Made some thin fins. I was shooting for 0.250 inches thick at max chord length of 4.75 inches, but the layup came out to 0.300 (before foiling), so they are closer to 0.280.
The prior fins were 0.350 thick. so this represents roughly a 20% decrease in thickness.
I was surprised at the difference. The fins were more responsive AND faster in my system. Recall my system turns the fin in toe-in, so there are no concerns about large angles of attack. All angles of attack are smaller than those in other systems. Also, some of the benefit may be from the lost weight.
The only trick is you need to make the foil stiff enough. I think I can go thinner…
Ever try aluminum fins (or some other metal), that is 1/2 thinner than your current?
My brother used to make aluminum fins, as thin as on a waterski, for use on his kneeboard, and they worked fine. 6" tall, 8" keel shaped, no foil of course, too thin.
He’s a goofball, always trying to make something work incrememtally better than stock. With the aluminim fins, he could just pass the stern of the skiboat, almost like a waterskiier/wakeboarder. With stock thick plastic, the drag held him back about 7-10’ behind the boat.
I am always interested in what you are doing, and as you know, I just can’t resist a fin discussion. Everything you said seems to make sense except the performance increase due to the thinner fin being lighter. A rough calculation indicates that the weight difference would be about 1/3 of an ounce or less. The amount of wax on your board would make more of a difference, as would the size of your last meal! In spite of the fact that I am often disagreeing with you I am interested in your rotating boxes. Does anyone stock them in NZ or do you mail order them? BTW, did you ever try that wax recipe?
I am always interested in what you are doing, and as you know, I just can’t resist a fin discussion. Everything you said seems to make sense except the performance increase due to the thinner fin being lighter. A rough calculation indicates that the weight difference would be about 1/3 of an ounce or less. The amount of wax on your board would make more of a difference, as would the size of your last meal! In spite of the fact that I am often disagreeing with you I am interested in your rotating boxes. Does anyone stock them in NZ or do you mail order them? BTW, did you ever try that wax recipe?
Well, light is always right in surfing materials…that being said, I pretty much agree with your point. So far I am selling US only, but if you are interested, email me, let’s talk. If you’re going to try it, I’ll send you a unit and a “thin fin”.
WRT the wax recipe, every time I get a board done I get 4-5 sticks of wax. So far, I haven’t run out yet, but it is filed under the header “when I run out of free wax”
I am looking forward to having a go on the new fangled machine. Would I be correct in assuming that the box has a standard ‘spring’ to it, meaning that it takes x amount of force to rotate the box? If so could the setup be tuned for a tighter or looser feel by altering the fin rake and area? For example an upright fin would exert less leverage on the box than a raked fin, and if a fin were partially 'balanced by building it with some of the area forward of the pivot point then a tighter spring reaction would occur. On the other hand a very raked fin with a big tip area would make the spring looser? Also I was wondering how much strain the box can be expected to take . . .would it be ok to put a twelve inch fin in it for example?
Also I am wondering about using a rotating box fin in conjunction with a tunnel. I find that tunnels work best in conjunction with a central upright fin. Up until now I have always put the central fin in front of the tunnel, because if you put a fixed fin behind the tunnel the board becomes too tracky. If I put a rotating fin behind the tunnel this problem would be eliminated. If I built the tunnel in the form of a flattened, wide ellipse rather than a circle, then the setup would resemble a thruster, except that the toe in would be achieved on the back fin rather than the side fins. There would be no toe in drag at all. I have often looked at those curved fcs side fins and wondered when they will figure out that the circle can be completed by joining the two. The trouble is that if you close the circle then you have to eliminate toe in on the tunnel. Now we can put the toe in onthe back fin. Have you tried parallel side fins with a rotating back fin?
I think that I need to grow a few dozen more sets of arms, as my mind is working faster than my body!
The finbox has rubber bumpers, which I provide in a variety of stiffnesses. Changing stiffness is a 5 minute procedure that replaces the bumpers which are worth about $1. Much easier to change bumpers than to change fins.
The longboard setups have not yielded any problems with 9 inch fins so far. It is VERY easy to make fins for the system if you make fins at all. We’ve not blown out a box or a fin yet. I recognize a 12 inch fin would put even more stress on it, but the box will take less stress than a fixed box (because energy is absorbed in the bumpers).
I haven’t explored, at all, use of the box with multiple fins. But my intuition says the tracky elements go closer to the tail (so that they tend to straighten the board when the turning gets too acute). My first attempt at a 3 fin board will have two rotating side fins, and a rear fin that rotates with substantially greater stiffness (and less range), because that is what my intuition tells me. I’ve been wrong before, though. I’m only really happy with a design when I hand demos to a dozen people I trust and they give me consistent feedback.
Keep hacking at it. There are certainly things left to discover in surfboard fins.
In looking back through hydrodynamic principles, it seems that if you stay below the stall angle, foil max thickness at 20%, you get
the highest lift:drag slope
the lowest drag
with a fin in the 5-6% thick range. Again, it is essential that the fin is stiff enough. For fins foiled this far forward, the dynamic range is smaller (ie: you stall easier), but you get both more lift AND less drag at smaller angles of attack. This works perfectly in my system, because the finbox minimizes the angle of attack. And it gets better the thinner you go, in theory (I gotta go thinner still, probably be another week or two).
Thicker fins would work better when the necessary dynamic range is higher, sensitivity at small angles of attack is less important, and fins are foiled max width a little further back (closer to 30%). Make certain predictions about how you would want to toe fins as a function of thickness in a thruster, too. Thicker fins, less toe.
