Barry 2019-04-05

Engineer Andre Moltschaniwsky of Lomocean Design looks at the question of foiling in power boats, a subject that has been brought to the front so graphically with the recent America’s Cup bots, both cats and monohulls.

It is fair to say that by now, most people out there with any kind of interest in boats at all will have a pretty good understanding of what ‘foils’ are and what ‘foiling’ is. This was illustrated very graphically for day after (painful) day last September as our screens were filled by the fantastic sight of two 72’ wing powered America’s Cup sailing cats blasting around San Francisco Bay on hydro-foils at speeds that a few power boaters can relate to, but probably not too many yachties.

The word ‘foil’ actually has a number of meanings and I was delighted to hear America’s Cup TV commentator and sailing legend Chris Dickson deadpan the expression “foiled by the fickle finger of fate” as race 13 was abandoned when the 40 minute time limit was reached, depriving Emirates Team New Zealand of the America’s Cup by a matter of only a few minutes. I had always heard a slightly modified, but less publishable version of the epithet, but being foiled in a competition that relies so heavily on a different kind of foil to achieve sheer, outright speed does have a certain symmetry to it…

So we know that foils can be used to make boats ‘fly’ above the water, but in technical terms, what is a ‘foil’? 

Leaving aside swords, very thin sheets of aluminium used to wrap food in and the verb for thwarting someone or something, a foil is basically what most people would call a ‘wing’ – and is the technical term for a lifting body that generates force on one side due to the flow of a fluid past that body. The fluid can be gas or liquid - air or water – and the density of the fluid, the speed with which it flows over the foil, the shape of the foil and the angle of attack (the angle at which the fluid flow meets the foil) all dictate how much lifting force it can generate - and how much drag is also produced. 

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