While the vast amount of data on a wide variety of racing boats is already out there, the potential complexity of the new VO 70s has increased the need to make such analysis. “In the days of the VO 60 it was relatively easy, there was one keel, one rudder and we just got on with the job of optimising the hull shape. Now we’ve got any number of appendages down below the hull and we have to sort those out before we move on to looking at how they will influence the hull shape and what the various options are from there,” says Steve Morris.
Ian Campbell of the Wolfson Unit in Southampton has considerable experience in the tank and is clear about the challenge facing designers and the need to tank test. “Tank testing means you can establish fundamental resistance properties very quickly and this allows you to see how these properties vary with basic parameters. Tanks are also good for experimenting with appendage arrangements. Anything that involves wave making resistance is also well served by tank testing,” he said. “It’s also possible to establish whether foils are being overloaded under certain conditions. This can help designers to decide whether the rig or the foils need moving to improve the balance,” he continued.
One leading design office already well versed in canting keel configurations is Reichel Pugh Yacht design, but, as Jim Pugh explained, tank testing still remains an essential part of their development programme. “A lot of the work you do in tanks is like checking in, making sure that you're working along the right tracks,” he said. Creating a test plan is very important indeed, where identifying what you're trying to learn is as important as the information you might gain. We aim to break our designs down into several parts, namely the appendages, their positions, the hull shape, the canting system and then a complete stability study versus performance. Having assessed each component of the package we then put the pieces back together. From there we then set about finding the best hull shape to get around the race track.”
But, while time-honoured tank testing is clearly an important part of the design process, a relatively new computer based tool, computational fluid dynamics (CFD), is changing many designers’ approaches. The process works by dividing up the hull’s surface into millions of tiny panels, typically around 1mm square, for which individual calculations are made on the resistance of each little patch. The individual results are then collated to form a map of resistances and pressures.
Although CFD does have some limitations when it comes to predicting waves and other free-surface effects, the process is good at assessing the differences in other areas deeper down from the waterline such as the hull and keel interaction, as well as the effects of the position of the centre of buoyancy. “Using CFD can allow designers to run through a wider range of variables, especially when it comes to varying appendage configurations,” says Campbell. “One of the keys to these new canting keel boats will be the balance between the dagger boards, the keel and the rudder. Getting the balance right between a negative leeway angle and positive lift on the keel is always an issue. Negative lift on the keel can mean big drag values. CFD is particularly good for assessing this. Where tank testing really scores with designers is when they are simply looking at the way the model moves through the water. Designers are generally very good with their eyes and tank testing provides a good additional visual guide as to what's going on,” he adds.
While the tank testing tools of the trade and the lessons learned in the mid 1800s may provide a better understanding of the issues involved, Steve Morris is under no illusion as to the effort required to recognise what might make a successful new design. “Tank testing is essential to establish the variety of combinations and trade offs. It’s a big job and the tremendous complexity of the VO 70 rule makes your head hurt!”