I draw a line about an inch or so along the leading edge and use that as a guide for the thickest part of the fin. Then I just start grinding away trying to create a smooth naca like foil. I don’t get caught up in the numbers, just go by what looks right. For the single foiled fins, I clamp them together like this at the end to get them looking like a double foiled fin.
I haven’t tried adding a concave surface to the flat side, but on the blue fins I will try it. They a have pretty thick solid panel, so I have a little meat to play with. The yellow ones will stay flat on one side for now but I may round the leading edge slightly later.
I added a little glass on the yellow fins to make the base fit into the box, they were slightly narrower. My sanding resin was so old it had hardened up, and my can of wax additive seemed dried up as well. I’ll need to get some wax additive or sanding resin to finish the yellow fins. When I foiled them they really gummed up my pads. I went through 2 pads getting them foiled. The panels were made with poly laminating resin and a very slow hardening mix. I wanted a lot of time to get the glass saturated. My jig saw blade got gummed up too.
Sharkcountry, Sounds like you’ve got it covered, I’m not caught up in the NACA numbers either. I’ve found if I don’t add enough catalyst it can take ages, so now I measure it every time. I’ve put a hot (warm) water bottle on a fin panel to kick it all the way through in cold weather.
These are my fin panels now.
So the bridged, box-like structure improves strength, allowing the use of thinner foils and lighter construction.
Thanks for your rehearsed questions ! Bill Wurts ( Stoneburner) and I have collaborated on this project for a year or so, email every day, sometimes a dozen times a day and despite being continents apart we have a great ability to discuss concepts and work through the problems of design, materials, construction, testing and comprehending the results.
I build the fins but Bill ( Stoneburner) is a great mentor, adding his scientific knowledge and common sense and he knows that if he asks the right questions, that will make me go out and find the right answers. Originally I wasn’t heading towards a double bladed fin but Bill didn’t shut the conversations down, he always led me to look further ahead. And now we have a unique concept that delivers a great result. So I thank you Bill.
Win, lose or draw, we always learn huh ?
Back to the refinement…
Funny, I misread your post and I thought you meant Bill Stonebraker, who was a big time board maker here in Hawaii in the late 60’s early 70’s. If he said they would work, then they would work.
Interesting. Didn’t Kelly Slater try something like that?
Not quite sure what to think, so much going on there. So many forces that those fins will be under at various times. I think that design has a tendency to want to go straight or track. A looped fin made from curved panels with more of a normal fin outline may be an interesting thing too. And also asymmetrical outlines, so that the inner side is wider or narrower may have a big effect on turning.
There’s something a guy here in Hawaii did about 20 years ago, an X shaped fin. I met him years ago when he had a shop on the north shore, he closed the shop, and the last time I saw him he was making paddles for surfboards and kayaks. Not sure if he still makes boards, but I saw a recent picture of Jock Sutherland riding one of his boards. That’s quite an endorsement.
Look up http://www.cleanlinesurf.com/race-hawaii-surfboards-used-8-2-ex-fin.html
Best of luck on them.
S R F D C O wrote:
Win, lose or draw, we always learn huh ?
You did the creating and testing. I mostly just asked questions and commented…
“We never lose. We learn.” ~ unknown ~
Sharkcountry I did a good search to find anything to do with multi element fins but that doesn’t mean someone, somewhere did something very similar or totally different.
I’ve been looking at how far each fin overlaps and putting more overlap at the base but almost none at the tips.
Initially I was doing carbon wrap fins and attaching them to a base that fit the FCS Twin tabs. But then made them with the tabs on the fin. Offsetting the tabs keeps the grub screws open and allows the base to sit on the plugs and the board for support, so more support than just the 2 tabs and the overall area is about the same as a standard fin.
Next I’m doing them with a Futures base which will be ultra strong.
Bill W / Stoneburner, I’ll send you a set or 2 for Xmas.
Over 1000 hits in 7 days. Wow.
Here’s one with a curved rail fin that scoops flow onto the main fin.
Hi SRFDCO -
This whole thread has my head swimming. There has certainly been stuff done in various high tech fields that validate the concept but I don’t claim to understand.
I do wonder where the division between something like MVGs to disrupt flow and double foils to direct flow via ‘slot effect’ gets blurry. I also wonder about stuff like the tuburcles on the leading edge demonstration I saw once that demonstrated the increased laminar flow effect of the tuburcles with increased angles of attack of the foil.
I don’t think you saw something I did awhile back with curved rectangular ‘feeder’ fins in front of a hoop. Will try to add a pic. Will also try to add a pic or two of some old slotted and ‘fenced’ windsurf fins just to stir the pot.
johnmellor wrote:
I do wonder where the division between something like MVGs to disrupt flow and double foils to direct flow via ‘slot effect’ gets blurry.
Will also try to add a pic or two of some old slotted and ‘fenced’ windsurf fins just to stir the pot.
IMO the double wing/fin, slatted wing and slotted wing have abstract differnces.
SRFDCO questioned me about the difference between double and slatted wings which, to me, have differences that are distinguishable graphically. But when he challenged my views about fluid flow around a slatted wing, it gave me a different grasp of slatted and slotted foils.
The double wing increases lift but is affected by drag at higher speeds. Air flows through the foils differently from slats/slots. Hans educated me about the “softening” of stall angle created by double wings when separation gap is correct. I will let him address that.
After some reading, my understanding is that wing slats and slots affect lift as AOA increases. A slat is on the leading edge of a foil and is fixed or movable/adjustable (aircraft). A slot is a fixed channel/opening within the foil. Your pictured fins are definitely slotted.
My current understanding of slat/slot fluid flow set me off on a slot fin design quest. It seems slot position within the foil, slot angle and slot shape are important. Slot gap figures in as well.
Below is my first fin slot design from Saturday. Since then, angle gap and vertical gap have factored in. I am looking at slotted-foil profile shape also. The number of apparent variables has grown one order of magnitude over the last 2 days.
Turns out my first slot design has issues with respect to fabrication.
Hi John your Feeder fin arrangement is similar to what I’m doing , it’s the same idea of having a forward fin that draws water onto the back fin, its the same as slats on the leading edge of an aeroplane wing.
By directing air or water onto the back of a fin or wing it slows the drag as the AOA increases so where a standard side fin starts to drag and slow down these fins don’t. It’s why they put slats along a plane wing or multiple overlapping slats on the front wing of an F-1 car.
The concept is new to surfing because I’ve incorporated both blades into a single fixed arrangement with high aspect fins around 5-6:1 that direct flow all along the fin in a precise pattern. Some of the test fins are sitting as close as 15mm, others out near 30mm, and the fins have a overlap pattern so they have more overlap at the base and not at the tips.
It’s taken a lot of trial and error to get the right distance apart, overlap, base lengths, materials and construction techniques, which had to be invented because no one makes hollow fins.
Bill W (Stoneburner) has gone into another area with slotted fin design now.
When I first looked at the examples, I didn’t notice the offset. I think that’s an important aspect of the design. The only way you’ll learn if they work is to try it. If it does, great. If it doesn’t work well, you tweak it some until you get to a point where you definitively know that it does or doesn’t work.
Don’t be afraid to switch the fins from what you started with to the inverse. My brother has used unusual fins backwards and didn’t notice any bad effects. Assym boards are now being made in what people call a reversed layout, because a goofy foot surfer rode a board made for a regular foot surfer and really liked it that way.
Hi Sharkcountry, oh yeah I’ve tried it all different ways to get it to this stage, initially I had the fins with reversed can’t and moved them all about but I’m zeroing in on a solid configuration.
Here’s a version that looked like a Flying Taco with no connection at the tips. It needs the tip connector to stop flex.
And a pic showing that the middle SLAT image is how the fin is getting the extra lift.
And an early version with a curved front fin and a straight edge rear fin mounted on a Futures base.
Making new fin designs takes ingenuity. Each fin can have a different base for FCS or Futures so I’ve made a wooden mold for each and add a carbon fibre base as required. Still testing the tuning of area and overlap and where the overlap creates a different feel during a turn.
I’ve found the twin blades create a definite secure feeling, not more speed but a feeling of easier balance, they’re great because of the stability.
Moving to 6mm wooden fins, the overlap channels the flow but too much at the base stiffens it up but more overlap at the tips holds the drive.
