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< + Dutch Yacht U.S.A.& CANADA $1-25 @ R.A-F. launches © _ Plastic boat-building Siti D> HOBBY MAGAZINE JUNE 1975 In the Tideway A eee scheme thought up by a Fleetwood MYPBC member could be attractive to all car Owning yachtsmen. It is a windscreen sticker (plastic film type) about 3% in. across, in blue, black and white, as pictured on the right. We offered to mention that individuals could be supplied with them at, say, 15p each including post, balance to regatta funds, giving the address; the result is that the Editor received a supply to sell under those conditions. … So if you want one, make a 15p P.O. out to Fleetwood MYPBS and send it to Model Boats. Contents Three small points we should mention. First, we haven’t discontinued “Introducing * which will resume again shortly. Second, the only currently required alteration to the Class A rule is the fresh water measurement; the others on page 237 of May issue are still pending. And third, in the epicyclic vane gear (p. 187 April issue) the output from the annulus is taken off the bottom vertical tube of the annulus assembly, and not off the rod on which the annulus pivots. A badly placed arrow in the sketches might give the wrong idea although it is fairly self-evident. Club and Regatta Notes Radio yacht regattas at Fleetwood on Saturdays are open to any visitor with any class of R/C yacht wanting racing experience; remaining dates are May 25, June 7 and 21, July 12, August 2 and 23, September 6 and 20, October 18. Certificated models from affiliated clubs are eligible for RM open races on May 26 and August 31, and the Northern District RM Championship on June 22. Details s.a.e. to G. C. Burgess, 4 Osborne Road, Altrincham, Cheshire, WA15 8EU. Cotswold M.M.C. is running a RC multi and Naviga regatta on June 8 at their water at the Cotswold Marina, South Cerney, Glos. The water is not wadeable but two rescue boats should be available; the water is about 100 x 300 yards. The club, incidentally, goes in for multi, scale, yachts, hydroplanes, and experiments, has a friendly membership willing to give or take advice, and would welcome anyone interested. Secretary is D. A. Lucoque, 4 Trelawney Avenue, St. George, Bristol. The Town and Country Show at Stoneleigh, near Kenilworth, over the August Bank Holiday (August 23, 24, 25) has asked Coventry M.B.C. to put on again a model boat display, working in a specially constructed pool and static on an inside stand. The club will run some type of speed event but needs the help of other boaters for scale. Anyone who can demonstrate on any or each day please get in touch with P. Heath, The Strettons, Church Lane, Fillingley, Coventry. An offshore event is planned for September 14 by Manx M.B.C., in Douglas Bay, Isle of Man. Anyone welcome. Details from B. Hooper, 8 Hamilton Close, Lower Foxdale, Isle of Man. This is the third such race and promises to be bigger and better. All forthcoming events of Mid-Essex Boat Modellers will be held at a new venue, Millbeach Hotel, Heybridge Basin, Nr. Maldon, Essex (on the B1026 MaldonGoldhanger road) and not at Doctor’s Pond. R/C scale June 8, R/C steering and novelty September 14, both 11 a.m., 15p per boat. Boat secretary of Keighley D.M.E.S. is now D. pur 25 Rockwood Road, Calverley, Pudsey, near Leeds. 275 New club is Bolton District R/C Sailing Club, covering wind, electric, and steam power, operating every Sunday afternoon, with shield races alternate Sundays. No i/c engines due to environmental restrictions. Details of membership (we hear there are already 27) and water etc. from the secretary, A. Roberts, 2 Caldbeck Avenue, Heaton, Bolton, Lancs. Change of secretary in Coventry D.R.C.M.S.; new one is D. Allen, 156 Middlecotes, Tile Hill, Coventry, CV4 9AZ. Change of date for Dover M.C. annual regatta—it has been brought forward from July 27 to July 13. Other details as regatta list in March Model Boats. Not perhaps quite a club announcement, but it is worth mentioning that the regatta being sponsored by Cleethorpes Council on June 21/22 is open to virtually any boat. Even if you don’t enter any races, your attendance with model(s) would be welcome; there will be a chance to have a spot of time on the water and there should be lots of interesting boats to see and jaw about. A special prize for the most interesting or attractive model present, even if not officially entered, is likely. R/C Sailing Points At the last M.Y.A. R/C Committee meeting two decisions were taken which radio yachtsmen will wish to note— Identification of Boats while Racing. It was agreed to adopt a system of brightly coloured flags made of thin polythene, measuring 6 in. x 4 in. minimum, which would be clipped high up on the backstay or at the masthead with crocodile clips. Colours are not related to frequencies and a flag would be collected by each competitor immediately prior to a race and at the same time as he takes his frequency peg. Split Frequencies. It was agreed that, provided they can be obtained, crystals with a separation of 25 MHz from the six spot frequencies should be used, rather than a 20/30 split. This is a recommendation only. Incidentally, any radio skipper planning to take part in a Naviga R10r race should note that the Naviga yachting committee has never adopted the alterations to the 10-rater rule made in 1968, intended to make sail area measurement fairer; the old 6000 factor is used in the formula, and the roach is measured. Any I.M.Y.R.U.rated boat would be likely to have too much area. John Cleave passes this information on, having discovered it while preparing to compete in Vienna. JUNE 1975 HE observation that deep keels on small sailing models are very important, made in the recent Splinter article, is quite correct, but the explanation suggested leaves something to be desired. It is true that the water deeper down below the surface is better at resisting leeway than the surface layers, but the analogy made with layers of carpet does not hold. The reason lies in the basic definition of an inviscid fluid (of which water is a good approximation); fluids cannot resist shear forces. This means that if the top surface of a fluid is pushed sideways the layers below it do not follow. In actual fact all fluids have a certain viscosity and are therefore viscous fluids and when a top layer is moved the lower layers tend to follow. This is best represented Fin Depth A deep fin does improve sailing qualities of small models, but how or why? Graham Bantock joins the discussion with interesting comment by imagining the fluid next to the hull surface. As the hull moves the particles next to it travel at the same speed and give some motion to the adjacent particles, which in turn have some effect on those next to it. This is the way in which a boundary layer forms. Eventually at a large distance from the hull the fluid velocity is zero (Fig. 1). Where the velocity of the fluid drops to within 1% of the hull speed it is said that this is the thickness of the of the orbits decreases with depth quite rapidly. If a wave of say two feet is travelling on the surface in deep water one only has to go down a few feet and the amplitude of motion experienced by, say, a model submarine would be only a fraction of two feet. Where the depth of the pond is small, i.e. it is necessary that there is no vertical motion at the pond bottom, the motion at increasing depth decreases even quicker. In fact the shallowness of a pond inhibits the formation of high waves, and in this respect can cause vast increases of model resistance around critical speeds, dependent on the length. Obviously the calmer the water that a keel operates in the more efficient it will be and because of the rapidly changing conditions near the surface shallow keels (shallow meaning around the same size as the wave height) Boundary Layer. In theory the velocity never drops to zero, however far from the hull one goes. This means that if we swap the hull surface for the air/water interface the water at the top of the pool has a velocity equal to the velocity of the air at the interface. The water surface has the same effect on the air moving above it, i.e. the lower “layers” of air are retarded in respect to the higher layers. Fig. 2 takes a cross section through the air and water near a pond surface. The thickness of the boundary layers for air and water will depend on the speed of the wind and also on the distance which each has travelled from rest, i.e. the distance travelled since the B.L. started to develop. Obviously the air has travelled a long distance and usually the B.L. in the air above water is in the region of say 10- are ineffective. Neither of these arguments, incidentally, has to do with the deep keel of Poppycock (see last issue) which is simply there to make best use of water depth in terms of stability. The Editor comments: The carpet analogy was the best we could think of to try to get over the point that movement of a layer of water tends to drag with it layers immediately adjacent. It was also hoped to show that producing movement of the top layer is easier than lower layers, since resistance to upward movement (or deflection) is easier. Take Fig. 3. A 3 in. plank level with the surface will, when moved sideways, push up a wave and a corresponding hollow will form. Sink the plank say 100 metres; really there is no way of telling because of the vast amount of turbulence present in the atmosphere. On the other hand in a pond on a relatively calm day (when there are no waves to complicate things) the B.L. is probably in the region of 0-3 in. This means that the top surface of the water is moving downwind at some speed less than the wind speed, so that effectively a boat only a few inches deep would be sailing against a relatively strong current and a deep keel is necessary for windward sailing. Downwind the converse holds, i.e. a deep keel would slow the boat down because it is penetrating water moving at a lower speed. Another reason which holds for yachts of all sizes, large or small (the previous argument obviously only holds for boats with a draught around 0-6 in. on a pond or less than a few feet in open water where the B.L. may be a good deal thicker due to the longer “fetch’’) is that wave motion at the surface disturbs the water lower down. Particle motion in a wave is orbital and the diameter FIG. VELOCITIES OF FLUID AT THESE DISTANCES FROM HULL | BOUNDARY HULL VELOCITY drive it at the same r.p.m.) or that a deeper hull or keel has better “grip”? , WIND SPEED FAR 7 FiG.2. F LAYER E NN merged produces more thrust (or needs more power to [FROM SURFACE ~/ –/ } K NI ye Nf 7 BOUNDARY —~ I THICKNESS FLUID NE INA SURFACE | el 3 in. and it will still form a wave and hollow, though longer and shallower because of diffusion of effort through the water above it. Does anyone doubt that it would be harder to move the submerged plank? In forming the vertical wave it acts against not only air pressure but the water above it—try it with your hand in the bath. Is this not a reasonable explanation as to why— or at least partly why—a propeller more deeply sub- | jLAYER 6 WATER y7 WATER SPEED =ZERO,FAR FROM SURFACE 289 TM aN —> “> FIG. 3. ri a {THICKNESS | |———>>” = 3; N :oe er ae ak N SS
