Tuning an S-boat


This is Brian Kelly's guide to tuning the "S" Boat, which came from the Quantum Sail Design Group.  It is not copyrighted, as Brian wanted this widely shared.  Brian was a top-notch sailmaker, whose loft was part of the Quantum group, and was an avid "S" Class sailor.  Brian is rumored to have singlehandedly won races in his S-Boat Eaglet, S57, on a number of occasions over the decades that he sailed.  Brian won an enormous number of races during his many years racing in the "S" Class and was widely respected.   Brian Kelly was inducted into the S Class Hall of Fame posthumously in 2011. 

Anything below in brackets [ ] has been added by Jeff Whittle for clarity or to provide additional information.


This tuning guide is based on my forty years of experience in racing "S" Boats, first as a crew for my Uncle Matt Burggraf aboard Eaglet, from 1964-1979 and from 1980 to the present [2006-2008?] as her skipper.  I have spent many years racing in the "S" Class with both the Long Island Sound [now Western Long Island Sound or WLIS] and Narragansett Bay fleets; during this time, I gathered much of the following information both from my own experience and from other skippers in the class.  I have organized the guide topically, moving from aspects of the mast set-up, to control of the sail shape, and then to the relationship between sail and helm.  It is my hope that this guide will be of help to all those who have sailed this boat by trial and error: certainly, the information here represents the legacy of generations of "S" Class sailors.


The "S" Boat mast is not blocked at the deck partner, and thus moves from side to side each time the boat tacks. In order to prevent this it is important to block the mast in the center of the boat by installing small wedges. I've also blocked it to within 1/2" of the front of the deck opening. Next, tighten up the lower shrouds fairly tight.  These are a heavier gauge than the uppers and are carrying more load supporting the mast than the uppers.  The goal is to prevent the mast tip from falling off in moderate breezes and to have the mast track straight.  The lower shrouds will be tighter than the uppers.  In moderate air (9-15 knots tws [true wind speed]), it is desirable to maintain a perfectly straight mast track; in light air, however, this will produce a slight windward hook at the top of the mast and in light air you may experience the top of the main breaks or luffs a moment before the bottom two-thirds.  This action was not problematical for us because in light air, we often had chop, and we would want to foot rather than pinch anyway.  Again, the lower shrouds should be tight enough to prevent the lower section from sagging to leeward and closing or restricting the slot between the main and jib.  Too much lower sag also induces [mast] tip hook. Sight for a straight mast track in 10 knots true.

[I always tighten the jibstay fairly tight before tightening any shrouds; this helps pull the mast forward which reduces weather helm.  In strong winds I've had excessive weather helm snap the tiller on an "S" Boat.  When I first adjust the lower shrouds, I adjust the main halyard to reach to the base of the lower shroud and make the main halyard down.  If I started on port, I then take the main halyard to starboard to see if it is the same length to the similar spot on that side.  If it isn't, I adjust the lower shrouds until the main halyard indicates the top of the mast is the same distance to both port and starboard.  Then I adjust the upper shrouds to straighten the mast track.  After the mast track is straight, check the distance to port and starboard using the main halyard.  If not the same, and the mast track is straight, loosen both upper and lower shrouds on the shorter side and tighten both on the longer side.  Then check tension on all shrouds and check that the mast track is straight.  Repeat until you find the right balance.  If this seems impossible, loosen the upper shrouds and start over.]

Our jibstay was always tight.  This is adjusted with the turnbuckle, which prevents too much deflection or sag to leeward.  To control the draft location, we constantly adjusted the jibhalyard tension in various breezes.  In moderate to heavy breeze, we would adjust the luff tension in the jib to assume a tension almost commensurate to that of the jibstay.  In light air we would ease the halyard until we saw wrinkles just developing around each hank.  Our upper runners were not too tight, but definitely under some tension.  The lower runners were very, very tight in order to help reduce jibstay sag.  The upper runners should bear more tension off the wind while reaching and running at which point they act as a real backstay; also, in heavy air, more tension should fall on the lower runners when the headstay is eased.  When you ease the headstay the mast tipis allowed to bend farther aft which causes the middle of the mast to bow forward more.  If the middle is allowed to bow forward too much this will cause the jibstay to sag and the mast to over bend.  You will know when this has happened as you will see over bend wrinkles emanating out of the clew on the mainsail and extending up toward the middle of the mast.  There is a direct correlation between the lower runners and the jibstay.


Controls for the draft of the mainsail such as the outhaul are very important.  In chop, both jib and main outhauls should be eased to obtain greater power in the lower sections of these sails.  Attention should also be paid to luff tension of the main, which is manipulated with the main halyard and cunningham and with the tension of the jib halyard.

The jib sheet should be adjusted only with tension enough to bring the leech twist coincident with the shape of the mainsail.  DO NOT oversheet the jib leech and close the slot between the two sails, you want the leech of the jib to have a nice crescent shape with the bottom batten parallel to the jib club/boom and the top batten slightly opened.

Main sheet tension and traveler position are critical.  I almost always keep my traveller on centerline and in very light air I have it above centerline, all the way to windward.  In light to moderate conditions I am always attempting to keep the top batten parallel with the boom.  As the breeze builds, I begin to drop the traveller a bit.  Say 4" to leeward in 14 knots, and 9" at 16 knots, and 14" to leeward at 20 knots.  Sheet tension should be adjusted according to the following indications: in light air, the top batten should parallel the boom; in moderate air, near parallel; and in heavy air, as tight as possible, with three wraps around the original Herreshoff main sheet winch, if htat's your hardware (by the way, Matt and I never used a winch handle, but rather sheeted by hand).  As a rule of thumb, always line the boom up in accordance with the corner of the transom: in almost all conditions, you want the boom to be just above the intersection of the corner where the transom and stern meet the hull.

The jib traveller is also a critical adjustment.  Guidelines for its tension should approximate that for the main traveller, that is, more towards the centerline in light air and moving outwards as the breeze builds.  This corresponds numerically to 11"-13" from the centerline in light air; 16" below cl [centerline] in moderate ir and 18"-19" below cl in heavy air.


The "S" Boat can have a good bit of weather helm in a stiff breeze otherwise she is an extremely well-balanced craft, which should not be over-steered.  Even in a blow, we steered Eaglet with three fingers.  The helm can be balanced properly through attention to traveller adjustments and sail flattening techniques, as I will illustrate.  Of the mainsail controls, the most important is the adjustable headstay.  Eaglet has a headstay lead on a twelve to one purchase system, which leads to the cockpit.  This control is manipulated in conjunction with mainsheet tension.  In light air, the headstay should be as tight as possible in order to straighten the mast, and thus bring greater power to the main.  This will also tighten the leech so that you will have to ease the sheet accordingly.  Remember, the top batten should be parallel to the boom.  As the breeze builds, ease the headstay in minute increments.  Watch the draft and the leech twist and feel the helm.  By easing the headstay, you are allowing the mast to bend more and more and by sheeting the main harder and harder, you also bend the mast more and more.  More mast bend will flatten the [main] sail, reduce the power of the main, alleviate the heel of the boat and thus relieve the weather helm.  But be alert, if the headstay is eased too much you will again see those over bend wrinkles emanating out of the clew up towards the middle of the mast.  The "S' Boat is a heavy displacement boat with a massive mainsail, the refined helmsperson will steer by alternating pointing and carrying way with bearing off and footing in order to achieve speed and power.  I have seen too many "S" skippers sail entire upwind legs strictly pinching and trying to point high, a techniquewhich slows the boat tremendously.  Sail full and fast.  Work the boat up when the sea and wave conditions permit or when you are trying to pinch off a boat on your windward quarter.

Always adjust your main and jib sheets in accordance with the variances in wind velocity.  It is vital to kepe the jib leech twist consistent with the mainsail shape in each velocity change.  One-half inch to one inch of jib sheet and halyard tension makes a tremendous difference so pay attention to these adjustments.   In conclusion, stay with your competition, and watch the fleet in order to gauge your speed and height.  Do not sail to the corners of the course: it's lonely out there.

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