A tale of foils and of a rather rapid International 14 design from Dave Hollom

We became involved in International 14s seven years ago when the guys at Composite Craft asked us to design a set of foils for the class (daggerboard, rudder and horizontal lift foil). The problem with the foils on a 14 is that, due to the speed range of the boat, they have to work over a wide range of Reynolds numbers (Res) and annoyingly they span the magic Re of 1,000,000. Above an Re of about 1,000,000 laminar flow becomes increasingly difficult to maintain and, to minimise drag, you have to start designing to maintain laminar flow. Below 1,000,000 laminar flow is easy to maintain but you have to design to minimise the negative effects of laminar flow at these lower Res. This makes designing foil sections to run over this range of Res challenging.

The lift from the horizontal foil is limited by the pitching moment caused by the overall weight being ahead of the lift created by the foil. That is why the crew trapeze as far aft as possible, putting their weight as near the foil as they can to maximise the lift for a given pitching moment. However, it occurred to us that if we mounted the horizontal foil on the front of the rudder rather than the rear, which seemed to be the favoured location at the time, because we would reduce the lever arm, we would be able to further increase the lift for the same pitching moment. And so it transpired and we were fortunate enough to secure second, third, fourth, fifth and ninth in the Prince of Wales Cup week that year.

Unfortunately, having the horizontal foil partly ahead of the rudder results in the front of the foil being unsupported and when landing off a wave the force deflects the front of the foil which increases its angle of attack. This increases its lift, which deflects the front even further and so on until the foil self-destructs. This is a classic case of divergence. The cure was to have a pair of leading-edge extensions on the rudder, one above and one below the horizontal foil, to support the front of the horizontal foil.