Friday, July 6, 2012

Harmful Cavities

Recently we looked at human-powered submarine races.  Today we take a look at another water craft which is certainly a sailboat since it floats and has a wing sail and generally has the elements one would expect to find.  But it doesn't exactly fit what I think of as a sailboat.

In the sailing world I am used to, masts are at 90 degrees to the hull and the water (well, sometimes my Laser mast is parallel with the water).  Not so on this boat.

And in the sailing world I know, it is a given that the hull is in a direct line with whatever direction the boat is headed. Not so on this boat - which has the hull (or, as they refer to it, the main fuselage) and beam at 20 degrees to the actual direction of travel.

All this in an attempt to beat the current world speed sailing record of 55.65 knots held by a windsurfer (assuming the windsurfers don't do so first).

This boat is actually the second (and hopefully improved) version of the original prototype which reached 52 knots.   Check it out here - the Sail Rocket.

Which brings us to cavitation - a problem usually associated with propellers.  Apparently it is also a major problem for the foils on boats approaching 60 knots when water turns to vapour due to the very low pressure on one side of the foil, creating a cavity or bubble which produces lots of drag and loss of stability.  Another way of explaining why that matters -  the pressure exerted on a surface by water is about 14 psi and the pressure exerted by vapour is almost zero,  a difference of nearly 10 tons per square meter.  Ouch.  The cure for such cavities is not a good dentist, but to find a way to "ventilate" - by getting air at atmospheric pressure into the cavities created by the cavitation and eliminating the drag-inducing pressure difference.

The good news is that since cavitation is not a big issue until around 60 knots, it is one less worry for me on my Laser.

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