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MODEL MAKER Practical a Fig Design 12) 20/21 28) 13 Pl4] ——\ XXX S © © N o 5116 | 17] Hints 18 | 19 |22 |23 |24 725 | 26127 29/30] 31 |32 g S. Witty offers some valuable {33 t 1} a BS \WE DGE r© VO SSSO88oS IO R. MID SECTION tips to all would-be yacht designers —also useful to home draughtsmen EARLY all yachtsmen have some ideas they would like to incorporate in craft of their own design, whether they are able to draw or not. A great deal has already been written on the basic approach to this problem and I do not intend to go over this ground again. This short article does not cover all stages of design but is merely an attempt to troubleshoot the main difficulties encountered by anyone willing to have a go. – A drawing board, T square, adjustable set square, dividers, compass, straight rule and a small No. 9 french curve are the MINIMUM requirements; without these you may as well chuck it! Don’t worry about splines, weights, long and costly french curves, etc. Long shallow curves can easily be drawn by using a rule to construct the same in a series of very short straight lines—believe it or not with practice I find this method as quick and just as accurate as those more usually employed. One of the best means of checking the fairness of a curve is by foreshortening the distance between the half-scale sections by relaying the line on a new datum so that the whole length is reduced to 20 in. or less. By doing this any flaws in the curve became much more readily apparent to the eye without having to look along it. In fact it is a good idea to draw the entire half scale plan and elevation of the canoe body foreshortened in this manner either during design or as an aftercheck. An attack of designitis is usually preceded by an urge to sketch strange unyachtlike shapes on odd pieces of paper accompanied by a ten330 dency to assume various attitudes of frozen horror while peering at models stranded on the grass. After several months of this the yachtsman may feel he would like to try his hand at this design business. While he must always retain “ in his mind’s eye” the weight, length and main features of the embryo yacht there is no doubt that the key to the character of the entire design must lie in the form and size of the midsection and nothing very much can be done until the shape of this is settled. Apart from the necessity of obtaining the proper equipment and instruments for draughting, the greatest stumbling block to would-be designers is the difficulty encountered in calculating the areas of the submerged sections and the displacement of the yacht as a whole. Note this applies only to ten rater and “‘M” classes, the “A” boats being a different proposition entirely. For the beginner the best way of predetermining the approximate displacement of the hull so as to be “in the parish” is to compare the drawn midsection with that of another design similar in weight and water line length to the envisaged hull. In any good book on yacht design you will find a certain section devoted to different ways of calculating the cross-sectional areas of the various submerged sections, the most widely used of these being known as Simpson’s rule. Unfortunately it is not just a case of drawing a hull and then simply calculating these areas to form a fair displacement curve but cross reference must constantly be made between the two. Since all sections except possibly the mid- JULY, have been able to construct by measuring off section are constantly being modified as the design progresses the calculation of these areas by the accepted methods alters from being tedious to being downright prohibitive. Alternative methods of calculating these areas are by drawing the sections on squared paper and “counting up”, a method which is both slow and very inaccurate, or by using a planimeter. the areas of the submerged sections as described. The area contained under this curve represents the volume of the canoe body and can be drawn to any reasonably handy scale. By bisecting this curve horizontally as before each column becomes a simple rectangle and the height of each of these (“A’’) is measured and added together (with a rule this time since the distances involved are too great to be taken off with dividers). The total is then reduced or increased back from the scale you decided on for the graph and multiplied by the full scale distance between the sections which in this particular case we will take as 4.5in. You will then have the volume of the canoe body in cubic inches. Since fresh water weighs approximately 62.42 lb./cu. ft. one cubic inch weighs 62.42 + (12X12X12) which equals .0361 Ib./cu. in, and the volume in cubic inches of the These instruments are costly and not nearly as accurate as most people imagine. Altogether I regard this calculation difficulty as the main stumbling block to would-be designers and as a result I have evolved a system which is quicker, less complicated and probably more accurate for the small areas involved than any of the more usual methods. This consists of (1) Dividing the half section (2) Bisect each square broken by the edge of the section with an upright or horizontal line so that approximately equal areas exist on either side of the curve. (After practice it will be found not really necessary to draw these lines in every time.) Small discrepancies in this area “suesstimating’” tend to cancel out and are in any case very tiny. (3) Count up all full square not cut by the line of the section and divide the total by two, ie. 36-2=18. (4) Next use your dividers to add up the LENGTHS of the half inch wide rectangles formed between the lines bisecting the curve and the next full squares. These rectangles are shown shaded for clarity and their lengths may sometimes lay horizontal if the curve is cut by a vertical line. After adding these lengths progressively on the dividers until all are contained holow the waterline into half inch squares. canoe body can then be multiplied by this figure to give the true displacement. This last system can easily be adapted for finding the displacement of the in wedge when the yacht is heeled 20 or 25 degrees, as is shown on the first diagram. By this method the “lifted out” part of the hull section is subtracted directly from the submerged part by the simple drawing of a vertical line positioned by the equalisation of the areas each side of the line as before. Since the areas involved in this case are larger and more awkward to judge, the results are not quite so accurate but as a near approximation is all that is required it does not matter a great deal. The height and width of the resultant rectangles are then measured and multiplied together to give the displacement increase at each section. By plotting these results on top of the displacement curve proper the heeled displacement curve is easily obtained and will give a useful visual idea of the hull balance, also these curves are used to ascertain the (Continued on page 337) between a single span of the points measure this distance off on a decimal rule and add this figure onto the halved total of the full squares, Le. 18+4.25=22.25sq.in. This gives the figure for the submerged cross sectional area of a FULL section and while seeming a little complicated in print actu- | ally in practice each section Fig,Z -SS. i can be calculated in less than one minute. This method owes its accuracy to the fact that all measuring and adding is done on the dividers, thereby eliminating inaccurate measurements on the rule, and can_ be applied equally well to the problem of finding the area under the displacement curve which you will WA bs A re b957 Fa 7 10 R. 331 DISPLACEMENT CURVE 23-9 LBS. SUEY, 4 HOLES ® pees 5B.A. SPIGOT LOCATING PLATE TO EPLATE, Pc. To ON 12″ = =| HEAD Es CLAMP \ PEyAe \ =e CYLINDER -— HEAD PLATE iti ARE TO ; E Fig. 2.—A plate style of jig used for . aligned correctly Fig. 3.—A_ sketch showing the rotating type of centre which is 1500″ CENTRES as fl 3 : 4125″ 4it \ey ss i \| aotA ia rs useful for supporting the cylinder 4 REQD. 5 AE DIA. Fi Z ! == & 2S Se CA ae sh ae ne and with the support of this nature the tool can form the angular faces as shown. There is no point in making a special tool for the radii as these are filed with the aid of a smooth member in a matter of ten minutes or so. If bar material is used for this component, then the flange needs milling before the turning stage— many readers may question this remark and suggest that after and not before is the best time to carry out this task. However, as the outside of the cylinder is tapered, difficulty in setting up for milling is envisaged, so by setting the bar in a vee block and The material is then held in the manner described above and the turning carried out in exactly the same way. The Liner I always had difficulty in securing a high grade of cast iron until I started to mould my own—porosity always appeared to occur just at the vital spot and meant scrapping the piece, so I obtained a few parts of old machine tools when I constructed my foundry and from then on I was not troubled with this problem. I still have blowholes, but this is generally due to my own mistakes and not to the material I am using. Obviously the cylinder must not possess blowholes if it is to remain gas tight, so immediately reject any bore that shows a suspicion of being of poor quality because you are simply wasting time carrying on with material of this type. A lapped bore is really essential for this detail— every reader acquainted with car and motor-cycle work knows full well the finish on these components is high class, so in the next article I will deal briefly with this operation and then even tyro readers can make an excellent job of this liner. Cast the pistons with a chucking piece to enable you to hold it is the four-jaw chuck, so all the turning is thus accomplished at a single setting and the grooves and outside diameter run truly. TO SUIT TAILSTOCK THIS DIA. IS A os RUN FIT ey beL hortce Whee BOLTS FINS,TO REMOVE ANY A =| OTATING PART TO SPIGOT Faeenne SUSPICION OF A CONE MADE BY THE CENTRE ‘TOOL FOR FIN GROOVES CLAMPS HOLDING PLATE hold the casting while the tool travels deeply into the wall of metal. A typical set-up is seen at Fig. 3, The Piston 355 \ chuck and turn, bore and face it at a single setting. The formation of the grooves is possible during the moulding but I would not advise it as they are easily machined—in fact this work is much simpler than casting, but you need a steady in the tailstock to milling the square, this is overcome. \\ ROTATING PORTION Saacices Vie a EA r Sale poeta > LOCATING Se a LA = L \PLATE while the fins are formed = N SY ast \ J / sso pea ain R oe | = a e\ bf fo Al machining the valve seats and holes which ensures both bores are VIS 7 12 3 % FACE PLATE For the gudgeon pin holes a similar set-up to that depicted at Fig. 1—the facing operation for the cylinder ports—is ideal, and will ensure that the holes are perfectly in line. I suggest you skim the inner diameter of the piston for a short distance until the tool touches the cross bosses during the same turning stage and use this as a locating medium when boring the pin holes. The subsequent holes then are always at right angles and there is no likelihood of the pin working rather tightly when assembled with the connecting rod. With care and the use of some soft thin strip to protect the hole, you can use this bore to hold the piston while the chucking spigot is faced off, but do not attempt to take heavy cuts or to part off this piece as this can tend to damage the hole if the clamp moves slightly. Gradually turn the spigot until nothing remains and then face the piston head to a smooth finish.
