hey guys, just wanting to get some info on how to shape the foil on a D-fin (see pic below)
(not asking HOW TO FOIL! I’m more after the right shape of it!)
I think I have a rough idea what to do but if someone can explain it to me better than what is in my head that would be great!
the fin is 21mm (7/8") thick all over now. 10" base, 8" high, to go on THIS board (see pic below)
does the fin have to be thinner at the point than at the base? how much? I want to keep the fin pretty thick throughout, Tom Wegener style.
am I right in thinking that if you look at a cross section through the fin it is kinda like a stretched teardrop shape? with the “hips” so-to-speak, towards the leading edge?
any info or better still… instructions! would be greatly appreciated, I realise how important the fin foil is to the board so I wanna get it right!
Nice looking board and fin. Post some more shots of the board if you have them. Good stuff.
Typically a good rule of thumb for fin foiling is to have the wide point be at 20% of the chord of the fin. This is just a good baseline and I’ve used it to shape some thruster fins with good success. With that D fin, that might change things a bit, but 20% should give you a reasonable foil. Maybe take a look at one in person and see if you can tell if the wide point is further back on a D fin.
I did a quick graphic for you to show you roughly where a 20% wide point would fall on your fin. Just measure the base and multiply that by .20 and that is how far back it needs to be at the base. Measure the thickness near the tip and again multiply by .20 and put a dot that far back from the leading edge. Then all you need to do is connect the dots with a pencil and that will give you your wide point to foil to. This gives you a good reference point to foil from/to.
Black line is the widest point of the foil. Just taper forward and back from there.
Just looking at that picture, it might be better to shift the wide point back to 25% or even 30% of the chord. I really don’t know how these traditional fins are foiled. Maybe someone with a D fin can measure how far back the wide point is as a percentage of the chord.
IMHO the vertical cord will work nicely at 25% off the leading edge. Most all traditional fins have very poor foils on them so you really cant learn too much from looking at older boards. For that matter lots of new fins that are made with a fancy wooden cord have a huge flat spot in the center of the foil. So my advice is just to go about the process carefully. Leave the leading edge nice and round and a little on the full side.
Someone who has foiled fins for a living for decades told me 25-30 % is what they find is a good all around position for the max camber line.
I prefer, as lawless mentioned, 20%.
I would not make any reasonably conventional fin with the max camber further forward than 20% or further back than 45% (and that far back only on a center fin for something like a tow-board).
The general rule is the further forward max camber, the more lift, and the more drag. When the WP is too far forward the whole thing falls apart with drag, and when it is too far back you just can’t get much lift.
With respect to the tip thickness compared to the base, some like the rule of thumb that the max camber is proportional to the chord length. The chord length is the fore-aft distance. As you get towards the tip, chord lengths get shorter, so by the rule of thumb the max camber should get thinner.
However, make it too thin and the lack of stiffness will dominate performance. Stiffness is a function of the cube of the fin thickness, so if you make the tip real thin, as the rule of thumb says it should be, it will also be real floppy, and that will affect performance. As a result I usually do not thin the tip as much as the rule of thumb suggestion says you should.
lawless: you can see more shots, in fact the whole sequence from start to up to date if you search for “robbo’s hollow wooden pics” in the “forums”
Blakestah: thanks for the great info as well, so if I keep the base thickness at 21mm (7/8") the thickness of max camber at the tip in your opinion should be? (measured 1 inch down from tip say to allow for rounding at tip top)
thanks again for the great info and very well explained methods guys, anyone else?!
Your base thickness is fine, tip should be tapered to 1/4". The foil I’d had the best success with had the chord at 35%, with a blunt leading edge. I used to taper the trailing edge to 1/16". I rode wood fins with that foil in large surf in California, and in 25’ Waimea. The fastest Guns I’d made “in the day”, had wood foiled fins as I’ve described. Take your time, and get both sides the same.
hi bill, keep in mind that this is a cruiser/old school mal intended for knee-chest high waves, is all your info still relative? also does it affect the foil shape if I want to have a resin bead around the fin when glassed?
Yes to the foil info. Any error in foiling will be exaggerated at high speed in larger waves. For smaller waves, at lower speed, you will benefit by having smoother backside cutbacks. The foiled fin will have less of a tendancy to stall when you make an abrubt change in direction. If you add a glass bead, be sure to foil the same as the fin. Blunt leading edge, sharp trailing edge.
Don’t forget to use a pencil & draw a line all the way around the perimeter - bisecting the edge. Nothing worse than trying to eyeball the middle of a foil after you’ve done one side & flipped it…
If you make the trailing edge thinner than a mm, or so, you may find unpleasant side effects from bumping against the fin during a fall…it cuts like a knife.
If you make the trailing edge thicker than 2 mm, you may find that your board no longer knows the words (but hums right along).
I’ve always gaged adequate stiffness in my hands, and compared to standard glass fins to assess if stiffness were adequate. 1/4" thick tip, as Bill suggested, would be a good start.
Moving the max camber point from 20 to 40% is an interesting exercise, one I recommend for those who really want to know the difference. They will all work, to some degree, and for you some better than others.
Finwork is great fun. I know of no other part of surfboard design where such little changes can produce such dramatic changes in performance.
Benny1 makes a good point, you need to be able to tell where you are side to side when foiling. My other “secret” process, was that I used no rope when glassing a fin on. I would use stagger cut strips of cloth, with the weave in a bias pattern. It’s way stronger. For the fin you have, use eight 8oz layers. You should be able to set the leading edge of the fin 13" to 14" up from the tail, and still not have any spin out problems.
To clarify the above comment about glassing on a fin. That would be 8 layers of cloth on each side of the fin. My method was to lay down the full layers first (3), then successive shorter layers, then hotcoat and sand. Gloss and polish to finish.
