Fin placemnt and Center of Lateral Resistance

In designing sailboats the designer takes into account the “center of lateral resistance” for the boat which is a function of the individual centers of lateral resistance for the hull, keel and rudder. The COLR is simply the vertical centerline of the profile of the blade or hullor template as we would say. The overall COLR for the boat determines the pivot point of the boat with respect to the center of effort of the sails. Designers like to get the COE of the sails just slightly behind the COLR to give the rudder some feel and a slight tendency to round up into the wind. There is a lot of good guidance here on Swaylock regarding multi-fin placement. But, I’m thinking that it is mostly rules of thumb based on trail and error. What if you took a board that you knew was exactly right with fin placement and measured the area of the fin(s) profile and referenced that to the length of the board expressed as a %of the overall board length. Seems to me that would take a lot of the guess work out of fin placement fore and aft. You could then place your twins or twinsers or thrusters in such a way as to maintain that CLOR. It could work for boards of most lengths. And it would take into consideration the area of the fin. As I think about it now, there is probably a certain sweet spot with respect fin area and how much is needed for total lateral resistance. I could see plotting a curve of fin area versus the weight of the surfer, perhaps with parallel curves to account for tail shape, concave, etc. You would still want to use the conventional guidance on the width of fin placement. Herb, many many thanks to you for all your work on fin placement and such. I spent about six hours yesterday reading everything I could find that you had posted on the subject. I am late to the game, but totally smitten by the supercharger fin concept. I look forward to hearing from you on this and my email on how to get supercharger fins.

thanks! man a new word concept to add to my lexicon colr colr colr CENTER OF LATERAL RESISTANCE wow ! I think I can feed my reverie all day with that one! thanks best hit of the week. ambrose in your debt

uh, that was an interesting response. At least it answers a few questions about the school system in his country. If someone out there has a serious answer about fin placement beyond the ususal rules of thumb, I’d be interested to hear about it. We all acknowledge that movement of just a 1/4-inch or so can make a noticable difference. Can we get precise? Can we measure something and extrapolate to the next board?

Fin placement definitely affects the feel of how boards ride. But, there is no one size fits all correct position, toe or cant. Different rail shapes and bottom contours react differently with varying combinations of position, toe and cant. Different surfers’ weights and styles also interact differently with these combinations. If you through in the variable varying types of surf, the matrix of the optimum fin positions, toe and cant becomes quite large. Experienced shapers spend years developing a mental data base that refer to for their various shapes and sizes. For the general public information is usaully given out as “rule of thumb”. But, as you can tell from advice given on this forum, there is an aweful lot more going on than just rule of thumb if you really want to dial in a board for a given set of circumbstances. That’s why we made Red X adjustable. You would be amazed at how much moving your fin positions by as little as a 1/16" can affect the ride of a given shape. Our first priority was base attachemnt integrity and our second was adjustability. It makes exploring fin positions a process that you could document.

Thanks for the help. I realize good fin placement relies on years of judgement. But I suspect that most surfers don’t really know if their fins are placed as well as they might be. They (the fins) maybe close and the board may ride well but because the can’t tinker with them (your fins excepted) they think all is well. I was just looking for the lazy engineer’s short cut formula, since I don’t have the experience needed otherwise. I think I can get close enough with the good guidance provided on postings on Swaylock.

edgamacation is the bulwark ok the satisfied Intelect.I for two am going off to go teach a 14year old to excercise his right brain and have fun riding the firsy wave of his life . though the educational system failed one gifted child… this one shall be redeemed and perhaps further refine the nose fin so it can become actually functional in the name of Casy JONES in the most humbling posture ambrose protector and counsler to neglected right brainz throughout the known universe… typing without glasses or smell czech can be illuminating

Well, OK. Now we are talking. I’m not sure what we are saying, but that’s OK. If you are teaching a kid to surf, then I apologize for any slight I might have made, real or imagined. Go out, buddy, and train the next generation, and do it well. Of course not at my home break. And also, I’d be curious to hear about the nose fin, if I understand you correctly.

Hey has anyone one of You seen this fin system before? You can adjust toe-in, cant, and also backwards-forwards. Looks pretty well made too. /Erik

Ok here’s a link, I’ts worth checking out.

OMG! That is exactly what I have been designing in my mind for the last month. Only they did it and did it a lot better. I think their jingle ought to be : “This changes everything!” Thanks for clueing me to this. I’m going to try it.