Great thread but I still havn't seen what I am after. I have been using about 4.5 degrees of cant on my standard thrusters. I have also been aligning toe-in to a point 3" each side of center @ 12N (12" back from the nose). My theory there is the angle is more relative to board length. Our waves are generally head high or less and rather fat. Most of the time we have wind chop also. I am gathering that I can increase the cant angle to say 6 degrees and this should help with the slow choppy conditions. My standard rails are med/hard @ about 25/75. Had one rider say board was super "loose" (6' 4" with 4.5degree cant, slightly less toe for drive but increase tail rocker). Its all a compromise in efficiency. Jim's dissertation on dynamics was really great information, thanks Jim! Now how about the cant, what effect does increasing and decreasing have when all else stays the same?? Krokus
Toe-IN: >My theory there is the angle is more relative to board length. Correct up to a point, but no Cigar! Not so much board length, but "Expected Wave Size". Longer boards surf bigger waves. Smaller boards smaller waves. Thats why the string on the nose to the fins has always been such a decent trick for aligning toe-in. More toe-in on the smaller boards for ......smaller waves. Less toe-in on the larger boards for ......bigger waves. If you were to create a Super Board that was 5'10" that you used exclusively for big days you'd want 'less toe-in'. Conversely, if you made a 7'1" that you wanted tuned for smaller waves,......'more toe-in'.
yep eric thats the camber move it forward for small waves back for big waves ... krokus if that was me in those waves id say 15 degrees of cant no tail rocker behind the fins. all that cant will lift the nose making it easier to throw up onto sections and over mushy bits but because itll lift the nose itll make the tail sit down, so flat behind the fins means that as the tail is being pushed down itll squirt ..it will be hell slipery in o h waves but in small stuff real drivy off the tail and great around sections where you want just go straight up after youve got around it...throw in a little more outline to compensate for tail rocker halsose have you tested those theorys???????as well as testing that theory i would also suggest a book from the libary on the physics of foils then it will make sense....you probably wont find a book like that ...maybe something on plane wings will do... regards BERT
Bert if I have it wrong , please explain. I do double duty as a student and sometimes a teacher. Let me have it....I can take it!
ok i will try my best ...buts its a hard thing to explain in few words...side fins on boards are basically like plane wings....so if you imagined a plane or a bird droping nose first and then pulling up real hard, those same angles of attack are being thrown at your side fins ...when you turn your either turning off one fin or the other ...anyway back to planes ,if a plane is flying along and then suddenly pulls up the angle of attack on the wings is increased ,which in turn increases lift but if the angle of attack is to great the wings fail to keep lifting because the air traveling over the top cant stay attached to upper surface and starts to break away in turbulence and cavitations ..its commonly known as stalling and when pilots are learning to fly they have to know how far they can push there plane safely or else itll fall out of the sky...in a surfboard its called pushing hard into a bottom turn if you push to hard your board slides out,(the other design aspects of a board have an effect here as well but for now its just fins were talking..) what you are doing when turning is changing the angle of attack on the fins...as you change the angle of attack the fins produce more lift but not lift in vertical sense ,but at 90 degrees to the foil ....thats what hooks you into a turn, get 2 boards identical one thruster one single the thruster will hook way easier coz of the foil on the side fin ... now when we change tow in were changing the point at where the fin will have enough angle of attck to actually start turning your board,,,,if your fins are parralel and your going straight and you have decent foils there will already be enough angle of attack to create lift ,only problem there is ,there pulling against one another...coz one fin is pulling or lifting one way and the other fin is pulling the other way, your board becomes super twitchy and responds to very subtle body movements .......but if you turn to hard itll slide out earlier coz of the greater angle of attack on the fins due to less tow in ..now if we towed our fins right in like 10 mm it would be like a plane flying along with the body straight but the wings tilted down, theres no lift at all, before the wings had enough lift to make the plane pull up ,the tail of the plane would be hanging right down ....so when we tow our fins in we have to turn our board further before the fins pick up the water and start to pull us around,but what that means is that we can push our board further again before the fins fail and slide out ... less tow = more sensitivity less hold more tow = less sensitivity more hold add to this one more thing and thats speed ,the faster you go the less angle of attack a fin or wing can tolerate before it fails (just like if you turn a corner in your car, go slow and stick to the road ,go to fast and you slide out,each individual water molecule has mass as well if its made to go around the leading edge of your fin to fast itll slide out crashing into its buddy and starting a chain reaction of turbulence and your sliding out of control) so you can see why a big wave board needs more tow in 1 is you dont want your board sliding out if you push it to hard ,,,, 2 is the extra speed means your fins are likely to fail at a lower angle of attack ... how was that halsose ? it took me years to get a handle on my fins and exactly what they were doing...so if my explanation helps you understand a little better that will be good .... regards BERT
Bert, I often agree with most everything you say. I paticularly like the part about too much tow and your fins are going to act like a planes wings pointing downward and plowing along. (Something the Future Vector diagrams seem to totally disregard) But, I would emphasize more about the lift coefficient pulling your rail down and giving you something to pivot about. Also, fins that are parallel to the string with a 50/50 foil and camber at 30% or greater back from the leading edge are going to track. That's why the old school fish used to be great down the line point break waves. But, were hard to get vertical on. Toe is used to setup some initial instability so the board wants to turn. Toe, cant, camber, rake, base area, depth and tip area all have to be considered when determining the average flow rate across your fins and rail. More cant, larger template area with forward camber 20-25% will generate more lift at lower speeds. That's why we make the XT-1's with a fairly vertical template and the camber forward and thick and use a tiny trailer fin to be used in wider tailed boards and softer waves. But, they will also create more drag at higher flow rates. The X-2s are made to be a higher speed fin with the camber farther aft and a smaller template. So, they are probably best used with less toe and cant(5-6 degrees)in narrower tailed boards. As the velocities increase to the speeds that tow in surfers are reaching you want less drag and more inherent stability so the fins get smaller the camber is farther aft a 20/80% leading edge curvature is utilized and the toe is reduced all minize drag and turbulence. So, considering how much of each of the variables is necessary is super dependent upon how fast you plan on moving through the water.
Something else the futures guys overlooked, was being able to have SOME, or how about ANY control over the cant of the fins. I've seen many installers putting those boxes in with no install fins. They just tape them off, and slam 'em in. Angle/cant?...anyone's guess. Center fins straight? Anyone's guess. Correct me if I'm wrong, but in this age of modernization of the surfboard, coupled with most production guys on the shaping computer(for accuracy i'm assuming), this seems to be huge step backwards IMHO. Cant was being measured accurately with glass-ons for decades. Shouldn't a fin system have cant that's at least checkable? I think so.
Thanks Burt, give me some time to digest this...... i.e. read a few times.. in the meantime , someone has already disagreed, and I've wanted to restate my example.....since I think my outrageous example probably caused problems..... what I know of toe-in relates to the original Twin Fin Fish. No toe-in , it would track. toe-in: it would strengthen the boards rotational pt. , as in a pair of ski's going slow downhill and being able to rotate at your hips to pt. the ski's. i.e. No tracking! As to my previous example it was an Extreme Example, perhaps the example would have been better stated as...... If you have two boards each 6'4" and want one for larger waves and one for smaller waves and the only variable you can change on each board is the toe-in, ....then the small wave board should have more toe-in, the big wave version less. Anything wrong here! I'm thinkin not, but my previous example was like two boards er ahhhh.... dont remeber, but like 6'1" for big waves and 7'1" for small waves and that probably caused more confusion than it did to clarify.
I'm definately no rocket scientist, I only relate my info to what I feel when I test ride my theories and my theory with big wave boards is that you need more toe in because the rockers and outlines are longer and if you want that puppy to turn you need to toe in those fins as for smaller waves i find the opposite, the curves are shorter and the outlines have more curve so you need to get some distance (squirt)out of turns over flat sections and if your fins are toed in to much it feels like you are pushing water. All that this proves is that there is more than one way to skin a cat. You can play with all the major keys (outline , rocker, foil, fins) to achieve different reactions thats why there will never be one surfboard or fin template. KR
kr is right on there...tom that was sick ..as i red through your post it was in logical sequence man thats brilliant to know that others out there really understand the dynamics of fins...i was impressed....another thing to consider on big wave boards is when you increase toe in even tho your inside fin starts to hook a little later because it takes longer before the angle of attack is enough to produce lift....your out side fin is actually producing some lift as well first...if your foils are good ...a plane can fly upside down it just needs way more angle of attack to produce lift....as far as the drag is concerned its not a bad thing coz you end up shaping other things in as well to produce some drag but give some control at those high speeds....as for as tow in surfing is concerned....there at the next level again with different needs .... ps i made some jigs that fit into the futures boxes to line them up...there boxes are great but there fins havent really got it yet ....i make my own and mold the bases to fit the boxes.... regards BERT