Yachting: Volume 49, Issue 4 – April 1931

Model Yacht Designing – Part II—A Practical Method of Designing for the Beginner. The Value By —‘AIN ALFRED TURNER, RN. of Metacentric Balance in Models ; N THE completion of a design it 4% sometimes happens that the model is & too heavy or too light for the intended } length and the following rule for correction may be useful if the sections are yourself are the model yacht which is acted upon by three principal forces. They are: (1) the weight of the whole outfit of the model concentrated just above the upper edge of the lead keel, as will be the case if she is well designed; (2) the vertical push upwards of the water on the underbody of the boat, which prevents the model from sinking to the bottom of the pond; and, (3) the force of the wind on the sails which tends to capsize the model. Sitting in the water will not assist in the illustration as the human body has insufficient bulk to support its weight in water in the same manner as a model. Therefore you should sit on a fence, or preferably on a hinged gate, and see what happens to you in respect to your drawn on squares: Weight of large model Distance between stations in large model (Sides of squares on large sections)? in small model small sections)? Weightofsmall ~ Distance between stations x (Sides of squares on model Suppose a six-metre model is required, with a water line of 37 inches and 21 pounds displacement, and that the balanced design on which it is to be based is of 32-inch l.w.l. and 12-pounds displacement. Draw squares over the sections of the small model (Fig. 17) and measure AB or ab, as convenient. It is more convenient, as a rule, to measure AB. Assume it is 8 inches. The distance between the stations will be proportional to the lengths on the water lines; e.g., if they are 3 balance. Let your feet represent the lead keel; the weight of the upper part of your body the force of the wind on the sails, and the pressure exerted by the gate on your nether parts the pressure of the water which prevents you from falling to the ground. It is then possible to balance yourself by your feet as you lean backwards or heel, as it were. If -you right yourself again, you are facing in the same direction as you were originally — provided that the gate has not moved nor opened as you heeled and righted yourself. Suppose, however, that the gate was hinged in a north and south direction with you sitting on it, facing due east, and that, during the process of your heeling, the gate moves on its hinges to a N.E. to S.W. direction. Then, in this case, you will find yourself facing the S.E. to N.W. direction on completion of the heeling process. Similarly, if the general line of support of the vertical force from the water on the model changes its axis of direction during the process of heeling, the model alters its course. It may flap its sails or bear up — it is a matter of the shifting of the general line of the water support, and not a matter of centers of effort or lateral resistance, which are of secondary importance. But the model is not so much like a man sitting on a gate as one sitting on the top bar of a fence supported inches apart in the small model they will be 3 x ag inches, or 3.47 inches apart in the largest one. Substituting in Rule 2 as it stands: 21 =—37 12° «33 (AB for large model)~ 64 The use of this rule enables minor adjustments to be carried out and it also enables a large model, such as a Class A model, to be drawn with precision from the lines of a small model, the sides of the squares being calculated, the sections and profiles sketched in by freehand, leaving nothing but the fairing of the large drawing to be done. This rule is mentioned as it is dangerous to proceed on the principle following. Suppose a model is right for length but 10 percent too heavy. It might be thought that squeezing the water lines together by 10 percent is all that is necessary (or, alternatively, squeezing the buttocks 10 percent closer). Such a method is risky and false. If the model is balanced in the first place, such a distortion will disturb the balance. Neither distortion will affect the trim of the A a4 my but “e- may spoil a good prove a defective one, of course, but considerations of ov metacenter _ one what In attempting to form a 1e can or cannot do. judgment on the actions of another person — in deciding whether his — ri right or : boat. They may im- wrong, consistent or in- \e ~ 8 / a an a 7 * Je N — \ A — out how you would be- | a al gta It will be profitable, therefore, to consider that you you sit ae i / J |7 LAS \ J/ ne ~~ ys happen. } fa Figure 17 facing east an ‘ the north upright moves to the east as you heel yourself —— the southern up- towards the west, then the right at the same time moving wet Me consistent, it is advisable to put yourself in his place and with the bar of the fence lying due north and south, the y TM~ Sa by two wobbly uprights. If each of the uprights can wobble to the east or west, bar of the fence must change 7 in its line of direction. So oe you, 4 Pa cafe be acing : towards tie S.E. Simi- larly, if the uprights move in the opposite direction, you a face towards the N.E. eelf sitting balanced inthis way on a smooth polished curved rail of a spiral stair99 GATE FENCE WOBBLY FENCE HEEUING FENCE 4 POSITION 2 POSITMNON 1. POSITION 41. POSITION 2 case. Then you know, although you need not try it, that you can slide from the top to bottom of the staircase, facing, in turn, all the points of the compass. But you may object that the model is not supported by a rail sloping downhill or uphill. It is not. Nevertheless, you can imagine yourself balanced on a horizontal smooth rail while you are pushed either directly or obliquely by a friend who, in pushing you, takes care not to topple you over, leaving the balancing entirely to yourself. Then you will see at once that the direction in which your body slides is governed, not by the weight of your feet (the lead), nor by the weight of your body (the sails), but solely by the axis of direction of the rail. In speaking, therefore, of the balance of a model in these articles the word is to be read in the above general sense, so that a ‘“‘balanced model”’ is one that does not alter its axis of direction during the process of heeling, retaining, as the French would say, its stabilité de route. It is sometimes said that a model is balanced if it is so shaped that the center of buoyancy moves neither forward nor aft during the process of heeling. This may be true or untrue — it depends simply on the axis of direction of the line of support from the water. Models with sharp ends or full ends can be designed in which the center of buoyancy does not move in a fore and aft direction, but it does not follow that they are ‘“‘balanced”’; they may, nevertheless, be troublesome to sail as the axis of direction is a bent one. Can the axis be straight? No—the disturbance caused by the keel distorts it and the most that can be done is to arrange for the shoulders and the quarters to be in line. Why is preference given to shoulders and quarters rather than to the midship section, the bows, the forward overhang, the stern, the counter or any other part? It will be seen in studying any ordinary example that, in balancing considerations, the shoulders and .. stances permit. The position of the upright metacenter can be determined quite simply by the rules already given and the model can be shaped around it so that the heeled metacenter may coincide with the upright position when the lee rail is awash. It will then be found that the same metacenter points out where the supporting force acts in the intermediate positions most accurately, thereby ensuring that the model is balanced at all angles of heel between the upright and rail under positions. The displacement curve in full lines is that formed by the upright position and the dotted curve that made by heeling ‘‘rail to.” The upright curve balances on the knife edge at Bu, the ‘upright C.B.”’ The heeled curve balances on Bh, *Norte: In a well designed vessel the shape of the midship section does govern the shape of the ends to a great extent. There is a continuity, or rather harmony, of form from end to end of the boat. Eprror. UPRIGHT =. we” quarters become of such importance that in comparison with them the term ‘‘master section” for the midship section is in actual practice a misnomer.* In actual practice, the model conditions can be represented by the example of a person sitting on a fence supported by two uprights, each of which moves over by a like amount in the direction in which the body is inclined. As you sit upright, the line of vertical support passes up through your erect body; as you heel yourself backwards the whole fence bends backwards under you, thereby illustrating the movement of the vertical line of support from the center line of the model, when it is upright, to some line well under the turn of the bilge when it is heeled. Now the metacenter gives you a direct indication of the extent to which the concentrated supporting force of the water has moved during heeling; therefore the axis of direction of the general line of support must be underneath this metacenter and as straight as circum- MEELED DISPLACEMENT CURVE —-. SUPPORT MOVES TO LEFT te 45n 4 &| [Bu Figure 18 DISPLACEMENT CURVE April, 1931 101 Yochting the ‘‘heeled C.B.” In a Class A model, spare no pains to make Bu and Bh coincide without making any modification in the form of the model that will disturb the balance of the’ sections under the metacenter. It is advisable to shift both Bu and Bh towards each other. This is most readily effected by studying the curves at points Sh and Su at the shoulders and Qh and Qu at the quarters. Lowering QA and raising Qu will bring the C.B.s closer together; raising Sh and lowering Su will have the same effect. Qh is lowered by clipping a little off the quarters at q, and Quis raised by swelling or lowering the buttock at q2. Sh is raised by swelling out the shoulders at S, and Su is lowered or decreased by raising the buttock at S:. If the model is of normal proportions, no difficulty should be experienced in making Bu and Bh coincide to the thickness of a line as regards accuracy in a Class A racing model and it cannot be emphasized too strongly that a maladjustment of one-half inch or so in a racing model has a very marked ill effect on its performance. It is most undesirable that this conception of balancing be confused with the usual process of finding the center of gravity of curved areas by balancing the paper patterns on the blade of a penknife. The former is considered to be the first essential quality of a design; the latter is a standard method used in mensuration, being quite as accurate or as ‘‘scientific’”’ a method as the use of figures — in fact, it frequently is even more accurate. Both these processes are called “‘balancing,’”’ but they have no connection with each other. To speak the truth it is necessary to know what it is. The writer does not pretend to have read everything that has been written on the shape of boats, but, having studied much of this material, he has been reduced to a condition of bewilderment. Horses have been known to laugh at men and their efforts to tame them, and perhaps yachts and models, too, would do the same if they could. Men like them all the better for it. It has been laid: down, among other things, that the first essential feature of the design is to place the center of buoyancy at 55 percent of the water line when measuring from forward. This may be right, and it may be nonsense; but unless it is explained why such a position is essential it is safer to regard it as black magic and consign it to the pot. The adoption of a raking midship section is sometimes advocated. Jt may be desirable or undesirable, and you are entitled to ask: ‘‘Well, how much shall I rake it and why?” And unless the answers are forthcoming, answers that you can understand, regard them as more black magic. The writer feels, however, that the one subject which has been garbled by some, hinted at by others, and yet burked in all cases known to him, is that of the balance of a yacht — or for that matter — of a model yacht as well. He realizes, at the same time, that in entering the subject he himself may not know the truth either and therefore cannot speak it. He can do no more, however, than give the results of his experiments. In all sciences and arts it is nothing but experience that teaches — not necessarily one’s own experience, but that of others, present and past. It is advisable, therefore, that the inexperienced designer should start at the beginning and stick to the simple guiding principles that can be understood. The shaping of the hull must be guided by four principles, as follows: (1) The hull must be as long as possible. (2) It must be as powerful as possible. (3) It must be as easily driven as possible. (4) It must be balanced at all angles of heel likely to be met with in racing. The present task of the writer is to prove these rather dogmatic statements. In the first place, the considerations of these principles are interwoven so closely that it is advisable to review the balancing problem and treat the other three as they arise, as it goes without saying that if:the model is unbalanced the other qualities will not save her, however long she is or however easy. One is apt to regard a model yacht as merely floating — it is so obvious that one is inclined to think no more about it. Actually, a model yacht is a body prevented from sinking by a force acting vertically upwards on the bottom which totals the displacement of the -boat. When the boat is upright, this force is distributed in uneven fashion along the bottom of the boat from one end to the other, yet acting at each point under the center line of the model, assuming, of course, that both sides are alike. When the model heels, the new upward displacement force (never mind any question as to whether it is (Continued on page 138) ~ 17? i4 xe atsags ip po’ – 5 7 Sod pt eo ae _ -~ £ Le