Yachting's next revolution? Pt2

In part two of this article we look at the applications of the Dynamic Stability System and go sailing

Thursday May 8th 2008, Author: James Boyd, Location: United Kingdom

This article continues on from part 1 published yesterday...

The upshot of the research was that the foiled boat proved faster than water ballast from 16 knots upwards and faster than the canting keeler at 20 knots and above. Bare in mind there are few occasions the boat will be sailing at less than 15 knots and these figures are also highly conservative. “You go back and look at all the rest of the figures and you can revise that down pretty quickly,” explains Welbourn. “The scale effects on foils we learned a lot about that on the tandem keeler with GBR. In the tank that was a rocketship uphill and slow downhill. And CFD said ‘we think it is going to be quick downhill as well’ . On the water it was the same at best uphill and it was a rocketship downhill. So the tank was completely wrong. This comes back to the scaling of relatively small foils working in tank conditions. So armed with that data you can come back and make your own assumptions on this. If it says we have a crossover at some point you knew it was going to be significantly lower.”

From here the team took a step even closer to the real world when they went down to Brisbane, Australia and fitted a foil to one side of a Boatspeed 23. “We built an overlength foil to give it some separation and get it down into the water where it should be," Welbourn continues. "It was just great - we went out in 11-12 knots. We were doing 9 knots with the foil out of the water. We tacked on to the foil and suddenly we were doing 12 knots.”

Because the foil was overlength it was adding too much lift to the extent that on the non-foil tack they would sit out on racks, while on the foil tack they would have to sit in. “We finally got a 12-13 knot puff and the boat accelerated past the chase boat and the GPS had us going past 16 knots and then it ripped off! The chase boat saw clear air under the hull at that point!”

Welbourn is at pains to point out that the DSS is not designed to create a flying boat (although that may be another evolution some way into the future). This would produce a wealth of additional problems to solve such as longitudinal trim and how to steer the thing... The DSS, as it is today, is attempting solve the dynamic stability issue of yachts and not create a Moth or L’Hydroptere. Welbourn is keen that it be simplicity itself, a ‘no brainer’ for people to fit it. “What we are trying to do is to get a signficant effect out of it, so all the instance angles effects, they are all controlled by the initial set-up on the boat and how the boat works around it. So it is fit and forget and if it breaks you still have a normal boat underneath you.”

Since then the foils have also been incorporated into a Welbourn-penned Open 40 originally destined for last year’s doublehanded Melbourne-Osaka race. While this build went ahead, the project was strapped for cash and the boat missed the race. The new Kamikazi has water ballast, as well as wings, but came out at just 4 tonnes despite being built in glass and foam. So far her top speed has been 24 knots...

Meanwhile in Europe they built their own 27 footer (above) fitted with the DSS. While this is a demonstrator, it is also a test bed. Welbourn admits the hull of the 27 is a little odd proportionally as it is simply a scaled down version of the 100 footer, and it is also not the most hi-tech construction - built from a machined-down solid block of polystyrene which was subsequently laminated upon. For example the board originally fitted was undersize (it hadn’t been resized during the scaling-down process) and has since been replaced with a longer one - this you can see in the photos and is why it protrudes slightly from the weather side when sailing.

The flipper alternative

As we have mentioned the board on the 27 simply slides through the centre of the boat, however there are other alternatives such individual flip-out boards. These would have the advantage of being able to be knocked backwards into their housing in the event of a collision. They could also alleviate the potential problem of being forced to design the foil as a continuous shallow curve (in order that the foil is near horizontal when the boat is heeled), although this is not a major problem.

A slight issue with the flip-out set-up is how to seal up the aperture left in the hull when the foil is deployed. With the slide-through foil, the end of it neatly closes the hole at the weather end of the case. With flip-out boards Welbourn reckons a mechanical cover could be fitted on large yachts while on dinghies it could simply have a seal like those used on daggerboard case exits. Quite what he would do on intermediate-sized boats is still subject to some head scratching.

While the foil is designed to operate at an optimum heel angle (12deg for the 100ft maxi), aspects such as the curvature of the board(s), placement fore and aft in the boat as well as the aspect ratio and shape of the foil are all dictated by the requirements of the boat to which it is fitted. At the extremes a boat designed to go upwind would have a long skimmy foil while the more you want to go reaching, the shorter and fatter it would get. Typically a good all-round foil ends up somewhere in between.

“My starting point of how you size it, span-wise and area-wise, was that I wanted to take 5 degs of heel angle off for upwind speeds,” says Welbourn. “I thought that wasn’t a bad starting point and I haven’t had any real reason to go away very far from that. I am still grappling with the aspects of how you relate the span of the thing to rig height and the area and span for total lift to the displacement of the boat. There are some loops you can go around on all that to get an optimum figure.”

As a rule of thumb the foil is designed to ride around the same distance below the water as the cord length. “If you are close to the surface it is not very effective, you are losing lift but it is not as bad as you would think.” On the 100ft maxi for example Welbourn says the cord of the foil would be around 1.2m with a span of around 2.5m, but on other boats of this length the foil dimensions would vary according to their displacement. Even so the foil for the 100ft maxi would generate around 18 tonnes of lift...

So would the foil ever be used partly out? Welbourn thinks not. “It is actually just like canting keels or water ballast: they are either on or off. The changeover on the performance boats, the difference between being powered up and not powered up is like that (clicks fingers) and suddenly you are off and you need everything you have got.” The only occasion when the foil might be brought in partly would be in big conditions (in a similar fashion to the Volvo Ocean Race crews who occasionally ease back from maximum cant when the sea state was bad).

While it would be tempting to add tweaky things like a gate at the end of the foil to act as an endplate, Welbourn has deliberately steered clear of such items to keep it simple. “If we were doing a Wally you’re not going to push it to extremes. You’ll have something that works all the time and it is either on or off and that’ll be the same on a cruising boat. On a race boat you’ll think about things a lot more carefully,” he says. “What we are trying to do is make something that is safe and seaworthy and actually puts 20% increase into chunks of your performance envelope, without sweat.”

A possible development for offshore race boats for example might be the inclusion of fore and aft water ballast tanks (useful for those occasions when you want your boat heavy - going upwind or to punch through waves). In this case filling either the forward or aft tanks could also be used to alter the angle of incidence of the foil from its normal 2-4deg of lift setting. Welbourn acknowledges that being able to manipulate trim is vital on high speed boats. “You don’t see a power boat without a set of interceptors or fences to control the trim. One of the things that I definitely wanted to do with the Bols was to make a boat that naturally trimmed a little bit bow up. That is slow, but it is a very safe mode for blasting through the oceans when you have shitty seas and you use the trim tabs to flatten it out. And there was a 10% speed benefit out of that, plus the safety factor.” But generally Welbourn believes the DSS can be dynamically self-limiting. “Overall on this hull it doesn’t need trim control devices because you have that all built-in.”

For example the faster you go in theory the less foil you would need to achieve the same lift. But with the DSS foil it starts lifting the bow thereby automatically reducing the incidence of the foil. “If the foil starts coming close to the surface it starts losing lift as well,” explains Welbourn. “It is not one of the biggest problems. Because the system is dynamically self limiting, most of the time you can just let it get on with itself. You don’t have to protect the foil or the rig or the loadings.”

So on the 100ft maxi for example in what wind speed would you deploy the foil? “When you’re doing 12 knots. That’s normally just after you’ve pulled the mainsail up,” says Welbourn in all seriousness.

When will a DSS-equipped boat be faster than a canting keel equivalent? Apart from a small period immediately after being powered up when the canting keel is superior, at all other times the DSS boat is faster, says Welbourn. While the foil is mostly responsible for this, in the light with the foil retracted the whole boat has the advantage of being lighter through not having to carry around the heavy keel canting mechanism. In reality the performance gain, particularly reaching in 20+ knot winds will represent an increase of up to 25% compared to the canting keel Welbourn estimates.

The lightweight answer

Another interesting side affect of the foil is that like water ballasted boats, or canting keelers the fact that the ballast is movable means that the amount of lead in the bulb can be reduced. A fan of lightweight boats Welbourn is particularly keen on this idea. “Ultimately bulb weight is only determined by the AVS [angle of vanishing stability] requirement. You can say we have as much righting moment from water plus wing (for the ultimate race boat) - you go for x amount of water which gets you uphill and gives you some mass, but it means that you can build a 20 tonne (100ft) maxi with only 6-7 tonnes in the bulb, which still passes all the AVS rules by miles.”

While Bols’ bulb was 11 tonnes on the larger 100 footer Welbourn reckons it will be 3 tonnes less. “That would get Bols down to 21 tonnes. Then your sail requirements come down and you are on the efficency spiral heading in the right direction rather than getting bigger and heavier and more powerful. You don’t end up with a huge monster.” Perhaps a reference to Mike Slade’s new 40+ tonne ICAP Leopard.

On board

And what did we think when we sailed the 27 footer? On Lake Garda it was early days and there wasn’t enough wind, however down at Sotogrande near Gibraltar recently, there was around 10-15 knots and a short chop, the boat seemed to live up to expectations. Upwind the boat seemed to struggle slightly but no more than any other lightweight boat would in the conditions. In these conditions it paid to crack off slightly, sail faster and get the foil working to make better VMG upwind.

But the boat really turned rocketship when cracked off on to a tight reach or downwind under genniker when we clocked 17 knots.

Most surprising about the foil is that it doesn’t seem to slew the boat around at all. In fact the stability it adds both fore and aft as well as laterally gives the helmsman a great deal of confidence to be able to throw the helm around, to steer around waves, to come up and take the speed back down as you would on a multihull or a skiff, without the feeling that the boat was going to round up any minute or plant itself into the back of a wave. When the foil was working properly and the boat fully arced up, one of our colleagues described it as “a bit like sailing on rails”. As with multihull and skiff sailing keeping the boat sailing fast seems likely to be the key to getting the best from the DSS.

Oddly the ride seemed drier upwind than it would be on a conventional boat, but off the wind was as wet as you would expect but with the water tending to come not from the bow but more from the foil as it occasionally exploded out of a wave. This is probably a function of the 27’s low freeboard through being a scaled down 100 footer. (NB: for artistic reasons there are lots of photos on the following pages of the boat with water exploding around it that may give the wrong impression - it is wet, but not THAT wet...)

So what applications do we see the DSS being used for?

As discussed there could be significant benefits for shorthanded ocean racing boats. While someone may certainly try it on an IMOCA 60 or a Class40, the boat we hope it ends up being tested on first is a Mini, which is traditionally the hotbed of design innovation. It would be a fun project to buy a relatively cheap fixed keel Proto and to retrofit the DSS to see how it faired against the canting keelers. Mmmm...

The system could also be retrofitted to conventional race boats - in fact in many ways it is best though of as a retrofit, a turbo for an existing yacht. This is soon to happen to Neal Pryde’s Welbourn modified Farr 52 Hi Fi. How easy retrofitting is on other boats will depend upon the layout, but often, particularly on bigger boats, the case for the sliding foil can be hidden away in the bilge.

As to how it will rate under IRC - the Rating Office have been kept informed at every stage of the DSS’ development, to the extent that they visited one of the tank sessions, and are fully up to speed with how the system works and its potential benefits, and will hopefully allow it a sensible rating.

A bunch of DSS-equipped boats on the race course could be most entertaining with their foils all sticking out, but Welbourn points out that while canting keels stick out to weather, the DSS foil is to leeward where the rig is already hovering over its footprint.

There is an obvious application in the cruising market - additional speed, while not priority is always welcome, plus the foil is more easily hidden on a cruising boat’s deeper bilges and there are the benefits of reduced heel and substantially dampened pitch and roll.

But we feel one of the biggest potential markets for the DSS is with large superyacht-sized sailboats where traditionally designing in enough stability is always a massive problem, as the keel simply cannot be deep enough to be practical. However on a DSS-equipped superyacht there is the potential to reduce the size of the keel bulb allowing the retractible foil to provide the necessary stability to allow the boat to be sailed in anger.

The DSS is the subject of an international patent and it should be noted that two law firms are backers of the project...

To find out more about DSS - go to their website: www.dynamicstabilitysystems.com

More photos on the following pages....

See video of the 27 here

What are your thoughts on this? Will the DSS be the next revolution in our sport? Email us here

Latest Comments

Add a comment - Members log in


Latest news!

Back to top
    Back to top