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. Second was the mechanical problem of designing a structurally sound and functionally satisfactory mechanism for placing sails where they could exercise the degree of control required for this purpose. Third was the mathemati­ cal problem of determining dimen­ sional relationships for this mechan­ ism which would permit moving sails as required without changing the distance between bowsprit head and mast head enough to impair the tautness of the jibstay. These are not trivial problems. Long and arduous work has gone into the set of coordinated solutions which were incorporated in a new catamaran launched in August 1964. The extraordinary performance of this vessel has prompted the author to share the secrets of this design The 30-foot catamaran designed and built by Colonel H. S. Morton is seen here at the dock wi th others. of boatbuilder Fletcher Johnson on the Chesapeake Bay.

Mm'ch -April 1965 19 duces a turning force as great or The forward end of the bowsprit fl exible halyard wire so that it can greater than that produced by the' is connected with the upper end of run over a sheave attached to the rudders. the mast by the jibstay, a heavy forward end of the bowsprit. As the turn continues, this lever wire on which the jib is mounted . With proper dimensional relation­ arm tends to i'ncrease, with the re­ Two "bobstays" from the bowsprit, ships (discussed in the next section ) sult that the turning effort gener­ one to each catamaran hull (Fig. 1) , the head of the bowsprit can now ated by the backed jib, with its hold the front end down and oppose be swung from side to side over the forward end over the wind\yard bow, the jibstay tension. The lateral entire range of positions from one increases at the same time rudder spacing between hulls also enables bow to the other, maintammg action decreases. The observed re­ the bobstays to restrain the bowsprit proper tension on all wires in all sult is a rapid rate of turning, which head against lateral motion. The pOSItIOns. The lateral position of does not appear to diminish appre­ whole forms a structurally adequate the bowsprit head is now controlled ciably, even when forward motion assembly of utmost simplicity. by the skipper through "sheets," diminishes or even ceases. It is fortunate, indeed, that two ropes that are similar to those that The aerodynamic - hydrodynamic very simple modifications of this control the position of the sails. problem can, therefore, be solved conventional design can convert it With proper dimensional relation­ by moving the head of the bowspri t into a " movable bowsprit" system ships between the elements of this over to the side if this can be done that is capable of providing the re­ mechanical design, the bowsprit without detrimental change in jib­ quired displacement of the bowsprit head ca n be moved far enough to stay tension. head without sacrificing the basic either side to enable the backed simplicity of its structural elements. jib to keep the bows turning, and The Mechanical Desi~n Problem 1. The solid attachment of the the process of coming about can be completed promptly and positively. A fixed bowsprit can be mounted aft end of the bowsprit to the super­ But the determination of " proper on a typical catamaran with an structure is replaced by a hinge or dimensional relations" is a mathe­ elegantly simple rigging of great pivot that allows the bowsprit head matical problem of challenging com­ strength. The bowsprit, which is a to turn about the axis A-A' (Fig. 1), plexity. solid member under compression, has with some leeway for vertical motion its aft end attached to the center out of the plane B-B'. Mathematics of the MovablE' of the transverse superstructure of 2. The two separate bobstays at­ Bowsprit the catamaran, at a point substan­ tached to the front end of the usual tially above the level of the forward bowsprit are replaced by a single Attention is invited to Fig. 1, decks of the two hulls. continuous bobstay that is made of which shows (upper right ) a side elevation of the bowsprit in its cen­ ter position, with the jibstay at­ tached to its front end a nd with its aft end hinged to the cabin structure. T he single bobstay passes over a sheave attached to the bow­ sprit head, a nd its ends are made fast to the two widely separated bows of the hulls. With a jibstay of fix ed length connecting the top of the mast to the bowsprit, the head of the bow­ sprit describes an arc of a circle about the axis of rotation, A-A', in the plane, B-B', perpendicular to this axis. At the same time the sheave on the bowsprit head, moving along the bobstay wire, describes an arc of an ellipse in the plane of the two branches of the wire. T his plane keeps rotating about the line join­ ing the fix ed ends of the bobstay and becomes more nearly vertical as the bowsprit head moves away from center. M athem atical analysis shows that this arc of the ellipse (generated by keeping the bobstay taut ) will not Colonel Morton is at the helm of his craft. In the background are Digest staff member J . W. Howe, Jr. (left) and layout artist W . C. Lucinski. keep the head of the bowsprit exactly

20 .\PL T echnical Digest in plane B-B' during lateral motion 38 0 r------~------~------.------~------_.------~------~------~ of the head. If it is in plane B-B' when the bowsprit is centered (see lower right of Fig. 1) it can be in the plane at two points of maximum lateral displacement; however, at all intermediate points it will lie above 34° ~------+------+------~~----~------r------_+------_+------~ plane B-B'. This means that the bob stay does not hold the jibstay length absolutely constant during lateral motion ; on 32° r------+-~~--+------1------~------~~----_+------_+------~ the contrary it relaxes tension dur­ ing lateral motion and only returns to the original center-position ten­ 30° ~------+------+-----~~------~------~------+------+--~~~ sion at full "allowable" lateral dis­ placement. The jibstay will not allow motion beyond this point on either side. 28 0 ~--~~+------+------4------~--~~~------+------+------~ A half dozen equations that define all essential dimensional relation­ ships will be given, but their deriva­ tion is too lengthy for inclusion in this paper. These equations and a

few graphs and tabulated values arc 24 ° ~ ______~ ______~ ______~ ~~ __~ ______~ ______~ ______~ ______~ all that are required to select com­ 0.34 0.36 0.38 0.40 0.42 0.44 0.46 0.48 0.50 patible sets of dimensions for any D/ R installation. Nine dimensions and an angle will Fig. 2-Elfects on the value of angle a of the ratio of D (distance between the bowsprit hinge he defin ed before giving the equa­ and the axis of the fixed ends of the bob stay ) to R (radius of the bowsprit arc) for three given ratios of R to W (lateral distance between the fixed ends of the bobstay). tions which r<'late them (sec Fig. 1) :

W = lateral distance between the {\' = angle between plane B-B' there is every reason to make M as fixed ends of the bobstay and plane of the bobstay large as possible without adversely secured to the two bows of wires when the bowsprit is affecting other factors. It should not the catamaran hulls. It is at centered. he made less than about 80% of W least equal to the distance and can be 90 o/r- or even 100% of These seven dimensions and the between center-lines of the W, with certain limitations on other angle a would apply to the bowsprit two hulls, and may be made dimensions. if it were permanently fixed in its greater if anchor points are In general, the dimension S, which center position. Only the two other near the outer edges of the measures a change in jibstay ten­ dimensions mentioned below have to hulls. sion, should not be allowed to be­ do with the lateral motion of the R = radius of the arc described come too large, but it is by no means bowsprit head ; but they are the two by the bowsprit head about necessary for it to approach zero. whose proper and acceptable values axis A-A'. This is not exactly There is always some slack in a d epend on correct relationships be­ the bowsprit length unless jibstay so that it can deflect laterally tween the other dimensions. the hinge lies in the plane a small fraction of its length, in B-B'; in general it does not. M = maximum lateral motion of order that its tension will not be D = the distance from axis A-A' the bowsprit head. The bow­ excessively high. to line C-C', joining the sprit head lies in plane B-B' It is not difficult to select dimen­ bobsta y fix ed end poin ts. in its center position, and sional relationships that will keep F = R-D again when it is a distance S well within the normal range of L = total length of the bobstay. M/2 either side of center. tension adjustments that are per­ (This is the only dimension S = the maximum distance above missible for a jibstay wire. not shown in true length on plane B-B' which the bob­ The general solution for this prob­ Fig. 1. ) stay allows the bowsprit head lem relates all dimensions to one H = dista nce from line C-C' to to rise, and is an approxi­ another In the form of ratios, the head of the bowsprit mate measure of the slack­ particularly the ratios of various when it is in its center posi­ ening of the jibstay. other dimensions to width W. This tion. Since the purpose of the movable is the basic dimension to which all V = height of plane B-B' above bowsprit is to give a back~d jib a other dimensions of any specific dr­ line C-C'. large increase in effective lever arm, sign are related :

Mm'ch -April 1965 21 R/ W = 0.75 R/ W = 0.80 R/ W = 0.8 5 1 +

( I )

1 (~r - O/ R O/ R ( 2 ) O/ R 0.50 Gfr 4 0.50 0.50 0.42 0.42 0.42 0.34 0.34 (tvr (~r(I -%r (3) 0.34 (rvr = (~r - (tv): (4 ) I I I I (tv ) (5 ) --t --+ --+ cos a = (It) , Fig. 3-Elfects of angle a on the ratios of R to Wand of D to R. and

L / W VALUES FOR BOB STAY LENGTHS WITH TURNBUCKLES AT MINIMUM ELONGATION (M/W = 1.00 )

D R R R 0.75 0.80 0.85 R W W W L M values which make 1.00 It will save the reader who proposes W W to use these equations a lot of time 0.34 1. 602 1.618 1.629 if some indication is given of the 0.36 1.557 1.573 1.584 practical range of values for these 0.38 1.5ICi 1.53 1 1.542 0040 1.47fl 1.492 1.503 ratios. 0042 1.442 1.456 1.467 It is suggested that calculations be 0.44 1.408 1.422 1.433 started for M / W = 1.00 since turn­ 0046 1.377 1.391 1.402 0.48 1. 3-!8 1.362 1.373 buckle adjustments of the bobstay 0.50 1.321 1.335 1.346 length L are sufficient to change this Percent in- ( to lower values if necessary. It may nease in L which reduces 1.6 1. 3 1.1 be pointed out that reducing M/W M / W from 1.00 by turnbuckle adjustment (lengthen­ to 0.90 ing L ) decreases S/ W four or five Lengthening the bobstay hy extending the turnhuckles deneases hoth W/ W and S/ W. times as rapidly as it does M / W. The only reason one would need to make M / W much less than 1.00 ,,

22 .-\P L Technical Digest pletely closed, so that any turnbuckle order of half as great as those pro­ plane B-B' is much smaller than adjustments will lengthen L and de­ d uced by changing L. SI W, since a typical jibstay is likely crease M I W and S I W. Either or both of these adjust­ to be some three and a half times Table I shows that the adjustment mepts produce changes in V and /l', W. necessary to reduce M I W from 1.00 as well as in M and S. This paper will not undertake to to 0.90 (with far larger decreases in Figure 4 gives the final informa­ set a maximum permissible SIW SI W ) is well within the capability tion needed to show whether M I W value, since most yachtsmen who of normal turnbuckle adjustment. needs to be reduced by lengthening may want to use the foregoing One other possible adjustment can L in order to bring S I W down to graphs and formulas will have a be made which has a similar effect: lower values. It shows why, as was good idea of what is acceptable. It if the two hold-down fittings by which previously stated, the smaller D I R may be said, however, that if Fig. 4 the bobstay ends are anchored to val ues are better. If D I R is large is used to select combinations of the hulls have two or more holes it may be desirable to reduce MIW DI R and MIW that avoid the high separated by a percent or two of by a few percent to secure much S I W values on the upper right, no the dimension D, the effects of in­ larger reductions in SI W. combination of dimensions on this creasing D are qualitatively similar The fraction of jibstay length by graph will permit any seriously ob­ to lengthening L but are of the which the bowsprit head rises above jectiomible change in jibstay tension.

PUBLICATIONS HONORS The following list is a compilation of recently published books and technical articles written by APL staff members.

C. B. Swartz and A. Michelsen L. W. Ehrlich, "The Block Sym­ Burning Velocity and a Geometric have been named by the President metric Successive Overrelaxation Interpretation of the Effects of of The Johns Hopkins University as Method," ]. Soc. Indust. Appl. Flame Curvature," Phys. Fluids .. 1965-66 recipients of William S. Math., 12, Dec. 1964, 807-826. 8, Feb. 1965, 273-280. Parsons Fellowships. Both men will R. G. Munn, F. J. Smith, and E. A. W. Liben, "Monkeys and Microelec­ begin an academic year of work at Mason (University of Maryland ) tronics," Electronics, 38, Feb. the University in the fall of 1965. and L. Monchick (APL), "Trans­ 1965, 90-93. port Collision Integrals for Quan­ ]. T. Massey and S. M. Cannon, Dr. Swartz, who is Supervisor of tum Gases Obeying a 12-6 Poten­ "Constricted Discharges in the the Logical Programming Project in tial," J. Chem. Phys., 42, Jan. 15, Rare Gases. I. Spectroscopic and the Computing Center, will develop 1965, 537-539. Electrical Measurements," ]. Appl. and teach a course in computer W. G. Berl, "Instability in Solid Phys., 36, Feb. 1965, 361-372. science, being concerned with the Rockets," Astronautics and Aero­ J. T. Massey, "Constricted Dis­ subject from the point of view of nautics, 3, Feb. 1965, 54-62. charges in the Rare Gases. II. the working scientist. S. D. Bruck and R. R. Rector, "New Analysis of the Macroscopic Mr. Michelsen will be associated Drip- and Leak-Proof Precision Properties of the Discharge," ]. with the School of Hygiene and Syringe," Rev. Sci. Instruments, Appl. Phys., 36, Feb. 1965, 373- Public Health and will investigate 36, Feb. 1965, 239-240. 380. biological effects of x-radiation rela­ W. E. Buchanan, "Navy's Transit J. T. Massey, A. G. Schulz, B. F. tive to protection, therapy, and gen­ Satellites Showing the Way in Hochheimer, and S. M. Cannon, eral health physics. He is a member Outer Space," Navy-The Maga­ "Resonant Energy Transfer Studies of the staff of the Equipment Re­ zine of Sea Power, 8, Feb. 1965, in a Helium-Neon Gas Discharge," search and Development P.roject in 7-10. J Appl. Phys., 36, Feb. 1965, 658- the Fleet Systems Division. R. M. Fristrom, "Definition of 659. -BOOKS APL COLLOQUIA V. Uzunoglu, Semiconductor Net­ work Analysis and Design, Mc­ Apr. 2-"Stability of Automobiles," May 14-"Progress In Quantum Graw-Hill Book Co., Inc., New by D. L. Nordeen, General Motors Electronics," by B. Lax, Lincoln York, 1964. Research Laboratory. Laboratory. S. D. Bruck, "Thermogravimetric May 21-"Computer Systems 1967," Studies on an Aromatic Polyimide Apr. 9-"An Interpretation of Mar­ by C. H. We isert, Applied Physics in Air and in the Vacuum Region tian Phenomena," by C. C. Kiess, Laboratory. of 10-2 to 10-3 Torr Using the Georgetown University. May 28- "Reccnt Progress in Field Cahn RG Electrobalance," V ac­ May 7- "Optical Holography and Emission and Its Applications," by uum Microbalance Techniques 4, X-Ray Microscopy," by G. W. W. P. Dyke, Field Emission Corpora­ Plenum Press, New York, 1965, Stroke, University of Michigan. tion. 247-278.

MaTch -APTil 1965 23