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Old 10-02-2012, 06:32 PM   #65
psneeld
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"What creates more resistance: a freewheeling propeller, or a fixed one?"
Bertram has seen conflicting data on the subject, and indeed many of you weighed in emphatically on both sides of the question. And indeed it is a bit of a trick question, because, as Ian McColgin noted, "... Like the answer to the question, "What's the best lead for a jib sheet?", the answer here is, "It depends."
First, a non-drag consideration. Some transmissions are harmed by allowing them to freewheel while the boat is moving. Not all transmission manufacturers' claims accord with boat owners' experiences on this matter, which simply tells us that many manufacturers' reps and engineers never went to sea.
The forces that contribute to prop drag when the boat is moving and the engine is off include: Resistance from the presenting surface area of the prop; turbulence behind the prop; and hydrodynamic (or aerodynamic)resistance of the prop. The first does not change much whether the prop freewheels or not. To the extent that it might diminish a little if freewheeling, the net's about the same, as there's some transmission resistance to counter. Turbulence is probably reduced somewhat if the prop is allowed to freewheel.
Dynamic resistance is nearly non-existent when the prop is held still. However, as anyone who's survived a helicopter crash can tell you, the power of a prop's autogyration is considerable. If a chopper loses power, the pilot will steepen the prop pitch a bit to get them spinning nice and fast and will then flatten the pitch to the point where they gain useful lift and are still spinning fast enough. The chopper can then be "glided" straight down.
On a sailboat with a fixed prop, the typical pitch is enough that the freewheeling prop is actually generating lots of reverse lift to resist the boat's motion. Most sailboats will experience drag from a free- wheeling prop equal to drag from a towed bucket a bit larger in diameter than the prop. The drag from a fixed prop is usually about equal to the drag from a bucket about 3/4 prop diameter if a three bladed prop and considerably less than that if two bladed.
Ian McColgin
S.V. Granuaile
A fine analysis, Ian, and it is seconded by this month's winner, Commander David E. Davis, USN. He writes:
Sorry but...it depends. According to the U.S. Navy Towing Manual, Appendix H, when you determine the strain for a towed vessel the propeller resistance is a function of the projected waterplane area of the prop, times a constant (3.737), times the velocity of the tow squared.
So the faster you tow, the faster the prop resistance increases. For example: for a typical destroyer the towing resistance attributed to the prop(s) at 6 knots is 34k pounds while at 8 knots the same prop(s) yields 61k pounds. This is based on a fixed controllable reversible pitch wheel. A controllable reversible pitch prop would be set max ahead to minimize resistance. But then some ships save fuel when steaming independently at higher speeds by simply trailing a freewheeling prop. This discounts any concerns re shaft seals, machinery lubrication, etc..
The bottom line is that there is likely a crossover velocity where the resistance of a fixed prop (braking effect) becomes greater than the force required to impart rotation to the entire gear train (in a freewheeling prop), but studies on the topic are scarce. Given the small waterplane area of most sailing vessel's props, the relatively slow speed through the water, and the overriding desire to minimize wear on the shaft seal and transmission, fixed is the way I'd go (at max ahead or better, feathered, if the pitch is variable).
CDR D.E. Davis, USN
Commanding Officer, Naval Diving and Salvage Training Center


I don't know...too much conflicting data and one study from MIT who makes money from publishing reports....just don't know....not that it probably matters worth a hill of beans for most of us
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