Throughout the 80’s those of us active in sailboard design kept looking for the most efficient designs for speed and early planing.
We found that glossed and polished bottom felt slippery at low or no speed, but that by sanding rivulets aka long parallel lines into the bottom of sailboards we picked up significantly more speed once the salboard was planed up.
In taking off from the beach, we devised a method of stepping onto the board and pumping the sail back and forth in order to fill the sail with enough wind to pop the board up onto a plane. The sailboard would lurch as we did this action than sit p onto the water much like a waterski does when it attans planing status.
By getting proficient at this maneuver I could step perform a beach start on a light wind day and get my board to plane on what would normally be a substandard day. Meaning, unless you knew how to pump your sailboard onto a plane, you would continue to lug forever.
We felt the benefit of increased speed from sanded sailboard bottoms long before the modern surfboard did. In fact it was Curren that came in and checked out our sanding of bottoms that was the first guy I knew that swapped it over to his boards. I mentioned we could substantiate this claim because we were clocking speed boards with radar.
As a further statement regarding testing or quantifying whether sanded bottom were faster, I had a conversation with Dennis Connors, winner of The America’s Cup and we compared what each of us were doing to facilitate earlier planing, sustained planing, reducing drag, improving laminar flow, and how drag coeffcients come into play with speed.
You can argue it all day long; sanded bottoms are faster. Greg Liddle has a good piece on it.
sheesh. in a transparent medium like water you can't tell at all whether the flow is laminar or turbulent. you can't tell at all just by looking at it. of course, that doesn't mean it doesn't happen.
And I suspect the differences in most situations are not going to be noticeable. There’s too much else going on.
We know that there is a point along the hull at which the boundary layer flow turns from laminar to turbulent. When the flow transitions from laminar to turbulent the boundary layer increases in size. Correct?
Turbulent flow is definitely visible to the naked eye, you can see it in the surfboards wake, and in the broken wave. I understand that it is possible to have turbulent flow that is not visible to the naked eye. And if, in fact, it is not visible in this situation how do we know that the boundary layer is turbulent and not laminar, in the case of the surfboard hull?
There is a good internationally available book with equations, graphs and illustrations with a chapter on this topic which probably agrees with MTB and DeadShaper. Aero-Hydrodynamics of Sailing, C. A. Marchaj. The author put out 2 books, the first is thinner and more available, the second is much larger, less available and corrected only a couple things from the first book, I don’t remember if the chapter about hydro boudary layer was modified in the second book, an intro page in the second book tells what was modified from the first book. Probably a must read, its common , libraries have it.
You cannot detect turbulent flow in water simply by looking at a flow. I think what you are looking at and interpreting as turbulent flow is actually vorticity (rotational flow, cause by separation) coming off the rails, tail, fins etc and mixing with air from the free surface of the water. While this flow that you are observing definitely looks “turbulent” it has nothing to do with a turbulent boundary layer. There are quite a few expermintal techniques to monitor flow structure, using small bubbles, lazers, dies, and many more i dont know of. I do know that you cant do it visually with the naked eye. When a wave crashes, the whitewash is a mixture of air and water, and it is definitely turbulent. BUT if it werent for the air that mixes in, you would not be able to see it. Turbulence in this case is unrelated to boundary layer turbulence.
Turbulent and laminar flow are very distinct, but dont confuse a separated flow with turbulence.
For the most part we used finer grits 220 was common. But we also had results with just about any sanded bottom in comparison to polished. We had one teamrider that raced with a board that sanded it similar to what Al posted here.
I always felt that glossed boards felt slippery at low speed…which, hate to say it, but after slalom and surfsailing, even fast days on my surfboards felt slow…kind of a bummer in some respects. But still hld to the fact that once the sailboards actually popped up on a plane they WERE faster. However, until they popped up on that plane, they felt ‘stickier’ than a polished sailboard.
The thing about the test tank…sometimes your eyes deceive you.
Right on…you guys are getting into a real complicated discussion here. I suggest you look at some of the very extensive earlier threads about boundary layers and such. Those threads get really deep into the subject including many things like water tension, etc. that you haven’t touched upon here.
I posted some stuff there, but fortunately for me I gotta go get in the shaping room and not worry about turbulence right now.
In world cup ski racing victory is often determined by a few 1/100ths of a second. The planing surface of the ski is riding on a micro-layer of water caused by friction.
Cohesion is the drag and must be overcome with different formulas of wax and bottom texturing. The stakes are high and tons of R/D has proven that a textured base
is faster.
Not sure exactly how the mountain relates to surf, but food for thought anyway.
In world cup ski racing victory is often determined by a few 1/100ths of a second. The planing surface of the ski is riding on a micro-layer of water caused by friction.
Cohesion is the drag and must be overcome with different formulas of wax and bottom texturing. The stakes are high and tons of R/D has proven that a textured base
is faster.
Not sure exactly how the mountain relates to surf, but food for thought anyway.
You cannot detect turbulent flow in water simply by looking at a flow. I think what you are looking at and interpreting as turbulent flow is actually vorticity (rotational flow, cause by separation) coming off the rails, tail, fins etc and mixing with air from the free surface of the water. While this flow that you are observing definitely looks “turbulent” it has nothing to do with a turbulent boundary layer. There are quite a few expermintal techniques to monitor flow structure, using small bubbles, lazers, dies, and many more i dont know of. I do know that you cant do it visually with the naked eye. When a wave crashes, the whitewash is a mixture of air and water, and it is definitely turbulent. BUT if it werent for the air that mixes in, you would not be able to see it. Turbulence in this case is unrelated to boundary layer turbulence.
Turbulent and laminar flow are very distinct, but dont confuse a separated flow with turbulence.
Thanks durbs, I have a few more questions that I don’t understand.
With regards to boundary layers how do we know that it is turbulent on a surfboard hull?
Separation does not necessarily create turbulence… the relationship between the two is that a turbulent boundary layer can help prevent flow separation, which is what causes stalling/drag in fins.
With regards to boundary layer being turbulent or laminar… there is a relation (uses Reynold’s number) that predicts when a BL transitions from laminar to turbulent.
I want to look some of this up before I say too much to make sure that I am relaying the information correctly, I’ll check out one of my books tonight and get back with some answers, maybe I can scan a few diagrams or two that might help. I’ll see what I can pull off.