We are continuing our tests of L/V foils. The constraints imposed for this series are:
– Foils made of straight segments with minimum (hydrodynamically clean) transition radius.
– Total foil horizontal projection not more than 400mm to respect the inboard exclusion zone (tip-to-tip measurement) in the A Class Rule.
The two main variables to explore are:
-Foil immersion (draft).
Findings so far have been very interesting.
In short, the two variables listed above interact in a way very difficult to quantify but with a pronounced effect on handling.
Deep foil partially retracted.
Notice tip just beginning to breach the surface
As you can see in the above picture, we have made the verticals very deep so that we can explore a range of draft values. The goal being to fine-tune foil depth to one that gives an immersed area such that the range of values of lateral resistance yields the desired coupling between leeway angle and lift.
To obtain the desired range of values of lateral resistance, foil area has to be such that it gives enough hydro sideforce at full immersion (hull in the water) to counter maximum aero sideforce (driven by righting moment) with a relatively low Cl.
Meaning that when ‘lowriding’ the vertical has to produce enough sideforce to keep leeway angle small.
But the area must also be small enough that it will require a high leeway value (large Cl but before the stall) to generate the same sideforce at optimum ride height. Thus permitting some sideslip to unload the horizontal before it breaches the surface.
However, if foil chord is such that draft must be very large to immerse enough vertical foil area, then leeway coupling will become too dominant as a means of heave control.
Meaning heave equilibrium will be reached long before the inboard tip of the horizontal foil reaches the surface.
Therefore, in situations where the speed is high but leeway is small, the boat will tend to fly too high. This in turn could lead to loss of ‘grip’ by the rudders/elevators.
So one of the interesting realisations to come from this testing is that chord is actually a critical value in the design of stable L/V foils. Our iterative approach, combining a range of vertical and horizontal foil designs at different angles, is proving to be a quick, economical and fun way to explore this new design space.
Check out the following video to see an unedited run on one foil in barely enough wind to be on the wire upwind. The acceleration is addictive.