pl Grtinentat Reifen FEBRUARY, Photograph shows Noyn, a@ very suc- cessful **amateur’’ designed 10-rater. 1959 sliding bodily to leeward under the sideways pressure of the sails. No doubt most readers have known this for many years, but if we proceed to ask, “Is the position of the C.L.R. important?” and “Where should it be placed?”, then matters immediately become much more complicated. We shall at once find that some writers, after referring to the fact that the C.L.R. moves its position whilst a yacht is actually sailing, acknowledge that consideration of the position SOME THOUGHTS ON MODEL YACHT DESIGN By R. H. Morrell T has often been said that “Yacht designing is an art rather than a science”, and there is, of course, much truth in this. I think that most model yachtsmen would agree that a really successful designer must combine a measure of imaginative creative skill with a knowledge of certain facts which can be scientifically detined. Now all this sounds very formidable to the amateur who is seeking knowledge of this fascinating but complicated subject. He may well be confused by much that he has read and find his mind in something of a jumble, with the distinct impression that the article written by “A”, which he has just been studying, does not tie up very well with what the recent notes by “B” seemed to be saying. This seems to apply particularly to the subject of keel design. The writer has found it profitable to sift carefully much that he has read, picking out the points which seemed to him important, and seeking to correctly co-relate them. It is in the hope that they may help other seekers to see the subject in clearer focus that these notes are written. The Mystery of the C.L.R. Almost all model yachtsmen know that there is a certain point on a yacht’s load water line (L.W.L.) at which, if sideways pressure be applied by a pencil point whilst the model is afloat in a tank, the boat will move laterally sideways through the water. Pressure applied at any other point will result in either the bow or the stern swinging away first, but at this one point the model may be pushed uniformly sideways. This point is known as the Centre of Lateral Resistance (C.L.R.), lateral resistance being the force which prevents the model from of the C.L.R., whilst the model is at rest, is an important point of yacht design. Nevertheless, one does not always receive a clear impression of the writer’s views as to what that position should be, and is perhaps left with the true, but somewhat unhelpful, edict that knowing where to place the C.L.R. is largely a matter of experience. It is, however, well known that if the C.L.R. is too far aft then the yacht will run off the wind as her speed increases, and this effect, if excessive, can cause her to “bolt”. Attempts to correct this by moving the sail plan aft will fail, for the model will be found to come up into “irons” as soon as the wind lightens. She will then hang “head to wind”, unable to hold a course. On the other hand, if the C.L.R. is placed too far forward the yacht will be “hard-headed”; always trying to come up into the wind. If it is attempted to counter this by moving the mast forward then we shall find that as the wind eases strength the sail plan is too far forward, and the model will fall away from the wind. Incidentally, this “hard headedness” or tendency to “gripe” can also be caused by the leading edge of the keel being too far forward, or by the leading edge being too nearly vertical. These points will be further referred to later. One way of expressing the position of the C.L.R. is by saying that it falls at a certain percentage of the length of the L.W.L. For instance, if the L.W.L. measures 100 in. and the C.L.R. is placed at a point 56 in. from the forward end of the L.W.L., then it may be said that the C.L.R. position is at 56% of the L.W.L. MM Nt MOOEL MAKER Whilst designers state that there is definite and specific point which can be regarded as standard practice, it is obviously helpful to have some idea as to what is likely to prove suitable. With this in mind the author has studied a few designs by well known designers, and comparative placings of the C.L.R. were found to be as follows (approx.): Class Name of Model 36in. Design given in “Modei Sailing Craft” “M” Saida McDonald 54 _ “M” Polaris Andrews 52.3 per cent. “M” Windsong “M” = Mithras Designer Tucker McDonald Position of C.L.R. as Percentage of L.W.L. 55.5 per cent. 54.4 per cent. 54.3 per cent. 10R. Selene (Pre-war) Tucker 10R. Diamond Daniels & “A” Thisbe (Pre-war) “Marine Models” (Sharpie) Tucker design per cent. 56.25 per cent. 54.7 per cent. 57 _ per cent. From these figures it would appear that postwar practice largely favours about 54 per cent., and the budding designer would certainly be on safe ground in accepting this. placing. Now the shape of the canoe body of a normal hull does not greatly influence the static position of the C.L.R.; the determining factor is the shape and placing of the keel fin (and skeg), and we will now consider this matter. radius, This obviates undue cutting away of the forefoot; it also avoids the necessity of the lower water lines of the lead being given an unduly blunt entry. The Positioning of the leading edge of the Keel This is of particular importance; indeed, some well-known designers appear to regard the actual position of the C.L.R. as relatively unimportant provided that the leading edge of the fin is correctly placed. For example, in Part 3 of an excellent article entitled “Model Yacht Design” (MODEL Maker, April, May and June, 1954) Mr. John Lewis stresses the importance of the angle and the correct placing of the leading edge of the fin, but considers the shape and position of its after edge to be relatively unimportant. In speaking of the advantages of keeping down fin area to reduced wetted surface drag he says: “However, it has been my experience with every 10 Rater that I have owned that a good hefty slice off the after edge of the keel has resulted in improved all-round performance. Not all these boats were of my own design or build.” Incidentally, Mr. Lewis does point out that excessive reduction of the lateral area can allow a model to sag away to leeward, and obviously moderation is necessary. Readers will appreciate that any cutting away of the after edge of a keel, leaving the leading edge unaltered, must result in an alteration of the position of the C.L.R. This clearly does not worry this eminent designer, so long as the leading edge The Importance of Fin Design Well, it is hoped that a little light has so far been cast on our problem, and in proceeding to consider the keel we must first return to the very important matter of the fin’s leading edge. The question as to the angle of the leading edge is very simple, so let us dispose of this first. Experience has shown that if the fin’s leading edge is at too steep an angle to the L.W.L., there is a very rapid shift forward of the C.L.R. as the yacht gathers way, and this will cause the boat to “gripe” badly. If an angle of something like 45 degrees is adopted this trouble will not arise. Similarly, too steep a leading edge to the skeg should be avoided, but this does not appear to be so critical, and angles of up to 60 degrees are often employed. It used to be thought desirable to provide the keel with a bottom edge that was well raked, and to blend the leading edge into it with a nice easy curve. More recent tendency is to make the bottom edge horizontal, letting the leading edge join it by a curve of very small 70 of the fin is correctly shaped and placed. It is of interest that Mr. H. E. Andrews (designer of Polaris, etc.) is also definite on the importance of the placing of the leading edge of the keel, but considers the length of the fin (fore and aft) to be a matter of opinion. He then says: “The fin determines the resistance to leeway, but it is not its area which does this, but the dynamic build-up of pressure on the leeward side of its leading edge when the boat is in motion” (MoDEL Maker, September, 1952). Whilst it is clearly necessary for the C.L.R. to be reasonably placed, we now begin to see that its exact position is far less important than the correct placing of the leading edge of the fin. The vital point now is to determine where this placing should be, and Mr. Lewis advises that the leading edge of the fin should start at a point 30 per cent. along the L.W.L. Mr. Andrews adopts another method by placing the forefoot of the fin about lin. forward of the mid-section, the fin’s leading edge being at an angle of 45 degrees to the L.W.L. This FEBRUARY, seems a quite different approach, but it will be found to give a similar result. The area and positioning of the skeg will, of course, affect the position of the C.L.R., and it is assumed that the skeg will be of normal size and positioning. A combined area of about 12 to 14 sq. in. for the skeg and rudder will probably be chosen. The Area of the Fin Reference has already been made to the variations of the fin area by the alteration of the shape and position of its after edge, and we now have to consider what will constitute a Satisfactory fin area. As with most points of design, we are faced with the necessity of compromise. On the one hand, an unduly small lateral area will encourage excessive drift to leeward, and make the model tricky to handle. Conversely, too large an area will considerably increase wetted surface drag, which is a considerable factor at the compara- tively low speeds possible in light breezes, when it can represent as much as 75 per cent. of the yacht’s resistance to forward movement. Many designers understandably tend to “play safe” by being fairly generous in the matter of lateral area—a view which is supported by Mr. Lewis’s experiments, already referred: to. If a model yacht will habitually be sailed on sheltered ponds, where light breezes prevail. here is a point worthy of special consideration. In turning to concrete facts and figures on this matter much help may be derived from the study of existing designs. The following examples should prove of interest and, although they are only approximate figures, they do serve as a basis of comparison. Class Name 10R. Designer Diamond (Sharpie type) Area of Keel (Fin only) “Model Sailing Craft” 127 sq. in. *10R. J0-11 Tucker 120 sq. in. *10R. Hyperion Lewis 106 sq. in. *10R. Mareelll Wilcock (Sharpie type) 83 sq. in. 1959 ot ATSO%OFLW.L. | 26% OF LW.L. gece ee Fig 7 Profile with leading edge too nearly vertical and too far forward. This boat would “gripe” badly. (Diagram exaggerated to illustrate point.) adopting the considerably reduced figure of 106 sq. in. This, of course, ties up with the record of his past experience in lateral area reduction. Finally, we meet the surprisingly small figure of 83 sq. in. given by Mr. Wilcock in Maree II]. The writer is extremely interested in this MopEL MAKER design, and hopes to build her during the coming winter. At first glance this fin area does seem very inadequate, and your humble scribe is still a little tempted to consider increasing it to about 93 sq. in. by suitably extending the after edge of the fin. Of course, it may well be that Mr. Wilcock has deliberately gone to the limit in reducing wetted surface area with a view to imparting good light weather performance to a type of hull GLR. AT 50% re ee | ‘ | 30% OF LW.L. ‘ABOUT 45° 60° OR LESS Fig 2 Length of fin, and placing and area of skeg, should be adjusted to put C.L.R. about about 54% of L.W.L. Adherence to the simple rules depicted will save needless disappointments. which is normally best favoured by a stiffish breeze. Whatever his reasons, the fact remains that Mr. Wilcock has been very successful with this design, which was runner-up in the 1956 Metropolitan and District M.Y.A. Championship, and 1957 winner of the President’s Cup of the London Model Yacht League. Bearing in mind that she is a sharpie, a type considered inferior by quite a few, there can be no doubt that she is a great success, and does her designer much credit. It is interesting to speculate as to whether the views of Messrs. Lewis, Wilcock, and Andrews on lateral areas portend considerable changes in the previously accepted standards, *MoDEL MAKER designs. (Continued on page 89) The considerable variation in these figures is a little surprising, but very informative, as they indicate differing points of view. First we have two boats by Daniels and Tucker, both first-class designers of great experience, and in these designs they favour areas of 120 sq. in. Fi ¢ A—AREA ABOUT 120 SQ, INS. B—AREA ABOUT 90 sa.’ins. XN or over. as Then we find John Lewis, regarded by many today’s outstanding 10 Rater designer, Long type fin “A” gives great directional stability, coupled with larged wetted surface. Modern small fin “B” offers reduced surface drag, 71 MI FEBRUARY, the surplus market. The meter, a beautiful instrument, cost 2s. 6d., and the switch 6d. The meters being of the thermo-couple type will not withstand excess current without burning out, so care must be taken to see that the battery is connected up with the switch at the position of maximum resistance. My particu lar meter reads to 350 mA. and the positio n of the 1959 of 1.250 specific gravity and allow to stand for about 12 hours. The plates will absorb some of the acid and the level should be made good. For a first charge the cell should be on for about 48 hours, after which the acid should be poured off and fresh gravity poured in. acid of 1.250 specific needle when one of the cells described is on charge, should be a 150 mA or 0.15 Amp. For the first charge with new plates, fill the cell to the top of the plates with sulphuric acid With old plates add the acid and charge until the cell shows signs of gassing. Cells made as described will be found to give excellent service provided they are not asked to give out current in excess of their capabilities. ( “I UCKER’S marks, though in salt she is on them. It would TOPICAL AND SALT DiSPLACEMENT TALKS es a ON FRESH oy, O NCE again we look forw ard to another season, 19 My Own case from rather a different viewpoint. 1958 marked a turning point in my life, as during it I retired from business and I have recently move d to the South Coast. Moving house, and all it involv es, has somewhat detracted from the amoun t of time I have been able to give to matters appertaining to model yachting. However, I hope that once settled in, I shall have more leisure. I also had the misfortune to spend much of December in a nursing home, suffering from a severe attack of bronchial pneumo nia: these circumstances combined to prevent my providing the usual “Talk” in the January issue, for which I must express my apologies. * A * conversation with * a leading skipper at Gosport during the A-class Regatta Week last August, leads me to put forward the following tentative suggestion for general consideration. As some lakes have salt water and others fresh, boats from inland clubs are above their normal sailing waterline when they visit seaside ponds, and vice-versa, As a result, visiting yachts are often out of tune and at a disadvantage. Now the difference between the displacement of a boat in freshwater and salt is approximately one-thirty-sixth. In the case of a 54 lb. A-class, this is 14]1b. Since measurement is taken in salt water, this means that the yacht is below her marks in fresh. It takes 1lb. approx. to put an A-class down 1/16 in., so in fresh water she is almost }in. below her be a simple matter, however, for boats to have an internal weight equivalent to exactly 1/36th of their total displacement (and included in it) to be carried exactly over the boat’s C.G. (as is done with the 2 1b. Crew Weight used in the I.Y.R.U. 6-m. model). If it was made compulsory to carry this weight on salt water but remove it on fresh, boats would always sail on their measured L,W.L. length. Although I have dealt with A-class, this really applies with equal force to all other classes, including those which do not have their L.W.L. measured, and those measured in fresh water. It is, in fact, a simple method of ensuring that boats have the same sailing waterline in both salt and fresh water. Against this idea, it may be claimed that full-scale craft go from fresh to salt water without change of weight, but it must be remembered that as a rule, these migratory boats are light, planing types, and dinghies. Racing for full-displacement types of yacht in this country at all events, is almost entirely on the sea, or in estuaries where the water is to all intents and purposes salt. Hence the problem only concerns models, and is a matter model yachtsmen can settle for themselves. Mention was made of the LY.R.U. 6-m. model class, and the 2 lb. crew weight which is added after measurement has taken place. In this respect, this class is unique because it follows full-sized yacht practice in measuring the craft before the crew is put aboard. In other model classes, no allowance is made for a crew. When the A-class was instituted, it was intended that these boats should be 1/6th scale models of a 6-m. class to the Y.M. Cup formula. Had a crew weight been included in the rule, it would have meant 34 1b. Perhaps it is as well that this was overlooked, as it would have meant even larger yachts than we now get! MI mL MODEL MAKER THE AMERICA’S CUP Proposal to use sailing models to test designs discussed by N. D. Hatfield, Hon. Sec., Y.M.6m.0.A. AS long ago as 1930 an article appeared in The Model Yachtsman under the heading ‘‘The relation of the model to the full-sized yacht’’, written by the most famous of all model yachtsmen, Mr. W. J. Daniels, in which he stressed that much useful information could be gained by comparing the sailing performance of models of prototypes built to various designs, but the same rating rule. He further claimed that the pecularities of the model and the full-size are absolutely parallel with each other. However, designers of full-size yachts have been slow in taking advantage of this, but there were exceptions, and Major Malden Heckstall-Smith and Admiral Turner proved without doubt that the study of model yacht performance can be of very great assistance to full-size designers. Nevertheless, since the war emphasis has been placed on the value of tank tests, and the sport of Model Yachting has never received proper and just recognition from either full-size designers, yachtsmen, or the yachting press. The daily press has treated it light-heartedly, and the general public are left with the impression that, far from being the serious and scientific sport that it is, it is a pastime for boys and old men. The struggle to achieve better recognition and publicity has been long, hard, and discouraging, but the complete failure of Sceptre’s challenge last September caused many people to doubt seriously the value of absolute reliance on tank tests, because it is very difficult to simulate the action of wind and wave on the model. A well-known and successful designer of many Ocean Racers, Mr. Guy Thompson of Burnham-on-Crouch, wrote a challenging letter to the Yachting Monthly in which he stated that, before building to one of his designs, he always built a model to the scale of 1} in./1 ft., and sailed it on the River Crouch. In every case the prototype displayed the same characteristics and performance as the model. He suggested that designs for future challengers should be so tested, and proposed the formation of an association of amateur designers for the sole purpose of producing a new challenger. This letter created a great deal of correspondence, some of which was published, including a letter from Mr. B. H. Priest, who, as one of our leading model yacht designers, is well qualified to support Mr. Thompson. Meanwhile, Mr. Daniels has produced a 12-metre design which he sent to Mr. Hugh Goodson, Chairman of the Sceptre Syndicate, and the “Big Boys’’’ interests must have been aroused because Mr. Uffa Fox, as President of the Y.M.6-m. Owners’ Association, telephoned to say that Messrs. Owen Aisher, Hugh Goodson, F. G. Mitchell, Charles Nicholson and others wished to have 12-metre models built, sailed, and tested at the Rick Pond, Hampton Court, using the Club’s premises as their headquarters, and that he would present a cup for the winning design in a series of test races. This idea was followed up rapidly by a meeting which took place at the National Boat Show on 5th January between Uffa Fox and representatives of the Y.M.6-m.0.A., at which it was agreed to proceed with the idea, and to invite any designer or builder of models or full-size yachts, amateur or professional, to take part. It was emphasised that the best scale to adopt would be 1/10th full-size, which would produce a model the same size as the ““A’’ Class, but professional designers and builders are not accustomed to using this scale, and as nine 1/12th scale models are already in existence, this scale may be chosen instead. The nine models, which were built for tank testing, include Sceptre, and she would, of course, prove useful as a trial horse, but it will be a pity if the larger scale is not selected because it would prove better for testing purposes, and although the “‘A’’ Class is not the same Rating Rule, it would be interesting to compare their performance with a 12-metre. The old 12-metre model class to 1/12th scale is extinct and was unpopular because of the complicated rule. A modern 12-metre model to this scale would be about 70 in. L.O.A., 6 in. L.W.L., and therefore easy to transport, but its revival as a Class could hardly be expected in view of the popularity of the simple ‘‘Marblehead’’ Class. The time factor is all-important, and at the meeting it was suggested by the model yachtsmen that trials should take place over a period of not less than one year, with experienced model skippers sailing the models in turn. But it is understood that at least three of the “Big Boys’’ wish to start building challengers in September, with a view to a challenge in 1961. We feel that this is too soon, and that the trials should not be hurried. But International prestige and big money are at stake, and there is a feeling that the Swedes or the Australians may get there first if we delay too long. However, whatever happens, this opportunity may gain for Model Yachting the recognition and publicity it has long awaited and so thoroughly deserves.
