Go Back   Starboard Forums > Ask Our Team

Reply
 
Thread Tools Display Modes
Old 12th December 2008, 04:20 PM   #11
Floyd
Member
 
Join Date: Sep 2006
Posts: 459
Default

PS
Using either foil on surface is defeating object.Our boards alreay do that.
Lift to drag ratio of fin on surface would be same as a planing surface.
Floyd is offline   Reply With Quote
Old 12th December 2008, 04:30 PM   #12
andretsin
Member
 
Join Date: Oct 2008
Location: Spain
Posts: 28
Send a message via MSN to andretsin
Default

I don't know about point b). I think it would be possible to work as I said, but maybe I'm wrong.
But point a) is not like you said. Front foil has shorter leg, so while it is in the surface and the board is almost horitzontal, the rear foil is deep inside the water. If you increase speed front foil can't go even upper because it is already in the surface. Maybe rear one would do. But only a little bit, until the pitch of the board is that so that the rear foil has zero attack angle, so it can't go even upper. Don't think about other boats. It's a radical different system, mixing planning in the front ski and foiling in the rear hydrofoil.
PS: I know the front ski works like normal boards. The advantage is that the overall wetted surface of ski+hydrofoil would be less than the one of normal board (i hope) and also drag comming from waves making. And, the best property is to have an allround board.

Last edited by andretsin; 12th December 2008 at 04:33 PM.
andretsin is offline   Reply With Quote
Old 12th December 2008, 06:24 PM   #13
Floyd
Member
 
Join Date: Sep 2006
Posts: 459
Default

Sorry I disagree.
Rear foil would have to be large enough to to "fly" at at most 10-12 knots.??? Without something to reduce its lift by 20 knots (or there abouts) it would break surface and start porpoising. There must be a system to "steer" rear foil in a vertical movement.(or reduce its lift).If your system worked it would have en empoyed on Moths years ago.
Good example is Airchair.Imagine keeping pitch same on that and then increasing speed. to 20k.If rider didnt lean forward (lowering ride height/lift by reducing angle of attack) foil would simply rise to surface lose lift ; sink and start again.
Keeping board up at front (or down) (ie maintaining pitch) without reducing lift or "steering" rear foil would create extremely high drag and in practice just wouldn`t happen.(ie system would not be balanced; rear foil generating loads more lift than needed but held in place by a smaller non foiling foil a few feet away ????.
If angle of attack is good for 10knots (to get flying) how is it changed to maintain "equilibrium" at 20k + ???
It would only work at a very specific speed when all forces are just balanced.If that speed (in theory) was 35knots it would never get there.If it was 18 knots it would do little more.
Your idea is very similar to Aquaskipper.Try towing one at 20 knots and see what happens.! Great at 8knots !
Our system would have to work form just under non flying speed of craft to 40knots + ?? Its a tall order and why its not been done YET.
Floyd is offline   Reply With Quote
Old 12th December 2008, 06:47 PM   #14
andretsin
Member
 
Join Date: Oct 2008
Location: Spain
Posts: 28
Send a message via MSN to andretsin
Default

Do you know how does Trifoiler works?
It's almost the same.
Front two foils controlled by respective skimmers, are keeping constant height in the front (this would be replaced by the front foil of my idea). Rear foil (wich is in the rudder), is fixed. When it goes at low speeds, the boat has trim to the back so the rudder foil has bigger angle of atack, giving enough lift. At higher speeds the boat is more horitzontal (tail up) and angle of atack of rudder foil is lower, so it produces the same lift althought the speed is higher. You must know that changes of 2degrees in the angle of attack mean huge changes in lift. Look at curves of lift coeficient in front of angle of attack.
I have to say that rudder foil in trifoiler is not fixed at all. You can trim it deppending on the wind and if there is one or two persons in the boat. But you are not changing it during the day, only before going to the water, so we can consider it like fixed.

Last edited by andretsin; 12th December 2008 at 06:49 PM.
andretsin is offline   Reply With Quote
Old 12th December 2008, 07:24 PM   #15
Floyd
Member
 
Join Date: Sep 2006
Posts: 459
Default

Hi
Some time since I saw a Tri-foiler (had a quick go in one around 1995;as a passenger) but if I remember rightly there were ome kind of sensors dragging in water;but things could have changed. Have a read of http://www.hobiecat.com/sailing/TriF...lls%201990.htm

Not sure we`d actually see any performance improvements; afterall speedboards are about same wetted area as a ski. (on of foilers front "skimmers")
Floyd is offline   Reply With Quote
Old 12th December 2008, 08:15 PM   #16
Floyd
Member
 
Join Date: Sep 2006
Posts: 459
Cool Worth reading ! No mention of boards though !

http://www.inqbator.org.uk/rs600ff/T...5/Default.aspx
Floyd is offline   Reply With Quote
Old 12th December 2008, 08:43 PM   #17
andretsin
Member
 
Join Date: Oct 2008
Location: Spain
Posts: 28
Send a message via MSN to andretsin
Default

Now i don't have time to read. I'm at work. I just wanted to ad that foils work at 4-8degrees of atack angle. So with only few change on the board pitch you would put the foil to angle 0 of atack (or -2 if you prefer, that use to be the angle they say there is zero lift) so no more lift will be produced and it won't go out of the water.
andretsin is offline   Reply With Quote
Old 15th December 2008, 10:07 AM   #18
davide
Member
 
Join Date: Oct 2006
Posts: 248
Default SailRocket

Quote:
Originally Posted by Floyd View Post
PS
Using either foil on surface is defeating object.Our boards alreay do that.
Lift to drag ratio of fin on surface would be same as a planing surface.
The new approach that might cry out for windsurfing development is http://www.sailrocket.com/

It has obviously its problems (I never seen before a boat doing a back flip at 50 knots!!!) but it it is incredibly fast and efficient. The boat is 45-52 knots in 18-22 knots of wind. Amazing.

The boat is very different from a conventional hydrofoil and ...I am no engineer but it might suggest a different kind of windsurf (much wider and very short, no rear fin, with the joint sliding across the deck to allow manouvers?)
davide is offline   Reply With Quote
Old 17th December 2008, 10:29 PM   #19
Lessacher
Member
 
Join Date: Sep 2006
Posts: 96
Default

Iworked some years ago a Formula Weedfin Duo 40cm with two wings, both are 20cm
long and 10 to 7cm wide. Both are 40cm. 8cm under the bottum. Angle for lift is
5°. A Starboard Formulaboard and 12,5mē sail with a dutch windsurfer made with
5-7knots wind 34km/h . He said that it was 4-5km/h faster than with a 70cm fin.
My fin is in the normal box. I tried it later with more wind. I made 43 km/h,but there made the wings so much lift at the end of the board, that I feeled a brake in front. I must find the point of balance, it should be 30 or 40cm more in front. No time to try it. Wolfgang
Lessacher is offline   Reply With Quote
Old 17th December 2008, 11:53 PM   #20
Floyd
Member
 
Join Date: Sep 2006
Posts: 459
Question Tiny wings ?

Hi Wolfgang
I`d wondered if the fitting of tiny fins(Arranged horizontally) (2cm length 1cm depth at most) to speed boards might reduce overall drag.Not by totally lifting rear of board out but by offering more efficient overall lift drag ratio .ie board would rise out slightly towards design Vmax but not to extent to raise board or seriously change angle of attack of board.(enough support to reduce drag of board?) Same as your idea on Formula but vastly reduced in area in for faster speeds.

Many slalom water ski`s have similar "foils" (on skeg)to adjust angle of attack of ski.(Think skiers call them "brakes".) They are used to increase drag in this instance so skier can brake better for turn. We`d be doing opposit. ??? Wondered if you`d tried it ?

Not explained it well but think you get idea ?
Floyd is offline   Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not 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


All times are GMT +7. The time now is 12:05 PM.