats and all props at all times. Each hull shape is different. Each prop is different. Different forces act at different speeds. The fact that serious experiments come up with conflicting answers, and the fact that trained experts differ in their views, is enough to convince me that nobody can give a definitive answer for all boats.
I've had boats where the prop wouldn't freewheel at all, no matter what speed we achieved. But in any case I always lock the shaft because the noise of the spinning prop and gears drives me nuts and because I can imagine bearings wearing out prematurely for no reason at all.
John V.
Dear Anon: I have seen the experiment you refer to. I don't subscribe to the theory that it is correct for all boats and all props at all times. Each hull shape is different. Each prop is different. Different forces act at different speeds. The fact that serious experiments come up with conflicting answers, and the fact that trained experts differ in their views, is enough to convince me that nobody can give a definitive answer for all boats.
I've had boats where the prop wouldn't freewheel at all, no matter what speed we achieved. But in any case I always lock the shaft because the noise of the spinning prop and gears drives me nuts and because I can imagine bearings wearing out prematurely for no reason at all.
John V.
John, where do you get the 10% of prop diameter for tip clearance from?
I'm curious because we just recently reduced the size of the aperture on my boat and just made it is small as possible around the prop.
Gary, Dave Gerr, well known New York naval architect, in "The Nature of Boats" says: There should be a clear distance--a tip clearance--between the hull and the propeller blade tips of at least 10 percent. Ten percent is a bare minimum; 14 to 18 percent is the standard ideal distance., though the bigger the clearance the better ..."
"Skene's Elements of Yacht Design" says: "The tips of the blades should clear the hull as follows:
"15 percent prop. diam. of clearance for auxiliaries.
"20 percent prop. diam. of clearance on motor sailers and powerboats."
And many more credible authorities say the same thing.
John V.
Post a Comment