Bullets or wind gusts, how best to benifit from them.

[Dave_S]

I'm sitting on the windward side of the water on a sandhill with 20-ish knots of breeze from behind. The water is semi-glassy in the lee of the sandhill. I'm watching the turbulent air land in patterns on the water. It seems to land hard and move and spread quickly initially then as a whole, the footprint looks to make slow progress across the water however the airflow on the water surface only seems to slow slightly.... I'm guessing but I'd say the air on a small scale is travelling across the water surface when it lands at 15kn then 20seconds later is doing 10kn then 50 seconds later is doing 8kn and dissipates shortly after. The whole time the footprint moves and spreads much slower..... Maybe 4-5kn. So the air flow in the turbulence is faster than the progress of the whole of the turbulent pattern which seems right.

I assume the air continues to rotate above the water surface and is repelled by the water surface and this gives the variation I can see. This must equate to some odd wind changes along the height of the sail and the diameter of the rotating air must be dependant on the cause and distance from the cause.

Long story shortened.... Is there a strategy for using these bullets or gusts to get the most from them and hopefully to catch the next one. They did land in a fairly predictable manner.

[Alan Mighty]

Originally Posted by Dave_S

Long story shortened.... Is there a strategy for using these bullets or gusts to get the most from them and hopefully to catch the next one. They did land in a fairly predictable manner.

My understanding is that the experts reckon sailing strategies are few to none.

The question has been studied by several, including horticulturalists who were interested in using barriers to protect plants from wind; architects interested in how barriers (e.g. another building or row of them) influence the wind loading on a proposed building; and of course racers intending to compete on a course that has cliffs, buildings etc on one side.

With respect to sailing, I've a copy of Alan Watts, _Wind and Sailing Boats_, 3rd ed., 1987, on my bookshelf.

Quite a few variables to consider. Let's make it simple at say that you're near Slaughter's Gutter, just west of the Big and Small Sandhills on Mulgumpin (formerly known to white 'pfellas as Moreton Island) on Quandamooka nation country. And let's pretend the wind is SE (but it might be ESE, SSE, or whatever)

First there are the windward complications. The wind makes both a horizontal and a vertical angle of attack on the relatively smooth surface of the sandhills. The windward slope of the sandhill gives the wind lift, so packets of wind rise. That causes complexity on the leeward side, seen as eddies of wind on the leeward side, as some slower packets of wind and some faster packets of wind slam down to hit the water. And of course the slope of the sandhills is not constant, neither from N to S or in time as the wind changes ever to slightly from ESE to SE to SSE or vice versa. Adding to the complexity is that not all of the windward side of sandhill is bare (some has trees, grass, etc) and the windward side is not one simple even slope (multiple slopes, fore dunes, after dunes).

Second, you can make generalisations about the windward side, but the only generalisation that is close to true all the time is that at a horizontal distance from the ridge of the sandhill equal to 30 times the height about sea level of that ridge, the effect of the sandhill ridge will be close to 0. Of course, you're only interested in what happens to the air over the water. And you're mainly interested with what happens around the height above water close to the centroid of your sail plan. And all that the best theory says is that that air will be disturbed - and disturbed in different ways depending particularly on the wind strength and wind angle.

Third, Watts and many racers spend time observing and recording. For many established racing routes, habitues have notebooks that record where and under what circumstances a racer can find advantage.

When I anchor at Slaughter's Gutter, I'm only interested in where the bullets make contact with the drink. And I anchor to windward of that (delighting in the fact that at Slaughter's, Led Myne sits to the wind most of the time because the tidal flow is small except at Spring Tides.


So ... I've been no help at all.

Look for a copy of Watts's book (or books, he enhanced and repackaged the same gems of information many times). And keep a notebook of your observations, because that's the only way.

[Apollo366]

Head up in the gusts< fall off in the lulls.

[Alan Mighty]

Originally Posted by Alan Mighty

Second, you can make generalisations about the windward side, but the only generalisation that is close to true all the time is that at a horizontal distance from the ridge of the sandhill equal to 30 times the height about sea level of that ridge, the effect of the sandhill ridge will be close to 0. Of course, you're only interested in what happens to the air over the water. And you're mainly interested with what happens around the height above water close to the centroid of your sail plan. And all that the best theory says is that that air will be disturbed - and disturbed in different ways depending particularly on the wind strength and wind angle.

Oops. Mea culpa. That para is about the leeward side of the sandhill. Sorry.

[thomm225]

Originally Posted by Dave_S

I'm sitting on the windward side of the water on a sandhill with 20-ish knots of breeze from behind. The water is semi-glassy in the lee of the sandhill. I'm watching the turbulent air land in patterns on the water. It seems to land hard and move and spread quickly initially then as a whole, the footprint looks to make slow progress across the water however the airflow on the water surface only seems to slow slightly.... I'm guessing but I'd say the air on a small scale is travelling across the water surface when it lands at 15kn then 20seconds later is doing 10kn then 50 seconds later is doing 8kn and dissipates shortly after. The whole time the footprint moves and spreads much slower..... Maybe 4-5kn. So the air flow in the turbulence is faster than the progress of the whole of the turbulent pattern which seems right.

I assume the air continues to rotate above the water surface and is repelled by the water surface and this gives the variation I can see. This must equate to some odd wind changes along the height of the sail and the diameter of the rotating air must be dependant on the cause and distance from the cause.

Long story shortened.... Is there a strategy for using these bullets or gusts to get the most from them and hopefully to catch the next one. They did land in a fairly predictable manner.

Yes. You watch the water carefully and steer the boat in such a way as to keep it's attitude/heel the same throughout the gusts as it was before they hit. (you have to anticipate a bit as it hits or just before. You get into a rhythm)

This is one of the reasons some skippers win more races on one design boats.

Oh yes, and have the right sails up and settings for the conditions this would include batten selections if you have full battens (luff to leech)

[Dave_S]

Thanks all.

Alan, I'll look that book up.

I've been reading about how to surf squalls without ending up in their wake which is interesting reading as well. That's what made me think there might be a similar technique to best use the turbulence too.

I'm watching them again today. They definitely leave a lull in their wake and on the windward side where the air flow cancels out so if you are sailing into the normal state wind it would be better to skip across the leeward side (I don't think I could make use of the leeward side without ending up in the lull) and steer out to the front edge of them and not turn into them.....this is what I assume thomm225 is saying by maintaining heal (I'm a catamaran). If you turn into them the benifit would be very short lived and you would be left in a lull. It would be very hard to use them because of their speed across the water would overrun a cruising boat pretty quickly. Maybe further out from where they are landing would be better?