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brynkaufman2
Joined: 10 Sep 2002 Posts: 383 Location: Kailua Oahu
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Posted: Mon Mar 12, 2018 1:53 pm Post subject: |
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My foil is at the very back of the board. I am a little bit back foot heavy so I would like to move it up just an inch or so. I think then I would be evenly balanced, but it would still be near the back.
If the board is longer it makes sense to have it forward even more but I think putting it in the middle would be too far forward. |
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ittiandro
Joined: 22 Nov 2009 Posts: 294
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Posted: Mon Mar 12, 2018 2:52 pm Post subject: |
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grantmac017 wrote: | Good way to learn how to replace a fin box, but never going to fly even one cm. |
OF COURSE IT WON'T FLY EVEN ONE MM (!) IF YOU BREAK THE FIN BOX !
DO YOU ALSO IMPLY, HOWEVER, THAT IT WON'T FLY EVEN WITH THE FIN BOX IN PLACE?
PERHAPS THE WORD “FLY” IS EXCESSIVE, BECAUSE MY INTENTION IS NOT MORE AMBITIOUS THAN HAVING THE BOARD RISE OR SKIM JUST ABOVE THE WATER IN A QUASI-PLANING MODE, NOT DARTING 1 M. ABOVE IT LIKE THOSE SOPHISTICATED AND EXPENSIVE CONTRAPTIONS.
AFTER ALL, IF YOU CAN PLANE ON A BOARD WITHOUT ANY SORT OF HYDROFOIL WING, SHOULDN'T IT PLANE EVEN EASIER WITH THE ADDITIONAL LIFT OF THE HYDROFOIL WING, PROVIDED, OF COURSE, THAT THE WING DOESN'T COME OFF?
THIS THE CRUCIAL POINT!
IT OCCURS TO ME, THOUGH, THAT IF FLIMSY, ULTRA LIGHT MODEL AIRPLANES MANAGE TO FLY IN ( AND AGAINST!) THE AIR, SOMETIMES TURBULENT AT THAT , WITHOUT BREAKING THEIR WINGS OR BEING SHATTERED, EVEN THOUGH THEY GO MUCH FASTER THAN A WINDSURFER IN LIGHT WINDS, PERHAPS THE HYDROFOIL WING OF A WINDSURFER MOVING IN LIGHT WINDS SHOULDN'T ENCOUNTER STRONG ENOUGH WATER RESISTANCE TO RIP OFF A US FIN BOX, UNLESS, NOT BEING AN ENGINEER CONVERSANT WITH THE MATHEMATICS AND PHYSICS OF HYDRODYNAMICS I UNDERESTIMATE THE FACT THAT WATER HAS A HIGHER DENSITY THAN AIR.. STILL DOES THIS REALLY MAKE A |
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dllee
Joined: 03 Jul 2009 Posts: 5329 Location: East Bay
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Posted: Mon Mar 12, 2018 3:16 pm Post subject: |
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Obvious, water is a much denser medium than air. How much is for google to know.
What we do know is that for straightforward single US box, around 13" blade or 11" pointer is near max allowable loading, for moderate sized pilots.
So, possibly low aspect FRP? |
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grantmac017
Joined: 04 Aug 2016 Posts: 946
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Posted: Mon Mar 12, 2018 3:47 pm Post subject: |
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Google "Reynolds number" and get back to me once you understand the concept fully. Once you finish that I can explain why your idea won't work. |
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dllee
Joined: 03 Jul 2009 Posts: 5329 Location: East Bay
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Posted: Mon Mar 12, 2018 3:56 pm Post subject: |
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Possibly, moving mast track forwards would create more lift so a more centered stance can be used. |
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ittiandro
Joined: 22 Nov 2009 Posts: 294
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Posted: Tue Mar 13, 2018 2:11 pm Post subject: |
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grantmac017 wrote: | Google "Reynolds number" and get back to me once you understand the concept fully. Once you finish that I can explain why your idea won't work. |
THANKS FOR YOUR REPLY.
I HAVE LITTLE KNOWLEDGE OF PHYSICS AND SCIENCE, BUT A FAIRLY GOOD REASONING POWER.
I GOOGLED UP THE REYNOLDS NUMBER AND I WAS ABLE TO TO UNDERSTAND THE BASIC CONCEPTS UNDERLYING IT AND THE RELATED ISSUES.
I DO NOTICE THAT THE KINETIC VISCOSITY OF WATER AT 20° C (9.7937) IS ALMOST 10 TIMES AS MUCH AS THAT OF AIR ( 1.5111) THEREFORE THE REYNOLDS NUMBER OF WATER IS ALSO ABOUT 15 TIMES THAT OF AIR.
THIS IS BASED ON A CHORD LENGTH OF APPROX. 8 INCHES, WHICH IS IN LINE WITH THE DIMENSION OF A HYDROFOIL WING ON A WINDSURFER.
I TAKE THE SPEED OF 22 KNTS TO BE THE SPEED OF THE BODY MOVING IN THE FLUID, IN THIS CASE THE BOARD, BUT I WAS NOT TOO SURE WHY IT IS REFERRED TO AS THE VELOCITY OF THE FLUID ITSELF, UNLESS WE TAKE THE RELATIVISTIC EINSTEINIAN VIEW THAT WHEN A BODY ( IN THIS CASE THE BOARD) MOVES RELATIVE TO ANOTHER STATIONARY BODY ( THE WATER) , BOTH CAN BE SAID TO MOVE RELATIVE TO ONE ANOTHER.
I KNEW THAT WATER HAS A HIGHER DENSITY OF AIR, BUT I HAVE NO IDEA, HOW TO ESTIMATE , IN PRECISE TERMS OF PHYSICS, THE STRESS ON THE WING CAUSED BY THE HIGHER KINETIC VISCOSITY OF THE WATER .
IN OTHER WORDS, HOW CAN THIS CAUSE SUCH A STRESS ON THE WING AS TO BREAK THE FIN BOX AT A SPEED OF ABOUT 22 KNTS/HR GIVEN IN THAT SITE?
TO FULLY SUBSTANTIATE YOUR CONTENTION ( AND TO MAKE ME UNDERSTAND) THAT THERE WOULD BE A TREMENDOUS LOAD ON THE WING, WE HAVE TO TRANSLATE THE KINETIC VISCOSITY FACTOR OF WATER INTO A FORCE ACTING ON THE HYDROFOIL., IDEALLY A FORCE EXPRESSED IN KG.
IF SUCH A FORCE IS TO BREAK IT, IT MUST BE SIGNIFICANT, AT LEAST THE WEIGHT OF THE SAILOR!
IT MAY BE SO, BUT HOW IS IT CALCULATED , BASED ON THE SPECIFIC DENSITY OF WATER?
HOW CAN WE CORROBORATE, IN THE LANGUAGE OF PHYSICS, THAT THE HYDROFOIL GLIDING UNDER THE WATER ENCOUNTERS A FORCE ABLE TO BREAK IT BECAUSE OF THE RELATIVELY HIGH DENSITY OF WATER? ( THANKS GOD, I AM NOT WINDSURFING TROUGH HONEY. OR I’D BREAK MY BONES, TOO…?)
Thanks for clearing this up
ITTIANDRO |
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dllee
Joined: 03 Jul 2009 Posts: 5329 Location: East Bay
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Posted: Tue Mar 13, 2018 3:07 pm Post subject: |
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Most ws breakage occurs from catastrophic accidents, not from normal use. And of course, more often than not, the accident does'nt immediately cause a failure, but it weakens the structure so later normal use seems to be the cause of failure.
Adding additional pilot weight is one factor, but just as damaging is adding repeated accidents, by both small or big pilots.
A case in point is the recognised limits of a normally installed US finbox on a ws board. While it's possible a 230 lbs. rider can use a 14" fin for hundreds of days, it's much more likely that a strong 140 lbs. rider would tear the box out within 10 days using a 12" fin.
We are humans, and differ greatly, so mathmatics can only be applied in a general sense. |
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dllee
Joined: 03 Jul 2009 Posts: 5329 Location: East Bay
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Posted: Tue Mar 13, 2018 3:13 pm Post subject: |
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And possibly, a norm of around 17 mph is a much more realistic speed for a foil, with less for low aspect surf designs and maybe 24 as the absolute top end for hi aspect narrow wings used in the real world. |
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grantmac017
Joined: 04 Aug 2016 Posts: 946
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Posted: Tue Mar 13, 2018 11:07 pm Post subject: |
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A US box can handle maybe a 36cm fin at most, a deep tuttle box (like almost all foils) can do a 75cm one and people still break them with foils. A fin box doesn't care if you are foiling 10cm or 100cm off the water, the forces are the same.
As for mono foils and why they aren't likely to work you will need to do some reading on tailless aircraft design. The main problem they have is pitch instability. As anyone who has tried wind foiling will tel you pitch control is very challenging.
Also you'd need to completely relocate the fin box forward enough to get the center of gravity of the board/sail/rider directly above the center of lift of the wing. Which will then result in little distance between the mast base and fin box, which reduces directional stability as well. |
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joethewindsufa
Joined: 10 Oct 2010 Posts: 1190 Location: Montréal
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Posted: Wed Mar 14, 2018 5:19 am Post subject: |
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since dedicated foil boards are now being made ...
am surprised more are not looking at the double fin option ...
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