It is commonly thought that bilge keel boats offer the benefits of shallow draught and the ability to dry out upright over low water, at the expense of performance. However, that is by no means the whole story – a well designed and twin keel boat may have on average better performance than some single-keel yachts.
However, it has to be said that many of the early bilge keel designs were not in any way efficient. These tended to have very shallow keels, that often had minimal aerofoil shaping, and were attached to the boat vertically. It's no surprise that these boats did not sail well, especially to windward, although they tended to benefit from good directional stability - a factor that has long been valued in cruising boats.
The importance of (heeled) draught
It was not long before designers started to look for improvements. One of the first yachts to be tank tested, long before computer design tools enabled designers gain a better understanding of their products, was the Macwester 27 in the early 1970s.
As a result the new boat gained much deeper keels, with draught increasing from the 0.75m of the earlier Macwester 26 to 1.2m – a 60 per cent increase. Equally importantly, the keels of the new boat were splayed out at an angle of around 10 degrees from the vertical, and given an efficient aerofoil shape. The angle of the keels to the vertical means that when the boat heels the leeward keel becomes almost vertical, and is therefore efficient at resisting leeway than a single keel at a large angle of heel.
Despite many similarities between the two boats, the sailing performance of the Macwester 27 was in a totally different league to the 26 and put a number of fin keel designs of the era to shame. We should perhaps consider the draught when the boat is heeled as being a critical factor in minimising leeway. This hands an advantage to a twin keel boat with the same draught when static as a single keel one – the latter’s draught will reduce, whereas the twin keeler’s draught will increase.
Astute designers continued to work on the efficiency of bilge keels - there was huge demand for such boats, in the UK especially, throughout the 1970s and much of the 80s. A further refinement was to increase the angle of the keels from the vertical to around 20 degrees. This ensured one keel would be pointing directly downwards when sailing with the boat fully powered up, while the other would be almost breaking the surface, where it would both be reducing drag and where its weight would be most effective at increasing stability.
The 26 foot Westerly Griffon, designed in Ed Dubois in the early 1980s, for instance, can be seen with its windward keel popping out of the water in wave troughs when well heeled. When sailing the boat there is also a familiar tell-tale thud of water when a wave top hits the under side of the windward keel. In only a dozen years efficiency of bilge keels therefore advanced enormously.
A further refinement was an asymmetric shape to the aerofoil section of the keel, such that the immersed keel would provide lift that would tend to ‘suck’ the boat up to windward. David Thomas’s designs for the UK boat builder Hunter Boats in the 1980s, 1990s and 2000s were among the most efficient twin keelers produced. These had efficient deep keels, with a high ballast ratio and small bulbs on the bottom of each keel to keep weight as low as possible. Combined with an efficient rig, these boats were frequently faster than larger fin keel designs, yet retained the benefit of excellent directional stability that was seen on early twin keel designs in the 1960s.
Today there are very few manufacturers of bilge keel yachts – there's a sense in which they have fallen out of the mainstream fashion, even though many of the later designs were efficient. The French RM range of 35-44ft yachts however bucks this trend - all the company’s models have twin keels. With the RM1260 winning the Family Cruiser category in the European Yacht of the Year Awards at the 2013 Dusseldorf Boat Show, maybe other manufacturers will again start to look at twin keel options.
Fin keel advantages
So what then are the advantages of a well-designed single keel boat over a twin keel model? It becomes impractical to design a twin keel boat with a genuinely deep draught, so while a twin keel vessel of moderate draught has the potential to outperform a sister ship with a single keel of equal draught, a deep keel variant will always be quicker.
In addition, a boat with a deep single keel will have less wetted surface area, and therefore less drag, than a twin keel model. This particularly has a bearing on performance in light weather, with the twin keeler at a disadvantage.
Would I buy a twin keel boat? Definitely, if it suited my priorities at the time and I could find a good one at a suitable price. However, many owners of later bilge keelers seem to keep some for a long time and those that do change hands often attract a premium price compared to similar fin keel boats.
For more useful boat-buying tips see How to Choose the Right Weekend Cruiser, Choosing the right family cruising boat and Getting a boat surveyed: an essential guide.