North Sails Trim/Tuning Guide – 1980’s 

Author – Unknown, Date Est. Early 1980’s


This tuning guide appears to be one of the first ever produced. If you ignore reefing the main, this is an excellent article on tuning and sailing the boat

A light, fractional rig boat like a Kirby 25 is not only fun to sail but is also difficult. You’ll always find a greater divergence between the good and the not-so-good sailors in a Kirby 25 regatta compared to the more common masthead boats. This puts more than the usual amount of pressure on the sail maker for two reasons – one, designing fast sails for a boat like a Kirby 25 requires more testing and research than a masthead boat and is much more complicated. Two, your sail maker has an obligation to help you properly tune your rig, set your sails and drive your boat to its maximum potential. We know that our North Sails Fogh sails are the state of the art for the Kirby 25 since we have put so much time and testing into the boat. The following guide is to meet the second part of out obligation to you.

Rig Tuning

The forestay is non-adjustable. When we were Fogh Sails we sailed the boat, and with Dick Steffan, decided on the optimum rake. This ensures one-design compatibility and means you have one less thing to worry about. We suggest applying extreme tension to both upper and lower shrouds, enough so that when sailing upwind in 20 knots of wind, both upper and lower leeward shrouds are tight. This rig tension is necessary to keep the forestay sufficiently tight and to prevent the mast from having to bend too much. The tighter the rig, the less backstay tension you’ll need to control both main and genoa shapes.


Mirage Yachts supplies all the necessary controls but we suggest adding a little purchase to the cunningham, and perhaps to the vang. you might try using coloured sheets to help your crew keep track os the spaghetti and you should strongly consider using Harken ratchet blocks for the spinnaker and guy sheets. That way you won’t have to use a winch for the sheet in all but the windiest conditions. Lastly, try installing a 3:1 purchase on the traveller. you’ll find your forearms suffering much less in the windy, puffy conditions. The extra purchase nullifies an excuse for not working the main up and down in the puffs and lulls.

The Sails

  • Main
    An excellent compromise or light and heavy air conditions, the North Fogh Kirby 25 main doesn’t do what other manufacturers do – bias the shape to only one condition. On a fractional rig boat a main too full is slow in light air ) the upper sections become very difficult to twist or flatten) while a flat main is slow in heavy air, (it inverts from backstay tension needed to keep the forestay tight.) The cloth we use is a 5.4 Bainbridge cloth tested and researched inside out from out long experience in the Soling class. We doubt you’d ever need more than one reef for racing purposes and we include all the usual ‘go fast’ gadgets – cunningham, soft cloth foot shelf, flattening reef, tapered battens and maximum roach.
  • #1 Genoa
    Since mylar is still outlawed in the class rules, (it’s expected to be legal in 1982) we’re forced to use the next best cloth, a 5.0 oz. yarn tempered material, to help prevent stretching over the usable wind range (0-20 apparent.) With a conservative flat entry for pointing and powerful enough sections (too much fullness causes heeling and slows a fractional rig boat) our genoa is designed for optimum steering. Since it’s difficult to keep a light fractional rig boat in the groove, we don’t design a super flat entry genoa. you may point higher with it, but chances are you’ll go considerably slower, especially in light air.
  • #3 Genoa
    We’ve purposely used a high aspect 7.25 oz. cloth for this sail to help it last and to maintain a smooth, fast, firm leech shape in winds of 20 knots and over. With a classic heavy air draft forward and straight leech shape, we’ve made the sail to maximum size (on the roach especially) to help in the marginal #3 conditions. The clew is very low since we know that it’s the best way to control the leech twist.
  • Spinnaker
    Forced to be good on tight reaches and dead runs, a spinnaker for a fractional rig boat is difficult to design. Ours is biased slightly to broad reaching since the Kirby 25 is slower with a spinnaker than a genoa on a very tight reach. Most importantly, the North Fogh .75 oz. Tri-Radial is easy and forgiving to fly. For those who sail in predominantly light wind areas, a .5 oz. Radial head od .4 oz. Norcon (mylar) spinnaker would be a great advantage.

Upwind Sail Trim

Light Air (0-5 Apparent)

By pulling the traveller to weather and easing the sheet you’ll develop sufficient twist in the main to not stall the unstable air flow. Remember a full, deep, tight-leeched main in light air is slow. When in doubt, induce more twist; try to keep the top tickler flying as best as possible.

The genoa should be as full and deep as possible, To achieve this, slack the halyard until wrinkles appear and allow your forestay to sag. Like the main, twist off the top of the genoa by easing the sheet a few inches.

Never try to pinch or point the Kirby 25 too high in light air conditions. Light fractional rig boats don’t have much inertia. Always try to foot and develop speed before trying for normal pointing angles. Flattening the mail by mast bend may be a good idea but the forestay becomes tight, and as a result this causes the genoa to become too flat.

Flying the spinnaker in this condition is very difficult because the apparent wind is so low. Try tacking downwind to improve your VMG (fastest course to the mark.) A higher heading increases your apparent wind making it infinitely easier to fill your spinnaker to lift and fall.

We’ve found that the Kirby goes best with weight quite forward in the light air condition, especially downwind. Upwind, by lifting the stern out of the water and by inducing about a 20 degree heel, wetted surface is significantly reduced.

Medium Air (6-14 Apparent)

  • Main
    This is the easiest condition in which to properly adjust sail trim. The top batten of the main should be approximately parallel with the boom, or slightly twisted off in the bottom wind range. Try to keep the top tickler just stalling. If it constantly streams you’ve probably induced too much twist. If always stalling you should ease the sheet and pull your traveller up. The boom should be carried in the center until it is difficult to keep from heeling. As velocity increases use more backstay to bend the mast and flatten the draft. Remember to always tighten the cunningham after bending the mast to keep the draft forward. As the wind velocity increases to 10-14 knows the outhaul should be tensioned and the sheet tightened to keep the leech firm. Flat water with 12-14 knots apparent is excellent pointing condition. you cannot point if the leech of your main id not firm (i.e. top batten parallel with the boom.)
  • Genoa
    As the wind builds through 8-10 apparent, increased backstay tension will tighten the forestay thereby reducing genoa draft, At the same time you should tension the halyard to keep the draft forward.

There is no question that the main and #1 genoa should be used in this medium wind range. In flat water, try pinching on all the puffs and head up before the puff comes to avoid heeling past 25 degrees. In choppy conditions, twist the leech of the main more and ease the traveller in the puffs to keep the boat on its feet.

Heavy Air (15 Apparent and up)

  • Main
    As the apparent wind builds from 15 through 20 knots, increase backstay and cunningham tension so flatten the main. To determine how flat the main should be use the “power rule.” If the boat feels underpowered, i.e. it’s not heeling more than 20 degrees (with all crew on the weather side) power up by easing the backstay sheet and cunningham in unison to deepen the draft, If you’re over powered (but you are feathering into the puffs) flatten the main until it’s almost dead flat. Then if you’re still overpowered (22 apparent or over) start easing the traveller to maintain control. At 25-30 knots you should have the cunningham maxed out, the mainsheet maxed out, the backstay maxed out (this makes the main look like a board) and the traveller eased so that you’re sailing only on the last 2-4 feet of the main. Even though the rest of the main will be back-winding, the Kirby will be faster this way than with a reef.
  • #3 Genoa
    The #3 should be used over about 22 apparent, but if you’re stuck with the #1 try moving the lead 6-8 inches back. This will flatten the foot and twist off the top, thereby greatly reducing the heeling. Upwind the #3 should always be sheeted tightly to keep the leech firm and untwisted. One inch of sheet tension makes a noticeable difference to leech twist. Therefore, take a good look at the sail and mark the sheet so your crew doesn’t have to look again. If you’re caught with the #3 in a dying breeze, ease the sheet, slacken the halyard and sag the forestay as much as possible. These adjustments will provide a little more power to keep the boat going. The choice between the #1 and #3 in the 17-22 knot wind-range depends on the sea condition, or whether you want to point high. With a #1 the Kirby 25 will point higher because the lead is farther inboard. But, in big seas the #1 will overpower you since you won;t be able to feather into the puffs (the waves will stop you dead. Also, remember not to reef until at least 25 knots of wind, If you’re not sure, don’t reef. you’ll point higher and with a little helm practice, go faster.
  • Spinnaker
    The earliest note highlighted the importance of adjusting the pole in light air. In heavy close reaching, it is imperative to ease the sheet as much as possible, even if to the extent of carrying a one foot fold in the luff. This ensures that the chute is not dragging you sideways. If in doubt, fly a genoa instead of the spinnaker on a windy close reach since on a fractional rig boat like a Kirby you’ll find little difference in speed.


From our experience racing the Kirby 25 we’ve learned that the boat puts a premium on steering, sail shape and crew work. A lot of masthead boats are easier to sail and therefore, hide faults. Therein lies the beauty of a boat like the Kirby, it forces you and your crew to become better! It also rewards you with noticeably better speed when you learn how to trim the sails properly. Hopefully, this guide has helped.