Fins...

I got thinking about this after reading John Mellors post about builders and designers. This is a question about fin design. Not so much to do with foiling, cants, outlines or toe ins but with how the base of the fin blends into the bottom of the board.

To try and make my point clearer, take a look a todays fighter plane and compare it to fifty years ago. Say a Corsair from World War II. It looks like a fuselage with wings tacked on the side.

Now take a look at a modern day fighter (excuse the aerodynamics) and wings and fuselage look far more “as one”. I think the idea of fins and a surfboard bottom that is more as one would also be interesting. Absolutely no idea how to go about this but I thought it was an idea worth sharing.

I remember reading about fillets (the blending bit betwee fin and deck) a while back. They do change things.

Here’s a link to a relevant patent but it does not give a lot of detail:

http://www.wikipatents.com/3965514.html

There seems to be a great case for refining fin position with adjustable fins and then putting glass-ons on that board model.

Red boards are faster

I believe that the design of the fillet at the base of a fin is a balance of two types of drag.

Without the fillet, you have that hard angled joint between your fin and the board. In this case, there is “interference drag” experienced. Both surfaces, the board and the fin, have a viscous boundary layer, and where the two surfaces are joined together, the two boundary layers interact with each other. This interaction results in more drag → “interference drag”.

By using a fillet to make the transition from one surface to another smoother, you can reduce this boundary layer interaction and thus the interference drag. But, you are introducing more frontal area (to the flow) so you have now introduced another form of drag (cant think of what to call it, maybe just “pressure drag”).

Basically, there should be a point at which drag is minimized by an optimum sized fillet. Any larger or smaller of a fillet, and you would have more drag.

I have neglected to consider turning performance here, i dont know how the fillet would effect the fins ability to produce lift. I would guess that as long as it is small, there would be negligible effect, but if too large it would reduce the fins ability to produce lift. This is just my educated guess.

besides that fluid mechanics explanation, glassing over a curved fillet would make the joint much stronger, as opposed to bending the glass at a sharp angle.

but oddly enough, the most efficient shapes yet designed for both airplanes and sailboats look like … wings tacked onto fuselages. Gliders, solar powered planes, racing sailboats - all the craft that are designed to wring as much as possible out of limited thrust at (comparatively) low speed are following that rule. Big fillets end up increasing drag, and having a keel or a wing just butt up against the hull works fine at preventing parasitic drag … there’s a thread here someplace that discusses a fin based on a high aspect winged keel, and I think it goes into whether fillets help compared to just having an endplate.

I’m guessing the fighter wing shape has a lot to do with mach+ speed, thrust comparable to the weight of the craft, someplace to put all the gas, and not wanting the wings fall off when you turn:). Stunt planes do just fine with high aspect ratio wings, and they’re a bit more comparable to short boards.

Off topic … seeing as the bumps on whale fins are being commercialized and stuck on everything from ships’ rudders to wind turbines … where’s my lumpy keel fin !?!

Why not use silicone caulk instead of glassons?

Silicone would make a fine fillet. Cheap and easy…ha.

Intro.

http://www.csmonitor.com/2008/0516/p13s01-stgn.html

Pictures.

http://www.treehugger.com/files/2008/03/biomimicry-fan-blades-mimic-humpback-fins.php

Source material.

http://darwin.wcupa.edu/~biology/fish/pubs/pdf/2006AnnalRevFluidMech.pdf

This material will be on the final.

In previous discussions of fillets, G. Loehr suggested wax. I thought it was a great idea.

Can’t find a link to this but Dave Parmenter did an article somewhere about the difference between glass-ons and plug fins. He felt the plug fins rode better… the implication was the fillet created drag.

I like Mark Spindler’s idea of silicone sealant to build up a fillet on a plug fin. If it did in fact create more drag, it would be easy to peel off.

The US Navy in it’s continuing search for more stealthy technology has extensively tested fillets and probably continues to do so. What is public knowledge is what we see and that is the Navy has retrofit some of it’s subs with filets at the base of the conning tower and the latest generation subs have fillets at every appendage junction. They are very smooth. Turbulence creates noise which takes the stealth out of stealth. Why spend billions on stealth motors and propellers when there is turbulence at the base of the rudder or conning tower? They have spent millions on sub design too lest we forget. Dr Fish is one of the many researchers. There is a lot of info on the web and if you piece the careers of the scientists and their reports together you can see a fascination picture of the direction research has taken in the last 35 years. Mainly towards studying biology and bio-engineering. Much of it found it’s way into the LauderFishreport PDF link I posted.

Wax probably would not last very long, but it might last long enough to make the point… I would not second guess Greg Loehr because he’s probably done this before many times. But if it’s proof you’re after try the wax first because silicone is a more permanent solution. Though not actually permanent at all as you John wisely point out.

My main point is to get out there and experiment. It’s not that hard, and it opens minds. Experimentation should not be left up to fin companies nor is it experimentation simply buying and using their products. That’s already been figured out and factored into their business model. In fact they dont want you to really experiment because it threatens their business models (and product list) and then they may have to come clean or make new products which will take money away from the CEO and CEO’s don’t like to give up money if they don’t have to. Or they may simply choose to steal them from you.

So I say experiment and stick it to the man.

It’s not really a revolution unless it starts with the people. Got wax? Start revolting.

lots of tech concerning foil root fillets,

from struts on power boats to

fins and keel foils on performance sailboats,

reynolds numbers are more comparable.

most now are without or with very small

fillets, but your mileage may vary, if you do

try silicon fillets:

smooth it with an ice cube

cleaner than a wet finger, leaves a glass smooth

surface, will pop off with a razor blade.

I suppose you could tech out and mold custon Ice cubes,

I find about three minutes in some JD works the trick.

Pete

Quote:

Why not use silicone caulk instead of glassons?

Silicone would make a fine fillet. Cheap and easy…ha.

I like Barnfield’s idea of using parafin. Easy to shape and pretty easy to remove. After that, I’d try the silicone thing (probably latex caulk as it’s easier clean up).

I’m with the “no fillets” crowd on this one. I remember reading in a book about yacht design that large fillets promote thetransfer of turbulance from one surface to another. And that less was better.

Now, I don’t know about nuclear submarines, but if you look at America’s Cup yachts (why isn’t called the Kiwi Cup yet?) all of their appendages- keels, rudders, and now the little winglets on the keels- have little or no fillet. America’s Cup syndicates might not have the budget of the US military but they don’t exactly economize on their R+D.

Over the years Mark, Herb, Rich & I have discussed fillets, micro finlets and several other drag reducing theories. My take on fillets are that with an angle of attack of zero a leading edge fillet will help to reduce a small amout of drag. Side fillets I believe compound boundary layer interference between to two planes and create more drag. The problem with added leading edge fillets is that it increases the base length of the fin. This in turn creates a larger area to pivot about as your angle of attack increases either side of zero. So, if you like drivey boards and aren’t a big proponent of super tight radius turns a leading edge fillet or a micro finlets will reduce your down the line drag. But, then again so will reducing toe and cant.

Quote:
Can't find a link to this but Dave Parmenter did an article somewhere about the difference between glass-ons and plug fins. He felt the plug fins rode better... the implication was the fillet created drag.

Brewer says fillets create drag, and plug-ins ride better too.

A good test would be to take a guy that’s been a dedicated glass-on guy his whole life, and really tuned into his boards…and switch him to box fins and see if he has any complaints…

…may be in Hawaii or in other big surf places

but putting a bit of roving in the base of the glass ons doesnt change anything in average surfing with ridiculous speed

but with most plugs, the fins doesnt adapt to the tail rocker and also the water pass under the base of the fin and this is not good for the drive

also the removable fins are standard

why the people is so pricky with the shape etc and then put a generic fin in they boards?

Great thread on this very subject last summer. Search for ‘‘Scoop on Glass-on Fillets’’.

maxmercy put up a link to a research paper that anyone interested in this subject should read.

Bottom line is glass-ons produce more lift (in terms of planing ‘‘lift’’), which is why they’re the choice of pros

trying to extract the maximum performance from a board in normal waves, and conversely, why tow-boards

work better with removables, where you don’t want the lift.

Deanbo, I posed your question to my father, a 36 year retired military aircraft mechanic.

He thought about it a few minutes and shook his head.

Nope, not useful on a surfboard was his opinion.

Something about the wings being useless at slow speeds. The air just slides off them.

The wings need 4 or 5 inch dams built on top of them just to keep enough air on the control surface at slow speeds.

His opinion surfboards don’t attain enough speed for this to be useful.

He then gave me the old speech about differences between aero and hydro dynamics.

Chris

Hey Tom, I always enjoyed those discussions.

Note to the general population:

Micro finlets are no longer in the speculation category.

In air they are a proven technology. In air they produce no measurable drag at 0 degrees AOA. From 0-14 they have tested to be drag free and effect free, but at 15 degrees they kick in and measurable drag is measurably reduced.

The NASA research is on the web for anyone able to find it.

Regarding fillets I would suspect that less is more iykwim.