Similarly, CFD predicts that the wheels


on an open-wheeled racing car produce downforce whereas in practice they pro- duce lift. Is this case of bumps on the bottom of boats a result of this apparent inconsistency in the CFD programs? I have always thought that if the results


from any computer program go against logic then logic and Bernoulli should prevail. I imagine that in some cases Bernoulli must be turning in his grave.


Foils And so to the foils, the design of which of course depend upon the speed of the boat. These boats have enormous righting moment and thus enormous power and so, if drag is not too high they should be capable of very high speeds and if that speed is high enough they may well encounter cavitation. Cavitation occurs when the pressure


becomes so low that the water boils. It is thus pressure dependant. Lift Coefficient (CL) is, in essence, a measure of pressure difference. The higher CL the lower the pressure on the low-pressure surface of the foil so, to minimise this pressure difference and thus delay the onset of cavitation, the foils should be designed to produce the required lift force at the lowest possible CL. If cavitation is a problem then the first


solution should be to make the area of the foils as large as possible. Double the area for a given lift force and velocity and you halve the CL. However, at lower speeds than those at which cavitation will occur, you will then have a foil that is larger than ideal for that lower speed and thus your drag, at these lower speeds, will be greater and you will sail more slowly or not point as high as you could. All design is a compromise. A way of reducing the peak CL without


incurring a drag rise at lower speeds would be to use an elliptical planform. An untwisted elliptical planform in a uniform flow will produce a wing where the CL is constant along the entire span. That is why it produces minimum induced drag for any given span. However, in this case the fact that the CL is uniform means that the wing can operate at a lower peak CL than if it was not uniform. Obviously, if you have a wing with a


varying CL along its span parts of the wing will have to produce a higher than average CL to make up for the parts of the wing where the CL is lower than the average… and those parts where the CL is higher will be more prone to cavitation. It is (deliberately) difficult to see exactly


the planform shape of each boat’s foils from earlier pictures but it does seem that most are straight taper which would mean that the central sections of the semi-spans are more prone to cavitation than the root or the tip. However, Britannia’s foils are very


tapered such that, in her case, the tips will be operating at a very high Cl. The roots will be operating at a low Cl and the Cl will rise continuously along the span to reach a


peak at the tips unless, of course, the wings have some twist to give washout towards the tip in order to lower the Cl along the span. Trouble is this will only give a con- stant spanwise Cl at one Cl. At all others it will vary along the span, thus it will only be correct at one speed. The only boat (currently) with some-


thing approaching an ellipse is Luna Rossa. Note, however, that the New Zealanders did initially appear with elliptical or semi- elliptical wings but promptly switched back to a straight taper… The design of the foil section will also


influence the onset, or not, of cavitation. Designing a foil to delay the onset of cavi- tation is similar, in some respects, to designing a foil to delay the effects of transonic flow on an aircraft wing, the so-called supercritical wing section. Both require peak velocities to be reduced to be as low as possible. I had the good fortune to work with


Herbert Pearcey, the aerodynamicist who is credited as being the father of super - critical wing sections, and we had many conversations on this and other subjects. The secret is to have as smooth a velocity distribution about the foil section at its designed CL as possible. Any peaks will send the local flow


supersonic and then separation will occur when the flow needs to go back to sub- sonic. This requires a lot of what is known as rear loading. Making the back end of the foil work harder reduces the lift on the front end and thus the local velocities in that area. Perhaps the same technique would work on the foils on these boats but so much will depend upon whether they reach a speed at which cavitation will occur. One area where velocities can be very high is at the flap hinge when the flap is deflected but there are techniques to minimise this effect. Another area where velocities can be


high and thus pressures low is at the junction of the swing arm and horizontal foil. Where you have two velocity distribu- tions superimposed upon each other, as you can have at this junction, the velocities can become very high which is obviously not what you want if you want to delay the onset of cavitation. One solution is to have a bulb that is


waisted, where the foils join it, as are the bullets on the tails of aircraft like the Buccaneer or VC10, so that the velocities around the bulb are reducing in the area of waisting as those around the foil are increasing so that they tend to cancel each other out. In the early days of supersonic flight


aircraft had great difficulty breaking through the so-called sound barrier because of these local increases in velocity where things like wings joined fuselages. The solution, pioneered by aerodynamicist Richard Whitcomb, was area ruling, the waisting of the fuselage in way of wings etc which gave the fuselage the famous Coke Bottle effect which is what we are talking


about here. Modern supersonic aircraft still use the same principle but in a differ- ent and perhaps more elegant way. Rather than waisting the fuselage to remove vol- ume where the volume of the wing and other components occur, the various com- ponents are now staggered so that they don’t interfere so much with each other. In an aircraft like an F15 first you have


the increase in volume caused by the cockpit but as that tapers away the volume of the wing starts and as that finishes the vertical tail begins and when that finishes the horizontal tailplane begins. They are all staggered so that the velocities of each are not superimposed on the velocities of another. The same thing applies here. Both the Defender and Luna Rossa are


the only boats using this principle at the time of writing. New Zealand’s early foils had the horizontal ahead of the arm without any bulb but on boat two they are behind the vertical arm but with a bulb. Luna Rossa’s are the other way around


with a bulb on the first boat and without a bulb on the second but with the horizon- tals ahead of the vertical. The foils don’t have to be separated completely in a longi- tudinal direction. Having the velocities on one increasing where they are decreasing on the other is probably ideal. Most of the AC75 foils have anhedral.


Wing tips lower than wing root. The idea of this, bearing in mind that to produce both side force and lift force on the same foil that foil has to be angled at about 20° to the horizontal, is to allow the foil to run nearer the water surface without breaking that surface. As mentioned, the nearer the water


surface the foil the less of the swing arm is in the water and the lower the drag. Up to now all the boats except New


Zealand’s raceboat have some degree of anhedral but New Zealand’s is almost non-existent. I wonder why? True, for the same span, there is less wetted area in a foil with no anhedral but then more of the vertical arm is in the water. You pays your money and you takes your choice. And nobody doubts the quality of Team New Zealand’s design tools. One of the foils on Britannia is a little


strange. It has a crank in each wing so that the anhedral is greater in the centre section and less in the tip. Bearing in mind that a straight foil will have less wetted surface than a cranked foil and that the net force in the required direction will be the same if the Cl is the same and that there are also an additional two junctions, each of which will cause drag – once again, I wonder why? So. For what it’s worth, in my opinion,


the boats with the best hulls are a toss-up between Luna Rossa, Patriot and the Defender and the best (current) foils are on the New Zealand boat and Luna Rossa. As a patriotic Englander I now find


myself on uncomfortable ground: I predict a final between (obviously) New Zealand and Luna Rossawith Patriot as a dark horse. q


SEAHORSE 65


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