Wind shear

As a skiff sailor I have to be even more seat of the pants, but I still found much of value in this thread.
I was the one who used the expression "Coriolis effect" but I'm not going to guarantee it's an appropriate use of it (and are too lazy to check). What is happening is that as the wind is slowed, it diverts from following the path of the isobar gradient line and heads towards the low pressure system. And it veers because if you face the wind, the low is on your left and the high on your right. Below, I'm going to call it the 'diversion effect' to avoid ambiguity.

The traditional mantra to twist in light weather was an apparent wind effect in a laminar flow wind and applied on both tacks. The surface wind is lighter than the higher one, so when the boat moves forward - at a constant speed at all heights : ) - you get a different apparent wind at each height.

But in these laminar flow conditions, it also makes sense there might be a divergence effect on top of that. I've certainly had experiences that I couldn't explain that would be consistent with it. And it's worthwhile knowing that I might consider vanging harder on starboard tack than on port in these often frustrating conditions (where you might have time for such adjustments).

Frank Bethwaite might be read as suggesting that once the wind goes turbulent (more than 7 knots), the divergence effect occurs suddenly at some height, with a definite boundary layer.

What makes me doubt that as being applicable to all turbulent wind conditions is that (on my cruiser/ racer yacht - with no VMG, just wind direction) I experienced the strange wind reading people are talking about here. So maybe there's something to it in some conditions.

He has two follow up books which are just as good; although they deal less with weather.

His writing style can be ponderous and to teach it to teenagers, I did a 'plainer English' version of the wind chapters he was kind enough to approve for me.

In Frank's first book is an interesting pic of a Tasar with no mainsail up. Instead it has about 5 streamers on a topping lift holding the boom up. When the boat is not moving the streamers are parallel. When the boat moves the top streamers are at a greater angle than the lower ones. This is a great example of the need for twist in our sails.

Frank was a great designer because he was relentless in experiment backing up theory. He worked out that rotating masts could have squared off aft sections, that wave humps could be reduced through a vee design, and many other great innovations. I watched him do hours upon hours of testing the first stages of the HSP project - he backed up all his theories with real world data. So in the spirit of the late great Frank this theory needs to be proven by experiment. This is how scientific theories are validated.

I advocate streamers on the topping lift on the mooring. Then jump in the dinghy and look from aft, or look up from under the boom. If this effect is true then you will get a consistent pattern of twist at zero boatspeed. If it isn't you won't.

I am intrigued by the anecdote about the gun sailors with huge wind sheer. I understand that angular wind shear (rather than wind shear by height which is the term's usual meaning) will happen but it is rare and I would say that it could be caused by a cool laminar drainage effect, a cool laminar layer down low and a different breeze above. We can see this happening with Cirrus clouds and it could happen within the height of our rigs. But even this example is uncommon.

My boat is not available today but I would be interested to do the experiment when I splash the little one in just over a week.

cheers

Phil

Hi Shaggy, Frank Bethwaite demonstrated this years ago, using a Tasar, so, not a tall rig at all.
Its why the boat needs to be trimmed "assymetrically", in light conditions. Mostly causes the conversation, when the helmsperson says the boat feels sluggish from one tack to the other, and the trimmers will say; but the settings are the same both tacks... facepalm!!
Telltales on the main leech at 2nd batten from top, and jib leech on your light air jib, about 25% from top of leech will help with the trim in the light stuff, when things just don't feel right..

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shaggybaxter said..

samsturdy said..
One things for sure Shaggy....you're not going to get dementure filling your brain with that stuff. You'll be
creating so many new brain cells your brain will be all muscle. Good on you.

thanks Sam! I had meningitis at the end of school, and had to relearn a lot of stuff, so maybe it's linked to that!
It was funny, I knew how to speak and write, but I couldn't recognise my family or remember anything I had ever done. I did quite a few years with specialists remapping the damaged part of the brain. Part of which was redoing my senior school again at night school, grrr.....
but it did give me the BEST excuse in the world for the next few years on forgetting to do things, and denying any knowledge of everything else!

The reason for the rambling story is you're dead right, by the act of learning something you actually physically are growing new neuron pathways, ie: adding muscle to the brain. You should charge people for consulting fees buddy!

Haha. Thanks Shags. Reading between the lines however gives an insight into a determination you have.
What you have achieved is entirely your own effort. Good on you.

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shaggybaxter said..

The Aus and NZ history section is just as interesting as the performance sailing tips.

Much as I liked and respected Frank, it has to be said that much, perhaps most, of his history is just wrong and grossly simplified into a caricature of stodgy keelboaters and northern hemisphere sailors versus innovative working class Down Under experts.

These days we've got so much material available on the web, like old newspapers, that you can look at bits like his piece on Una, for example, and see that not only is Frank wrong, but what he writes was actually directly opposite to what really happened. For example, he says that the 'establishment' were against Una, a little 18ft (ish) catboat imported to the UK in the 1850s. In fact, the "establishment" loved Una - the Prince of Wales got a Una boat and so did Lords, Admirals and other aristocrats. The Royal Yacht Squadron announced a race for Unas, and the Prince of Wales later ran a regatta for them that Queen Vic came down to watch. In other words, rather than being looked down upon by the big boat crowd (as Frank claims) the Unas were embraced. For more, see

sailcraftblog.wordpress.com/2016/05/20/sailcraft-pt-3/

Similarly, the skiff sailors of the 1800s weren't all rough, tough footballers who worked on the docks. They included leading lawyers and businessmen. In fact, as in their New York ancestors the sandbaggers, the social mix was one of the attractions. Many of the leading lights in the "skiffs" were also involved in big yachts, and at times small yachts were faster, lighter and much more innovative than the "skiffs", just as the British and German dinghy designs were often more advanced that the Aussie ones.

The claims that skiffies were also into development at all cost is utterly wrong - the 18 Footers, for example, started banning lighter boats as early as 1908. The Germans, in contrast, had fewer rules and faster dinghies so the implication that Aussies led the way is very wrong.

Sadly, like many other writers Frank bought into a history where stodgy sailors have held back development. The enormous amount of new information we can get these days shows that sailors were actually pretty damn sensible, that most major sailing countries did lots of development, and that design development is held back for what are normally pretty good reasons if you look at the issues like cost and the fact that most people don't really care about going faster. The good guys versus bad guys story, sadly, has got such a hold of us that it's harming the sport.