Additional Light air Tuning Suggestions (Unknown sailmaker)

A few things with regards to sailing the B25 in Light Air. If your venue is less than 8 knots consistently you will want to purchase a fuller genoa than the AP#1. It is important to have a deeper cut, most B25's have flat sails- this is not fast in less than 8 knots. Ease the shroud tensions to 27 on the uppers and 20 on the lowersusing a standard LOOS gauge  Crew weight to leeward and by the shrouds. The B25 has a ton of wetted surface in the back and not enough sailplan to drive it in light air. So you want to get the STERN out of the water as much as possible. Put the crew below if you have too. Make sure the helmsman is in front of the traveler and out of the stern of the boat. Traveller up all the way on the track and ease the sheet a bit for twist. Backstay ALL THE WAY OFF, important as that is the BOAT BRAKE. Genoa cars all the way forward and trim to about 3-5 inches off the spreader depending on sea state. Upwind- FOOT FIRST do not try to pinch in the light stuff. Foot, get speed, then stick it. Downwind in the light stuff is a bear if you don't have a masthead kite. Heat for speed and then burn off pressure. The B-Boat will not sail dead down in anything less than 8 knots. Trimmer and driver must be VIGILANT. Trimmer should be in the companionway and OUT OF THE STERN. Get the driver out of the stern as well. Don't be afraid to heat for speed and then burn.

Headstay length should be between 29'-5" and 29'-7".  

For medium air, (8-15 kts) your rig tension should be about as follows, using a small Loos Gauge:


  • Uppers = 29
  • Lowers = 22

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Mast rake should be about 18". The method here is to hang a plumb bob from the main halyard. It should hang about 18" aft of the mast, measured at the boom. Ensure that your mast bend is compatible with the luff curve that is built into the mainsail. Here is a rule of thumb that will help you. If the sail is built full, you'll want a bit more mast bend, which you achieve by slightly more tension on the uppers and less on the lowers. If it is a fairly flat sail you'll probably be okay with the number given. If the main has a lot of roach, you can reduce the rake. This will ease the extra weather helm that the large-roached main adds. Be sure to adjust the backstay accordingly. Once you're comfortable with the location of the mast and mast bend. You can go sailing and fine-tune the rig. With the boat powered up in 8-10 knots of breeze, with a genoa and a couple of people on the rail, the leeward shrouds should be soft, but not moving around. If they are moving you should take up a turn at a time in either tack until they stabilize. Once you have the rig dialed in this way, you can tighten the uppers and lowers 1 to 3 turns for heavy air, but be sure to ease them back off when the wind goes light.

tuning, from north sails

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