myiW Current Conditions and Forecasts Community Forums Buy and Sell Services
 
Hi guest · myAccount · Log in
 SearchSearch   ProfileProfile   Log in to check your private messagesLog in to check your private messages   RegisterRegister 
Flat bottom vs double concave hulls
Goto page Previous  1, 2, 3, 4 ... 11, 12, 13  Next
 
Post new topic   Reply to topic    iWindsurf Community Forum Index -> Windsurfing Discussion
View previous topic :: View next topic  
Author Message
techno900



Joined: 28 Mar 2001
Posts: 3601

PostPosted: Sun Aug 30, 2015 1:06 pm    Post subject: Reply with quote

I was just sent this and thought that it may add a little detail to this tread. Nothing new here for those that have been around for a while, but maybe helpful for some others. Primarily, planing short board information. "Short board windsurfing design 101"
http://users.telenet.be/fanaticfanart/Windsurfing/Tricks/webPages/boardJargon.htm
Back to top
View user's profile Send private message
dllee



Joined: 03 Jul 2009
Posts: 4714
Location: East Bay

PostPosted: Sun Aug 30, 2015 7:58 pm    Post subject: Reply with quote

Catamarans non planing and D-2 hulls are similar. Long, narrow, but very deep waterline, maing more wetted surface depth wise, less width wise, and more length wise.
That is the fastest displacement sailing possible.
Back to top
View user's profile Send private message
konajoe



Joined: 28 Feb 2010
Posts: 506

PostPosted: Sun Aug 30, 2015 11:46 pm    Post subject: Reply with quote

I know. I said that there were no displacement boards. It would be more correct to say that ALL boards are displacement boards. They all have to displace a weight of water equal to the combined weight of the sailor, rig, and hull when standing still.

Here's a picture of a D2 board planing. http://www.opendivision2.org/?attachment_id=246#main I think it's safe to say that all boards will plane.

OK, Ittiandro. I also believe that you would be best sticking with what you have. The best gliding boards you can get are narrow, high volume boards with large centerboards. Forget about bottom shape. If you want to make things more stable by making it wider, the glide will suffer.

My next choice would be to look for a cheap, used, Mistral One Design or higher volume Equipe or Equipe II, or a MegaCat. Those boards with the boxy, straight rails are epic. And when you rail them, they cut through the water even better.

But if you have to have a wider board, I'm pretty sure you'd be better off with a Kona One instead of a hull that isn't specifically made for windsurfing. The windsuppers I saw sailing along side with the KonaOne failed miserably.
Back to top
View user's profile Send private message
joethewindsufa



Joined: 10 Oct 2010
Posts: 1093
Location: Montréal

PostPosted: Mon Aug 31, 2015 5:02 am    Post subject: Reply with quote

@ KONAJOE

reminder: Ittiandro HAS a Mistral Competition SST
Back to top
View user's profile Send private message Visit poster's website
whitevan01



Joined: 29 Jun 2007
Posts: 605

PostPosted: Mon Aug 31, 2015 6:32 am    Post subject: Reply with quote

ittiandro wrote:


Boardsurfer,

I do agree that in some ways water resistance increases with the wetted surface: certainly a 65 cm wide hull offer less water resistance than a 1 mt wide hull. From the standpoint of physics, however, the forward motion of a hull through the water wouldn’t exist without water resistance.
In physics, especially in an upwind course, but also to a certain extent , in a downwind course, forward motion of a sail craft through the water is the combination of two vectorial forces: the wind pushing laterally (leeward) on the sail and the water which counters this lateral pull with an equal force in the opposite direction ( windward) by the 3rd law of Newton. The combination of these two forces results in a different median direction, which is forward motion. It wouldn’t happen if there were no such lateral resistance! To this you have to add, of course, the pressure differential between the two sides of the foil ( the sail) and in the end, from this perspective, the craft moves not because of the “ push” of the wind on the sail as we commonly think, but the PULL, analogous to the upward thrust of an airplane’s wing. The airplane wing is “ sucked up” while the sail is “ sucked” forward.
It is exactly what happens when you squeeze a wet soap bar with your hand and the soap bar flies off your hand and forward. In terms of physics, the soap bar, squeezed between the inner palm of the hand and the outer fingers, is subjected to two forces: the inner palm pushing laterally on one side of the bar and the fingers pushing laterally on the opposite side with an equal force . These two forces do not neutralize one another but they are expended in a combined forward motion. This is why the bar flies off your hand, just as the board moves forward..
But, with the qualification in physical terms I provided above, you are right, too, when you say quote More surface under water => more wetted area => more friction => less speed unquote. In fact, if you look at a catamaran, it moves so much faster because the wetted surface is minimal, relative to its size, being limited to the narrow edges of the hulls cutting through the water. However it wouldn’t move forward if it were not for the lateral resistance of the water against them..

Ittiandro


sorry, so much wrong here, I will just address one issue. Nothing gets "sucked", everything gets pushed. Pressure is force/area. so, that means that the high pressure side of a foil has more force/area than the low pressure side. thus, the sum of the forces means that the net force is in the direction of motion. This is always true, Newton's second law.

As for Newton's third law, the reaction force to the sail's lateral force is not the force of the water on the board, this is such a common misconception of the third law force couples. the board pushes against the water, the water pushes against the board. the wind pushes against the sail, the sail pushes back against the wind (air). I put a force on someone's face, their face puts that same force back on my hand.

well, I guess I'm ready for my students next week!!!!
Back to top
View user's profile Send private message
adywind



Joined: 08 Jan 2012
Posts: 665

PostPosted: Mon Aug 31, 2015 6:42 am    Post subject: Reply with quote

I'm with Ittiandro on the windsup vs racing longboard debate. No matter how fast a board is its speed won't excite me in light winds after sailing shortboards regularly in 30+ kts. The only exiting aspect I see in the racing longboards is the racing one. With no competitions involved I'll choose the wide; flat; uncluttered and soft deck of the windsup anytime. I can paddle it when there is no wind at all, cruise on it A to B and back, do freestyle tricks or just plain lie on the soft EVA and relax /try this on a longboard /. I'll go as far to recommend a windsup without a daggerboard. Ittiandro is not a beginner and using the windward rail to point upwind won't be a problem for him . Daggerboards add an additional cost and are a huge nuisance . I never had a problem pointing upwind on my just 9' , all around soft rails, tri fin and NO dagger windsup with a 5.8 being the biggest sail I ever used on it.
Here is the Ittiandro program the way I envision it, involving a windsup, BIC Core 293 and a bottle of my favorite beer:
0-5 kts-sup time-paddling or sleeping on the deck Smile
https://youtu.be/Wb6-6OY2Kj4
5-15 kts-windsup time-cruising, free styling 5.0-6.0 sails
https://youtu.be/yCCMoxHsEMQ
15-25 kts- BIC 293 time-planing with 6.0-8.5 sails
https://youtu.be/eBCvBFEfppk
25+ kts - Leffe time yeaah!



image.jpg
 Description:
 Filesize:  125.54 KB
 Viewed:  4435 Time(s)

image.jpg


Back to top
View user's profile Send private message
Sailboarder



Joined: 10 Apr 2011
Posts: 656

PostPosted: Mon Aug 31, 2015 7:16 am    Post subject: Reply with quote

whitevan01 wrote:


sorry, so much wrong here, I will just address one issue. Nothing gets "sucked", everything gets pushed. Pressure is force/area. so, that means that the high pressure side of a foil has more force/area than the low pressure side. thus, the sum of the forces means that the net force is in the direction of motion. This is always true, Newton's second law.

well, I guess I'm ready for my students next week!!!!


While I agree with you that nothing gets sucked, there is merit in letting people use this image. Most beginners think that the only thing going on is direct wind pressure against the sail. In low wind, they will oversheet the sail and wonder why they are slowing down a lot. They however have to maintain laminar flow on both sides of the sail, to achieve 'suction' leeward and pressure windward. If they don't, and stall the sail, I read somewhere that you can loose surprisingly more than 50% of total force. (If I recall right, it was measured by the man who gave his name to the famous tower, Gustave Eiffel)
Back to top
View user's profile Send private message
konajoe



Joined: 28 Feb 2010
Posts: 506

PostPosted: Mon Aug 31, 2015 8:23 am    Post subject: Reply with quote

joethewindsufa wrote:
@ KONAJOE

reminder: Ittiandro HAS a Mistral Competition SST


Was there only one version of the competition SST or was it like the Equipe that had a low volume, early version. If so, I wonder which one he has?

Someone stated that Ittiandro is not a beginner. He has said that he is not making tacks and jibes on his competition SST. Otherwise, that board would be perfect. Keep that in mind.

KonaOne has a soft deck, and the footstraps are removable. The person suggesting that you may not want a centerboard doesn't have any longboard experience. And this thread is about what Ittiandro wants to accomplish, not what other people would do in certain wind conditions (ie: sit on the couch unless it blows 20).

Are there really folks who think that sails work because the wind pushes on them instead of generating lift perpendicular to the direction of the fluid flow? Wow!
Back to top
View user's profile Send private message
jingebritsen



Joined: 21 Aug 2002
Posts: 3323

PostPosted: Mon Aug 31, 2015 8:27 am    Post subject: Reply with quote

the problem with sitting out sessions that are not very exciting is what to do for cross training. cycling, esp mountain biking is a great one. i don't have much of that inside of a 2 hour car trip, so i long board wave sail.
_________________
www.aerotechsails.com
www.exocet-original.com
www.iwindsurf.com
http://www.epicgearusa.com/
http://www.seanski.com/


Last edited by jingebritsen on Mon Aug 31, 2015 12:03 pm; edited 1 time in total
Back to top
View user's profile Send private message
techno900



Joined: 28 Mar 2001
Posts: 3601

PostPosted: Mon Aug 31, 2015 9:15 am    Post subject: Reply with quote

The Mistral Competition should be a great board for light wind cruising, but it does take some skill to enjoy the ride, tacking and jibing. Going to a more stable board may be more fun at the moment, but committing to mastering the Mistral is the obvious route for most windsurfers, even those with limited time.

I have always told novices that before they go on a reach of more than a hundred yards, attempt 10 tacks and 10 jibes, and then go cruising. Our tendency is to do what is comfortable, easy and fun, which slows the learning curve considerably.

If the majority of ones sailing is sub planing, get a dagger board.

For Konajoe - here is one definition of a "planing hull" =
Quote:
planing hull
A boat hull designed to ride on top of the water rather than plowing through it.


I don't think the D2 boards are planing. Half their hull is plowing through the water. They are at the transition point, where a real planing hull would begin to flatten out (nose dropping, tail rising) and gaining considerable speed.
Back to top
View user's profile Send private message
Display posts from previous:   
Post new topic   Reply to topic    iWindsurf Community Forum Index -> Windsurfing Discussion All times are GMT - 5 Hours
Goto page Previous  1, 2, 3, 4 ... 11, 12, 13  Next
Page 3 of 13

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You can attach files in this forum
You can download files in this forum

myiW | Weather | Community | Membership | Support | Log in
like us on facebook
© Copyright 1999-2007 WeatherFlow, Inc Contact Us Ad Marketplace

Powered by phpBB © 2001, 2005 phpBB Group