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Re: Canting on fins?

Posted: Thu Jul 19, 2012 5:54 am
by Johnny Rotten
lobodomar wrote: I share your questions, would be really nice if someone with more knowledge/experience could bring on more info.

My take for now is that canting is more useful at low speeds, when the heeling moment generated by the lateral force component is not enough to keep the board flat. In this case, the vertical component provided by canting helps in providing enough heeling moment.
I'm with ya and it still explains Paulo's explanation (whom I'm certain has experimented with this) that it keeps the board from sticking (by keeping the heelside from edging into the water at lower speeds)

Quick math - Rolling Moment about centerline of board.
all fin length = 40cm Assume Center of pressure < 20 cm
Board width = 70 cm
Cant = 5 degrees

Since the board width from the center line is roughly 2 x the center of pressure of the fin, adding cant gives you more than 2 x the moment arm for your force. Since there is going to be some foot pressure at low speeds that won't be manageable by fin lift, by adding cant you are doing what you can to preventing the board from edging and doing more of this with less fin , so this makes sense to me .

at a cant of 5 degrees 8.7% of your total lift is going to this heeling moment so these aren't insignificant angles. However, When I see videos of guys riding shadow fighting to keep a board flat at high speed it doesn't seem like it's being put to good use in that situation.

lobodomar wrote: I would really like to experiment with fins (for example have 2 pairs of the exact same fin, one pair canted and the other not), but good fins are so expensive :cry:
Fin cost makes building my own raceboard economically not feasible. so I'm
currently working on molding my own fins and making them while I sleep for like 10 bucks worth of carbon. If I can get the process sorted where I can accurately mold out a 50+ cm foil, I can just cut them down and pot them at various cant angles, rake angles and lengths. To sort out the impacts of these variables.....
So far I've handmade a long length of reasonably accurate NACA 0010 foil by stepping the points every 1/16th of an inch on my router table out of foam and hand sanding the small steps smooth and glassing. I've got the plug smooth and shiny and ready to make a mold.
mold should be done in a week or so.

Won't be super high performance cause anything I can build has to have straight leading and trailing edges or it gets really complicated to make accurately but I should at least be good for testing things like rake, length and cant. and giving me an unlimited supply of fins for repairs.

Once I'm happy with the process, I can get a proper elliptical foil accurately CNC'd or buy an existing performance fin and copy it knowing I won't be throwing my money away.

If it works, I'll have a bunch of identical (albeit possibly shitty(ish)) fins at various degrees of cant laying around until my race board build is complete, if we still dont' get any anwers we can talk experiment at that point.

Re: Canting on fins?

Posted: Sat Dec 21, 2013 9:04 pm
by C A R A F I N O 2014
Hello Johnny R:

Would you mind to explain to us about RAKE on the fin. It is my understanding that the less rake you have, the more lift you get. Is this accurate?

BTW, are you the Johnny R that is in the Haiku Canary making windsurf and other boards since way back when?

Re: Canting on fins?

Posted: Sun Dec 22, 2013 1:12 pm
by Johnny Rotten
C A R A F I N O 2014 wrote:Hello Johnny R:

Would you mind to explain to us about RAKE on the fin. It is my understanding that the less rake you have, the more lift you get. Is this accurate?

BTW, are you the Johnny R that is in the Haiku Canary making windsurf and other boards since way back when?

For the speeds that we run which are low compared to jet aircraft and the medium we run in (water) you are correct.

Rake basically shortens the wingspan for a given wing area and decreases it's aspect ratio which ultimately will reduce the lift for a given amount of drag

In addition the pressure distribution caused by higher amounts of rake will cause the water to flow down towards towards the tip increasing the fin loading at the tip and encouraging the tip to be the first location for a stall to initiate. Since the tip has the highest leverage on the craft a tip stall can destabilize craft if the lift let's go here first.

All jet aircraft you see have rake as it delays the onset of shock waves. Even for subsonic flight as the plane approaches the sound barrier, the flow on the curved surface of the wing is faster than the plane so may begin to generate shock waves. Shock wave formation creates a lot of drag.The speed at which this occurs can be delayed by rake making the plane faster and more efficient. The tip stall problem can be reduced with wing taper, twist and winglets.

Experiments run by hoerner ( a famous aerodynamisit) show the ideal lift drag ratio for low speed craft occurs somewhere between -1 and 5 degrees of rake afterwhich you start to lose efficiency.

However the most important thing to consider for a kiteboard when setting the fin rake is what angle does the board spend most of its time while being ridden. To prevent smashing the nose into waves and to reduce the wetted area at speed, the board is most often angled upwards a little bit, you need to set some rake into the fin so that it is operating in it's efficient zone at THIS range of angles.

As a follow up to my last post, I've built a bunch of fins however, sadly my raceboard sits half finished in my basement waiting for the weather to warm up so I can lay it up. (the wind and waves this summer were AWESOME so not alot of progress occured on the board building front) As a result I have no results to back up the theory presented above from a practical perspective

As for canary.....I wish......different Johnny R :(

P.S. Deeply respect your build and fabrication skills....People have little idea that actually building this stuff is when the REAL enigneering happens.