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Old 13th October 2010, 06:09 PM   #21
Farlo
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Simple physics, yes and no. Lateral force on mast base can't be so strong or you could never go upwind and ride the fin. Then it doesn't matter so much how high is the mast track. Lowering sail center of effort by a few centimeters may help, but I guess sail design and proper rigging are far more important. Recessed deck follows the trend towards flat boards & backward volume. You need some surface ahead to get going but once on a plane it should disappear. Another few years and you will obtain... a Kitesurf.
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Old 13th October 2010, 10:13 PM   #22
Ken
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Stability and control are different things and not even sure about your statement.

Think its this sort of reasoning that original poster is talking about.
Where the sail connects to the board, the closer to the surface of the water equals more stability. I have an iSonic 111 and a HiFly 105 Move, both almost the same in volume (108 vs 105). The 111 is wider - 68.5 cm vs 63 cm for the HiFly. The HiFly is 16 cm longer. The biggest difference is the thickness of the deck and the HiFly is much more "tippy". Width plays a role here but if you take this concept to the extreme and made the HiFly deck much thicker and shorten the board without changing the volume or width, it would be even more unstable at rest because you moved both the sailor and mast further off the water's surface. The higher off the surface of the water you place the mast and rider, the greater the likelihood of turning over or falling off.

With that said, I think that the thickness of the board has more to do with the stability than the slight recess of the mast foot connection, but both do help with stability.

It's a little more difficult to measure the stability differences while moving at 20+ knots, but I believe it is still there with the thinner board and lower mast foot.

Stability and control are different, but they are closely related. Unstable = less control and Stable = more control. I guess I would use the word stability when talking about slogging and control when planing. This whole thing can get pretty murky if you think of a formula board that is very stable at slow to medium speeds because of the board width and thinness, but can have huge control issues at high speeds.

It is simply a matter of physics.
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Old 14th October 2010, 05:31 AM   #23
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Everything is a matter of physics.

This is a matter of some simplistic physics with some aspects missed out and lots of assumptions thrown in.

There are just so many variables missed out;how far off water feet are (which transmit more power than mast foot). Centre of lift of board; centre of lateral resistance;centre of buoyancy.Total CoG (not just boards)

Stability and control are almost mutually exclusive.Narrowest least stable boards have far better total control.

We do what the marketing depts do. We chose particular aspects of the physics and quote them to justify the bits that have been added.

Windsurfing is full of half truths and sadly the objectivity that is demanded in physics (in the real world) is sadly lacking in the world of windsurfing.

I`m afraid windsurfing is full of mis- conceptions. Fair enough this might not be one of them but its certainly by no means "simple" physics. (there`s no such thing)

How many times do we hear

"Cut outs reduce drag" (They actually increase it !!!)
And others !!!

We accept so much subjectivity; opinions and here say.
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Old 14th October 2010, 07:36 AM   #24
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The interface between aero and hydro dynamics is always going to be an experimental field with so many shifting parameters nothing can be taken for granted. Anyone interested could have a look at the books by Marchaj on this subject. Sailing Theory & Practice runs to over 400 A4 sized pages while Aero-Hydrodynamics of Sailing runs to 685 pages. While written in 1964 and 1979 these remain a great reference.
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Old 14th October 2010, 05:46 PM   #25
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Quote:
"Cut outs reduce drag" (They actually increase it !!!)
Euhm, I'm very curious how you explain this ?
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Old 14th October 2010, 09:23 PM   #26
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I agree, it's extremely complex and the designers are still experimenting to find a better way. You don't find better boards every year, but they do keep getting better over the long haul.

Assuming cut outs reduce the wetted surface, one might conclude that there would be less drag?

Evaluating board performance will always be subjective - there are too many variables for it to be objective. This leaves us with no other option but to accept subjective opinions.
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Old 15th October 2010, 12:15 AM   #27
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Everything is a matter of physics.
As someone a bit interested in philosophy (of mathematics...but still) I actually beg to differ. Remember that physics is the STUDY of nature (simplistically speaking). It's not nature itself. So a more appropriate way of describing thing would be that the behavior of windsurf boards can be described and analysed in terms of physics. But one can also look at it purely from en empirical point of view. Ie go out and sail see what works and have some other form of more or less systematic way of "transferring" the experience to new board shapes (something which could very well be a tacit type of knowledge).

And it is in fact FAR from clear which is best. As a scientist myself, I of course want to believe in the scientific way of working and that working from general principles of physics (hydrodynamics etc) would be the best way of doing things. But since the matter is so unbelievably complex I think the more tacit form of understanding in practice has proven to be better. Sometimes this form of knowledge can of course be informed from general physical principles, though. But I seriously doubt ANY board designer works purely from physiscs. I doubt even sail boat designers do.

Legendary bike designer Keith Bontrager formulated it well when he said that design is part art and part science. A combination of creative thinking and disciplined thinking.
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Old 15th October 2010, 12:30 AM   #28
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Cut outs do not reduce wetted surface area;they change its position.

A board going at a given speed needs a certain amount of area to support the load.Reducing area at rear means board will just find required lift elsewhere to find equilibrium.

One element of drag is directly related to the length of the trailing edge;its why aeroplane wings;our fins (for speed) have straight trailing edges. (or curved but not cutouts) Cutouts increase trailing edge length and hence increase drag;

Modern boards use big fins ; big fins need a wide board to provide the leverage to prevent excessive railing;fin lift. At some point as board increases speed the rear of the board starts producing too much lift;we see this as control problems;tail walking etc etc.(ie board is producing more lift than loa

Cutouts make it possible to build a wide board but with a small surface area.A wide board without cutouts would have less drag but severe control issues.

They dont work by simply reducing drag.Its much more complex.
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Old 15th October 2010, 12:39 AM   #29
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I agree with your point Ola but we shouldnt then try and use the physics to collaborate/simplify/justify what is in essence informed experimentation.
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Old 15th October 2010, 02:43 AM   #30
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Quote:
Cutouts make it possible to build a wide board but with a small surface area.A wide board without cutouts would have less drag but severe control issues.
So you mean that cutouts are improving more control than direct speed. The total wetted surface stays the same. The cutouts simulate a thinner board in the tail while planing, so more control.
I'm following this.

I don't understand why a longer trailing edge increases drag. Is it because the smooth and continuous tail curve gets "interrupted" by the cutouts - when planing - and in this way also disturbs the hydrodynamic flow ?
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