The jib has a slightly higher angle of attack than the main because it generally has slightly less draft, and so has an entry angle that needs the slightly higher angle of attack.
Trimmers who desire "racing trim" pay constant attention to the telltales, draft position, luff tension, wind strength and shifts, boat speed, jib slot effect. My answer to these questions is highly speculative, since I've not seen any quantitative research on the topic or any treatment in the technical books.
The jib redirects the wind flow so that mainsail drag is reduced. The jib redirects wind on the lee side of the mainsail jib and main form a ''slot'' for the air passing between so as to reduce mainsail drag. An easy way to remember it is the sail moves in the direction of the fluttering telltale.
Jibs When discussing overall sail trim, it makes sense to start with the jib slot effect for two jib slot effect reasons: Jib and mainsail combine in the same way as the staggered wings of a biplane.
The turbulence causes the upper telltale to dance forward while the middle and lower stream aft. Airplane designers know all about the slot effect. The mainsail increases the jib's lift.
Use the spreadsheet for entry and exit angles to play with exit angles. It is appropriate here, nevertheless, to at least state the cause and effect of "the slot'' because it is an important part of sloop performance. The main needs to be sheeted at 3 or 5 degrees in order to maintain its own angle of attack of about 17 or 15 degrees to the oncoming wind.
The jib itself creates lift and provides a portion of the boat's forward momentum. Our most valuable sail trimming aids, there should be three sets of two telltales, one on each side of the sail, evenly set along the sail near the luff.
Qualitatively, the main is sheeted more closely than the jib because the effect of the jib is to place the main into a headed wind. Close hauled apparent wind at about 30 degreesthe jib deflects the wind by 7 or 8 degrees.
The sail should be hoisted so that there are no horizontal jib slot effect emanating from, or vertical wrinkles along, the luff. For downwind sailing there are also genoa foresails "jennies"which are large jib sails, and the so-called gennakers, which are genoa-spinnaker hybrids. This is a tactic jib slot effect to decrease pressure in an overpowered sail, opening the sail's leech up to spill excess air out, but normally we want to maximize power by making the top and bottom uniform in the presentation to the wind.
This will work well in winds up to around 15 knots, but as wind strength increases, the lead blocks will need to be moved aft. This factor is called the "slot gap index".
There’s more to getting the most out of your mainsail than just the sheet
Backstay tension is adjusted for more or less headstay sag. When wind moves aft and the sheet is eased, the lead must go jib slot effect to compensate and close the upper leech. In light airs, the slot is probably slightly more effective, and in heavy airs it is also probably more effective.
The airplane wing provides vertical lift; the boats' triangular mainsails provide horizontal lift, whereas the foresails provide drag for downwind movement. Ever wondered how it is done to perfection every time? This is an accurate setting for the lower sail, but the upper usually needs tweaking with the sheet.
Way back in the s Prandtl was able to write that "the total induced drag of a biplane is smaller than that of a monoplane of the same span and of the same lift.