Nielsen’s new rudder profile (left) and NACA predecessor (top). Similar to Nielsen’s new keel (above in green, NACA in red), the new shape can be split into three, a round entry, a long mid-section with near-parallel sides transitioning into a concavity aft (the keel entry is much finer than the rudder as it operates at much smaller angles of attack). The CFD illustrations suggest the new sections allow laminar flow to reattach sooner in spite of a shape ‘reversal’. Nielsen argues that aft of max thickness flow is turbulent anyway, minimising the extra drag of a concavity but allowing water from each side to reconnect as two parallel flows rather than clash at opposing angles with a convex section. Writer Øyvind Bjørdal has fitted this pair of Nielsen rudders to his ultra-light Chi (issue 465)
shorter-span wider-chord shape. With older-style keels this relationship between length and thickness means you always end up with a long, more or less straight- sided section over most of the chord length. It’s not really a wing profile at all. ‘The final point is that with a narrow,
high-aspect shape you can of course distribute weight better, especially if you are incorporating a bulb.’
Wrong shape, section, area… or all three Customers who come to Nielsen to discuss foils most typically complain about diffi- culties getting back up to speed after tacks. ‘They start to drift sideways, and have to bear off more than they want to build speed before being able to come back up and point. Upwind there will often be a very fine line between pointing too high and too low, that very narrow “groove”.’ On many well-known production
designs the helmsmen find it quite challeng- ing to keep at target speed, and then it falls off very quickly with the slightest mistake. The result is that many skippers spend far too much time sailing well below target. ‘I had a recent example, a 38-footer,
where the upwind target was increased from 6.6-6.7kt to 6.8-6.9kt by changing the keel and rudder design and slightly increasing foil area; but more importantly it became a lot easier to maintain target speed across a wide range of conditions. ‘In a race an improvement like that will
change your whole situation… plus the trimmers may also get criticised less often.’
Round nose more forgiving So what does a fast but manageable profile look like more precisely? ‘A good all-round profile will still have a gently rounded nose to make it easier to steer the boat without stalling with small changes in angle of attack,’ Nielsen explains. ‘A round-entry will always be more for-
giving. A fine-entry profile creates less drag but for most crews this profile is vulner - able and stalls easily (we sometimes risk this on a keel – see the illustration above – but very rarely on the rudder). What you want is a fast profile that at the same time maintains its control effect – even when slightly outside the perfect angle of attack. ‘This compromise can then be finessed
to suit the customer, though we now have a new baseline shape that we use for the majority of our cruiser-racer clients.’
Closer to 100 per cent ‘So to wrap it up,’ Nielsen says, ‘with opti- mised keel and rudder profiles you will sail closer to 100 per cent for more of the time. And whatever your level of racing. ‘That gives you an important edge,
especially when it comes to competing under measurement rules. You will more easily sail to your handicap. Let’s say that with your old keel you are averaging 85 per cent of your target speed. ‘With a new and improved profile, with
enough practice you may be able to aim for as much as 95 per cent – and at a slightly higher-target speed. And let’s not forget that your boat will feel easier and be more fun to sail.
‘It’s quite strange, most people who
compete expect to buy new sails on a regu- lar basis, yet nearly all continue to use the same foils year after year. Few stop to consider that – albeit on a less frequent basis – improving our foils may be just as good a way to spend our money.’
So what does it cost? Nielsen always starts out by talking to customers at length about the experience of sailing their boat. Then the design work takes place, and when a new foil is eventu- ally proposed three or four sets of fairing templates are made depending on span. ‘If there is a big difference between the
old and new profile, then foam may be needed to create the new shape. If the dif- ference is small the new shape is created using epoxy mixed with micro balloons. Finally the leading edge will usually receive an extra layer of grp with the trailing edge finished off using a carefully moulded “sharpened wedge” in grp or carbon.’ In terms of the practicality and cost,
rudders are normally replaced with a new blade but most keels are capable of being sufficiently modified, unless the plan shape itself is changed. For a typical 38-footer a new rudder including stock will cost 7,000-8,000 euros while just reprofiling the keel fin on a similar boat will cost 3,000-4,000 euros. Taken together this outlay is similar to
the cost of a new suit of racing sails… and you don’t have to do it every season. Caspar Nielsen was speaking to Øyvind Bjørdal
SEAHORSE 51 q
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