Mainsail - Genoa interaction

We just described an extensive procedure for trimming the main, focusing somewhat on the back of the sail. It's obvious, however, that the main doesn't function in a vacuum. All you have to do is move forward a bit to see that it interacts greatly with another important sail called the genoa. The area that separates the main from genoa is called the slot. The width of the slot (see below) is controlled by a number of different factors:
- Traveller position
- Mainsail depth
- Sideways mast bend
- Genoa lead angle
- Genoa sheet tension
- Mainsheet tension
 

© North Sails
When you adjust these controls, you are fine-tuning the interaction between genoa and main. In medium and heavy air, your goal is to set up the main so its forward section "lifts" (shows the first backwind) evenly from boom to headboard, at the same time as the genoa's windward telltales lift. Ease the traveller to induce lifting, then readjust the sheet as follows: If the top lifts before the bottom, the sail is too twisted, so trim the mainsheet. If the bottom lifts before the top, ease the mainsheet to induce more twist. Lots of backwind in the mainsail means that the slot is too narrow. Correct this by:
- Easing the jib sheet slightly
- Trimming the main
- Flattening the main
- Changing to a headsail with less LP
- Moving the genoa lead outboard and/or back
- Making sure the mast doesn't sag to leeward in the middle