Helpful Cavities

Exploring the problems in sailboats approaching 60 knots (I can dream) we saw how cavitation is a real problem.

But apparently this phenomenon, when pushed to the extreme can be a real boon - at least for some objects like torpedoes and maybe for boats.  I refer to supercavitation.

As you will remember, cavitation is liquid turning into vapor (bubbles) due to a pressure differential - for instance bubbles forming on the low pressure side of a foil when that reduced pressure goes below water's vapor pressure. Huge drag ensues.

In supercavitation you turn a real drag into a huge boost by "simply" making the bubble large enough to encapsulate the entire object, thus drastically the amount of wetted surface and drag.  The trick to doing this is to inject additional gas into the bubble until it gets large enough.

The photo above is from the University of Minnesota "cavitation and bubbly flows research group" (as Dave Barry would say, a great name for a rock band).  It shows axisymmetric ventilated supercavitation with ventilation increasing and cavitation index decreasing from top to bottom, but I am sure I didnt have to tell you that.

At any rate, the military superpowers have long been interested in supercavitation as a way of making torpedos and other underwater projectiles go very fast. The Russians made the Shkval VA-111 torpedo which allegedly goes 500 km/h.  DARPA has the Underwater Express program which envisions a submarine at 100 knots and according to some sources a one-quarter size model has already been tested off Rhode Island.   Tillerman, have you noticed any unusual currents lately?

And now a private company claims to have a developed a supercavitating stealth watercraft.

And finally if you have a hankering to build one yourself, Scientific American can give you some hints, Although they do mention that it will require "a deep understanding of the fluid mechanics and hydrodynamics of this novel flow regime" this shouldn't be a problem for my readers (note I did not include myself).

Heat Saps

Sap.  In plants it is life giving and invigorating - how curious it is that the same word describes what heat and humidity can do to the energy of a no longer quite so young Laser sailor.

Summer in Abu Dhabi is not the exactly the same as in Europe and while the worst is in August, last weekend was tough with temperature of 45 C (113 F) and a high dose of humidity, making it feel much worse. Our club sails every Friday year round and that is great – most of the time.

But, I have to admit the heat took a toll on me last Friday.  We sailed 2 races and I did well in the first one, but it tired me out.  I had made a big mistake, wearing hiking tights because I thought the wind would be strong and the inserts behind my thighs would make me less tired. But the wind was only 10-12 knots and the extra insulation from the hiking suit more than offset any positive effect.  Between races I floated next to my boat but the water is warm and refreshes very little.

I considered calling it a day after the first race, but went ahead and sailed the second race, doing poorly. And sailing back after the race I could have taken it easy, but I couldn’t resist trying to catch a boat ahead and went into semi-race mode. Testosterone thinking.

I had been careful to stay hydrated during the afternoon with both water and sports drink, but by the time I got back to the slipway, having caught the other returning boat, I was running on fumes.

Pulling the Laser up the slipway took my last reserves and I had to lie down on the hull a few minutes - I was feeling lightheaded.  I de-rigged slowly and everything eventually sorted itself out once I got in the bar with a cold cider.

But, I will be sailing a two-hander over the next couple of months in order to ease up a bit on the physical effort in the heat. Plus I have been meaning to improve my acquaintance with a stayed mast, jibs and spinnakers and this will be a good time to do so. 

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.

Submarine Races

We sailors rightfully take pride and inspiration from the fact that we navigate our craft using only nature - the wind and our own physical efforts - and we routinely disdain stinkpots and other forms of practical locomotion (except of course when we need them, but let's not quibble).


But, even our noble sport might be disdained by another set of sportsmen who use only their own muscle power to race a water craft - I refer to human-powered submarine races.  Maybe this sport is not quite as developed as ours, but don't tell that to the engineering students who compete every year. 


The rules are simple - "teams must design, build and race flooded submarines piloted by one or two scuba divers, who must be fully enclosed within the hull of the machine. All propulsion power must be provided by the diver during the race (i.e. no energy storage devices such as flywheels or batteries are allowed), but otherwise the design rules are open to whatever innovation teams decide to use." 

I guess David Bushnell and his fellow Turtle crew members could claim bragging rights as having competed in combat conditions without scuba equipment.


Long dominated by the Americans who have held the International Submarine Races since 1989,  the first European championship event was recently held at Gosport where a Canadian team from École de Technologie Supérieure streaked to a world record in their submarine Omer 8 over a 13 meter section of the course where they were clocked at 7.03 knots. (I assume we can rely on this report, although it does not say whether the official timer was Rolex, IWC Schafhaussen or other). 


You can see a ten year history of the ISR Championships with some videos of the submarines streaking along, albeit not quite as fast as the above mentioned current world record. 


And for you lucky US readers, please reserve your calendars for June 24-28, 2013 when the ISR championships will be held once again in Bethseda, Maryland.