by Rob Kothe
With the switch to multihulls for the America’s Cup the focus has swung to multihull development at both big boat and small boat level.
Ashby A-Cat dancing
Australia’s Glenn Ashby, who was the BMW Oracle sailing coach for the 33rd America’s Cup in Valencia and is a seven times A Class Catamaran World Champion, talks about recent developments in the A Cat class ahead of the star studded Australian Nationals which begin today on Lake Cootharaba on Queensland’s Sunshine coast.
Ashby explained ‘The foil development in all multihull classes, over the last few years, has probably seen one of biggest areas of performance improvement and certainly that is the case with the A Cat.
Six times A-Cat World Champion Glenn Ashby
‘The newer curve foils are made so the tips of the foils are at the maximum position, 750mm from the centre line of the boat, the tips can´t get inside that box.
‘All the boats are right on that limit and the Dutch DNA boats, which are the newer boats that have come onto the A-Cat scene, delivering 40 (boats) over the last 12 months worldwide, run a slightly tighter radius with their curve boards. Their centre board cases are out wider in the hull to allow for a little more curve.
‘The DNA and the Marstrom curve foils are the post popular foils at the moment, certainly in Australia.
‘I know the American and the European guys are working on different things all the time but here in Australia generally the Mastrom foils are used.
‘The curve part of the foil, essentially the bottom part where the tip is going through the water, is a little more horizontal than the top part of the foil, so the tip load of the foil is a lot greater than what it is with a straight foil.
DNA curved foil
'It has got an even lift distribution. What it does is dampens the pitching of the boat as it is using that tip for the lift whereas the top part of the foil, which is near the top part of the hull, is giving you the lateral resistance to stop you going sideways.
‘What you end up with at a higher speed, is a foil that stops you going sideways as it should but produces quite a lot more vertical lift, which unweights the hull or reduces the weighted surface area of the hull going through the water.
‘The foil takes quite a big load component going through the water. The displacement of the hull becomes a lot less and the boat is faster through the water because of that.
‘In a lighter breeze when the foils aren't going through the water very quickly, I still think that the straight centre boards are probably better in under about eight knots. In my personal opinion, in over eight knots of boat speed the curve foils start to come into their own and it is a quite a sharp curve up to where they are better, around 14 to 15 knots of boat speed downwind.
‘You have got to measure one set in for the regatta and a lot of the boats have got curve cases to suit the board so it is quite difficult to have it run a double case for the different events.
‘There hasn´t seemed to have been a huge penalty for the curve boards in the lighter conditions.
‘Because you run the windward one in the water as well, unlike you would on a larger multihull with an asymmetrical foil, we are running more symmetrical foils just for the fact you have to do so many tacks and jibes.'
Ashby continued 'The windward one probably does a little bit of work for you stopping you going sideways as well, rather than running a lowered one in the water and rising the windward one all the time. That´s why you can get away with the curve foils in light conditions.
‘This year’s Worlds in Italy was quite a light event but essentially there was a big debate about straightened foils versus curve foils. I opted for straight foils in my boat in Italy, to try and see what the difference was.
'After learning with the involvement with America´s Cup generally under lighter conditions a straighter foil will give you bigger lift to drag ratio in lighter conditions, I figured that being in Italy at that time of year it would be most likely be pretty light. It worked well (Ashby won his seventh world title) and I think the straight foil option was a good option.
‘At the end of the day in lighter conditions the straight foils are good but as soon as you are up to more than eight knots of boat speed the curve foil really starts to work. There is a crossover around that period anyway, where straight or curved, it doesn´t really matter what you have in the boat.
‘We had done quite a lot of development with the masts and essentially they have gone stiffer and stiffer sideways, and softer and softer fore and aft.
‘We are running sails with deeper seam shape and bigger luff curve sails that you can get rid of the shape and the power up wind. The upwind performance has probably stayed almost the same.
‘On downwind performance; you rotate the masts to 90 degrees to try to bend the mast more on the sideway axis which is a lot stiffer now than it has been in the past, so the sails are deeper and carry more grant so you generally end up sailing slightly lower for the same speed or slightly quicker for the same angle.
‘They are a slightly better package downwind that what the rigs were two to three years ago' concluded Ashby.
Glenn Ashby 7 times A Cat World Champion on last day of 2010 titles in Italy
Of course while equipment is vital, talents talks and reports from the practice rounds this week on Lake Cootharaba suggest that downwind, in virtually any boat, Ashby's skill level is superior to that of anyone on the water.
We look forward to an interesting week's sailing, Sail-World.com will be covering the event in detail.