Go Back   Starboard Forums > Free Forum

Reply
 
Thread Tools Display Modes
Old 17th June 2011, 11:08 PM   #21
Ken
Member
 
Join Date: Aug 2006
Location: Dallas, Texas USA
Posts: 799
Default

Floyd,

The board on the beach was not a good example, I grant you that.

Lift does come from two opposing forces and the boards momentum is one force when combined with the air under it, provides lift. Once the momentum drops, so does the lift and board.

The fin, converts lateral resistance to forward momentum as you say, which in turn allows the wetted surface to be reduced as the speed increases, which increases the apparent wind under much of the board. I really don't know how much lift if any is happening here, but it seems that there has to be some.

Wide boards with big fins tend to tail walk or take off more quickly than narrower boards with smaller fins. The difference is speed. They all will tail walk or take off at some point. When a fin is too large for the speed and board, it wants to "lift" to the surface, after all it is a foil. This in turn causes the board to become unstable and tail walk, which I assume is a result of air passing under the board and the fin trying to raise to the surface. If the fin always remained perfectly vertical, there probably wouldn't be much instability, but that is not the case in choppy water. Your example of towing a board with or without a fin is only valid if the fin remains perfectly stable and vertical, which would probably never happen.

I know we are going around in circles here, but nevertheless, I have enjoyed the discussion and am always eager to learn and or debate and issue.

Good sailing.
__________________
Toys:
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.
Ken is offline   Reply With Quote
Old 18th June 2011, 01:42 AM   #22
Floyd
Member
 
Join Date: Sep 2006
Posts: 459
Default

Ken
Agree with you about all points except tail walk.

Tail walk is a result of board producing more lift than load;it lifts up;lift drops ;it drops . In effect board is too wide and it can no longer reduce its wetted area to a point where balance would be achieved. Its why boards with high tail rocker (ie wave boards) do not tail walk. They just carry on reducing wetted area; planing on a smaller and smaller area. Its exactly why cut outs were introduced; a way of reducing area at rear of board but still maintaining width for leverage on fin.There is no way on earth a vertical fin can produce lift against gravity.It must be inclined to produce actual lift. Bigger incline ;the more the lift.(ie at 45 degrees lift would be distributed half against lift ;half against leeway..
And even when inclined it can only produce lift in one direction.ie it must try and flip board.It needs an angle of attack to produce any force.Inclined away from wind it would pull back under(actually aiding gravity!)(ie if inclined down wind it produces negative lift !!!);inclined to wind it would try and climb out of water. ie both instances rotating board in same direction !!! Sounds crazy but its how it is !!!(look up Bruce foils)
My take on things anyhow.

Last edited by Floyd; 18th June 2011 at 02:00 AM.
Floyd is offline   Reply With Quote
Old 18th June 2011, 09:36 PM   #23
Ken
Member
 
Join Date: Aug 2006
Location: Dallas, Texas USA
Posts: 799
Default

We are now at the point where I can only speculate, and have little knowledge to support anything else I could say. I will accept your take on things. Another good discussion.

I appreciate you "cut out" explanation, it helped me to better understand the design benefits.

Good sailing.
__________________
Toys:
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.
Ken is offline   Reply With Quote
Old 20th June 2011, 10:43 PM   #24
Unregistered
Guest
 
Posts: n/a
Question Failed engine here 2 , piston broke

Fins on sailing craft

The use of angled foils for powered craft is relatively easy.They provide reducing SA with speed and hence greatv efficiency and high top speeds. This is far from the case with sailing craft. Generallyt for recreationaL purposes (eg Moth foiling;windsurfers) a T foil is utillised which gives better efficiency (lift/drag) to a planinbg hull but does not reduce its SA with speed (as angled foils would) Hence no better top speed than a "normal" windsurfer.

If angled foils are tried as used let say on a hydro foil (ie foils angled outwards) the down wind foil will ,rather than give lift , pull the hull under with negative lift.(because of leeway/slip angle) If foils are angled inwards (not ideal;as craft rises its doing so an a reducing width base ) the windward foil will now give negative lift !! This can (and is) utilised in foiling cats/trimarans; where the windward foil rather than lifting craft is used to counteract leaning forces. (As in Hydroptere.Windward foil holds hull down; leeward foil gives lift and a T foil on thge rudder gives added lift to balance craft fore/aft)
I beliueve when inward angled foils are utilised with the windward one opposing heeling forces and the downwind giving lift they are known as Bruce foils.

It can be seen from this that when a foil (our skeg) on a windsurfer is angled downwind it will under its own forces try and return to vertica(Negative lift)l. As it angles upwind (railing) it will satrt producing true lift against gravity.
  Reply With Quote
Old 21st June 2011, 12:13 AM   #25
Farlo
Member
 
Join Date: Aug 2006
Posts: 527
Default

Ha ha, nice argument. Do you agree that over a certain speed the hull may be wet but not the rail, and the grip would come predominantly from the fin? Just for fun...
Farlo is offline   Reply With Quote
Old 21st June 2011, 12:37 AM   #26
Unregistered
Guest
 
Posts: n/a
Thumbs up Failed Engineer

What ever speed you are doing there is always some displacement into the water and therefore the rail must be in contact to some extent.IMO it appears rail is exposed but probably water has been shed away but even if it has it would be playing it part. (if it sprays off at high speed trhe momnentum exchanged would be massive)

Putting it simply board never just touches surface ; even at really high speeds (way higher than board is capable of) there would still be a channel of some description.Without it no momentum exchange can take place.
  Reply With Quote
Old 21st June 2011, 12:42 AM   #27
Unregistered
Guest
 
Posts: n/a
Thumbs up Failed engineer

But yes I think you are correct that faster the board the more the relative contribution the fin makes.After all the board is planing higher but never so high to just "skip" over surface. (As it appears to do)
  Reply With Quote
Old 21st June 2011, 01:37 AM   #28
Ken
Member
 
Join Date: Aug 2006
Location: Dallas, Texas USA
Posts: 799
Default

Ah, but a board in chop with enough speed does "skip" over the surface with no displacement for a short time. However, I agree with your point. If you watch speed sailing runs that Antoine Albeau and others have made on very flat water, the boards don't seem to skip as you say, and it looks like displacement is consistent.

When they do "blow up" or leave the water, the crash usually begins with a spin out. Small fin with too much force. However, I do believe that at some point, it's possible for the apparent wind to get under the board and lift it off the surface of the water. This isn't too likely given the narrow, short boards plus the heavy sailors with weight jackets all designed to keep the board on the water to prevent "take off" (and to allow the use of larger sails).
__________________
Toys:
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.
Ken is offline   Reply With Quote
Old 21st June 2011, 05:20 AM   #29
Floyd
Member
 
Join Date: Sep 2006
Posts: 459
Default

But Ken the only reason it bounces is because of water its moved !!! Newtons third law;action and reaction !!! Failed engineer explains it well !!!

PS Dont think he`s talking about bouyant displacement;he`s talking about water displacement through motion !!
Floyd is offline   Reply With Quote
Old 21st June 2011, 01:52 PM   #30
Farlo
Member
 
Join Date: Aug 2006
Posts: 527
Default

Yes we all agree that there is water displacement at high speed and the lift comes primarily from that. There might be phases where the hull doesn't touch water and air speed may help, but it won't last. Now depending on surface tension and sharpness of the rails, water may not have time to wet the edges but will rather be sprayed around. And depending on its outline, the tail of the board may have no grip at all due to the way water is released. IMO this is more likely to happen when going downwind over chops.
Farlo is offline   Reply With Quote
Reply

Bookmarks

Tags
None

Thread Tools
Display Modes

Posting Rules
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Isonic 133 what fin size for sail size. Nick F Ask Our Team 3 31st May 2010 05:58 AM
Sail Size vs Mast Size/Weight robwootton Windsurfing School 3 24th March 2009 05:32 PM
Board Sail size? zakinjo Ask Our Team 1 7th March 2008 02:44 AM
Fin Size/Sail Size Roly Gardner Windsurfing School 2 7th February 2008 04:27 PM
Max sail size for a board viking Free Forum 6 26th November 2006 07:57 PM


All times are GMT +7. The time now is 12:29 PM.