Boundary layer theory 6 Jul 19



16º 27.052s 145º 22.049w

Sun Jul 07 2019

With apologies to Roger Hehr, Texas A&M Ph.D. candidate, wind tunnel test guy, and my test pilot for numerous Unmanned Little Bird programs (until I escaped in 2014).

Theory says that at the surface of the water the wind velocity is zero, but it immediately begins to increase as the height above water rises, ultimately to some steady state value once true “free stream” conditions exist. If I got that wrong, I’m hoping Roger will chime in and sort me out. In any case, one can easily demonstrate while kite boarding that the boundary layer and diminishing velocity profile certainly affect kite performance. It’s instructive to observe the “good” kite surfers just a few hundred meters from where we’re anchored, blasting up and down the smooth water just out from and parallel to the beach. Once they launch their bodies out of the water and the board is planing, unless they need additional power their kites assume a fixed height above the water surface.

When the wind gusts then tapers back off, the bar must be manipulated to modulate the angle of attack of the kite, but leaving the kite at a specific height above the water does at least ensure a fairly steady state wind condition in the absence of gusts. My own kiting skills progressed to the point this morning that I was able to travel upwind to the left and keep the kite pretty well sorted out. Going to the right, especially upwind, seems to be a challenge where my brain and body are struggling to make the connection, but progress continues. Isabel is a couple of lessons behind me in skill level, but she also is making great progress and can tool along to the left both down and upwind for a considerable distance. Like me she is also finding going back to the right a bit more challenging, but we’re both confident that if we spend enough time (and money), and with the excellent coaching of Adrien, we’ll get this sorted out. Once we’ve got our own kit we’ll be able to add a new dimension to our cruising lives, and increase the fun factor by heaps and bounds. We both love to sail, but we do spend a lot of time relaxing in anchorages so we might as well have several “parked” activities to feel passionate about. Kite surfing will soon join SUP paddling and snorkeling as fun anchorage pastimes.

Today it seems we’re gluttons for punishment. About the time we recover from our kite boarding lesson, Aline is going to join us at JollyDogs for a dynamic apnea training session. That’s where we’ll work on our free diving skills, descending to about 8 meters and seeing just how long we can remain before panicking and launching back to the surface. Both of us managed about 3 minutes doing the static apnea training in the shallow beach water, but that only required lifting our head to get a breath. This next round is going to be more of a mind game, as well we’ll be generating more CO2 simply by diving down so it’ll be interesting to see if either of us can even break 60 seconds. It’s a good skill to acquire, as the ability to hold breath and motor along for multiple minutes will greatly enhance our ability to sort out anchor chain wrapped around a bommie, or to spear fish and return with our quarry. If we survive the afternoon lesson, it may then be time to begin the spear fishing class. We’re loaded for bear with both a long JBL gun and more of a pistol JBL gun, and it would be good to selectively kill our dinner rather than take pot luck on whatever bites the hook.

One of the “downsides” to spear fishing in these waters is how interesting your kill is to nearby sharks. Apparently it’s quite important to get the kill to the surface and into the dinghy before Mr Shark makes dinner out of it. Aline recently had to fend off a shark or two after shooting a nice fish, and she ended up with a “shark kiss” as Adrien calls it on one foot. Neither Isabel nor I are interested in enjoying the affections of a shark, so we’ll hope to avoid that situation.

Cruiser parties are pretty predictable, but we do have a lot of fun. It’s reminiscent of a bunch of engineering geeks or motor heads standing around yakking about their favorite “whatevers”. For us, topics of conversation range from the weather, to engine or water maker problems, to how much booze costs around here, to the most protected anchorages, to “who made that dip and can we get the recipe”, to “hey, can we have a tour of your boat? Last night was no exception, and of course after a night full of dead calm conditions followed by almost gale force winds and torrential rains, everyone is downloading the most current weather forecast so we can yak about it on the VHF radio. Several forecast models are available, and invariably people pick the forecast that matches what they want to believe or need for their own personal cruising plans. Back at the giant aerospace company we called that “dial a data”. From my point of view it’s like my pal Chan Morse used to say; it’s like a man with 3 watches who never knows what time it is. Another one of my test pilot pals who died tragically in a crash, Chan uttered various witticisms that kept me entertained.

Wups – I gotta go! John and Simon on Halcyon what me to watch while they flip their mast top anchor / tri-color / flashing light on and off. They’re screwing around with the circuit breaker panel and wiring; naturally something we discussed last night.

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