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Tuesday, March 19, 2013

Sweet Spot Part 1

Optimising a system such as a sailboat is a fascinating process. Once the basics are right, picking the best settings for different conditions is an art in its own right. Varying degrees of scientific thinking and 'gut feel' based on experience seem to be called for and an open mind is definitely an advantage.

Even with very sophisticated tools, the core of the problem is the large number of interconnected variables. As with any endeavour involving the scientific method, all bias toward trying to prove a concept that is arbitrarily appealing must be 'checked at the door' if the results are to be objective. Then data gathered on the water has to be sorted, filtering out extraneous noise, so it can be used to validate the initial predictions. 
If done correctly, the discrepancy between observations and predictions will illuminate the designer about where predictions were mistaken. 
If the source of the discrepancy can be identified correctly, then the next lot of predictions should be closer to reality.
Those with an interest in such things will know that arriving at quantitative predictions requires a discipline and constraint quite at odds with the creative thinking process that generates the concepts in the first place.
To get useful numbers, many assumptions must be made so that each calculation discriminates between only a small subset of variables. Getting the assumptions right is vital and that is where real observations are of great value.

As the process is repeated, the calculations will get better and more predictive of actual behaviour.
On a macroscopic scale this process has been going on in design over generations.
Computational fluid dynamics and tank testing are a great example of how one tool has been vital in honing another to the point where the older one is almost redundant (almost!).
On a smaller scale, each individual project goes through this cycle. Each boat, once the hull shape, rig design and foil package have been locked in, goes through a process of discovery on the road to delivering its full performance potential.

Every sailor will know an example of a boat that just seemed to respond to a certain setup that was not the obvious first candidate. A bit more rake, a bit more jib twist, traveler down a few inches... The winning teams are the ones who accept the observations and figure out how they fit in the overall model.
Complaining that 'it isn't supposed to be faster like that', won't change the reality.
Just replicating the fast setting without understanding why it works is dangerously limiting since its effect may vary with different conditions so the advantage it confers cannot be relied upon. 

Part of what makes our game so engaging is that there is a very stimulating interplay of different forces, at the interface of two fluids, acting on different pieces of equipment that all have multiple dimensions of adjustment.  

This post was prompted by the work we are doing now to make sense of the observations we collected when Paradox last competed.
As explained in the post following the Gosford regatta, Paradox first raced with very conservative foil settings. 
For the recent regatta we dialed in maximum toe-in angle, giving the most aggressive lift profile in the fleet. 
Now that we have tested the two extremes in the range of this important setting, we can plot how lift and drag interact, giving us an informed guess at where the optimum might be found.

During the regatta we also tested two different sails from two well known sailmakers. Since Paradox is much more forgiving than other A Cats, it does not require as much mast rake. With a more upright mast the newer high-clew sails open up an unnecessarily large gap under the foot of the sail and we suspect this leads to significant aerodynamic losses. Once we settled on the lower-clew sail (last day), we replicated known fast settings for diamond tension and spreader rake and obtained a sail shape very similar to other leading boats. 
Having eliminated the rig as a variable (with the exception of rake angle which remains an area of investigation) we could be reasonably sure that variations in performance came down to foil settings. 

Details of the degrees of adjustment of our foils and their effects on performance will be explored in Part 2.

This great shot by Karen Parker

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