January 24, 2012

About propellers

MOST PEOPLE who sail also do a fair amount of motoring.  The thing that makes motoring possible is the propeller, so it behooves sailing people to know more about propellers than most of them do.

In the long history of ships, the propeller is a fairly recent invention. The first screw propeller was used in England in 1838, and the first vessel to cross the Atlantic with the aid of a propeller was the British ship Great Britain in 1845. The development of propellers has been advancing ever since.

Auxiliary sailboat propellers usually have two or three blades, and most powerboat props have three or four blades. Each blade is twisted so that if it turned in a solid medium it would bore its way through like an auger bit. In water, however, the blades act more like the wings of a plane in flight, gaining "lift" as they turn, and their rate of advance is reduced by slippage.

Interestingly, a propeller with no slip would displace no water and therefore generate no thrust. It's similar to the principle that governs the lift generated by a sailboat's keel — if there were no leeway at all, the keel would not generate lift.

The Complete Boating Encyclopedia provides a greatly simplified definition of a propeller as "a pump, submerged in the fluid it is pumping. Normal rotation sucks the water from ahead of the propeller, accelerates it, and discharges it astern, creating an opposite reaction that pushes the boat forward."

Two-bladed screws are the most efficient because each blade moves through water that is least disturbed by the passage of the other blade. But to get the blade area required, the blades must be comparatively long, and often there isn't sufficient clearance between the propeller shaft and the bottom of the hull. Therefore, three or more broader blades of smaller diameter are used instead. Nevertheless, some very fast small powerboats, as well as sailboats seeking to reduce drag under sail, use two-bladed props.

A large-diameter, slow-turning propeller is usually more efficient than a small one turning at high speed, although an exception is made to this rule for boats operating at about 35 knots or more. In small craft, "slow-turning" means fewer than 1,000 revolutions per minute.

The problem for sailboats is that a big, slow-turning prop creates an awful lot of detrimental drag when the boat is under sail alone.  So it is usual to compromise, with a less efficient, smaller, faster-turning prop that allows better performance under sail.

Incidentally,  the amount of slippage experienced by a propeller has always amazed me.  On auxiliary sailboats the slippage usually amounts to between 40 and 55 percent.

Today's Thought
You shall have joy, or you shall have power, said God; you shall not have both.
— Emerson, Journals

"What did her father say when you asked him if you could marry her?"
"He darn near broke my arm."
"Did he hit you?"
"Hell no, it's just that he was shaking my hand so hard."

(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)

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