Keel design for shallow water

2015-01-21By Andrew Rice
Keels stabilise sailing yachts, but limit the boat to deeper waters.

As luxury yachts grow in length and volume, so their draught also needs to increase. But the deeper the draught, the fewer harbours and marinas become available. Balancing the need for shallow draught with the design requirements for performance is tricky.

The lifting keel seems to offer a decent compromise, and is becoming increasingly popular. However the technical challenges are considerable, and then there’s the added cost. One of the most impressive and extreme examples of lifting keel technology can be found on the recently launched _Hetairos _(known during her build as Panamax), the 60m ketch designed by Dykstra Naval Architects and Reichel/Pugh Yacht Design, and built by Baltic Yachts in Finland.

With the keel fully down, Hetairos draws nine metres, although there is also an intermediate keel position of six metres draught for navigating shallower waters. In the fully raised position, the keel will draw 3.5 metres.

Furthermore it is possible to raise the keel from nine metres to the six metre position while the boat is sailing and the keel is under load. This operation adds such immense loads and demands that the keel trunk is constructed incredibly strongly.

CFD analysis for the keel of the Philippe Briand-designed yacht P2

Naval architect at Dykstra, Erik Wassen is rightly proud of his involvement in designing Hetairos, but doesn’t believe that the ability to raise the keel while sailing is necessarily something that other owners should aspire to.

‘It adds a lot of complications, incredible complexity to the bearings and the structure,’ he says. ‘The cylinders need to be much stronger than if you don’t have that requirement. But it will be interesting to see if we have similar requests in the future.’

According to Wassen, the owner of Hetairos was particularly keen to have the ability to raise the keel while sailing, because he plans to go racing with smaller boats, around racing buoys in just seven or eight metres of water, less than the yacht’s maximum draught. As Wassen says, the complications go beyond the purely technical: ‘When we start raising the keel during racing, we also wonder how the yacht’s handicap will be affected.’ It will be an interesting test case for the rule makers.

As yachts grow to the proportions of Hetairos, some interesting problems arise as Jim Pugh of Reichel/Pugh points out: ‘As boats get bigger, with the increase in displacement, the loads become so much higher. The larger boat means heavier scantlings and higher loads making for a higher overall structural weight and lower ballast ratio. Compared with a smaller boat you’re actually losing (ballast ratio) stability. This means you have to look at other ways of gaining stability – a canting keel, water ballast or a lifting keel.’

For_ Hetairos_, the lifting keel is also supplemented by up to 24 tonnes per side of water ballast.

The Philippe Briand-designed yacht P2 as seen racing during the Loro Piana Superyacht Regatta

In the case of Vertigo, the 67.2m Philippe Briand ketch built by Alloy Yachts, the design team investigated the possibility of a lifting keel but in the end decided on a simpler solution – a 5.1 metre draught fixed keel with a carbon composite daggerboard that can be lowered through the bottom of the keel to increase draught to 9.1 metres.

‘When we started looking at the possibility of a lifting keel,’ recalls Briand, ‘we found ourselves venturing into an unexplored area. Today I believe Kokomo, at 59m, has the largest lifting keel ever. But it is very uncommon at this size. After discussions with the owner and everyone involved, we decided not to go for so much complexity.’

Akalam, a 32m yacht designed by Íñigo Toledo of Barracuda Yacht Design, is similar to Vertigo: she has a fixed 3.6 metre draught keel with a daggerboard that takes the draught to 5.5 metres.

The possibility of canting keels

But what of the canting keel? It’s had a chequered history in the world of grand prix racing such as the Volvo Ocean Race and the Vendée Globe where we have seen numerous breakdowns of canting keel technology. The number of life-threatening incidents should be enough to put any safety conscious cruising sailor off the idea.

Jim Pugh, however, says it would be unfair to dismiss the concept entirely. With Reichel/Pugh having designed the likes of Alfa Romeo and Wild Oats

‘They’re certainly worth looking at for the massive gain in stability you can achieve,’ says Pugh, although he admits they are expensive and high maintenance, and require a constant and reliable power source (and back-ups) such as a running engine available to power the keel from side to side.

Like Reichel/Pugh, Finot Conq is a design office perhaps best known for its work in the grand prix race world, but which now finds itself in increasing demand from ambitious superyacht owners looking for high-performance cruising yachts. The French design house beat off strong competition for the right to design a new 30m yacht with the simplest – yet most ambitious – of briefs from the client: to design ‘the world’s fastest 100-foot cruising yacht’.

Hetairos‘ lifting keel reduces draught from 9m to 3.5m. But fully raised, the keel protrudes above the boom

The resulting FC Cube 100’ would seem an obvious candidate for a canting keel, but as Finot Conq’s David de Premorel explains, they decided against it. ‘We did look at having a keel that was both lifting and canting, but it would have been a big weight penalty.

‘Since the intention for the boat is to do some of the big offshore races, you need a minimum AVS – an angle of vanishing stability – of about 105 degrees for a boat of that size. It’s something that is indirectly specified in the sailing instructions of these races, and also a basic safety feature for the boat. The problem with a canting keel is that, once you’re canted, your capsizing angle decreases.

‘If you want a canting keel and the same minimum capsizing angle, you need either a deeper draught or more bulb weight to compensate.’

In the end the canting keel option was rejected on safety grounds, but also with the problems of maintenance making it less attractive too. Instead, the FC Cube 100’ is being built with a lifting keel giving a 5.4m maximum draught.

‘We would have loved to have an even deeper maximum draught,’ says de Premorel, ‘but the lifting movement of the keel is limited by the height of the hull and deck. If you limit yourself to a certain figure when the keel is up, it mechanically limits you to a certain draught when the keel is down.’

The only other option would be to install the lifting ram above deck, and apart from the technical challenges, for most superyacht owners this would be too much of an aesthetic sacrifice.

The FC Cube 100’ uses a lifting keel that takes her draught from 5.4m to 3m

So what other options are out there? If you can’t achieve sufficient righting moment with one keel, what about having two? After all, we’re used to the idea of two masts. Briand doesn’t dismiss it as such a silly idea.

‘Twin keels have been done in the America’s Cup for some time, and it’s a configuration we studied when we’ve been involved in design projects for the Cup. So we know a bit about this, and yes, this could be a solution for bigger boats.

‘Maybe one time we will do it; the only problem we have with that is the ability for tacking and manoeuvring, as having one keel forward and one aft has a big effect.’

However, as Briand acknowledges, if the ‘tandem’ keel was worth considering for America’s Cup racing, and all the tight manoeuvring that this kind of racing entails, it should be good enough for the more sedate world of blue-water cruising.

‘I wouldn’t be surprised to see this in the future,’ he says. ‘We certainly wouldn’t rule out the possibility.’

While the tandem keel remains just a concept at superyacht level, one alternative configuration that is already making its way in the superyacht world is what Briand refers to as a ‘centreboarder’, otherwise known as a ‘whale body’.

The FC Cube 100’ claims to be ‘the world’s fastest 100ft cruising yacht’.

Instead of a standard keel configuration with a lead bulb attached to the end of a long fin, a centreboarder sees lead ballast incorporated into the bilge to provide the necessary stability. A lifting centreboard then pivots up and down to provide the lateral resistance whilst sailing, fitting into a recess in the hull for minimum draught when not required.

Briand is well acquainted with this concept. ‘We have done probably more than a thousand production boats like this so we know the naval architecture of this configuration. But this is not the kind of solution I have considered for large yachts, because compared with a lifting keel, it has a lot of downsides. It leads to a heavier boat. And the efficiency of the centreboard is also in doubt, because it requires an opening in the bottom of the hull and creates some drag. It is a much less interesting solution as far as the performance of the boat is concerned.’

Nevertheless, there is a growing demand from owners who are prepared to compromise ultimate performance for the ability to reduce draught to its absolute minimum. Malcolm McKeon of Dubois Naval Architects relates the story of Nirvana, a 53.5m, 2007 Vitters-built ketch.

‘The owner wanted to go world cruising with his family, and in order to be able to anchor near the beach, he didn’t want more than three metres of draught,’ he says. ‘We thought, as the design developed, we could convince him that it was unusual to go that shallow, and that we would persuade him to increase the draught to 4.5 or five metres, which is a more conventional fixed draught for a boat that size.’

When it became obvious that the owner really wasn’t going to accept a draught of more than three metres, the design office started looking at a variety of lifting keel and centreboard ideas.

‘There are two ways of achieving stability – with increased draught or increased beam. So with the extreme shallow draught, we opted for more beam and at the same time all the ballast was placed internally; in this instance we had to use 50 per cent more ballast than we would have done on a boat of this length.’

The hull and keel of Hetairos underwent computational fluid dynamics (CFD) analysis with a typical bending stress distribution analysis of the keel fin while heeled

Nirvana’s generous 11.6m beam is about a metre wider than it might otherwise have been; with the pivoting centreboard down, the draught increases from three metres to a whopping 10m.

‘The efficiency of the yacht under sail is exceptional,’ says McKeon. ‘The centreboard is a very high aspect ratio foil so it was made out of high tensile stainless steel to withstand the extreme loads. We tank-tested the design to confirm sailing performance.’

One of the additional benefits of the centreboard is how well it dampens the seasickness-inducing roll of a large yacht downwind, and while the pure performance will never live up to a lifting keel alternative, the success of Nirvana has now led to a variation on the centreboard theme with the 57.5m ketch built by Royal Huisman,_ Twizzle_, and to a third-generation 56m centreboarder currently under construction at Alloy Yachts.

The keel mechanism on Hetairos, and the design and construction of the keel box, is an extraordinary feat of engineering

Iñigo Toledo is also a fan of the centreboard concept and sees considerable hydrodynamic benefits compared with a conventional keel.

‘A fixed keel in a big yacht is limited by draught – the keel has to be short in height and long to accommodate the amount of area required [for lateral resistance]. With the daggerboard you get something higher aspect, deeper and narrower which is more efficient, more like a glider wing,’ he explains.

‘Also you have to build [a conventional keel] with a certain thickness and geometry so that it holds the weight of the ballast. There are structural constraints, whereas when you make a daggerboard you can actually just make the most hydrodynamically efficient profile.’

Toledo also believes the specialist nature of daggerboard manufacture results in a higher quality fin.

‘When people make daggerboards they somehow make much more effort to achieve a really good finish, more than when the keel is part of the hull. When you order a daggerboard from a composite materials company, the result is much better.’

McKeon sees more centreboarder superyachts, and Toledo agrees: ‘I would say in the future you will probably find 50 per cent of boats with fixed keels and 50 per cent with some kind of movable appendage,’ says Toledo. ‘Probably about 10 per cent will be lifting, and maybe the other 40 per cent will have daggerboards.’

The compromise between draught and performance is the perennial challenge of yacht design

Quite a prediction, given that every designer we spoke to acknowledges that a centreboarder will always struggle to match the performance of a lifting keel equivalent. What happens when you decide to take your shallow-draught cruiser to a regatta?

‘The problem with regattas is that the comparison is too fair and too cruel,’ admits Toledo. ‘You find out exactly where you are performance-wise. Some owners just don’t accept it.’

Better then to stay away? McKeon offers an alternative view.

‘What I think is great is when owners participate in these regattas, they can experience the full performance potential of their yacht. Some owners when they’re cruising are nervous about pushing the boat and how much one can safely heel over, whereas during a regatta the yachts are pressed a lot harder and they achieve more speed and ultimately the owners have more fun sailing their yacht.’

The compromise between draught and performance is the perennial challenge of yacht design. Whatever kind of configuration you prefer, Briand encourages all owners to take an interest in the appendage package of their yacht.

‘When you design a racing boat, it’s the first area you study,’ he says. ‘However, because appendages are underwater and never seen, they’re easily forgotten, but the appendage package is a big part of how the yacht performs. It is hugely important to determining the final quality of the yacht.’

Originally published: September 2011.

Kos Picture Source, Rick Tomlinson

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