America's Cup- Where have we seen anything like this before?

by Derek Kelsall on 13 Jul 2009
As the two America's Cup contenders take to the water at the same time on opposite sides of the Atlantic, the sailing websites and the forums are awash with news and speculation on the two race machines lining up for the next AC battle next year. A 90 ft trimaran and a 90 ft. catamaran. Amazing machines the like of which we have not seen before – or have we?

The fact that we have two very different craft is in itself, a great prospect for debate. For all of the 50 years of my involvement with catamarans and trimarans, there has been the same debate. Which is best for offshore, which is best for performance etc. In that time, one would expect there to be a clear role for each but the debate goes on.

The plan view of the trimaran is square. The catamaran has very slim hulls and a central, above water structure in Y plan form with rigging under. This reminds me vividly of the work we were doing in the seventies and early eighties. Obviously not as extreme or not in the same category power wise or budget wise but there are many similarities.

Kelsall 93ft Trimaran
93ft. race trimaran, called William Saurin, was launched in France about 1980. Designed for short handed offshore racing, she had some mininum accommodation space and a modest sail plan etc. In general concept however, the new AC tri is the same.

By this time, I had reached the stage of deciding that twin hulls offered the best way forward for offshore performance. This was reflected in our next race design.

Kelsall 63ft Race Cat
A 63 foot catamaran, with long slim hulls and a basic Y configuration central structure above the WL, with rigging under, was launched in the Isle of Man a couple of years later. The name was VSD after the French sponsoring news paper, Vendredi, Samedi, Dimanche. The lightweight central accommodation was built around two aluminium tubes from stem to aft beam and with ss rigging under. This structure took the high forestay loads, runner and sheet loads.

Both vessels were to the design and build method we had devised. In comparison, the budget for the projects was minute, reflected in the low tech approach which was applied to both.

The trimaran was the largest racing tri at the time. At 11.5 tonnes, it would be considered light weight even by todays standards. She held the 24 hour record for a couple of years. The general arrangement was a little different to the typical race tri of the time but not radically so.

The catamaran addressed the fact that the power of the sail plans being fitted to multihulls was ever increasing. Where the trimaran became established on the basis that the outriggers countered the wind pressure and the main hull took the weight of the boat, with the taller and more power ful rigs, the wind pressure was sufficient to lift the main hull clear of the water much of the time. If the main hull is not taking weight it is an interference what does not assist in the performance.

Since 1992, my web site has featured an article I wrote in the eighties and updated after I think it was the 1996 solo Atlantic start I observed. I have seen no reason to change the article since, in spite of the number of very successful trimarans that have raised the offshore sailing speeds so remarkably.

I am still convinced as others gave said, weight will be a major factor and in my experience we could always produce the equivalent catamaran at less weight than the trimaran. We had chance to prove this when we designed and built a 40 ft. tri for Lake Geneva. All up weight was 1,100 kg. This owner’s next boat was a catamaran to same basic specs. The catamaran weighed in at 850 KG. Both used carbon/kevlar epoxy skins on PVC foam core sandwich.

If I was a betting man, my money would be on the cat.

My Question
There is one basic factor which we did on William Saurin very effectively and which I would now apply to either cat or tri for racing. It is the position the mast relative to the cross beams. I would put the foot of the mast between the beams. This can obviously apply to many cat and tri designs.

Reverse Bow
The reverse bow, I see as a wave piercing device. Wave piercing for best performance, is a device which needs to be designed for the prevalent sailing conditions. Just how much reserve buoyancy and the position of that reserve buoyancy will determine the conditions in which it is most effective.

William Saurin and VSD can be seen under articles on www.kelsall.com

The cat v tri debate can be seen under articles on www.kelsall.com