Above: 1986 and Entrad sails very quickly with her no4 rig paired with a very thin and short (about 1m immersed depth) no3 foil. This photograph is taken from Frank Bethwaite’s definitive work, High Performance Sailing. Julian Bethwaite is seen sailing as front hand. Centre: how many times do you hear that someone’s foils ‘have cavitated?’ Cavitation generally occurs in the bottom 7/8th of the foil, ventilation is what you can see right at the top. Ventilation is quite common, cavitation on a rudder is extraordinary and rare. This is actually the rudder used on the former world speed record holder Yellow Pages. The reason I am showing it is that despite all the care and hard work that went into a very clever design, including the elegant elliptical tip, you still have a vortex spinning off that tip. However, this vortex is tiny – 99% of rudder profiles would be generating a massive spiral of cavitation at this speed and at this AoA. Above right: if you look at the very front of the disturbance there is a small spot. That spot is a 1mm in diameter and 0.1mm thick vinyl dot that has been placed on the foil as a demonstration. So look after your foils and in particular look after your leading edges
The sailors are left to fine-tune only by adjusting centreboard height. Initially the whole thing was masked
because of the 20-year-old spinnaker design; basically it was too fat so the sailors where constantly over-running the chute. Five years ago we re-tendered the spinnaker, and there was an almost imme- diate increase in top-end downwind speed. The old barrier that was 25.6kt fell quickly with most of the gold fleet going down- wind at or above that; the new top end is closer to 28-30kt, at ever lower angles. When we re-did the FRP 29er foils, ini-
tially the kids did not like them. They had to relearn everything and people hate change even though change is inevitable. As always, the question from the 29ers is now why we waited so long (it had to do with the supply of alloy).
Feathering/gybing? We switched onto feathering foils in the early 1980s (in the 18ft skiffs). My sister Nicky used a feathering 470 foil in the 1988 Korean Olympics. In the 49er you can’t switch foils (or sails or masts) mid- series plus the boat very quickly jumps up on top of the water (it planes upwind 99% of the time) so a gybing foil is relatively pointless. Lots of merit in a 470, or a 505/Int14, which is why in these last two classes they spend a lot of time and money running gybing options. In the 505/Int14 you can even carry a feathering foil in a gybing case (a tongue twister). So what am I talking about? If you
arrange the structural centre of the board to be in front of the hydrostatic pressure line (CLR), then the foil will feather. If the structural component is behind the
hydrostatic pressure line it will gybe. Given in a 49er at 10kt the yaw angle is likely to be less than 0.6°, then 0.3° of feather is a lot! In a 470 (or a 505/Int14 in low wind
speeds) yaw is probably 3° so you want to gybe, because the increase in drag from tip inefficiencies (vortex) is less than the reduction in drag from yaw. In a 49er (and 29er and 18ft skiff) you want your board to feather because you’re up on top of the water anyway, therefore any yaw drag is minimal and it’s minimised further due to the increased speed and therefore side-load carry capacity therefore less yaw drag, plus a feathering foil reduces tip (vortex) losses and lifts CLR which in turn increases RM and therefore Sail Carrying Power (SCP). Possibly the most interesting variable
was the rudder. We all had these rudder frames that put the skiff rudder 400- 500mm or further off the back of the boat, but as we did that efficiency dropped. In the end we instead put the LE of the rudder 50mm under the back of the boat and that allowed us to chop 100mm off the span (length). Sure, on starting lines the crew was very probably more in control of the boat than the skipper, but once we got going the drag reduction was noticeable. A centreboard operates at approx
0.5-2° Angle of Attack (AoA) for 99.999% of the time; if it operates at any more than that for any more than 0.5 second you capsize, period. A rudder operates at 0-1.5° AoA 90%
of the time but often it’s operating at 5/6/7° AoA and occasionally at 12/13/14° AoA (when rounding a mark, or avoiding another boat) and you want it to stick; 95% of the time it is carrying about 10% of the side load that the centreboard does. At design wind speed a rudder should be
approx 28% of the combined underwater area. With a 49er we have made it around 29-30%, and that is with the centreboard right down. As soon as you start lifting the centreboard the rudder % goes up. We do that so we can be very confident the rudder will always stick! The 29er is higher again.
So some simple maths will tell you that
the loading of the rudder most of the time is a quarter that of the centreboard. So, by definition, the rudder has to be a different section to the centreboard. The rudder wants to be (but often can’t
be) very low drag but has to, when asked, operate at very high AoA and generate even higher CoLs – at that stage you don’t really care how much drag (CoD) it’s mak- ing. But similar laws apply for spanwise drift; the section’s max camber is notably further forward, the taper toward the tips is still very defined and very calculated but the taper happens at a much faster rate. With respect to the specification of the
foils, we apply to every foil a load that exceeds the maximum load that foil should ever be subjected to. In the case of the centre- board, when two people are jumping like crazy on the tip trying to get the boat back up in the final race of the Olympics. In the case of the rudder, it’s the load
that would break (shear) an M8 stainless steel pin at the lower rudder gudgeon. Then we apply a lesser load, but still
one significantly higher than the operating load, and measure the deflection between that and a near-zero load, and the range of that measurement is small. No board (or rudder) on a 49er or 29er gets a measure- ment plaque until it has passed those tests. How each builder achieves that is up to
him. Given the tight requirements, the variations are minimal. The biggest issue is the sailors evolving. They are getting so good at it, and the foils are so consistent that we are starting to reach the limit of the current cast alloy rudder stocks. We will probably shortly be moving to an extruded rudder stock, which will not only be tougher but lighter and less expensive.
* High Performance Sailing by Frank Bethwaite (Adlard Coles)
q SEAHORSE 67
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