The pocket between the jib and main pre-compresses the air. So the air going towards the leeward side is already going faster prior to it hitting the mainsail. Then the shape of the mainsail causes the sail to speed up even more making the pressure differential between the windward and leeward side of the mainsail higher than it would be without the jib and causing more force to be imparted to the sail and hence greater speed. The other thing the jib does at the same time is it takes the air currents which are generally turbulent and swirl around a bit and straightens them out a bit prior to them hitting the main, this creates a more laminar air flow and threfore more efficient air flow over the main.
Does that help?
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Dave Bonin
1981 Nacra 5.2 "Lucile"
1986 Nacra 5.7 "Belle"
Winnipeg, Manitoba, Canada
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Rigging Question(s)
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Posted: Jan 04, 2011 - 06:54 PM
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Posted: Jan 04, 2011 - 08:43 PM
That is also my understanding, the jib accelerates the air. I'm not 100% sure, but it may also work the same as leading edge slats on a wing. The gap energizes the airflow & allows it it to remain laminar over the main wing at a higher angle of attack, & remain attached for a greater distance along the wing.
The jib also creates a lifting surface with its curved shape. Biplanes were an eveolutionary dead end, though both wings created lift. It may be that builing a taller mast, longer boom creates more penalties than the double wing effect.
I believe the jib adds a significant amount of extra lift. I sailed my 5.7 on jib alone one windy day, & if a brisk walk is 3 mph, I had to be getting close to 10 mph on a reach under jib alone.
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Posted: Jan 05, 2011 - 08:16 AM
the "slot effect" of the jib is hugely debated
the theory looks quite pretty but when cats have a limit on their sail area, like a-cats and moths, they just seem to point higher, more lift? and go faster, less drag? WITHOUT a jib
it's only when you have unlimited sail area that you might as well bang some more cloth up in the free space between the mast and bow to catch more air
this report from 1973 shows it's pretty much always been debated
http://www.arvelgentry.com/magaz/Another_Look_at_Slot_Effect.pdf#search=%22jib%20slot%20effect%22
but my boats were designed with jibs so i'll leave them on, they seem to work;o)
Edited by erice on Jan 05, 2011 - 06:16 AM. -
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Posted: Jan 05, 2011 - 12:21 PM
Yeah not sure about how the jib affects pointing, except that the extra force forward of the center of gravity provided by the jib would tend to pull you away from the wind.
The only thing that I can say about the slot effect is that it may be debated for flexible sails but I have seen many simulations and models of ridged structures very similar to a jib that are intended to do the same thing and work very well. Specifically I'm talking about hydroelectric turbine stay vanes and wicket gates and they certainly increase the efficiency of the turbine upwards of 20%, without the effect of adding more "sail" into the situation. But like I said nothing else works quite the same as a flexible sail, as long as it helps me go faster the physics don't really matter.
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Dave Bonin
1981 Nacra 5.2 "Lucile"
1986 Nacra 5.7 "Belle"
Winnipeg, Manitoba, Canada
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