Fin theory,tom,halycon and others

General Discussion
1

Ok its finally done,a few finished experiments and some observations,

i want to discuss halycons mental template ,concaved high pressure side foils and leading edge assymetrical foils …

ive mentioned you tom ,coz i beleive you have the best understanding of foils ,out of anyone ive ever come across ,not only do you understand whats happening ,but you can put it into words very concisely and make it easier for others to understand …so im hoping you can see through my attemps to explain and maybe find some simple terms to put it in…

firstly halycons mental template …

AWSOME!!!

what else can i say they go off…

when i first met rich ,we both worked out we had come to the same conclusions about certain templates and foils even tho we were half a world and a generation appart…

my very first business when i went out on my own was shaping and fin making…

coincidently the name of my fin business was speed fin…

now my trademark fin was a fin similar looking to the mental template but the cut away was lower and cut closer to the leading edge so they had way way more flex at the base ,like the current speed fins but a mental look …

the side fins werent very functional at all ,but as a tail fin they worked great in combination with normal sides…

eventually as my production increased i stoped pushing that package as i felt i had to educate every customer to the benifits of the cut away…

so then i meet rich and he has a similar template ,we discuss certain things ,rich had the flexing side fins fully worked out and overcome the problems that i experienced with the cut away side fins…

so rich gives me some templates and i go to work .my first set is like the mental template ,but slightly more rake in the tip…

i stick em in my normal short board …

im blown away by how fast and free my board feels especially through turns…

im thinking (na that cant make that much difference ,i must have made a mistake on the setts ),i go in and change fins ,put a smaller set of standard fins in ,now my board is slower and stiffer???

my first conclusion was i made a mistake with the positioning of the fins in the boxes when i moulded them and made the mental set looser…

i go and make 2 indentical setts except one has the cut away,same foils same position …

same results …

now i think i know whats doing it ,so i go and make another set of something even more extreme …

a fin with both the trailing base cut away and the leading edge base cutaway…

looks weird …

WOW!!! even faster and more free feeling in the turns…

now a comment tom made to me in a previous discussion is the answer…

tom spoke about a layer of turbulent water that runs along the bottom of your board ,and as i used to race power boats and was contantly dealing with prop foils and other design issues ,i had many examples of what he meant…

if your in a power boat when you look out the back you see this layer of turbulent rolling swirling water thats releasing off the hull of the boat…

that layer of turbulence is being caused by viscous drag or surface drag ,

in your board you go from laminar flow at low speeds(paddling speed) to turbulent flow at high speeds ,now turbulent flow is not a bad thing coz that layer of turbulence insulates the bottom of your board from the main body of still water and acts almost like ball bearings under your board,helping speed ,turbulent flow is good (like the fuzz on a tennis ball the holes on a golf ball or the scales on a fish )it helps promote turbulence at lower speeds,

now while its great for the bottom of your board ,its a nightmare for your fins…

if a plane flys through turbulence it can lose altitude rapidly as the wings cant function effectively in turbulence …

so if your fins have to deal with turbulence they dont function as well ,lose hold (like the plane falling)more drag ,especially at angles of attack…

the cut away in the leading and trailing edges lets the turbulent water pass under the fin ,so less of the fin has turbulent water pushed up against it in turns …

the part of a normal fin in contact with the board wont act efficiently ,have more drag ,less hold…

the cut away allows the fin to operate in a zone away from the board in cleaner greener water making it act more eficiently …

i have one more experiment that should work even better ,that is make a fin that looks like a red x before its put in the box ,maybe even 2 super strong supports that have little area ,so the fin sits 12 to 15 mm off the board …

that should let as much of the turbulent water as possible under the fin allowing the fin the cleanest possible water to operate in …

i was gonna write about the concaves and leading edge assymetrical foils as well but ill save that for another thread now…

tom i firmly know now that halycon has been onto something for years and the turbulence on the bottom of the board is the reason why…

so tom can you pack that lot into one of your simple meaningful sentences…??

regards

BERT

2

Bert,

   I've been lagging on my evaluation of Rich's fins. The two primary reasons have been a healing lower back and too much work. I got out for the first time in a month last Thursday and surfed Friday and Saturday as well. The back is on the mend. The fins have been fitted for Red X boxes. Now, I've just got to get a day that's not all blown out like yesterday and today. 



   If you look back at my response to blakestah, in the 5 fin bonzer to thruster conversion thread, about why you get more drive out of a Bonzer in powerful sectioney waves you'll see I mentioned reaching down through the turbulence. 



   Minimizing root cord length (the length of the attachment of the appendage to the body) has been a primary design focus of many grand prix level yacht designers for the past decade. The area where the fin meets the bottom of the board is the area of greatest drag due to the boundary layer of skin friction turbulence and surface turbulence. The longer the root cord length the more drag is induced. For a while designers went to elyptical plane form keels feeling they still needed longer cord lengths deeper down. That's the same logic as the cutaway fin templates. Today grand prix yacht designers are going with foils that are extreme in their aspect ratio. They are basically deep foiled struts to support the ballast bulb at the bottom. What I've found from sailing these keel shapes is that they work great when you have boat speed. But, they stall much earlier than the old full form keels at slower speeds. So, the max cord length you choose for your cut-away fin template should be dependent on how fast and flowing you intend to surf the board under consideration. Another issue that arises as you minimize the root cord length combined with larger max cord length deeper down. The torsional loads that the attachment mechanism experiences goes up dramatically. This is an area where Red X will shine. I mean if our boxes can handle a 7-1/2" Bonzer center fin, they will handle the new loads of this design direction very well. 



    I like where Rich is going with this template, I just want to make sure I don't blow out my back again try to find it's limits.
3

Be careful about suspending the foil.

The foil has to deal with keeping high and low pressure water away from each other until they can re-attach at the trailing edge.

If you suspend the root chord, water at the base will flow under the fin - basically, you will get tip eddies at the base.

Fins have narrower tips and progressive rake to deal with tip eddies and stall - when you create a cutaway, the trailing edge needs an appropriate rake to avoid the tendency of water to want to go from the high pressure side to the low pressure side.

In other words, I think you both need to keep the base attached and you need to have a rake on the trailing edge between the rear base root and the end of the cutaway - avoid a horizontal line there.

I’ve thought about this a lot, and I really think Rich is already in the right ballpark. So I’m having him make me a fin - the only problem is all the surfing all-stars lined up in front of me! I also got a chance to see the fin making compound last week - sweet stuff!

4

Next template will te called, “The Ultimate”

It will be evolved for the attached photo.

The new Resin Research / Carbon-fiberglass fins are the best I’ve been able to produce. Had the 8’3" panel bottom Freeline out yesterday. It was ON with the new Mental/Spinner set-up.

Photos to follow if I have time before I leave.

I’m out of the shop for a couple of weeks.

But as Arnold said so well, “I’ll be back!”

Mahalo, Rich

5

Big fan of the mental/spinner combo that Rich makes.Just dialing in a whiteside for my

longboard and it already has me won over.Tough being a fin junkie but Rich’s fins are well

worth having in your quiver.Not a gimmick.Well thought out fins that work.

6

OK Guys I’ll bite… I’m very interested in fin set up right now. Have some ideas that are getting fired up by this discussion. Can you guys give me an opinion as to the distance from the hull the surface turbulence extends??? Different bottom contours and speeds will tend to generate different depths I’m sure but how deep is enough to get the base out of the turbulence??

I saw on the speed fin site a reference to “optimum toe-in” of 3 1/2 degrees. Is this the recommended set for their system??? or could this be used as a basic starting point?? Does two sided foil need different toe -in than flat/single side foil?? All very interesting questions. I’d like to do some experimenting. Where can I see some pics of Rich’s fins??

Thanks

Krokus

7

Hey Krokus,

I posted three photos this afternoon around three o’clock of thruster set of new carbon-fiberglass Resin Research epoxy “Mentals.” They’re down the thread line some but I’m sure you’ll find them.

Attached are is the 5" Mental / 6.125" Spinner set-up on Jeff’s 8’0"x22.0" Schroedel. That’s Doug with the board under his arm.

Jeff checked in earlier in this thread so I though I’d post of photo for reference.

By the way Jeff thanks for the boost and the great photo of that Marlin that I posted above.

Mahalo, Rich

8

Rich,

I saw those when you posted them before! Too cool. Is that cloth inlay in the lay-up?? Looks GREAT! Any thoughts on the “turbulence” question I posted?? Have some FCS stock I would like to take the grinder to just to see what it does. I see the cutaway but are the bases also thinned length way too?? Workin nite shift …need…info…must sleep…

Krokus

9

the turbulence getting away from the fin…brilliant…in 79-81 Mccoy and i did eaxctly that for a different reason we think the the more you get away from the board the harder the water more drive so we cut the base away and put way more area in the tip and put in deep into the water;loose from the narrow base and drivey with tip deep and long…im listening… understand most and some things your buddy said…lost me as it got some of your terms…the bulb on the tip of the fin we worked with my board seemed to fly at the time but we went another direction deep with lots of area deep…would like to try the bulb again it does have some merit

10

At low speeds there is no turbulence just laminar flow. As the speed increases the depth of the turbulent zone increases. But, it never gets too deep.Think of taking a hose and spraying it across a pool of standing water. Some of the surface is disturbed. But, it remains still pretty quickly as you measure deeper. On hulls and vee bottom boards there will also be less turbulence in the fin area because the flow is shedding away from the center farther forward than a board with concave which is influencing the turbulence back towards the fins. So, the answer to your question is from none to maximum of maybe a little over an inch in a worst case scenario.

Now you need to consider the materials physical characteristics and what removing material close to the base will do to your flex characteristics. The more material you remove the flexier your fins will become and you’ll end up with a different turbulence problem.

11

Can’t you just keep the foil on the thicker side near the base, and stick with high modulus materials (ie: 30% glass filled nylons, or polyester/glass, or carbon fiber)?

I would think this would especially work in the thruster case where forces are substantially less than for single, and the aspect ratios are lower.

I did a little experiment recently with polycarbonate (Lexan). It has moduli in the 300k psi range. Not nearly stiff enough for a single (without glass filling). Don’t know how the Starfin and TurboTunnel pull it off. I lost a lot of drive relative to a glass fin of the same shape. OTOH, my fin is bulletproof…

I’m a big fan of really stiff fins.

12

a small/narrow well designed titanium or magnesium alloy spine would stiffen things up but it would not be very cheap or easy to make…after the fin is made cutting/routing out a slot and epoxying the spine in place might make it a bit easier.

13
Quote:

the turbulence getting away from the fin…brilliant…in 79-81 Mccoy and i did eaxctly that for a different reason we think the the more you get away from the board the harder the water more drive so we cut the base away and put way more area in the tip and put in deep into the water;loose from the narrow base and drivey with tip deep and long…im listening… understand most and some things your buddy said…lost me as it got some of your terms…the bulb on the tip of the fin we worked with my board seemed to fly at the time but we went another direction deep with lots of area deep…would like to try the bulb again it does have some merit

Cheyne, the bulb works in a way similar to the wings of a starfin, by reducing induced drag. Airplane wings now have wingtips and others have small bulbs (smaller aircraft). All do the same thing…less drag.

A George Downing fin.

14

We’re already running a 50% glass filled nylon material in our fins. The flexular modulus is 2,160,000 psi. I’ve been working with a company that is developing and ultrahigh modulus material. But, it’s only available in compression moldable form so far. Star got away with unfilled polycarbonate by making really thick foils.

15

ok this is swaylocks…

blakestah!!! i see your point about creating another region where the high pressure can try to move around onto the low pressure side ,and your suggestion about reducing the chord at the base made perfect sense…how ever even if the base did indeed remain flat i think it would still out weigh the negative effects of the base of a normal set or box fin dealing with the turbulent flow from the hull … the picture of a base i saw was somewhat like half an oval at the base…

finding something strong enough to support it would be the next challenge…

at one stage i went through a phase of doing more vertical looking templates where the area was moved around into a taller fin ,i was cracking fins just from normal use coz of the extra load, so that definatly is an issue…

krokus i think if you took some out of a fcs but didnt add any to the top youd still see an improvement and the loads shouldnt really change at the plug ,but the tabs would feel the extra stress,???

the base section is really not working that efficiently anyway ,coz its in the turbulent zone,so id say if you took 10% of the base away ,you wouldnt lose 10% of the hold because of reduced area…

as far as the depth of the turbulent zone , my estimates are 12 to 15 mm ,why i say that is ,i have so many variations on the mental style template ,ive got a box full in the back of my car ,sometimes im standing there for ten minutes just trying to find a match before a surf…

the experiment i mentioned above was done about 3 months back but ive been waiting to post till i got the ones with the front leading edge base cutaway as well,they were the fins i felt would have asured me the theory was valid…

anyway ive got a stack of different fins with more and less base cut away ranging in chord length at the base from 50mm to 10mm and in depth from 5mm to 40mm

i found once you have at least 10mm of base cutaway from your board to the base of the fin the difference is very noticable but once you go over 40 mm your just losing fin area and hold and not increasing the benifits of distancing the fin from the turbulent zone…put it this way i couldnt feel any improvent over 20mm besides losing hold …

thats why i estimate 10 to 15 mm turbulent zone may be 20 at the most,

also i hope you agree with this one tom as its seems to contadict what you said above ,

i beleive the turbulent zone is at its deepest (in regard to the fins)just after the flow goes from laminar to turbulent,where as ,as the speed increases ,the turbulent zone reduces in depth,coz the size of the eddies being created by viscous drag are smaller at the surface of your board,even tho they may be bigger afterwards but by that stage youve left them behind in your trail and your fins havent had to deal with them/it,

as far as the strength of the base , i might try standard fin panel for my next ones and add some divinycell to thicken the foil above the box , currently i use 9mm ply so by the time there glassed theyre about 11mm even tho the box is 7mm so far i havent had any strength issues ,well a few cracks at the base , so im at the limit with them now…

ill try normal fin panel and see how far i can take it…

and tom ill go have a look at that bonza post now …i have a bad habit of not reading everything unless it interests me ,in the title…

regards

BERT

ps when this thread runs out of steam ill do some on the other points i discussed about concave and assymetrical foils…

16

Bert,

   I believe the area of most intense turbulence is the area of <span style="font-weight:bold">shear</span> between the water that is being carried along with the skin friction of the board and the water in it's natural flow direction of the wave. This boundary layer is very close to the bottom of the board under 1mm. The turbulence becomes less agitated the deeper you go. I think we're on the same page. But, bottom contours, skin preparation, velocity and displacement all have a huge impact on how deep that turbulent zone is that still has significant influence on the efficiency of your foils. I did not go into detail previuosly on the gradient nature of boundary layer turbulence,  which may be the reason you think we were in disagreement. 



  As far as removing material from FCS tabbed fins and how that will affect flex. well let's just say I'm not impressed with there current flex characteristics. So, how could I advocate compromising any part of their structure. Also, with how wide the tabs are apart, you won't be able to shorten the root cord length much on most of their templates.
17

no problems there tom ,your totally on the ball as always…the fin i see now would have to work best in red x coz no other system could handle the loads , plus i could see a red x fin appearing and you wouldnt even have to retool just add an insert…to rearrange the base , what i was talikng about above is taking it to extremes , but what rich has already done and my chink out of the leading edge is more than an improvement already…i just like to know how far i can take things before it becomes unworkable …

regards

BERT

18

Hey ya’ll - This is great. Here is my little question: How do you guys feel about “fairings” at the base to help join the flows form the bottom of the board and at the base of the fin? There was a post a little while back with a link to an areodynamic site relating to increased drag at the joint of the body and the wing being reduced by filling the hard angle in with a “fairing.” Love to read what you think. Personaly I have been working with “flex” fins - thanks to insiperation form Dale S. and his buddy Greenouhg - for years, and I struggle with integrity-strength issues. I love the guy with the box-o-fins. That is the direction to go. I have had high flex fins that were great in smaller surf - lots of drive and energy return, but they eventually failed in large heavy surf. I am now working with the “paddle” fin. so far so good. More later.

Peace to all

19

Taylor,

    Go back and read MVGs & Superchargers thread from a few weeks back. MarkSpindler, HerbSpitzer and a few other discuss fillets (fairings?) and flow characteristics pretty carefully. I believe on the leading edge they may have some benifit. But, right now we're talking abount minimizing root cord length which not what happens when you fillet. Along the side profiles and aft I believe they just add drag.
20

Thanks for the response tom - I understand about the root cord length, but never-the-less, it seems if “filleting” helps, even though I understand you feel they increase drag, if they did help with combining flow, then it seems it would help, even with a minimal root cord length. Correct me if I am wrong, but wasn’t the discussion about the “MVG’s” about starting a break in the flow, or in some cases helping to “put it back together,” to help the overall flow over & around the fins? I am all about a minimal root cord length, but I am still intrigued by the aplication of the “fillet.” Thanks Taylor