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post #6 of Old 01-28-2013
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Re: Can someone explain roach to me?

Aerodynamic theory explains that the more perpendicularly the airflow crosses or leaves the trailing edge the more efficient the total aerodynamic effect.
A roach extends the sail’s area in the upper section so that this area more approximates or approaches the above. (and because it more rapidly transitions the ‘tip’ geometry toward that desired 'perpendicular to exit flow' geometry)

Iceboats carry radically raked masts to approximate the same - leech almost dead perpendicular to the horizontal.

The latest modern racing rigs have extended 'flat head' mainsails for the same reason --- more perpendicular flow ‘off’ of the leech; for more maximized efficiency and more optimized lift. The more efficient the mainsail, …. the jib becomes more aero-efficient, too … when sailing ‘aerodynamically’.

The absolute master of aerodynamic design for similar wind speeds, does the same ---- the albatross’ wing.
(So don’t be at all surprised when someday in the future you might even see mainsails with extended ‘primary feathers’ along the top of that big ‘square-ish’ & ‘flat top’ mainsail.)

BTW ... fabric sails dont have 'thickness', and therefore much of what you learned from sailing books about what causes ‘lift’ doesnt happen in that manner, and yet the Wright Brothers original wings were essentially sails with essentially NO thickness …… hmmmmmm.
More confusing to sailors, and some pilots, some of the airflow circulates 'around' a wing/sail (even ‘thick’ wings) ... there is a flow 'component' on the 'windward side' that is going --- FORWARD, due to the circulation flow ‘around’ the wind/sail!
Aerodynamics is definitely NOT an intuitive science, and your (US) high school 'science' teacher's explanation of 'lift' was almost DEAD WRONG

Last edited by RichH; 01-28-2013 at 06:27 PM.
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