Trade-offs: Modern Toed/Canted Multiple-fin Systems

General Discussion
101

Your last post makes a lot of sense, thanks.

But if you meant do I have a physical model in mind, that is having some idea of what makes a surfboard go, which you can relate to design, and maybe take forward, yes I guess I do.

I'm curious, how does your physical model differ from Hendrick's work from the 1960's?

http://www.rodndtube.com/surf/info/Hydrodynamics.shtml

Once your model is perfected, how do you envision your model will influence design?

By 'how' I mean, how will a surfboard designer, amateur or pro, use your model to make better surfboards?

By "forward" are you thinking software?

Really, just curious.

 

 

102

I'm glad to read that you've read Hendricks, I know you've pointed to the articles in the past. Perhaps you should start a new thread reviewing his work. Maybe even updating it, who knows, take it to another level. I know I'd find it interesting.

I think Hendricks' model (the four papers ) is great. I'm inclined to disagree with his emphasis in some areas, and perhaps on occasion, his language too. For example I think the role of drag in design is paramount – not that you have to minimize it, but that you've got to get right, perhaps optimize is a better term, which may include increasing it for a given the design. This relates back to one of the ideas I tried to convey in the original post of this thread. I also believe the role of absolute speed is overemphasized, it's more about acceleration. But on the whole, given when Hendricks wrote the series, it was and remains a great departure point and source - I recommend it. I do not however see it as the end of the story, or complete in anyway. And I suspect Hendricks might agree - that isn't the way these things work.

By forward I mean the ability to not only analyze old, but new or just proposed design elements, (and surf related gizmos), whether they be mine or that of someone else.

As for the rest of your questions, for me the forum is first and foremost about ideas. Admittedly its hard not to form opinions about people, good or bad, but even then in the end, it's still about ideas.

My guess would be that if anybody is holding their breath waiting to see their name credited for some idea or influence they've had because of something they wrote on Swaylocks, they'll be turning blue real soon. And for a good reason - Swaylocks is an informal collective – ideas often tend to emerge as part of the forum process itself. I don't foresee, nor count on having any personal impact on the industry. Most of the time I'm just delighted when someone takes the time to read and possibly think about what I've posted. As well as being delighted that they have taken the time to offer some of their ideas, or opinions.

kc

ps

...by the way, if this is where this thread is now headed (as in what are my goals and aspirations), I vote to let it die, or fade away – then again it is an open forum, and votes sort of mean squat. If you want to know more about such things, PM me. I won't reply, but at least we won't be boring the shit out of people...well, at least no more than usual.

 

103

Boring? Dude, I get bored real easily and Im still here.

But if you meant do I have a physical model in mind, that is having some idea of what makes a surfboard go, which you can relate to design, and maybe take forward, yes I guess I do.

To me, this statement is the antithesis of boring. Pretty bold I might say. Unfortunately, its also in stark contrast to your last post, which is very humble.

And it doesnt have to impact "the industry" to be impactfull. Most guys here give and take impactness (word?), and most here arent in the "industry". Does the shortboard revolution ring any bells?

But Im glad to "hear" that youre not working on some 'mindless' robotic system which will attempt to make human involvement in surfboard design obsolete. But Im sure there are clever folks out there trying to make it happen. I mean, robots ARE the future, right? (good advice: learn robot repair...its the future...unless they make robot robot mechanics.......damn you robots!!!)

If youre bored, you can go back a few years and read my own opinions about the importance of acceleration. Unless of course, youre on the north shore trying to 'control' it. Or at J-Bay trying to maximize top speed. See, now youve made me really confused.

Threads die on their own when the forum or Mike decides. Not to worry tho, some anonymous spammer with a hidden agenda will bring it back to life two years from now...just dont hold your breath or you will turn purple.

PS - if you do become world famous, throw us a little bone will ya? C'mon now.....

104

For all the amateurs here's a little story on Mr. Woo's credentials.

http://www.khnl.com/Global/story.asp?S=6468511

He's one of the first guys to actively master the APS3000 in Hawaii at Progressive Shapes.

I just wanted to admit that I've purchased, installed and used David Blake's fin rotating system

per his recommendation I put it on the 7'4"x21.5"x3" slight vee bottom, flat deck stubbvector Parmenter inspired compsand model below: Blue dow core, 1/8" paulownia bottom, 1/8" balsa deck fairly flat rocker.

Fins did exactly what Dave said they would.

Trimming the board straight ahead was very fast(almost like a single fin) making flat sections easy because of the low rocker and fins facing straight ahead instead of canted out like a normal thruster. But putting such a wide flat decked soft railed shape on edge the board carved a tighter turn with as much ease as some profiled shaped more high performance designs. Actually turned into the face and back of the shoulder better than the original which is also provided for reference. Made that fuggly in betweener funboard design actually fun...

There are better designs like the Griffin five fin concept but Dave's design lived up to it's promise. Unfortunately I mounted them straight onto the blue dow with no inserts for support and the boxes came out after a couple months like the rest I did with no inserts.. My bad just laziness.. Lucky I didn't lose anything like I did on the probox blow outs..

I like just trying stuff out finding out for myself what's and whats's not BS for how I ride a wave. Personally never understood flex fins ( I even amde and rode a feather single fin fish in 72 till it snapped off). Being a fat *ss (5'8"x200lb) like me who turns late and with all his weight into it doesn't help Although iin 72 I was 135lb. Flex or plastic fins always seem to wash out on me. Only the carbonlites, the foam cores and ceramic-like vectors seem to give me the drive I need prior to discovering Griffin's hand made G-10.

I always like Chipfish's famous Swaysaying "Build It, Ride It, Then Report Back Here"

But remember have fun dioing it too otherwise what's the use?

 

 

 

 



105

Great post Bill, now that you have clarified your model it fits more with what I imagine.  

 

 

I agree with you about the upward force of the water supporting and counterbalancing the weight of the rider, but I don't agree (and this may just be lost in translation, i.e. I am misinterpreting your model) that the splitting of the water happens somewhere in the center of the board, I think hit happens closer to the inside rail. Furthermore, for boards with concave, the split is closer to the rail than for boards with vee. Likewise, the position of this split moves closer to the rail as the board is put more on the rail. 

 

I guess what I am trying to say, and make clear, is that the splitting of the water is dynamic, and it depends on certain design factors like bottom contours and rail shape, and it depends on certain rider input factors like the angle at which the lifting surfaces are positioned to the flow. 

 

[quote="$1"]

[quote] However, if the flow is allowed to ‘spread’ by equal amounts on both sides of the stringer, both side fins will generate the same amount of thrust. (yes/no?) and both are propelling the board forward. [/quote]

 

Yes!  I don't think the thrust would be exactly the same but pretty close depending on the specifics of the turn.   See my newer drawing below.  It is hard to draw to proper perspective so hopefully everyone will understand the drawing.  The tail is shifted into the wave so the fins are all viewed at odd angles.  The tail fin doesn't have Cant.  As you can see there is significant flow hitting the side fins on the inside sides.  The compression ( for lack of a better term at the moment)  of the water between the fins and board bottom provides exceptional lift to counter the riders down force.  The Cant and Toe In allows the water to expand rapidly (for lack of a better term) out the back of the board, such that tremendous thrust is gained with little drag.

 

[img_assist|nid=1044473|title=Water Flow 2|desc=|link=none|align=left|width=640|height=480]

[/quote]

 

I really like this illustration, I just wish the the fins were drawn in the same 3-D representation that the rest of the system drawn (right now they are just 1-D lines). 

 

If we could see, or just imagine, the faces of the fins, we could see how the cant and toe (the main subject of this thread, I think???) interact with the flow. 

 

I don't agree (or should I say, don't understand how) that the outside fin has as much positive effect, or provides as much thrust as the inside fin. It seems that the flow impacting that fin is much less intense than the flow impacting the inside fin. 

 

[quote="$1"]

Janklow, this is why I wouldn't use the term tip drag as I don't think there is very much drag compared to the other forces.  And while there is surely some drag that term sounds too negative for all the positive stuff that is happening.

[quote] Your argument leads us to believe that increased ‘shedding angle’ B will increase the net AOA  (A) for each side fins and  generate increasing thrust. ( sounds  logical)

If each side fin is producing equal amounts of thrust while the board is traveling straight ahead. Does that mean thrusters improve the glide or top speed of a board, while traveling straight ?

If you tweak the bottom of the board to create more lateral flow (shedding flow B) at the tail and increase the Toe angle of the fins accordingly, are you flirting with a ‘perpetual motion’ design?

The extreme example of  fins toed  90 deg  would provide max thrust in the forward direction IF aligned with flow at the tail. Hard to swallow, huh?

 

Regards,

-bill

[/quote]

I can't answer for Kcasey and please excuse my butting in....... But I don't subscribe to often stated opinion that tri fins are slow going straight.  Besides the fact that one is vary rarely going perfectly straight, if ever, I think a properly designed and set up tri fin, is always faster then a single fin.  When we were making the switch from single fins to tri fins and the fin positions hadn't been fully sorted out yet there were many problems.  So as I have mentioned before, I just kept grinding the fins off on the same boards and each time changing only one or two factors.  I tried every possible combination multiple times on multiple boards till I felt they were surfing well and more importantly, I understood what each change or factor actually did to the performance of the boards.  Did it all without the math too!  Ha!

 

I am not much into the math, I am more into "tank testing" and my tank just happens to be exceptional for this purpose.  One of the things we all complained about was that the boards were too fast and were very hard to get to slow down for tube rides and such.  Pressing down on the tail of a single fin stalled it.  Pressing down on a tri fin accelerated it!  We all got used to this in time and were able to add more rocker and narrow up the tails a bit more and change our surfing to suit the added speed of the boards.  Once I had the fins sorted out, I never had complaints about tri fins being slow going straight or otherwise.

[/quote]

 

While I don’t agree that tri fins are as fast as singles when going straight, I think that it is a moot point because who goes straight on a tri fin (who goes straight at all)? 

Besides that I totally agree that tri-fins, or side fins, more specifically, make surfboards faster, and easier to control. That is, I think that side fins allow the rider to translate the change in their center of gravity, and apparent weight (by apparent I mean how you can momentarily change how much you weigh by squatting down, or squatting up) to changes in direction and speed. When you push down on the tail of a single fin (uncanted, untoed, on the stringer, and not much rocker or rail curve), the angle of attack of the fin doesn’t change that much, so it doesn’t translate to much change in direction or change in speed, hence the need to engage more of the rail to turn. On the other hand, when you push down on the tail of a board with side fins (which have cant, toe, and are often accompanied by more rocker, and rail curve) the AOA of the fin(s) is changed more dramatically, and the rider input is more efficiently translated to changes in direction and speed. 

This brings me to another idea that I have been pondering, but have not read much about. That is, that rail curve and rocker curve really represent ONE curve, in 3D space, (even though we generally think and talk about it as two separate curves) and it is this curve and it’s relationship to the positions of the fins that determines how the board is turned, and how speed is produced. With the same fin orientation (cant and toe), but a different rail or rocker curve, the rider will have differing ability to change the AOA. 

What I mean is that it is more about the angles between the rail/rocker curve and the fins, than it is about the angle between the fins and the centerline, because the rail/rocker curve is what determines the orientation at which the fins can be situated. 

106

Dude, are you even reading the posts of this thread?  You are making "new" assertions that have already been made and mis-quoting other people here, particularly BBs comments about flow splitting. 

And just to clarify..

even though we generally think and talk about it as two separate curves

"we"?  I sure dont look at it that way.

The centerline is a design reference for building a design, maintaining plan symmetry, accurately positioning fins etc.   

Finally, youre dissection of singles v. thrusters. All things being equal, what goes faster? A sailboat with one sail or two? And what if the second sail is more effectively positioned on the boat (to create thrust) than the first sail?  And what if the boat with only one sail had a poorly positioned sail to begin with?  And what if both sails of the boat with two sails were positioned optimally, as compared to a two-sail boat with only one sail positioned optimally?

107

 

Instead of getting your panties in a bundle, why don’t you try to make a clear arguement about the ideas I have posted rather than silly semantic issues, I am not a professional writer, nor a professional surfboard designer. I am just a dude trying to understand a rather complex situation. I guarentee that my mental model is wrong in certian aspects. 

I have read the thread  a few times, but it is possible that I missed somthing someone said, or misunderstood something, so if I contradicted something I would like to know.

Keep your little ego on the beach, and stick to the subject.

Thanks.

108

Aloha,

    While this seems off the subject it’s directly related.

obproud: This brings me to another idea that I have been pondering, but have not
read much about. That is, that rail curve and rocker curve really
represent ONE curve, in 3D space, (even though we generally think and
talk about it as two separate curves) and it is this curve and it’s
relationship to the positions of the fins that determines how the board
is turned, and how speed is produced. With the same fin orientation
(cant and toe), but a different rail or rocker curve, the rider will
have differing ability to change the AOA.

I COMPLETELY agree with this. The 3D curve that the rocker and template make up is one. Thinking of it as 2 seperate things just makes it a little simpler to measure and reproduce. How much water the rail directs past the fins is a huge factor on chosing fin placement and fin size.

Let’s use a 5’10 as an example. You could make one for use in 2 foot surf or 25 foot surf. The rocker and template for the 5’10 made for 25 foot surf would be flatter and have much narrower and straighter template because you would be traveling in such a long arc. The curve on the face of a 25 foot wave is much larger. Also the fin placement would change as well as the fin size.

The 5’10 for 2 foot surf would have more curve in the template and rocker. As well as a fin set up for turning tighter arcs.

Another thing that will influence the fin set up is the shape of the rail and how far up the board the edge runs. Imagine if you sanded off ALL the edge of the board all the way back to the tail. The board would have no drive at all. You would need the fin/fins to make up the drive. Run the edge all the way up the rail to the nose and the board becomes tracky. Where the edge fades in relationship to the surfers stance is key to producing user friendly boards.

So back to the point I was trying to make in my first post. Rocker, template, bottom design, foam flow will dictate or should dictate where you chose to place your fins.

At least it does for me. =)

 

Mahalo,

   MW

109

So, I just checked the surf and no waves. One thing that crossed my mind while driving to the beach. I’ve always thought of the template as the effective edge in the water. Snowboarders use that term " effective edge" for how much actual edge is engaged in a turn. The medium that they board on ranges from ice to powder. They use no fins. Have you ever seen how far on rail a snowboarder can get in powder without fins. There has to be something there. Prob. the template and flex of the board and the sharp edge. (My apologies for using snowboards for this example) I always try to keep an open mind and take information from every source I can.

There have been attempts to make surfboards finless and although you may be able to ride a finless board. It would be far from out performing a normal “thruster” fin configuration. I have been actually trying to reduce the amount of fin area on the boards I make. Rocker and template changes were the first things I did. Moving the trailing edge of the side fins closer to the rail was just one of the things to combat drift or cavitation on a bottom turn. Concave through the tail also helps redirect the flow of water through the fins more effecient. (Using my normal shortboard as an example: 60 X 18 1/4 X 2 1/4.)

Also just to touch lightly on this. I believe the foil on the side fins help keep the rail engaged in a turn. And just for the record I’m not a fan of concave fins. Flat/single foil seems to be the right “balance” for the amount of lift needed to keep the rail engaged. More lift is not always better and it does not always equate to speed. Balance has always been the key for me.

Mahalo,

 MW

110

Sorry for the last post here. KC has posted a new thread and I should have posted the last comment there. (Rocker, template, finplacement and stance.)

 

MW

111

Aloha Obproud and thanks

[quote="$1"] Aloha Zeffenell

SNIP

In my drawing, there is a central line drawn around which the water "splits".  It wasn't meant to be absolutely rigid nor was I intending to say that all the water flows exactly this way all the time.  I was mostly just trying to establish it's existence first.  I am in agreement with you that this splitting of the flow, doesn't always sit in the dead center of the board.  The split is mostly regulated by where the riders downward force is located on the deck side.  Depending on the rider then, the splitting of the flow will be happen in different places during different rider positions or manuevers. 

SNIP

So, if the rider is pressing the rail deep into a bottom turn he is having to overcome the buoyancy of the board and the lifting forces of the water underneath the board.  He will also have to sink the tail deeper then the nose to engage the rockers arc so as to turn the board.  This will cause him to shift his downward force further back on the board and more to the inside rail.  

 

The reciprocal or counter acting upward force from the water will then also shift underneath this area of the board where the downforce is.  And my drawing, if I were redrawing for this situation, would then have the red lines shifted so that the center of the split, would be off center of the board and somewhere under the downward force.

 

But even in this situation.  The water flow would still be upward, backward and outward as my original drawing shows.  Just the center of the split and it's angle on the board would be a bit different.  And I don't think it would be as different as some might think.

[/quote]

 

[quote] I agree with you about the upward force of the water supporting and counterbalancing the weight of the rider, but I don't agree (and this may just be lost in translation, i.e. I am misinterpreting your model) that the splitting of the water happens somewhere in the center of the board, [/quote]

 

Yes you have misunerstood me. Please read my post to Zefenell above again. 

In a previous post I thought I made it clear that.......

[em]"I am in agreement with you that this splitting of the flow, doesn't always sit in the dead center of the board.  The split is mostly regulated by where the riders downward force is located on the deck side.  Depending on the rider then, the splitting of the flow will be happen in different places during different rider positions or manuevers".[/em]

 

I also said....  [em]"... the center of the split and it's angle on the board would be a bit different."[/em]

 

Hope that makes my position a bit more clear.  I understand the potential for missing important data, in long posts dealing with complicated subjects.  So I take no offense Obproud.  I try to keep it simple and clear but it is often very hard.  Plus it is hard to know what others have ingrained in their belief systems and then how they will interpet the language used.  Especially if that language begins to unravel any existing, and well protected, belief systems.  Surfing and surfboard design is chock full of old wive's tales, that linger on for decades.  Sometimes they might be accurate in the day they were first spoken, but decades later materials and techniques will have changed that now make the statement technically obsolete.  Yet it will still have great social relevance, due to the power gained by those quoting it, seemingly making them authorities on the subject.

 

[quote] I think hit happens closer to the inside rail. [/quote]

 

I quess my question to you is ....... How close? 

 

If you are thinking very close to the rail or at the rail, you are probably in agreement with the majority of people.  They usually think the split is at either at the hard edge or apex of the rail.

 

As is often the case, I don't agree with majority!  Ha!  Gets me in trouble all the time. :-)

 

Lets get this very clear here.  The splitting of the WATER can only happen if there IS WATER. 

 

So..... if the wetter surface of the board is only half the board and that half is running nose to tail at an angle well off of the centerline, then the splitting of the water can only be happening in the wetted half, much of which is NOT centered on the board. 

 

So..... of course I wasn't saying what you said I was saying in your quote....that the splitting [em]"happens somewhere in the center of the board"[/em] (your quote, interpeting what you thought I said).

 

As I noted, these discussions are difficult without a lot of illustrations to help.  But to be clear again.  The split is not locked to the center of the board!  It is dictated by the riders downforce.

 

Now lets move on.  Since you have to have water hitting the inside of the inside fin and photos clearly show water is also hitting the inside of the outside fin....... then the splitting in the fin area simply  must be happening between the fins to have water going both ways.  And since the distance between fins is typically about 10" to 12" then it is highly likely that the split is happening fairly near the center of the board in this AREA!  And surely not out at the rail else the water would be hitting the outside of the inside fin and the board would be spinning out!  I emphasize AREA to answer those who will notice that the water hitting the outside fin on the inside side is actually water that is angling there from farther forward up the board. After it hits the fin it is moving at an almost reversed angle.  Lets not go crazy trying to disect this dynamic just to counter my GENERAL statement unless there is something significant that needs to be brought to the table.  The water that is splitting to the outside from more near the fins is just passing behind the outside fin.  The flow is still there.

 

The location of the splitting flow would then angle forward (as in my 2nd illustration) just below the green water line out to where the water initially impacts the front of the board.  The primary water supporting the riders downforce (not counting fins for a moment) is in this green water and the focal point of that force is centered within it under the riders downforce. 

 

Now the fins.... A substantial amount of counterforce is also being added by the fins. It is hugely significant!!  And why I often say you really surf the fin cluster.  Like knowing the CENTER OF EFFORT in a sail plan on a sailing yacht and the CENTER OF RESISTANCE in the hull, keel & rudder the FINS, HULL & RIDER all need to be aligned, balanced and utilized properly.

 

[quote] Furthermore, for boards with concave, the split is closer to the rail than for boards with vee. Likewise, the position of this split moves closer to the rail as the board is put more on the rail.]

 

I guess what I am trying to say, and make clear, is that the splitting of the water is dynamic, and it depends on certain design factors like bottom contours and rail shape, and it depends on certain rider input factors like the angle at which the lifting surfaces are positioned to the flow. [/quote]

 

I think I already said in a previous post, that the splitting was dynamic so I guess then I agree with that part of your statement.

 

But I don't think "certain design factors" as you said, really have all that much effect unless you are talking about deep channel bottoms etc.  Simple common type concaves, rockers aren't significant factors in where the split is taking place.  As I have stated previously, it is mostly regulated by where the riders downforce is, not by the shape of the bottom.  If that bottom shape causes the rider to stand in a significantly different location, his downforce will then move.  And the upforces to counter it will move on the bottom as well as the split to some degree.

 

This will also be true even if the rider just chooses to stand in a different position on the same board.  No revelation to be discovered here really.  A zillion small changes in "other" things will do the same.   But this will quickly become as Bill T has noted... "counting Angels on the head of a pin". 

 

[quote] Likewise, the position of this split moves closer to the rail as the board is put more on the rail [/quote]

 

I think I addressed this in my answers to Zefenell above by saying....

[em]"So, if the rider is pressing the rail deep into a bottom turn he is having to overcome the buoyancy of the board and the lifting forces of the water underneath the board.  He will also have to sink the tail deeper then the nose to engage the rockers arc so as to turn the board.  This will cause him to shift his downward force further back on the board and more to the inside rail.  [/em]

 

[em]The reciprocal or counter acting upward force from the water will then also shift underneath this area of the board where the downforce is.  And my drawing, if I were redrawing for this situation, would then have the red lines shifted so that the center of the split, would be off center of the board and somewhere under the downward force."[/em]

 

Don't disconnect the position of the board..... from the riders input and then over emphasizie changes in the boards shape.  The rider dictates his position, good or bad and the resulting downforce.  If he puts the board more on the rail, he is changing the position of his downforce and along with it the upforce and its associated split which will move accordingly.  Whether or not it is as you imagine or similar to mine is another issue.  There are many forces to consider at this level of detail and we quickly get into counting Angels again.  If some design feature causes the rider to stand in a different location we can acknowledge that but so can a lot of things. How each of those effect a boards ride, unless extreme, is another subject.

 

What I was trying to address in my comments on this thread was a bigger picture issue.  That is, the quite common, lack of understanding regarding the general water flows and forces that are under a board that give it life.  I am not sure how well I have done in clarifying things or dispelling old myths.... but I gave it my best shot!

112

Thanks Bill. I think I may have missed that post, sorry about that, there is a lot of information to digest here, and I can only retain so much at a time.

I am not trying to challenge you, I know that you have a clear understanding of what is happening, and I am trying to form a clear understanding. My hope is the my mental model will approach yours. I am also a contrarian, and your explanations seem to ring true with me. Thanks for the time.

Give me some time to digest and I will be back with more questions, and clarifications.

113

The pattern of flow can be better understood by using one simple rule: flow happens along the path of least resistance.

I recall from my hands-on experiments that the split occurs at or near the wet line. One side of the split is the thin sheet of spray that eventually comes off the side of the board, the other side of the split is the wetted surface under the board supporting the rider, flow off to the side as well but cannot easily be seen. The direction is of flow on either side of this split is approximately a mirror image.

What Ive just described above in three sentences is very counter-intuitive and nearly impossible for most to visualize theorize and predict. It certainly surprised me when I first saw it!

Mahalo again for taking the time to make the drawings Bill. When I have the time I will make some drawings as well.    

114

Thanks Bill. I think I may have missed that post...

You quoted "that post" in your own post response to Bill.

Common etiquette: when good folks are having an discussion, its important not to mis-quote them. Its ok to mis-understand. But mis-quoting is really uncool. I dont have my panties in a bunch, really I dont.   

Here's a tip: I often will think for days before posting. During that time, I will read posts several times to make sure Im thinking on the right track.

115

ok I've been guilty of the asking a question which has already been answered. My excuse is that I'm finding understanding this thread hard work, and not everything is sinking in on first reading.

I wrote:
[quote="$1"]
Could someone explain to me why the thruster has more drive than the twin-fin?
[/quote]

but blakestah had already posted earlier
[quote="$1"]
1) the rail fin is oriented relative to the centerline (think of how a sailboat tacks with

a mainsail oriented relative to the keel)

2) When forces are generated on the rail fin, they will want to spin the rider out of the turn (turn the nose too fast). The rear fin gets into a near stall configuration, and it has a longer lever arm, so it stabilizes the rail fin in a position that allows it to generate thrust. This is why a 3 fin board can generate thrust much more easily than a 2 fin board.  The outside rail fin is mostly a nuisance. You turn on 2 fins at a time. 
[/quote]

yes that makes sense to me now, an aircraft enthusiast explained to me that an aircraft that had stalled wasn't really flying and so i suppose even in the situation where the rear fin was stalling and not sailing forward the extra stability allows increased side pressure on the front sailing fin - which is a bit like having a stronger wind on a sailboard sail? This also ties in with the statement LeeV made on his drive defined thread http://www2.swaylocks.com/forums/drive-defined and I think Lee rides various sorts of single fins

[quote="$1"]
...I may be waaaay off but in my mind "drive" simply means the ability to make large radius turns. You use this design trait to "drive" around a section or "drive" down the line. You can add "drive" to a board by reducing the curve in the outline, reducing the rocker in the tail, and using fins with longer bases.
[/quote]

and crafty had wrote:

[quote="$1"]
One could easily argue that a three finner is more 'thrustier' because there's two fins providing thrust, the inside rail and center, as opposed to just one in a twin
[/quote]

yes I think the feedback i experience during surfing thrusters tells me that there are situaitons where the rear fin hasn't stalled and is still sailing and despite the rear fin being double foiled I remember from my sailboarding days reading about theory of how even a flat uncambered plate will sail.

so I suppose my earlier repeat of the theory made by others helps to explain why the 4 fin has more drive than the 3 fin (my extremely limited experience of quads tells me this is the case)
[quote="$1"]
I think this thread has explained to me why the twin fin is faster than the single fin (short board comparison only) - the fins on the rail contain the water that would otherwise escape sideways?
[/quote]
a four fin has two fins sailing with both fins preventing escape whereas a thruster has 2 fins sailing but only one the front fin containing the sideways escape from the "plough effect"

sorry if I'm just repeating what everyone else has said, i'm just trying to tie together

observations (from myself and others) and theory from others and understand it.

However i'm still partly confused, I think there is something to the theory/observation that a stiffer board can accept more sideways pressure without getting out of control, but it is possible to balance the front and rear foot pressure such that a loose board is highly pressured sideways without overturning - this takes a bit more skill - i made this observation on the McCoy thread:
[quote="$1"]
but when stepping down on to a looser board until its wired it feels squirrely with lack of drive
[/quote] {I mean drive comes back when loose board is wired}

cheers,

Mike

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pollution removed

117

Aloha Obproud and Crafty

You guys both need to step back and take a deep breath. 

Crafty I appreciate your support, I really do, but I am so often misunderstood or misquoted that I have gotten used to it a long time ago and rarely if ever take it personally anymore.

And Obproud… I didn’t take offense to your questions and I don’t mind if you challenge my ideas.  That is all fun stuff.  We all have different ways of working through new or unfamiliar concepts.  And sometimes we don’t express ourselves in the best ways to get the best results.  No big deal!

To everyone…  The community first and then 2nd the concepts discussed is what is most important.  If feelings get stirred and they will, take a break or put a cork in it.  And if your approached on a personal level, don’t take it to a then greater more offensive personal level.  It is impossible to win these things and in the end the community suffers most.  Especially when people get shy to discuss or challenge ideas for fear they won’t be doing it in the “approved” way.  In the end, everyone is deprived. 

Most forums are pretty raw communities and it is rare if they are as civil and community minded as Swaylock’s is.  Especially when considering how generally whacky, ego driven, irresponsible and yet overly self confident surfers are!  It is actually pretty amazing how long Swaylock’s has survived considering the pool us participants are drawn from.

Any discussion of commonly misunderstood subjects like, water flow over a board and fins, should be red flagged from the start as one that has a high likelihood of hurting feelings or offending someone simply because a certain amount of opinions or positions can’t be easily proven.  Everyone participating has to be both on their best behavior and wearing their thickest skin. 

A major part of the pleasure (and foolishness) in surfboard design is that for most, design accuracy and precision craftsmanship, really doesn’t matter anyway, since the real joy is just “being a guy making boards” and dinking around for awhile, with something they and others think is cool.  This is really where the magic resides.  Few ever get that good, get that rich or get any real respect for their efforts.  It is a terribly shallow enterprise in which luck and being in the right place at the right time is often way more important then talent, skills or design expertise. And most of those in the media, telling the stories and those consuming the products would be hard pressed to tell the difference anyway, even if they had the necessary expertise to know.

So Crafty, I don’t care if others don’t understand me or accept my ideas, or even read my posts carefully.  I don’t expect to convince anyone overnight, if ever.  I know (there’s that self confidence thing) that I spent the time to figure things out and have applied them with great success for decades.  It is a pleasure to share these things from time to time with others if they are interested.  Or sometimes just bantering the ideas around for the purpose of argument, can be fun if that is what they need and I have the time.  It is all good if everyone stays civil and friendly.  Otherwise I have a lot of other stuff I should be doing.

And Obproud… I am exactly 5’6" tall… so be careful where your going with that stuff!

 

 

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All that makes sense to me MrJ. Sailing analogies makes the most sense, to me at least. One of the reasons why its difficult to understand this stuff is because diagrams/sketches are much easier to digest. Explaining complex physical things with words is quite a challenge. Like I said before, when I get the time I will put sketches here and things will be much easier to understand. I least thats my hope. Cheers. 

 

 

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Very well said Mr.B! Aloha.

 

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Aloha Crafty

I think the split is a bit below the wet line or green water line.  The water above the green water line is loose, thin and moving at a different angle then the green water.  I call it “overspray” and it is pretty much no different then if it were more near the rail and just blasting off into space.  But because it is still sort of attached to the board it looks to be of more consequence then what it really is.  You could put a ridge on the bottom that would separate this flow earlier from the bottom and there wouldn’t be much of a loss.

In the green water, the flow is different because it is being constrained by all the other water around it and also the boards bottom.  So the green water flow is acting differently then we would imagine as the overspray skews the view. The overspray angles radically out to the rail because even though the board is moving forward the over spray is out of the green water flow moving to the rear.  The air that surrounds the overspray is moving backward too but doesn’t have enough force to alter the flow like the green water does.

So the green water constrains the spreading of the split and drives it backwards much more so then what the overspray seems to indicate.  Somewhere along that green water edge there is a change from green water to overspray and while the exact width this change consumes can be argued, it can’t be argued that it is taking place.  Once the overspray escapes from the tension of green water forces, it goes off on its own independent tangent.  But somewhere just below that breakaway point the flow is much more toward the tail of the board then one might think by looking at the overspray and extrapolating linearly backwards.  The breakaway is exponential to the extreme.

Somewhere in the green water is where the amount and mass is sufficient to support the riders downforce.  Wherever this downforce is, is where the split is centered and the spreading is taking place from there.  I think the center of the upforce and of the split is well below the green water edge seen on the bottom of a board in turn.

 

Well said Crafty.

Great! 

Maybe we are beating a dying horse here but regardless of the popularity of this subjet, (does everyone’s head hurt?) the subject is absolutely pivotal to all other aspects of board design.  If we don’t get this one generally right, it is impossible to understand everything else in proper context.