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UNIQUE BOWSPRIT Assures Greater Control for SAILING CATAMARAN

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UNIQUE BOWSPRIT Assures Greater Control for SAILING CATAMARAN UNIQUE BOWSPRIT assures greater control for SAILING CATAMARAN H. S. Morton The recent "small boat explosion" that is rapidly filling our lakes and coastal waters with new pleasure craft has been characterized by in­ creased interest in sailing craft of all kinds, particularly the modern catamarans and trimarans that have evolved from outrigger canoes and other similar craft long used by Poly­ nesians around the Pacific islands. Single-hull sailboats depend for lateral stability on either the weight of metal keels at considerable depth below the surface of the water or on broad but shallow hulls of consider­ able weight. A catamaran achieves stability by means of two very nar­ row, separate hulls of light weight and low resistance to motion, con­ nected by a transverse superstructure above the water. This makes the overall width of the craft from a third to a half its length. Trimarans have a third (and usually larger) The forward end of the catamaran, showing details of the movable bowsprit and its relation­ hull in the center. ship to the two hulls and the cabin. But in all cases the multi-hull craft, compared to a single-hull craft Their growing popularity stems from circumstances. The use of auxiliary of the same overall length or sail the fact that they provide seaworthi­ power to extricate the craft from a area, is a far lighter and faster boat. ness and passenger comfort in vessels situation resulting from the inability of shallow draft and much less to control it by sail alone is anath­ weight. ema to any true yachtsman. H. S. Morton, a consulta nt to the Labora­ tory since his retirement in 1958, came to Light weight, however, has its After more than a decade of ex­ APL in 1946 as Supervisor of the Bumble­ bee Warhead Group. He received his M.E. disadvantages too, the greatest being perience with his first 30-foot sloop­ degree from the University of Minnesota in a serious impairment of the craft's rigged catamaran, the author has 1913. During World War II, Colonel Mor­ ton was intimately associated with both the ability to "come about" promptly designed and built a second, similar development and the eventual world-wide and positively, a maneuver that re­ craft on which a simple modification use by the Allied Powers of the proximity fuze that was developed by the Laboratory. quires "heading up" into the wind of the bowsprit* has made an amaz­ He is also the inventor of the highly suc­ cessful bombing system known as Toss for the purpose of changing from ing improvement in positive control Bombing. Among other honors bestowed one "tack" to the other. Loss of when "coming about." on him, Colonel Morton has received the Legion of Merit and the U .S. Navy Dis­ positive control during any necessary tinguished Public Service Award, and he is maneuver is frustrating at any time, an Honorary Officer of the Most Excellent -x- Bowsprit is a spar that projects ahead of Ol-der of the British Empire. and can be dangerous under some the bQw of a sailing craft. March -April 1965 17 The Process of "Coming About" can propel a sailing craft along any to lateral motion (due to the center course except directly against (into ) board, or keel, or merely to extended Before describing the nature of the the wind or within some thirty or lateral surface) than to forward unique bowsprit design that so forty degrees of the direction from motion that the boat moves forward radically improves controllability, it which the wind is blowing. with very little lateral drift. is necessary to point out the typical Progress in a general direction In coming about, a sailing vessel behavior of catamarans without this against the wind is achieved by pur­ uses the rudder to turn toward the device. Consider a conventional suing a zig-zag course called "tack­ wind direction, and it loses the driv­ sloop-rigged sailing catamaran hav­ ing." The process of heading-up ing force of the wind while still some ing a single tall mast that carries a into the wind in order to change I 5 to 20 degrees "off" the wind triangular main sail, and having a from one tack to another is called direction. It begins to lose speed, bowsprit whose forward end is con­ "coming about." and the rudder (whose effect de­ nected to the top of the mast by a On a tack the fore-and-aft sails pends on speed ) starts to lose its " jibstay" on which the forward edge are set at a small angle to the longi­ effectiveness as soon as the sails be­ of a triangular "jib" (a sail of YJ to tudinal axis of the boat, and the bow gin to luff. Only the momentum of ~ the area of the mainsail ) is sup­ is pointed at a somewhat greater the vessel keeps it moving forward ported ahead of the mast. angle to the wind direction so that and gives the rudder some measure When any sailing craft with such the wind presses on the "aft" side of control to keep it turning through a "fore and aft" rig is headed di­ of the sails. Although the trans­ this remaining 15 or 20 degrees into rectly into the wirui, the sails "luff" verse component of force on the the wind, and an equal amount off­ (i.e., flap like flags in a breeze ) and sails exceeds the "forward" compon­ wind on the other side, until the exert no driving force. The wind ent, there is so much more resistance sails refill on the opposite "tack." A C \ • \ t PLAN VI EW \ MAST \ W JI BSTAY HINGE I BOWSPRIT \ /" \ e ---- - -+-- B' M I \ HINGE \ ;'OWSPRI,- " CAB IN --- B -~~ 7"BOBSTAY -- C ~A' ~ HULL I • A t -----------M --------+ C S S BOWSPRIT ~ ~ B' B e ------ - ---T -_e--B' I T R 'rl=-V I o~s,~' / j ~ D ~ /0 • • ----------W-----------+ A ' I Fig. l-Geometry of the catamaran movable bowsprit, showing all relationships with hulls, flexible bobstay, cabin, and mast. Dimensions used are defined in the text. 18 APL Technical Digest A catamaran suffers a two-fold Using the Jib to Turn the sure is still on the leeward side of disadvantage during this period: be­ Catanlaran the boat's centerline, is directed ing so much lighter than a keel more backward than sidewise, and boat, it slows down far more When all sails are 1uffing, the crosses the centerline a very short rapidly; and having two long hulls forward motion of the craft dimin­ distance ahead of the center of instead of one, it resists turning ishes rapidly, and with it, the power underwater resistance about which more and takes longer to complete of the rudder to maintain the turn­ the boat turns. Because of this short the turn necessary to put it on a ing motion toward the wind, a lever arm, the pressure on the sail new tack. For these reasons cata­ motion which is opposed by both is relatively ineffective when the marans often fail in their efforts to wind and wave. At this point skip­ forward edge of the jib is in the come about, especially if there are pers sometimes "back" the jib by center. waves of any size tending to resist pulling its aft end to the leeward The remedy is, of course, to move heading up into the wind. side so that the wind presses against the jib to a point where it will have In the absence of auxiliary power, the forward side of the sail. a much larger lever arm and greater it then becomes necessary to "wear This produces a backward thrust turning capability. To accomplish around," which means to turn away that causes the forward speed to fall this we move the head of the bow­ from the wind and make an almost even more rapidly, but it also has a sprit from the center line of the complete circle around onto the new turning effect that assists the rud­ boat to a point near or over the tack. This involves the process of ders in making the bows turn into center line of the windward hull, "jibing"-a sudden shifting of wind the wind. If this increased turning thus placing the center-of-pressure pressure from one side of the sails rate more than offsets the more of the backed jib on the windward to the other when the wind is rapid loss of forward speed, the instead of the leeward side of the directly behind. This is not a desir­ "backed" jib does some good; too boat's center line, while it is still able maneuver when the wind is often, however, the net benefit is trying to "head up." This, together very strong. small and is often insufficient to with the direction of the line of overcome the adverse effects of wind thrust, which gets farther and Achieving Complete Control by and wave. farther away from the boat's center Wind Alone The reason for this is obvious. line as the boat turns, gives the jib The line of thrust generated by this four or five times as much lever arm Three separate problems had to be backed sail, whose center of pres- as it has when centered, and pro- solved before a rigging could be designed to overcome these dis­ advantages. First was the aero­ dynamic and hydrodynamic problem of determining how sails can be used to provide positive and ade­ quate control, by wind power alone, when rudder control becomes weak or nonexistent through loss of for­ ward velocity.
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