... With a Cat with Low Aspect Keels
Those of you that learned the sport racing Hobies and Prindles probably know this stuff, but for the rest of you cruising cat sailors....
No Back Stay. This means that the forestay cannot be kept tight unless you want to turn your boat into a banana and over stress the shrouds. Although swept back, they are only designed for the side force and a trace of forward pull. Real tension on the forestay comes from mainsheet tension.
Why must the forestay stay tight? Sag allows the genoa to become more full, since a sagging forestay has the effect of injecting more sailcloth into the sail. The draft moves aft, the slot is pinched, drag increases, and lift does not. This is OK off the wind, but not to windward, since heeling and leeway (sideslip) increase, which also increases drag. Going to windward is about lift:drag, not just power.
How do you keep from easing the mainsheet in strong winds? Ease the traveler a little, being certain to keep the main outhaul tight (a full main pinches the slot). Reef; it's better to keep a smaller sail tight than a larger sail loose. You will see monos with the main twisted off in a blow. Ignore them, they are not cats. Use the traveler instead. It is also physically much easier to play the traveler than the main sheet.
Sheeting Angles. Depending on whether you have stock keels, you may or may not have enough area to support large headsails or have them positioned correctly for balance (the keel mod was written up in Practical Sailor). Either way, the genoa lead angles must be in the appropriate range. Too tight, you sail sideways. Too loose and you can't point. I covered it pretty well in the link below. The goal is be able to crank the jib in tight and flat without pinching the slot.
Sheeting Angles and Keels
In general, 7-10 degrees is discussed for monos that want to pinch up to 40 degrees true, but 14-16 degrees makes more sense for cruising cats that will sail at 50 degrees true. Try to match narrow angles and you are kidding yourself, just like the folks that believe tacking down wind is faster than wing-and-wing. It isn't, just look at the polar and watch the racers.Before you go making changes or getting sheet envy, set up some temporary barber haulers and experiment. I did.
Again, you are not a mono, you have less keel, and slightly wider tacking angles will be faster for you. You also do not have a hull speed limit; let that work for you. Just don't get tempted off onto a reach.
Watch the fore/aft lead as well. You want the jib to twist off to match the main. Typically it should be right on the spreaders, but that depends on the spreaders. If you have aft swept shrouds, you may need to roll up a little genoa, 105% max.
Leach Tell Tales. On the jib there can be ribbons all over, but on the main the only ones that count are on the leach. Keep them streaming, all but the top one. Any telltales on the body of the main are confused by either mast turbulence or jib flow and won't tell you much. If they suck around to leeward, you are over sheeted.
Clean Bottom. It's not just speed, it's also pointing angle. Anything that robs speed makes you go sideways, since there is less flow over the foils. I like a good 2-year paint, but I'm too lazy to scrub. West Marine's PCA Gold has been working well. Micron 66 is excellent in saltwater.
Push Hard, but reef when you need to. You will have the most lift vs. windage when you are driving hard. That said, I reef at about 20 knots apparent. It's not about speed, it's about pointing. If you don't want to push hard, then reef earlier, but keep the sails in tight (traveler down a little). The boat will also be more controllable than with twisted full sails.
Pointing Angles. Pinching doesn't work for cats. Get them moving, let the helm get a little lighter, the result of good flow, and then head up until the feel begins to falter. How do you know when it's right? Experiment with tacking angles (GPS not compass) and speed until the pair feel optimized. With a genoa and full main trimmed in well, inside tracks and modified keels, and relatively smooth water, I can tack through 100 degrees GPS with the boat on autopilot. Hand steering I can do a little better, though it's not actually faster to windward. If I reef or use the self-tacking jib, that might open up to 110-115 degrees, depending on wave conditions. Reefing the main works better than rolling up jib.
Advanced tip: If the actual upwind goal is to one side, you may want to pinch on one tack and foot on the other. The Speed Polar will give you tips on this. You still want to tack through the same angle, but the target is to one side. Some folks call this wallying, particularly when done in shifting winds.