Nigel Irens is one of the world's most renowned multihull designers, having been responsible for the design of several record breaking trimarans such as, Ellen MacArthur's B&Q-Castorama, Thomas Colville's Sodeb'O and Francis Joyon's solo Round the World record holder IDEC. More recently, Nigel was a consultant to the Alinghi design team.
He has been talking to BYM News editor, Marian Martin, about some features of the Deed of Gift protagonists, starting with the catamaran v trimaran differences.
Nigel Irens: Essentially both boats take the loads on a central structure. On the Oracle boat that structure is a hull that floats in the water when the vessel is at rest; Alinghi has chosen not to put the envelope of a hull around that structure, so clearly it mustn’t be allowed to touch the water, because that wouldn’t do a lot for the speed, because of the drag from the tips of the struts that come down. So that’s why the longitudinal vessel structure has to be held quite high in the air and the mast step, which is on it, is quite a bit higher than on the Oracle boat.
So how does this change things? There’s a fundamental difference, at this point between trimaran and catamaran configuration. Because Alinghi’s central structure is above the surface of the water, when the boat starts sailing it doesn’t have to heel very much in order for the windward hull to come out of the water, only a few degrees. Obviously, both boats are going to be sailed on one hull, most of the time, and as the Oracle centre hull is floating you have to heel that boat a lot further to get that hull out of the water. That is a point you can consider to be in favour of the choice made by Alinghi, because when the Oracle boat is on one hull, the windward hull is really quite high in the air and the higher it gets the faster moving air passes over it, so there really is quite a bit of aerodynamic drag there, which won’t be the case with Alinghi. So that’s one plus for Alinghi; on the other hand Oracle, has built a hull that has quite a lot of rocker on it, which is consistent with the needs of the structure because you want the beam to be fat in the middle and thin at the ends, and that is actually quite a good shape for tacking. As the boat comes through the tack, or gybe and the power comes off the main hull lands back in the water and, because it’s a rockered shape like that the main hull is quite good at turning. So, I anticipate that the Oracle boat would be quicker through a tack. Obviously, that’s a strategic position; the legs of the race are long and, on paper there’s not a lot of tacking to be done. On the other hand, if the two boats are very similar in speed, which I think is quite possible, then they could get into a tacking duel. If one boat thought they were faster at tacking than the other they would probably look for a tacking duel in order to exploit that advantage. On the other hand, there is a motor to power the Alinghi deck, so that may make up for a boat that is perhaps not as easy to tack as Oracle.
One aspect I find very interesting is that we are absolutely not privy to what the Oracle boat weighs and, if we find the boats are quite similar in weight, then I believe we will find they are quite similar in speed and that, contrary to first speculation some time ago, there might be much more of a race here than we thought. I was imagining that one boat would be faster than the other and that would be the end of it and rather boring from a racing point of view. Now, unless the Oracle boat has come out way overweight, which we don’t know, then it could be a lot closer than was first anticipated.
Is that based on seeing the new version of the BOR boat?
Nigel Irens: Well, we don’t actually know anything about the performance of the old version or the new one. There’s been a lot of speculation, but the fact is that if you analysed the probable performance of both boats you would be likely to conclude that they would be similar. My personal feeling is that the difference between a trimaran and a catamaran is unlikely to be a determinant factor.
Some people have commented that the new Oracle boat was pitching a lot on its first test sail. Was that simply because they didn’t have the boards down?
Nigel Irens: I don’t think you can read much into that. All boats will pitch if what we call “the rate of encounter”, which is the combination of the rate the boat is moving forward and the rate the waves are coming towards it, becomes just right. If you find that resonance frequency then the boat will pitch; take a motor boat where you come of the top of one wave and the trough of the next is right there the boat will pitch, but you can do something about that by going slower, or you can speed up so that you just beat that rhythm and don’t get into that situation where it gets worse and worse.
The BOR boat seems to be a lot longer than it was and a lot longer than Alinghi. How much of an advantage is that going to be?
Nigel Irens: I can’t say how much, but - apart from the obvious advantage that, in basic terms, longer waterline means faster boat - there is some advantage in that a longer boat is less sensitive to longitudinal trim issues. Clearly, if the sail plan is trying to force the boat fast in one direction the limiting factor is when it wants to cartwheel forward and a boat that is longer is going to have it bit more resistance.
Do you think one boat will have an advantage in a certain sort of weather and vice versa?
Nigel Irens: Yes, I think that’s inevitable, but from here I wouldn’t like to say what those conditions would be. In any case, I think that if one boat has an advantage in a certain condition that could be easily overruled by better tactical choices and you’d be into a close race; I don’t think it would be a completely determinant factor, just another of many.
We’re told that Alinghi will now start to be developed once they start getting feedback. Do you think we’ll end up seeing a completely different boat?
Nigel Irens: Oh yes, development is inevitable, but I don’t think we’ll see a really different boat. When you go out sailing on any race boat, you come back with a list of things you’ve got to change, but I think it will be obvious right away if something major has to be done. Obviously, the S-foils are very experimental; in a way you could say they’re the most radical part of the boat. There are limitations with a C-shaped board in that when it’s fully down you get the maximum lift upwards and when it’s fully up the last bit in the water is not much use for anything. In the case of Oracle, if the conditions were such that the foil wasn’t needed, the boat would primarily be using its centre hull centreboard to resist leeway. Alinghi doesn’t have that and, when the conditions are not suitable for looking for upward lift, you still need lateral resistance. A simple way to look at it is that, on Oracle, there’s a centreboard to resist lateral forces and a C-foil to create upward forces, but Alinghi have been obliged to combine the two into one. It doesn’t work to have two boards in the same hull, because they mess each other up.
Would you like to guess what crew will be needed?
Nigel Irens: I really couldn’t say, but, if they are intending to use the engine, the weight of the engine will have to be compensated for, otherwise you’re doing things twice. It is a pretty light engine though and if you can save crew weight that’s a good thing and grinders are very heavy and the manual mechanism is quite heavy, so I think the engine will definitely be a plus point.
What about righting moment; less people must mean less righting moment and I believe catamarans don’t have the righting moment of trimarans?
Nigel Irens: Yes, but catamarans are not usually so wide and what makes it possible to build such a wide boat is the central spine structure.
Could someone adapt this type of design for a round the world boat?
Nigel Irens: Absolutely not; our thinking is that, when you are going offshore to face all kinds of strong winds and sea states, the stability curve of a catamaran is too violent. In lay terms, a catamaran is at its most stable just as the hull is leaving the water, thereafter the capsizing sideways stability diminishes very quickly until it falls off the cliff. The trimaran tends to be almost like a keel boat in a way, so it is definitely the way to go if you’re racing offshore.
The Alinghi hulls look very fine; is there enough forward buoyancy?
Nigel Irens: They are fine and in multihulls it’s always been a bit of a cleft stick. The layman’s view is that you must have lots of buoyancy to stop the boat capsizing forward, but to have that buoyancy you have to make the waterline a bit blunt and the result is that you get more drag than you get lift, so you actually cause the boat to trip over. That was the lesson learned from the mistakes of the '70s and '80’s though, interestingly, the Tornado – a most successful boat - was designed in the '60s and the man who designed it, Rodney March, realised then that you don’t create the capsizing force if the bow is fine, so you don’t need the buoyancy.
Oracle has now gone to wave piercing bows, can you explain the advantage?
Nigel Irens: Well, I think the bows it had before could be thought of as more suitable for offshore sailing, where there are big waves. As I’ve said, you must keep the bows very fine, in order to reduce the tripping forces, but having sufficient buoyant force there is important. So, in recent years, people have elected to keep the entry fine but make it taller, so that if you are tearing along in the ocean and go into the back of a wave, you haven’t slowed down too much but you have considerable correcting force. In flatter water that is not quite the same and, if you put a big area of hull forward, there is quite a lot of structure involved, so quite a lot of weight and the bigger it is the more of a target it is to waves coming from the side. So, by making it a wave piercing bow, you reduce its side profile quite a lot and therefore the windage and the structural loadings. I’d say they’ve changed from a classic offshore bow to something that is more associated with inshore sailing.
The match is likely to have wind limits, any ideas what they will be?
Nigel Irens: I have no idea. I think Alinghi will be scratching their heads, at the moment, because they haven’t sailed the boat. When they do, they may say “Jesus, we need really low wind.” on the other hand they may say “This can cope with a lot more wind than we expected.” I wish I knew the answer, but I don’t. It does sound though like something there could be another court case over and I do wish they could get off this court thing. We’ve got this far and I think a lot of people are interested to see what happens and looking forward to a race, so I do hope they don’t fall into a quagmire of further litigation, that would be really boring.
Some people have been talking about cavitation being a problem on these boats. Do you think it will?
Nigel Irens: Cavitation is always a problem. Where any hydrofoil, by which I mean a rudder, or centreboard, or foil itself, passes through the water there is drag associated with it. If you make it very pointy at the front there’s less drag, but if the water meeting it gets to a steep angle – like if you pull the rudder very hard towards you – then the sharper it is the less tolerant to that and the water can’t reach the back edge of it, so two things can happen; either air is pulled down from the surface, which is called ventilation, or the water actually boils, which is cavitation. As you know, water can boil at room temperature if the pressure is low enough and what happens is that, when the water can’t get round the back surface quickly enough, the pressure drops so quickly that it boils. From the designer’s point of view, the dilemma is to make something that is low enough drag, but not so pointy that as soon as you make it work hard it stops working. Obviously, if the boat goes faster the problem increases and it would be nice if you could start reducing the size of the rudder as the speed increases, but there is experience of this cavitation problem in these fast multihulls, because even the little 60 footers are capable of speeds up to 40 knots.
People tend to think of foils as being ones that lift the boat out of the water, but we call all these devices lifting surfaces, whether they provide horizontal lift or vertical lift. If, for example, you got it wrong with the rudder when you were going around the windward mark with huge speed up, as you were about to bear away you’d find that not a lot happened and you’d have to slow the boat down to re-attach flow. Of course, the big danger is with a lifting foil. If you are actually foil borne, that is the whole boat is lifted out of the water, you can get into a state of unstable equilibrium and, if something happens, like it cavitates, then the boat would suddenly tip forward quite violently and do what people with powered foil borne craft refer to as “a crash”.
What you have to remember though is that everyone has been taking about Alinghi not having enough time on the water, but they have had really. This is a development of the Alinghi 41 and they tried out things like the S-foils there, so they have been learning pretty fast and they aren’t going to be going out there and doing it all for the first time. The 41 has been perfect for that because, if you wanted to build a big boat like this and get it right, you would undoubtedly build a smaller boat first. Alinghi already had exactly the right boat, so that’s been very much to their advantage.
One final question; some people are excited about the possibility of one of these boats bettering the 50 knot speed record. Do you think that is likely?
Nigel Irens: I very much doubt it, these are very different sort of boats from one you would design for a speed record. In fact, I think their top speed could be less than a G-Class boat, because they will both have to depower so early.
