Left: the 18-footer Singapore Airlines in Sydney in 1977 with a young Julian Bethwaite aboard as forward hand. The construction of this extremely light design was a great deal more advanced than her spinnakers – hard to work out the gybing and dousing processes and that old Super 8 footage really does not give many clues. Opposite: because of the strict one- design nature of the modern 49er the boat has provided its creators and class man- agers with the perfect platform with which to undertake full-scale foil research to an exceptionally detailed level of precision
and as far as the span and chord go not much had changed. Section shape, taper rate and LE considerations… that was a different story! Early in the piece 49er foils came out of
FRP moulds and people were pushing for an edge (excuse the pun). We had washers being placed under the edges of the moulds to fatten the LEs (very British), others were asking for the moulds to be ‘over-clamped’ so they had a thin board. But interestingly I think that every gold medal 49er foil was made by Bethwaite Design. With the demise/retirement of the two
Daves (49er builders Ovington and Mackay) and the change of guard to John Clinton (Mackay Boats) and Chris Turner (Ovington Boats) some sanity prevailed in 49er delivery and there was a much needed push to standardise on the foils. Also, at this stage you had the 29er
will generate. This is why 2-3mm at the bow of any boat is critical in hull shape… even if the bottom of the sea is 10m+ away. Back to foils. We sort of came up with a
20-25 multiple as a camber-thickness rule. I accept 100 per cent that there is no
theory behind it, it’s more empirical than anything, but it was our (Dad and my) belief that we should assume an 18ft skiff centreboard at 40-45mm thick would be having some influence on the water 1m+ away in a sideways direction. And the faster you go the greater this influence, because the water has less time to ‘rise’ (in other words, with nowhere else to go). Think about submarines, they are
‘allegedly!’ fastest at 250m (850ft) down… It’s all surface effect. Think about ferries, and the pressure waves as they navigate rivers and estuaries changing the shape of the foreshore as they do so. So, in the famous Entrad photo (page
65) from Dad’s first book*, that #3 centre- board is half the percentage camber of the #1 board and about the same length as the rudder (1m into the water). It’s 1986 and we were, ‘they say’, pushing past 30kt. In my previous article I commented on
the self-tacking jib added to an 18-footer resulting in a 20% reduction in foil length. It also resulted in a reduction in mast length, a reduction in pole length and a reduction in sail sizes. The reduction in mast height led not just to a reduction in weight, but we were
also able to get rid of the rig hydraulics (weight and complexity, gonnnnnne). Everything became simpler and lighter. I became a dedicated minimalist! We also got into Leading Edge (LE)
fanaticism as well as sectional demons, and this is where +/-0.5mm makes a differ- ence. In the early 1990s 99 per cent of the 18ft skiffs sailing were B18s and 100 per cent of the foils were made by Bethwaite Design, so we had a golden opportunity to play (I need to stress we played on our boat, and only then if we gained we trans- ferred that to others). Sure, there were gains to be made in sections but the big gains were all in the LEs, both radius and LE profiles. We had already created a primitive (but
surprisingly effective) program, right back in 1984, on what was not too dissimilar to modern Excel. Into that we entered pre- dicted boat speed plus crew weight, from which we got righting moment (RM). We knew the height of the CoE in the rig which then generated spanwise loading and from that we generated percentage camber, chord (width) and span (length). Very interestingly, chord remained sur-
prisingly constant. Then as span reduced aspect ratio (AR) reduced; and as you go faster and faster that’s a good thing. The big variables became span, camber, LE radius and taper rate. Jump forward 10-12 years and that same program spat out the 49er foils…
appearing with its extruded foils, and they offered an extremely interesting insight into spanwise drift. We made the 29er foils approximately 6in (150mm) longer than they needed to be, to make it easier to get up after a capsize, just as Bruce Kirby made the original Laser foil 1in longer (for more grip in light airs); so with the 29er foils we also made them about 0.5% smaller with respect to camber and with a really fine LE. It worked far better than expected. In
spite of the fine LE and finer section the 29ers never complained about ‘slip’ after a tack; in practice they never bothered to pull their centreboards up and were still clocking quite extraordinary speeds, way above what you would expect. The takeaway with respect to spanwise
drift (with the 29er) was that the foil’s shape was now more precisely defined using a very accurate extrusion tool, so by definition virtually a 100% consistent section along the length of the board, plus it enjoyed a very accurate sectional symmetry (particularly with respect to the LE). The result was we ended up with far lower drag. So we decided to revisit some of the work we had done in the 18ft skiffs – by experimenting ‘live’ on the 49er fleet. At this stage there were still 49er crews
plumping and thinning their foils by approx 1mm so what we did was trivial by compar- ison. We quickly honed in on an all-round good section, and that became our base (we are talking +/-0.5mm here, quite fine and detailed refinements). We then developed a series of very accurate LE tools that allowed
SEAHORSE 65
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