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Old 17th January 2007, 06:05 PM   #11
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Default RE: Cutouts - fact or fashion

Although I do not know much about hydrodynamics IMHO, the main target for cut outs is to improve control at high speed and reduce drag since both are related. Reduced drag might also help in putting a hull on a plane earleir...maybe.
Obviously it will depend on a correct design for it to work properly. It might even improve or change slightly the way a given fin works.
The idea of setting different possible depths sounds good however in the real world I believe it might be not very practical.

This subject seems to me a good application for fluid dynamic softwares. I do not know if they're in use in the windsurfing industry but I know from expeirence that some time should be spent to validate it ( in that applicatin ) otherwise results would be meaningless.

My two cents.
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Old 17th January 2007, 09:26 PM   #12
geo
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Default RE: Cutouts - fact or fashion

Yes of course, but the point is in how drag is reduced and/or control is enhanced.
And, of course, in how fluid dynamics software models are conceived, which requires to understand the physical facts as the first thing.
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Old 18th January 2007, 05:38 PM   #13
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Default RE: Cutouts - fact or fashion

Knowing why and how is always best however, sometimes not really necessary. Provided you validate your models and metrics you can come up with say version A x version B and choose the one with the best results.
The hardest part is to validate your models specially when there are lots of variables which are evalueted subjectively. One can say in which area a new board is better but cannot objectively measure it.....(yet). The task and job would be really huge but it has been done in other engineering areas.
Simulation programs are target to reduce the number of prototypes required to check a new design.
As far as I know the windsurfing industry has been improving its products through massive experimentation. Sometimes trying things that shouldn't work but that end up working in a different hull, for instance.
Working with simulation would in time reduce experimentation a lot and therefore the money spent in so many prototypes.
Example: in the auto industry a few years back sometimes one would have to actually crash 10,15 even more prototypes to get the desired results. Nowadays they crash only 1,2 to confirm simulation.
Hopefully all the money saved would mean cheaper products .....maybe
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Old 20th January 2007, 06:06 AM   #14
erling
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Default RE: Cutouts - fact or fashion

I am not all convinced that cutouts, contribute directly to board efficiency. This is because the cutouts on most boards leave an irregularly shaped planing surface, with reduced aspect ratio.

A positive effect of cutouts may be that they remove lift from the back of the board, thereby bringing the center of board lift forward. This is important as the center of board lift normally move towards the back, as speed increases.

In other words, a benefit may be that we can keep a well balanced stance in the foot straps, from marginally planing to top speed.
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Old 20th January 2007, 08:08 AM   #15
steveC
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Default RE: Cutouts - fact or fashion

I'm not sure that simulation testing is less expensive than prototype testing, however, I can't solidly validate this opinion one way versus the another. Yet my thought is that prototype testing is probably the most versatile, and ultimately less costly, especially if a number of candidate designs or versions being experimented with is numerous. What it really comes down to is the number of focus concerns. In reality, I think a practiced tester is better positioned to discern the range of performance in a practical way to the designer, particularly if varied testers are used.

Regarding erling's comments, I have to think that cutouts aren't targeting an irregularly shaped planing surface, as most designs are clearly balanced off the centerline. Of course, the actual shape of the wetted area can be viewed as irregular shaped when considered solely off centerline, but the sum of both is balanced overall. No asymmetrical concepts in today's offerings. Nonetheless, I have to think that overall lift is affected by cutouts. Similarly, I would think that overall range of performance is crucial to an optimum result, so cutout concepts must be efficient and productive across a broad range from light to strong wind conditions.
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Old 21st January 2007, 07:47 PM   #16
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Default RE: Cutouts - fact or fashion

Guest,

Knowing how and why is best, period. It may not be necessary, but it can also save lots of time and energies. I think that knowledge of physical facts that are behind board performance is even more useful than models, due to the high degree of subjectivity and the complexity of our sport.

Erling,

that is what I was saying: IMHO a superwide tail with no cutouts is the most efficient, and the need for cutouts is more related to longitudinal stability than else. As for the irregular surface, it should not be a problem as the trailing edge of a cutout would behave just like a tail's trailing edge. Only, I guess the angulation between centerline and the side of the central portion of the hull (the one between the cutouts) could be critical as it could not have a sharp release and therefore generate turbulence and drag.

steveC,

I agree about the idea that proto testing is probably the best way to go, both economically and practically. One can get new ideas even from a screwed up proto, plus I don't think there can be a sophisticated and complete enough model for numeric simulation of a sailboard (+ rig + sailor) hull's behaviour. This makes clear ideas about the physics even more important.
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