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Merlin Rocket FoilsGuy Winder - June 2004 |
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Of the 3 classes we build in any quantity the Merlin has the most interesting foils because there is little control over their size and shape leaving scope for design and development. The factors that govern the performance of a foil are :- Size. Airfoil section. Profile. Stiffness. Construction. Method of manufacture Finish. Fixed or gybing centreboard? The Turner Tales mould we bought came with a centreboard case mould which pretty well dictated the board area, thickness and profile, all well proven, so we took that to be the starting point for our own . Our rudder, based on Robert Inglis�s design had been used successfully for years so that was our starting point. We therefore have proven designs but why are they the size and shape they are, and is there potential here for cost effective performance increase? I refer to a book by Frank Bethwaite, �High Performance Sailing� which attempts to explain racing dinghy performance in layman�s terms. Frank describes many practical tests, including the resistance testing of full size hulls as well as performance prediction so could help in the formulation of long term rule change proposals. Airfoil section and surface area When we hike out we attempt to hold the boat upright against the side force generated by the rig and foils, so the harder we sit out the more side force we can generate up to our maximum (not much in my case nowadays). Without foils the side force would simply push the boat sideways but the combination of centreboard, rudder, and hull act against the water to considerably reduce this sideways movement, the leeway. The greater the surface area of the foils the more side force they can generate at a given speed and angle to the water flow, so we can use a small foil operating at high angles or a large foil operating at smaller angles to the water flow. Larger foils have more wetted surface so have greater skin friction drag. Smaller foils operating at higher angles suffer disproportionate increase in drag as the angle approaches that where flow starts to break down and stall. For an accurate, perfectly finished foil, the angle at which this flow breakdown occurs is dependant on the airfoil section. A section that can generate high side force at high angles and slow speed is likely to have high drag at high speed and at a small angle to the water flow. Since foils have to operate at high and low speed we use a use a modified NASA 4 digit section which gives excellent all round results. We have experimented with laminar sections but the results have been very difficult to evaluate and have not shown the improvement required for us to invest in production moulds. It is possible to calculate the optimum foil area for a given speed with the crew generating a known side force, for a known section. The problem is that the conditions change and a boat having completed a tack or at the start, will generate maximum side force as the crew sit out hard to roll the boat upright whilst the speed is low. Similarly the article on starting in the Spring Magazine talks of �Holding the boat on station, or �slow forward� in a close hauled direction with sails flapping, identifying how much leeway is made through the range of conditions�. These are conditions for which a larger centreboard and rudder are required and these are the sizes that have become established as the norm rather than the optimum once the boat is up to speed. Frank Bethwaite found the measurement of dinghy leeway difficult because it was almost non existent on his test boats. Calculations suggest our centreboards may be over sized for light crews and I am currently using a small board. My results this season are much improved so far but it is early days. Profile Once the area is established, what profile is best? Long and narrow, Spitfire wing, straight taper with square tip, or with wings? There are a huge range of options but Merlin rule limits length and since the board has to present a low drag shape when part raised, the Spitfire wing still looks good and is well proven. Sailing up wind causes increased pressure on the leeward side of the centreboard and reduced pressure on the windward side. This causes water to leak round the tip from leeward to windward and in doing so starts a vortex trailing from the tip, sapping energy so creating drag. The control of this vortex is the reason for winged keels and winglets on the tips of aircraft wings, but where wings are impractical the tip profile can be designed to reduce the vortex. Nature appears to have developed efficient shapes for bird wings and dolphin fins and these are worth copying. I had hoped to make model gliders with different wing profiles to compare performance and at the same time amuse my grandson, but the dog thought the first one was a bird and chewed it, and time has run out. Gybing centreboard? On the face of it, angle the centreboard 4 degrees to windward and the boat sails a course 4 degrees higher without pointing any higher. Unfortunately driving force is reduced so speed is lost. Perhaps the best way to understand the effect of gybing the centreboard is to consider an extreme case where the board is angled 30 degrees. Sailing with the sails freed 30 degrees opens the slot which is potentially beneficial to the rig, but the boat in effect makes about 30 degrees of positive leeway causing the �knifing� Merlin hull to be dragged at 30 degrees to the water flow resulting in huge drag. As the sails are sheeted closer the drive is reduced to a point where the boat stops and makes leeway only. My experimental boat, Stiletto was originally built with a cam operated gybing board. The gybing angle could be changed by replacing the cam with one having a different throw but I found that the smaller the angle, the better the boat performed. The Merlin appears to make little leeway so with a fixed board the hull is also making a small angle which may well generate valuable side force for a very small increase in drag. Further disadvantages of the gybing board are that the slot gasket is distorted by the angled board and some means of locking the board straight is required when off the wind. Construction As the centreboard generates side force it flexes to windward, resulting in maximum compression stress in the outer fibres on its windward side and maximum tensile stress in the outer fibres on its leeward side. The outer fibres (at maximum thickness) take the bulk of the bending stresses so explains the benefit of sheathing a lightweight core with a high strength material such as glass or carbon which gives a lighter, potentially stiffer board. This facility to control stiffness enables the foil maker to offer a soft board which flexes in waves to match a stiff mast, or a stiff board to work with the current soft Merlin rig. Stiff rudders give the precise control we need to get the best out of the boat and since rudder weight is uncontrolled the lighter the better. Most modern rudders are carbon sheathed for stiffness and strength over a lightweight foam core. Producing sheathed foils by hand is a highly skilled labour intensive process, hence the high cost. Winder Boats have steadily developed the moulding of foils over 16 years and now rarely make hand shaped foils. Moulds are taken from master foils, made as accurately and finished as perfectly as we are able. The cost of making a mould is therefore very high, but once in production the cost per finished foil is greatly reduced and the cost of the mould is eventually recovered. A further advantage of moulded foils is that it is a simple process to vary the thickness of the sheathing to vary the stiffness without affecting the shape. The core of a sheathed wood foil has to be very carefully shaped to allow for variation in sheathing thickness. Finish Bethwaite describes the very significant difference in performance between a highly polished rudder and one rubbed with 1200 wet paper. Perfectly fair, highly polished foils are fast, inspect them regularly and repair and re-polish damage, particularly at their tips. |