Fresh start?
Traditional NACA foil sections have been twisted, carved and otherwise abused over the years. Prompted by enquiries from mainstream race clients of his Copenhagen studio, Caspar Nielsen just started again, culminating in new CFD optimised profiles he argues are capable of outperforming many older NACA standards
Sails are hard to ignore. They’re right there in front of your nose all the time. But what most of us forget – or might not even accept – is that there is another set of ‘sails’ working away beneath the water. In more than one way these water-sails
mirror those sticking up in the air. Keels and rudders might be invisible but it is hard to overestimate their impact on how your boat performs. Of course, in most cases, you cannot really tune or trim them – they just work the way they are designed, good or bad. But today better profiles are steadily becoming available, and replace- ment of existing foils can come at a cost significantly less than a new set of sails. Casper Nielsen, naval architect and sail
designer, is the founder of C Performance in Copenhagen. His speciality is matching rudders and keels to the abilities and needs of his customers…
Very different profile ‘Most of us know what a classic NACA profile looks like,’ says Nielsen. ‘A fairly
50 SEAHORSE
broad nose, the thickest part roughly 30-35 per cent aft, and a soft convex shape running back to the trailing edge. Less well known is that modern CFD
calculations have made it possible to develop much better-performing profiles. Typically these new shapes exhibit a thinner forward profile, with the maxi- mum thickness pushed further aft to about 45-50 per cent – that maximum thickness being maintained for some distance aft before narrowing and switching rapidly into a distinct concave shape before reach- ing a very fine trailing edge. All in all a very different approach.’
More lift-producing area ‘There are several advantages to the new designs,’ Nielsen explains. ‘The most important of course being that they pro- duce more lift and less drag. This happens because with maximum thickness pulled further back, and extended for a while, the result is that we keep our laminar flow over a bigger area. Another advantage is that, when stalling does occur, the flow will usually reattach faster. ‘When water flow has to pass some-
thing, turbulence will always be created on the leeward, low-pressure side of the profile. Actually, during normal operation rudders and keels do not really create much turbulence relatively speaking, but this changes dramatically with increasing angle of attack. ‘In our new CFD-derived profiles major
turbulence first appears at a higher angle of attack, making these shapes particularly good for use in rudders. Generally the flow sticks a little longer and when the angle is reduced laminar flow will reattach faster.’
Concave shape better directs flow As the illustrations highlight, the new profiles change very rapidly from thick to thin in the aft section creating a concavity. The reason for this, Nielsen explains, is that once the flow has passed the thickest part it’s actually not that critical what you do with your section. ‘The boundary layer, the flow nearest the surface of the foil, will by now have changed from laminar to turbulent flow. This will happen no matter what you do. So we can pull our shape in pretty quickly without much loss. ‘But by using the aft concavity we can
create a significantly more parallel section approaching the trailing edge, the effect of which is to give the flow the same direction on both sides of the profile, certainly more so than the older designs allowed. ‘In a traditional NACA profile the flow
from the two sides will crash as soon as it passes the back end of the profile (trailing edge). Speaking of this, it is also a good idea to make that aft edge very sharp, which ensures that the flow leaves the profile abruptly. A rounded trailing edge means that the flow will break less evenly along the span, and increase turbulence.’ (As we know from years of experience in dinghy and small keelboat classes, when a super-sharp trailing edge is not achievable then the best alternative is an angled edge at about 45°. This still helps the water reunite more smoothly and also leads to a reduction in vibration and flutter).
Another reason why high-aspect keels are better We know that high-aspect fin keels are more efficient than wide keels for reasons of induced drag, plus, says Nielsen, ‘there can be less wetted surface overall for the same lift, which means less friction. ‘But, importantly for the purposes of
this discussion, a fin keel gives us the opportunity to design a more efficient profile, which would be impossible with a
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96
