- Description of contents
July 1983 80p_..< ~ (LR. £1.15 %, U.S.A $3:26) De MAGAZINE Build this. Re Launcth eam Launc * ~ com oy with Twin ies sto Acting Steam Engine “School Workshop Destroyer. Unfortunately the hull factors required for Vic Smeed’s early planing are ones which create considerable drag when the hull is in the displacement condition, and on the other hand the best shape for best displacement speed will be unstable if driven hard enough to start planing. A planing hull will almost inevitably be a bad sea-boat at low speed, which is one ODEL YACHTING has been going on for some time — it is undoubtedly the oldest form of working model — and apart from occasional jumps it tends to progress fairly steadily. By ‘jumps’ is meant the introduction of such things as Braine gear, vane gear, synthetic sail materials and radio control, each of which has had a far-reaching quick effect on sailing models. and relatively The fact that in between such innovations progress is made in performance etc. may surprise non-yachtsmen, given the centuries of development in sail propulsion which have elapsed. In fact, of course, progress is fairly slow and the difference in performance between, say, a 25-year old A Class yacht and a modern one is in general terms slight, and even a 50-year old boat would not be far behind, while in some circumstances it could well beat a modern model. Perhaps it is unfair to pick the A Class, where a remarkably good rule has tended to keep performances reasonably even; in the lighter 10-rater and smaller Marblehead classes, which are both less restricted, designs today’s are very different and performances are higher. One of the reasons for progress is a wider understanding of the forces acting upon a sailing vessel, particularly in the realm of aerodynamics, and another is the change of emphasis apparent in full-size sailing/racing. No longer are a few large and expensive yachts built each year, but thousands of smaller craft, and the more people there are actively concerned with a sport, the faster new ideas are tried and the better are the chances of improvement. What happens in full-size yachting influences models (e.g. the Bermuda rig) and, to an unsuspected extent, model developments _ influence full-size craft. Many of the famous full-size designers are or have been modellers and they tend to keep an eye on what's happening with models. In Part Two Vic Smeed discusses volumes and displacement for racing are approaching model functionalism, since there is a tendency for accommodation to be as rudimentary as the particular class rules will allow. Modern 6metres are pretty stark, for example — just a shell with a light deck and an open cockpit with the minimum of ‘furniture’; they must be very uncomfortable! Note that no mention has been made of any dinghies or suchlike smaller yachts. This is because all model yachts are basically displacement designs, relying on ballast carried low down for stability and sail-carrying power. A dinghy relies for these very largely on the crew, whose total weight might well exceed that of the yacht and who are (is, for one-man boats!) able to dispose that weight to the best advantage, which may mean ‘sitting out’ or hanging on a trapese, sitting at the stern to encourage the hull to get up and plane, and so on. A displacement hull moves through the water, continually displacing its own volume of water and maintaining a constant average waterline. A planing hull is one in which, due to the shape of the hull, forward movement creates lift, so that as the speed increases so the hull lifts out of the water and planes across the surface. This situation is not confined to ‘class’ racing models, since innovations which become common practice in racing spread into non-racing models, even if only because the conception of what a yacht should look like changes as full-size or racing model yachts develop. Fifty years ago most people were likely to think of a yacht as a vessel with canvas from bow (or bowsprit) to counter, influenced by the huge J class which got most publicity. Now the conception is more likely to be a vessel with a hull longer than the foot of the sail plan, albeit with a enormous spinnaker. genoa and/or an Many newcomers are prepared to accept a strictly functional model as the norm, but a far greater number hanker to build what they conceive to be a yacht, which usually means some representation of accommodation, even if it is only a cockpit. In this respect, full-size yachts designed primarily 374 Fig. 1 reason for dinghies getting into trouble when in unfavourable sea conditions with too much wind to be able to plane safely. On the other hand, a displacement hull will normally be sea-kindly, and it is possible with subtlety to introduce sufficient hints of planing factors to allow the hull to plane under the right conditions, without too seriously affecting its sea-keeping qualities. It is in this area that much of the improvement in performance of models has been gained over the last twenty or thirty years. Some of the main features aiding planing are light weight, a flat, wide and shallow hull, broad at the stern, and a minimum of rocker, which is the curve in the bottom shape of the hull seen in profile. Off the wind — that is, travelling downwind — such a hull might well be extremely fast, planing in quite a light wind, and it could well do the same on the reach, i.e., across wind. However, its windward performance (beating towards the wind direction) is likely to be poor because normally a yacht needs to heel to a sailing angle, which might be an unsteady one with a flat, wide hull, and because to sail well when heeled a yacht must have hull balance, which with a broad flat stern would be difficult to achieve. If you were to add a mast and fin to a cylinder of wood, however much it helped the cylinder would still float on the same waterline because as it heels the volume immersed would be equalled by the volume taken out of the water, Fig. 1. The wedge- shaped sections are in fact called the inwedge and out-wedge, and, since the weight of the model doesn’t change (ignoring any slight downward pressure Calculating the volume can be carried out from the sails) it will simply rotate about its by various means, and there are estab- centre-line and the in-wedge is equalled by the permitted sail area. lished procedures such as Simpson's Rule the out-wedge. in naval architecture. Any procedure, how- Of course it is possible to ‘design’ and build a yacht by eye and let the arithmetic take care of itself, but it is rare that such a Now consider a rectangular section hull ever, requires the rather tedious stage of (Fig. 2). As this heels, the wedges are not working out the below-water areas of hull equal; the in-wedge is greater than the out, sections — usually the load waterline is and since the weight of the yacht hasn't divided into 10, i.e., 11 stations, each end changed, it will rise in the water until the one being of course zero, leaving 9 to be principle attributed to Archimedes once worked, Fig. 3. In the absence of a plani- more obtains. If you remember, this says meter (a tool for measuring areas) the usual that a floating body displaces a volume of method is to trace half of each section (no water equal in weight to its own weight. So point if a change of position of the body (the hull) squared paper, either join. or 1cm/mm, tries to immerse a greater volume, the body and will rise until the new immersed volume Whole 1in. or 1cm. squares are totted up and noted, then the smaller squares in displaces the same weight of water as before. in calculating both sides!) on to simply count the included squares. broken whole ones counted, Fig. 4. Any Note that our rectangular hull or block square will rise equally along its length, i.e., its cut by the section line showing immersed volume is evenly disposed along more than half a square is counted, say less than half ignored. It is in fact quicker than it its length because its shape is constant. It sounds and surprisingly accurate. its centre of buoyancy The volume is worked out by multiplying (C.B.), which will be in the middle, and its ends will obviously balance each other. The will pivot about the sum of station O and station 1 by the distance between them, then stn. 1 plus previous cylinder will not rise, but its C.B. will clearly be central and its ends stn. 2 x distance, and so on. As only half of each section has been totted up, adding 1 balanced. It is possible, incidentally, to have and 2 gives the average area of the piece of a the hull in distance hull wedge cross-section exceeds the in which in-wedge, outwhich case the hull will settle lower in the water until volumetric equilibrium is reached. Consider cylindrical now front a hull half which has a merging between the stations, times the between stations gives the volume of that piece of hull. Add up all the separate pieces and you have the total volume. Most of the architectural rules are into a rectangular rear half. When this heels, the variations on this simple procedure. fore half will want neither to rise nor sink. obtained by using a factor known as the An approximation of volume can be The rear half, however, will tend to rise. The block coefficient. This is the ratio between result is that the bow will be forced down. actual volume and the volume of a block Lx This is what is meant by hull balance — or, B x D, Lbeing waterline length, B waterline in this case, beam and D depth of hull from waterline to hull bottom. The block coefficient will vary imbalance — and in these circumstances it doesn’t take much to see that an unbalanced hull won't be as fast as a similar hull which /s balanced. There are according to the fullness or fineness of the other effects which can form the subject of not future discussion. efficients of one or two published designs A few lines back Archimedes’ Principle hull, but for a yacht will be around -5. It is difficult to work out the block co- of similar shape to one intended, as all that was mentioned, and for those who have is necessary is to know the L, B and D and never displacement of the considered how the weight of a published example. vessel is calculated, it is basically quite a Multiply Lx B x D (in inches) to get the block, simple process. If a floating body displaces and the actual displacement x 27.7 to get the actual volume; divide the actual into the block to give the coefficient. a volume of water equal to its own weight, all that is necessary is to calculate the immersed volume of the hull (i.e., the same as the volume It may all seem a lot of trouble to establish of water displaced) and the displacement, but if the yacht floats on convert this volume to weight using the its designed waterline, the calculations in weight of water in the calculation. If working in Imperial measures, the volume in cubic inches is divided by 27.7 and the weight towards the end of construction and result is pounds displacement. Most older know English-speaking indifferent modellers still think in inches and pounds etc., but the calculations are if anything easier in metric (ccs. and gms. etc. if more familiar). regard to balance are valid and it will be possible to adjust the amount of ballast that many possible performance reasons have for been eliminated. If the design is to a class rule (other than 36in. Restricted or Marblehead) length, draught and freeboard and thence model is successful unless it is designed by someone of considerable experience who has used the mathematical approach suffiently often to know exactly what he is about. If a design is based on an existing yacht, model or full-size, then there is perhaps more chance of success, except that there between are what essential makes Fig. 3 | } iva —— 2 —} a a LWL 6 7 10 7| full-size will be right, but there is a minor snag here in that something may look right to an inexperienced eye which clearly looks wrong to anyone with experience. From the figure given earlier it will be realised that 27.7 cu.in. of water equals 1lb. weight. This is fresh water; most people know that it is easier to float in salt water, and the displacement of 1lb. in salt (sea) water is 27cu.ins. In other words a hull calculated for fresh water will float fractionally higher in salt, although the difference is slight enough to be ignored in model sizes. However, if working toa rating rule, the difference between 27.7 and 27.0 as factors has a significant effect on calculations and the fresh water figure is therefore usually specified. If we take a yacht of, say, 30in. load waterline length and 6in. max. |.w.|. beam, the horizontal area on the waterline will be perhaps 110 sq.ins. Adding an extra 1lb. weight will sink the hull until 27.7 cu.ins. more have been displaced, i.e., 110+ 27.7 means that the hull will sink almost exactly Y,in. lower. If 2lbs. excess is added, the yacht will be about ,in. lower in the water. Because the hull shape changes with depth, as a glance at the sections shows, the in-wedge/out-wedge relationship will alter, so that what was a balanced hull on its correct waterline is likely to become more and more out of balance (even though it still floats level) the further from the designed weight it becomes. If, therefore, you built a kit, or to a plan, and, say, instal radio heavier than allowed for, the perform- ance of the yacht will suffer unless weight can be saved elsewhere, or the ballast weight reduced so that the finished model is very close to its designed displacement. This particularly applies to a hard chine hull, where the sailing characteristics can alter quite dramatically if the model is overweight or, indeed, under-weight. (To be continued) Squares more than 50% counted 0 differences good vessel and what makes a good model. It is said sometimes that if a thing looks right it incorrect displacement will affect waterline _,_. a in A and RA, 10r and R10R, and M and RM. el LOGBOOK club secretaries were provided with master certficates from which photocopies should be taken Model Yachting Association News people reading different things into the same TC. are two new publications now available for all those anxious to keep up to date on the rules. One is simply a record of amendments to the Radio Racing Rules, following R.Y.A. changes and specific alterations to model rules. Rather than reprint the entire radio rules — an expensive words. What we have now are three sections each based on the appropriate areas of the earlier rules, but eliminating ambiguity and taking into account all the alterations which experience over the last few years has shown to be desirable. The opportunity has also been taken to clear away procedure, both for the M.Y.A. and individuals wishing to buy them — the amendments are set some of the dead wood and some of the unnecessary additions which have found their way in, in recent years; for example, in vane racing, the business of not steadying a yacht on a new out in a form allowing existing rules to be struck out and the amended ones pasted in. All sets of R.R.R. sent out in future will include the amendment sheet in the £1 purchase price; possessors of current sets, which should bear the date '1980° in the bottom corner {and probably include a yellow separation sheet) can send 50p for the new Competition Rules, Vane tack by applying the pole to the quarter seems to have got lost, though perhaps this is covered by “no other part of the yacht, sails or gear may be touched unless she is stopped by hand before tacking.”’ Inevitably it will take a little time for skippers to shake down to the new wordings. The Competition Rules do in fact cater for both Racing Rules and Rating Regulations, which asa vane and radio, but where there has been a grey package replace the Sailing Rules and General Rating Regulations. The earlier rules and regulations, established in pre-radio days, were alittle confusing (or could be) when attempting to apply them to radio yachts and the conduct of radio races, and attempts to widen their scope were area amendments. The second publication is a more elaborate affair, since itis a containing the 16-page booklet with card cover it has been made clear whether a rule applies to all boats or events or specifically to radio or vane. Hopefully anything clashing with conduct of races under the separate radio rules has now been eliminated. One minor quibble in the Rating Regulations is that ‘all Certificates shall be upon forms obtained from the M.Y.A. Publications Secretary.” In fact, clearly likely to lead to a tangled mass of verbiage with, inevitably, the possibilities of different of blank certificates for completion by club measurers/secretaries. The idea of this was (and is) that it saves the M.Y.A. paying for extensive printed stock, saves correspondence and postage and avoids a skipper having to wait to register a yacht because the club has run out of forms. It is still necessary for clubs to obtain Declaration Cards and 36R and 6m certificates from the Publications Secretary, incidentally. As mentioned in this column several issues ago, it is now possible to certify a yacht in more than one class. The obvious example is a Marblehead which could conceivably be registered as M, RM, 10r and R1OR, some possible O.0.Ds. who would not accept many of today’s bumpers. Generally speaking, this rearrangement of rules should be a big help in reducing uncertainties. It also includes some points which racing skippers should take aboard, so whether you sail as a casual skipper or a keen competitor you should bring your rulebook up to date by sending for a copy. Price is £1 including post, or £1.20 to include the amendments to the Radio Rules, from the M.Y.A. Publications Secretary, V. Smeed, P.O. Box 6, Rickmansworth. Herts. WD3 4RE; remittances payable to the M_Y.A. 1983 MYA Regatta Calendar Event 12 12 12 18 19 Mayoral Cup Drury Anchor Broadland Trophy Builder Cup Mickey Finn Nat. Championship 19 19 19 19 26 26 Little Portugal Cup Red Rum Trophy Model Boats Trophy Hinds Cup Met. & South Dist. Championship Northern Dist. Championship 26 26 26 26 Hove Corporation Trophy Whyte Trophy ‘Tucker Trophy Open Event Northern Dist. Championship and Northern Team Championship control. Many thanks to Sandwell Council for their enlightened approach to our sport. Thanks to Messrs. Selby and Cooper, Redhouse race control is now very slick indeed. To explain; Dave Selby has spent many hours programming his Sharp computer to control all the events held at Redhouse with instant compilation of best times, average times and points scored. FSRE lap scoring can be completed if necessary without any lap counters, however under Naviga rules this is not possible. Moving away from Redhouse, congratulations are in order for Cwmbran club who successfully staged their all-electric and second leg of the FSRE national championship. With the inclement weather it became very difficult to organise, however the event finished on time and how nice July 1983 Class Venue Fleetwood A Bournville M RM _ Broads M Mickey Finn A RM_ RM RM _ M 36R = R10R_ RM RM RM A Birmingham 3. 3 3 9 M.Y.A. National Championship Spastics Cup Open Event Midland District Championship & MacDonald Trophy Met. & South Dist. Championship Bournville 10 Gosport Cleveland Woodley 10 Northern Dist. Championship 10 10 17 17 Turner Rose Bowl Open Event Met. & South Dist. Championship Northern Dist. Championship & SE. Essex Clapham Birkenhead Hove & Brighton Northern Team Championship & Fred Parkes Trophy . Milton Keynes 17 Leicester 17. Chippenham 24 ~=Wilkinson Sword Trophy ~=Avocet Trophy Met. & South Team Championship Fieetwood ‘Thornhill Trophy 24 24 24 : One area which has defied a number of attempts at resolution is what constitutes an efficient fender or bow bumper. A yacht is now required to have ‘’an efficient fender on her bow, which must satisfy the Officer of the Day.” Again, interpretation may take a while to settle; there are R36R RM _ Chelmsford Gosport Duplex Chiltern 575 Birmingham 10R Hove & Brighton Mickey Leeds & Finn Bradford 36R Bournville RM RM A M South Wales Gosport Cleethorpes RM _ New Forest RM _ King’s Lynn RM Woodley Windsor Trophy R1OR_ Fleetwood Nylet Trophy R1OR Woodspring it is to race on the 60x 30 course. Incidentally, itis pleasing to see more new faces especially the younger element who after all are the future of model boating in this country. One final point raised concerns the problems of restricted racing, vis-a-vis motor, props and selected cells. People are becoming disenchanted with some aspects of restricted because of the ever-increasing specialisation of this class. Many think it high time some form of code governing the use of cells and props be introduced; perhaps some form of price restriction. This would eliminate the problem of super selected cells and highly developed commercial metal props. Naturally, there are those who would argue that development would be stifled and it would be less interesting because of the lack of inventiveness. Besides, there would RM __ Leicester always be a top dog, or so the argument goes. 1983 must see some form of consensus on the future development of restricted, for instance plastic props only? On the international front it has been suggested by a number of people well established in racing the European Scene that the Euro electrics are now well favoured fortheintroduction of a Driver's Championship, Grand Prix type, and worked ona similar system to Grand Prix racing. Whether this would be endorsed by Naviga is another matter however the international circuit is now very well established with over six countries holding all electrics. Logistically it would work, commercially it would be interesting to speculate, whether it is internally viable that’s another ball game. See you next time. 381
