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ti Mg is a new design developed by way of Hornet to make possible an increase in both power and speed. Compared with the latter there are several minor changes in the dimensions, all of which make for an improvement in the balance, notably increased waterline and sailing length, reduced beam and an easier and slightly deeper midsection. Also the greater draught allows a considerable improvement in the pendulum stability. The main difference, however, is in the design of the forebody, which is wider and tends to match the afterbody more than previously. To facilitate this and in order to cut down on the amount of hull out of the water, the stem is rounded in “Myth of Malham” style, being just full enough to allow an increase in bearing length while not blunt enough to be stopped ‘by a head sea. Such fullness as there is will in any case be of little moment when being driven hard downwind as all previous designs with this type of afterbody tend to sit up rather than bury. Viewed from above, the forward deck takes the shape of a long half ellipse which itself is a shape relatively easily driven through rough waters as well as being aesthetically good looking. Shown on the accompanying diagram together with the lee waterline is a line known as the “Welch Axis”. Basically, this is a line through the centre | se” Mp | CLIO [ “s.witty i proportions. Although some yachts go to windward faster and closer than others, I have never seen a fin and skeg, vane-guided model which is overbalanced—that is, that tends to run off when accelerated by a sudden increase in the strength of the wind although this sometimes used to happen in the case of the long skeg Braine yachts. In the case of the vane-guided yacht it is mainly a difference in the degree of un- (ULAR ww HATA Meee ee m © 987 SRT oo KT EY) C eee – – Z. maa eu TLL ‘ of volume of the immersed body and varies with the angle of heel, falling on the centre-line when upright. Excluding other forces the hull will tend to follow the line or curve formed by the Welch axis. If this has an outward swing the hull will tend to point up too readily, whereas an inward swing will tend to make her fall off, although in practice this does not happen, due to the unavoidable partial unbalance of the sailplan, C. of P. movement, etc. The form of the W.A. in fact constitutes the main basic difference between Wasp and the Hornet and Dynamo designs, although both the latter are still very fast in the right hands. Note that in the axis shown on the diagram the local shift caused by the inclusion of the fin and skeg rudder is not shown. An analysis of the W.A. diagram would seem to indicate that a wide transom could, in certain circumstances, help to improve the dynamic balance by accentuating the inward swing. However, no theoretical system of balance takes into account the upsetting effect of the very considerable aerodynamic forces which play upon a wide transom when heeled, so that in Wasp this width has been kept to moderate ou fee ae al a Peper Ne i ee 5 ‘ ae paveee ee Law 134 MARCH, 1960 Diagram at right shows the Welch Axis as dis- eo cussed in the text. Fullsized copies of the drawing below are available price 10s. 6d. post free from MODEL MAKER Plans Service, 38 Clarendon Road, Watford, Herts. | WELCH AXIS”) LEE WATER LINE HEELED TO DECK EDGE eee eee A NEW MARBLEHEAD balance and even if the hull itself has perfect straight sailing qualities the unbalance of the sailplan will give her the necessary tendency to point up when hit by a slam. The quicker this alteration in course a eee BY S. WITTY out roughly around the mast position. Providing then that the hull structure balances at the C. of B. (volumetric), as happens in this case, the math is simple. Taking moments around the C. of B. can be made to suit the increase in wind force the For For faster and more powerful she will be and less likely to be knocked on her beam ends. The ability to do this is easily attained with any yacht, balanced or not. The real difficulty is to avoid overdoing this action so that she is left full enough to be able to nip smartly back on to her original course, and only good dynamic balance can ensure this. For this reason a short skeg (longitudinally), being easier to turn, makes for a faster if more lively boat. the vane gear, etc., 23 x 0.5=11.51 lb. in. the sailplan, etc., 5.5 x 1.25=6.875 lb. in. Moment to balance lead 4.625 lb. in. The weight of lead is 14 lb., so to find the mini- mum advance required 4.625 -:- 14=0.333 in. As there is more solid wood and painted area aft of the C. of B. in the appendages this distance has been increased to 0.4in. in order to compensate and to allow the hull to sit with perfect trim. As vane gears grow in complication and gadgetry so the weight increases, being anything up to a pound these days, so it is difficult to know exactly what advance to give the lead in order to preserve fore and aft trim. For an “M” Class the weight of vane gear plus rudder post, tube, etc., comes out at around half a pound, while the rest of the fittings, plus mast, sails, etc., weigh about 1.25 lb. and balance So far we have dealt mainly with the performance to windward, so a brief word regarding the downwind capabilities will not be out of place. By and large Wasp will be about as fast on the run as the Hornet design. As the latter is not usually beaten downwind except by reason of trim this should be good enough, while an improvement in directional control afforded by the rudder being mounted further aft may assist in picking up even more points. The ») ability to plane cannot be considered to be a racewinning factor in the “M” Class as considerations of balance and general all-round performance suffer too much if a true planing section is used, so that in , the design shown the mid-section is, just flat enough to prevent rolling on the run and to allow her to sprint when hit by a slam. The sections used in the fin are as sharp and thin as possible with a reasonably low lead position and are such that the centre of pressure movement is reduced to the minimum. Although most skippers understand the action of the airflow over the sailplan and that a yacht is sucked rather than blown aS Nae XS Bet] Foresaic [Hst] MAINSAIL forward, some do not realise that the fin operates in a very similar manner, if in the opposite direction. Just as the sail derives the greater part of its drive from the suction on the lee side, so the fin develops the larger part of its lateral resistance by the generation of low pressure areas on the windward surface aft of the maximum camber of the section. This effect can be seen when a yacht is sailing at extreme angles of heel by the localised hollow in the garboard stream at the root of the fin. The sailplan is quite orthodox except that the foresail is carried a little higher than usual. This is partly to allow for a kick-strap fitting as shown on the plan, but if a conventional loose foresail is used both sail and hoist may be dropped about two inches. Due to this kind of individual variation the given position of the mast is only approximate, dependent on the exact trim, weight of vane, etc. Even so, the final position will be within the normal inch limits of that shown. 80. as 42°: 13-4″1.39° |52 5|61 25°17 5°: 63″ |66-25) 2 WMD | 93-STM413-47×30-9] 43° | 4975 17-574 51-2] 547 PRD | 27 S0l2°<24-6"| — — 37° | 4071 15-5" 142~ | 457 = sear 4 On the whole Wasp is more attractive than her forebears and the much improved balance allows her to take full advantage of the increase in sailing length. 135 a I\|\|\| : Do-it- Yourself Figo a sine JIB TACK FIXINGS JIB CLEW FITTING SAILMAKING As used with spring ring, Pin or hook may beused for racing and Le ies cruising craft Sie + IT PART FOUR rir By J. S. Drury piedhe Maing tga t ching te Ome ae a r eae = Fig, 12 METHOD OF MAKING CLEW FITTING FOR MAINSAIL Denil) 550" Now we last mentioned the foot (shoemaker’s, no joke here!), but before taping the sailfoot let us consider the attachment n to the boom. The tack is no problem, either Fig. 10a or 10b, the latter only needed for an “A” or the 3rd suit of a 10r. You will observe that I have omitted to show the eye and jib stay in the interests of clarity, but as you know the jib stay runs from the masteye to the end of the jib-boom through the luff rings, tightened by a separate bowser to the jib uphaul. I have shown the approximate position for the eye of the hook to jib rack (Y); this style of fitting is just as effective as a radial jib with kicking strap and king post, and so much simpler. Provided the separate jib stay is kept at the correct tension for the weight of wind the leverage provided by the rear-mounting of the hook keeps the leach as taut as you wish; it is usually best on the easy side. You must, however, have the clew on a sliding fitting for effective flow control, obviously, and the rig shown by 10c is simple to fit and to rig and unrig (simply slip the loop in the outhaul over the hook A, which is a fitting on sail, and slip through wire bar, adjusting cord outhaul with bowser). Now if you refer back to FOOT PLOT you will see why the apron curve was ended short of tack and clew, to prevent the hem of the apron from fouling the wire runner on the boom. Sure, you can set this wire higher to ensure clearance, but sail area is measured to boom, and why give away sail area? Aprons should be small, 3in. is ample, or it will catch on the boom. Hem this next. Now for the tape. Coat this thinly with adhesive, and starting always from the tack, and leaving + in. overlap, lay over the curve, which it will follow without trouble if care is used, keep the lower edge always in contact, and avoid force, leave 4 in. overlap at clew. Turn over, stick the corner fillets, and then the end of tape atop. You will have to trim the tape at an angle as you will see to finish parallel with other 140 side at lower edge. Now finish all stitching; the various diagrams show the lines of this clearly, and it is pretty easy, as it is not vital to keep quite straight! Now for the fittings. If you are using plates, stick the little slips of material on the sailcorners; for the plates themselves brass is best (not dural, please, unless you can get it anodised, or can get 1/16 ali. rivets, or both) about .012 to .015in. half-hard or annealed. I think the diagrams are pretty well selfexplanatory, but a few notes on procedure. The slight rolling in of the lower edge can be done after folding, it makes no odds, and the best thing to fold over is a bit of scrap metal about 1/20thin. thick. Fold over, drill just to pass rivets, say 1/16 in., and the lower hole rather bigger, about 3/32, slip off former, clean hole edges inside, slip over. sail, prick the holes and slip off, insert eyelets in sail as described later, slip back on with a trace of adhesive, and rivet. Use copper, or 3/8 R.H. screws annealed to red heat and cooled in water and nipped short. Rivet must always be only just long enough, and have the head flatted off, so that it will sit square on anvil and not bend about. Experiment with one first to get knack. DO NOT try to make holes in sail first and drill the plate to fit, or you will have fun! The ring fitting for tack shown at Fig. 11c is very sound; I have used it for 20 years without regrets. The eyelet and cord loop or hook is strong enough for the tack of all but a big boat, but the plate is chafe free and well worth the little extra, Fig. 12c for the clew, the plate is a “Yes” for an A, and I would use for 2 and 3 suits on a 10r, but the 10c design for anything else. It would be excellent to use 12e design for the clew of “A” class jibs. When fitting big eyelets it is good to use a pair of red fibre washers, as may be obtained from any garage or Halfords, between the metal ones and the sail, to preclude chafing, and Terylene bits for the He Ales oe gualles ones. adhesive faces all trace of fitting. With cloth all holes when may be best pierced, and for aes with —> ——— punch out about 2/3 size and ease out with awl after for the big ones. To fit small eyelets, see Figs. 14a to d. Best thing for block is a bit of soft wood, such as a bit of apple box, or an old cork bath mat or low grade hardboard will do. It is almost supernatural, the perversity that an eyelet can develop if you try to insert it into a ready-made tight hole (and the hole must be tight or the eyelight will draw out in use)—and having got it in, it will fly out and get lost in your garments the instant you take your eye off it to pick up the pliers. The “drill” Dl —> € 1: the latter material it is best to —— eyelets for the Terylene, but wit te Te – MARCH, 1960 : ae P =A Stitchd Le Lo ALTA S hiae CE Pint ofp ht cH Battens. Diagram should be pretty well capable of speaking for itself. Celluloid, about .020in., is best, as it can be cut with scissors. Rub sharp edges off with fine sandpaper to avoid cutting sail or tape, and drill end 1/16in. You can use as many or as few and of different lengths to get just the effect you need for the sail and boat, but at a guess you Fig. 14e shows correct way for pliers and drift is just the same. Always fit sail hooks as at xx, and then if one breaks off, as they do sometimes, you can fetch off the clench with a drill, i.e., countersink into the wire so to speak, and the broken hook will lift off and the eyelet, now minus its clench, will find one sufficient for an “M” jib, and 2 for each mainsail pocket, 3 for a 10r and up to 4 for an “A”. It is well worth a little experiment with the sail, holding above a bench as described earlier for testing for “if blown”, and varying till sail takes nice flow. Stitch through holes and leach and tie in knot. It is easy then to cut knot and replace. Battens 4 in. wide are ample, and }4in. for batten tape gives plenty of stitching clearance. One row is enough for all except an “A” and as the battens are. secured at the leach they can’t slip forward and for a bit. trouble with batten pockets. Always keep battens easy in pockets, and }in. short. Now convention shown will cope with the little varmints. To clench, turn over and drift from the bottom, can be slipped out and replaced. T’other way of original fitting, and you will be in Clotson’s shoes Just tap all eyelets both sides on a hard wood anvil, with a light hammer, to finally secure, when the sail is finished, and by the way, the ones with coloured rims often split if this is done, so as I said earlier, stick to brass, except where, in a prototype model, there is not much strain, when the “tap” can be omitted. I have sketched a cardboard jig, for taping and holding headboard and sail when stitching together, a wire loop for the correct positioning of sail hooks, for if fitted too close to the luff they will bind in the jackline in light weather, about 1/20 in. is about right, and luff ring. An alternative method of headboarding is to eyelet a celluloid shape to one side of the sail instead of cutting the sail off; this is satisfactory for a smaller boat, though not so neat. The two holes for the break the pocket open, which is the usual cause of places the battens at about right angles to leach, which makes the sail difficult to roll up without damage to battens and foot tape, so PLACE THE BATTENS PARALLEL WITH FOOT, avoiding all bothers, it has been well tried out and the sail sets just as well, if not better. All the above is applicable to cloth or Terylene, but in the next stages we must diverge. After finishing off sail, not forgetting to rub down the mix and soft metal fittings, both sides as one side dries. perhaps half-an-hour in usable condition, This will seal all stitching. We are, of course, writing of Terylene sails at present. To mark name, etc., use Indian Ink, waterproof quality, after doping edge. To mark No.s draw out number full size in ink on white paper (or use old sail), place BELOW sail, seems to give best set of sail. These are well worth having, but use good big 4in, holes for easy rigging. Ali. 1/16 thick is good for this job. fitted Fig 13 Do not flood over, and remember the mix will only keep for uphaul hook are to allow of use of whichever one Large eyelet stitching a bit with the spoon handle, make a of about 2 parts boat varnish, 1 part white spirit about 2 per cent. Terrabine, and with a small brush apply over the tape and all stitching and clean sail area to be marked with trace of meths. to degrease, and mark edge of No. with indian ink, filling in with black ‘“Japlac” mixed with white spirit and terrabine as for dope. This will dry quickly, stay on sail and avoid any Fibre | @ heavy look. FITTING SMALL EYELETS INTO MATERIAL When doing the other side ensure you get it right away round and up. Nothing annoys a scorer more than having to stand on head to read the numbers as a boat passes him! New cloth, Union Silk, for racing models. These may be left as they are, waterproofed, or doped. If untreated they require that constant care be taken during racing to take up stretch and ease off for shrinkage as they get wet, and they must be carefully dried before stowing away, or they will rot. There is’a lot to be said for waxing or doping, Masking and we will move on to this next month. 141
