Written by Jud Smith for Sailing World
The J/70 is a bit narrower than some of the other sportboats out there, so it’s a little tricky upwind because it’s more tender—it goes from being a boat that’s pretty underpowered in light air to a boat that needs to start depowering once the wind gets around 8 to 10 knots. It’s got a big jib, but the main is relatively small. The main has a nice roach profile, but it’s still not a big main by any means. In fact, there’s only around 3 feet of mast above the headstay, which is not a lot.
As a result, it’s a struggle for power in light air, constantly working hard to power up—you start by making the main full and loosen the rig tension, so you can sag the headstay to power up the jib. By 8 to 10 knots of wind, you’re already starting to depower. At 17 knots, you’ve already hit the rig’s max depowered setting. So the boat quickly goes from one extreme to another. As a result, everyone’s building versatile sails—relatively flat jibs and relatively average depth mains so that both sails can be flattened in a breeze and yet be powered up in light air. That requires a correctly set-up rig. Get it wrong, and you’ll struggle. In a way, it’s like a lot of other boats with single swept-back spreaders and a backstay.
Whether I’m coaching J/70 teams, watching J/70s race, or setting up my own boat, the first thing I always look at is the amount of mast pre-bend. That’s because, if you get the pre-bend wrong, the rig will be wrong—in every condition—which affects mainsail shape and headstay tension. The nice thing about the J/70 is that rig tension gives you a lot of control over headstay sag. That makes the J/70 work kind of like the J/24, but even more so.
When I’m coaching a J/70, one of the things I key off is the tension on the leeward shrouds. In breeze, as you’re pulling on the backstay, you’re bending the mast, which slackens the leeward upper and, if anything, tensions the leeward lower. That’s because there is a fulcrum action around the intersection of the headstay and the mast. The load on the headsail is the force sagging the forestay as the wind speed builds, while the combination of shroud, backstay and mainsheet tension are helping to support the headstay from sagging too much. The spreaders and combination of upper and lower shroud tension work together to keep the mast from overbending while tensioning the backstay to maintaining the correct amount mainsail depth and headstay sag for each windspeed.