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Catalina/Capri 25/250 Sailor's Forums

General Sailing Forum

Drive vs Lift?????

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Gloss
Master Marine Consultant

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USA
1916 Posts

Initially Posted - 05/10/2004 : 11:31:25

Hi there,
I'm currently in the middle of reading an article on furling Headsails in Blue Water Sailing (I know, it has expensive, fancy boats that most of us only dream about, kinda like reading Playboy) and the author had the statement "The headsail provides drive while the back of the mainsail provides lift"
So what is the difference between drive and lift? and why just the back of the mainsil providing lift?
I'll bet the racers know the answer to this one.
Thanks

Frank Gloss
Formerly 89WK/TR
85 Ericson 32-3 shoal draft "Molto Bene"

Edited by - on

ClamBeach
Master Marine Consultant

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3072 Posts

Response Posted - 05/10/2004 : 14:45:52

I've not heard of the mainsail provding 'lift' before.
I think lift job belongs to the rudder, keel and hull shape.

Edited by - on

Arlyn Stewart
Master Marine Consultant

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2980 Posts

Response Posted - 05/10/2004 : 17:43:28

I'll take a stab at it. Whenever apparent wind is generated, its because the sails are producing more than simple resistive effort. Air flow over the foil produces a low pressure on one side and high on the other and the lift force is transmitted to the spars holding the foil.

If I were guessing, I'd think the writer was characterising drive as resistance forces and lift as foil produced forces.

Edited by - on

Steve Milby
Past Commodore

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USA
5913 Posts

Response Posted - 05/10/2004 : 18:27:51

<blockquote id="quote">quote:<hr height="1" noshade id="quote">"The headsail provides drive while the back of the mainsail provides lift" <hr height="1" noshade id="quote"></blockquote id="quote"> I think either the author has used his terms loosely, or the meaning of the quotation has been clouded by taking it out of context. Both the headsail and the mainsail drive the boat, and both sails generate lift. I think he means to say that, when the wind moves over the surface of any fore and aft sail, it generates a positive pressure on the concave (windward) side of the sail, and it generates negative pressure (lift) on its convex (leeward) side of the sail. The combination of all the positive and negative pressures that are generated by the sails make up the sail’s “drive.”

Consider the shape of the normal 150% headsail. The leading edge is attached to the headstay. Then, the sail has a deep pocket. The sail flattens out as it
moves aft. When the wind blows across the sail, it strikes the leading edge of the
sail first and is split. Part of the wind blows over the concave side of the
sail, and part of the wind blows over the convex side. The wind pushes on the concave side (positive pressure), and it creates a vacuum on the convex side (negative pressure, or “lift”). As the wind passes over all the surfaces of the sail, it
creates vectors of force (on both sides of the sail) that are approximately
perpendicular to the surface of the sail. Because of the shape of the curve in the
deep pocket of the sail, most of the vectors of force in that part of the sail are more powerful and directed forward. As you look aft along the sail, the curve becomes very
slight, and the vectors of force become weaker and point almost towards the beam.

If there was no lateral resistance below the waterline, the boat would just drift downwind. The shape of the hull and the shape of the keel create lateral resistance to the pressure that is exerted on the sails, and they translate the lateral force of the wind on the sails into forward motion.

When the author of the article said, "The headsail provides drive while the
back of the mainsail provides lift," I suspect that what he meant to say is that, on a masthead-rigged boat (i.e., a boat on which the headstay terminates at the top of the mast) the headsail provides, relatively speaking, most of the drive of the boat, because the headsail is much bigger than the mainsail on such a boat. The boat’s ability to point to windward is greatly affected by the way that the back part of the mainsail (the leech) is trimmed. Moving the mainsheet traveler to windward increases the pressure on the windward leech of the mainsail. Because the leech of the mainsail is aft of the CE and the CLR, the added pressure on the windward leech of the mainsail slightly leverages the boat to windward.

<blockquote id="quote">quote:<hr height="1" noshade id="quote">I've not heard of the mainsail provding 'lift' before.
I think lift job belongs to the rudder, keel and hull shape.<hr height="1" noshade id="quote"></blockquote id="quote"> Generally speaking, “lift” is created whenever a fluid (either air or water) passes over a curved surface (a sail or a keel). The sails are often said to generate lift to leeward, while the keel and underwater surfaces of the boat generate lift to windward.

Edited by - Steve Milby on 05/10/2004 19:53:28

Oscar
Master Marine Consultant

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USA
2030 Posts

Response Posted - 05/10/2004 : 18:56:09

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ClamBeach
Master Marine Consultant

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3072 Posts

Response Posted - 05/10/2004 : 20:02:14

Here's a good article that I think explores the original topic..
"The headsail provides drive while the back of the mainsail provides lift"

http://www.wb-sails.fi/news/99_1_AeroShape/Aero.htm

(cut-n-paste to browser)

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