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

General Discussion
61

Aloha Zfennell

 

I wasn’t the one asking about how far the pressure field propagates.  But for the record I don’t think it is very far for several reasons.

The bottom area of the board that is inducing the pressure and the force if it are both relatively small compared to the medium that they are in (the ocean) There is nothing really containing the force other then water, air or the boards shape.

The water under the board is only “captured” (if at all) for a split second and quickly begins to leak off in all directions easily depending on AOA.  Concaves, Channels, Fins, etc will slow or direct the leaking somewhat and can be an advantage if set up and used properly.

62

Hi Bill B,

Thanks for the reply.

What you say makes sense.

My question was an attempt to further clarify your position regarding the ‘plow’ effect.

Specifically, how deep do you believe it propogates?

 

If  your board with side fins is travelling straight ahead,

Are both side fins aligned with the flow ( 0 deg AOA) ?

 

…Or are they only aligned with the flow at their base (while traveling straight ahead)?

 

I ‘believe’ that both Bill T and Tomatdaun  feel the flow pattern described in your diagram quickly straightens out to a more for-aft direction with increasing depth. Thus it is not the flow seen by most of the fin.

 

Blakstah, is definitely in your camp regarding the 'plow' effect being a factor.

However, he does acknowledge both behaviors, so I suspect he still straddling the fence regarding the actual depth of the effect.

 

Is that a fair assessment?

 

I realize it is a bit unrealistic to limit all of this discussion to simply  traveling straight ahead.  But it’s been hard for me to get past this particular point.

 

 

Regards,

-bill

63

Im sure if surfer mag published this article today it would go over about as well as a unic in a whorehouse.

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

64

 

One aspect of it.  And, actually, not the one that contributes most to performance.

WRT the depth of the “plow” field, the people who have made fins that twist in toe-in have evaluated this question empirically, at least for the types of waves most surfers see. IIRC the toe-in straightens over the first 2-3 inches of depth. These fins definitely have decreased drag for their fin area…there are some other interesting changes in performance, too, and I have not kept up with their development to know if all the finer details were resolved.

 

65

Aloha obproud

Sorry for the delays in responding.  My busy life doesn’t lend itself to being able to carry on timely conversatons here on Swaylocks.

I agree that there is a visible flow (spray) of water crossing the board diagonally.  This is visible as I have mentioned before, in many photos.  But I don’t think this is the primary flow but rather mostly a lot of displaced water that isn’t of as much consequence as everyone thinks, in spite of the very common belief otherwise and the wealth of photos that seem to back up that belief.

We have to be careful here discussing up and down forces because they are not always relative to gravity or horizons.

The primary flow or pressure is lower and, depending on various aspects of the actual turn, the force is counter balancing the surfers downward force on the deck of the board.  Therefore, somewhere on the bottom, there is a point where the counter force is centered.  This brings me to another part of the interactions that I have never mentioned here nor seen discussed.

At the intersection of the fin and the bottom of the board there is created a point where the escaping water toward the rail is stopped by the fin.  It is late and I am tired so I don’t know if I can explain this as well as I would like.  But the water force hugely builds in this corner or wedge between fin and bottom.  The surfer feels this as only the fin or the rail biting.  Which is partially true, but it is more like him having no board under him and rather two inverted Vee forms on each foot and him being able to run across the water due the pressure force that builds in each vee.  This example wouldn’t really work of course.  But maybe you get the picture.

The rider, by turning, is building extra upward force toward the bottom of the board, to counter balance his extra downforce due to the turn.  He is able to “hook” this force into the wedge of the inverted vee.  The forces and flow that matter are centered around this area and also forward and back of it. Cant among other things, adjusts this wedge.

The spay pattern running across the bottom of the board looks more consequential than it is.  But it is mostly just “overspray” trying to get away from the board.  But the board keeps drifting into that overspray making it look important.

It is like whacking the water with a stick.  A vertical blow will send water overspraying out each side equally.  And the water’s counter force will be centered in the middle of the stick and upward against it.  The overspary will escape to each side without attaching much to the stick.

But if you hit the water at an angle and keep the stick moving in that direction, the stick will catch up with the spray on that side and the spray will attach to the stick and look like it is some important part of the primary flow pattern.  But the primary force will not be where all this spray “visually” seems to suggest it is.  The primary force will still just be upward under the stick and, in this case, just slightly off center do to the off center downward force of the angled strike of the stick. 

There will also be an equal flow of overspray off the other side of the stick. But, due to the angle of the strike, it will be angled back into the water and will not be visible, nor will it show up in all those cool pictures in Surfer Magazine.

I understand your view of “Trim” and the upward flow up the face of the wave.  Maybe I should have said accelerating trim.  That is, a trim where the forward motion glide down or across the wave is significant and greater then upward side ways movement of the wave face’s upflow.  But these flows and AOAs are constantly changing and are very hard to set in stone unless a moment in time and circumstance can be frozen and then analyzed.

In a turn, the primary flow is on its flat side.  As my drawing shows. As it can also be when the rider is using the upward flow on the wave face to gain altitude.  My drawing would be correct in both cases I think.

As I mentioned above the water is being forced upward and sideways into this side of the fin and out the back.  This is why you need a fin, to resist the sideways drift of the board in the turn.  The inside flat side is then the high pressure side. If it isn’t so, then the board would spin out. And if it is as I theorize here, then the flow would not be straight into the leading edge but angled to the flat side as shown in my drawing.

A turn and the flows associated with it are complicated and changing throughout the turn. It is very hard to discuss in a forum like this
[/quote]

I agree somewhat.  Trimming as I was using the term, are those points in a ride where the board is not being forced significantly against the water flow.

I understand, and don’t disagree with your description of the flow and AOA and the power the that can be gained out of this up the face flow.  But If the board is accelerating as it should be in a turn, then the AOA is reduced substantially I think.

I think side fins are toed to initiate turning and better align them with the common flow across the bottom of the board.  Cant is such to capture the upward flow, ease tracking, set the feel of the wedge effect I mentiond above and to release the outer fin earlier and probably a bunch of other stuff I can’t think of at the moment.

Great discussion.  I will try to find time to respond to others comments and theories.  I am not leaving anyone out intentionally, just have a lot going on at the moment.

66

I figured that the dust devil that hit my car today was the fault of the Chinese ..... Bill ... thanks for clearing that up .....

67

 

Aloha kcasey

 

So unless I am mistaken you seem to agree with me that there is a large upward, sideward and rearward flow water flow.  If so, it makes it a lot easier to get a clear grasp of what's happening around the fins.  But later you still question why I think the force is the way I do.  Am I missing something?

 

[quote] SNIP

On turning however...

You can't increase mass by turning. Perhaps you meant weight, or apparent weight, or just something akin to pushing in the right direction, which ever that direction happened to be. [/quote]

 

Yes that is what I meant.  Sorry.

 

[quote] SNIP

During a power stroke on a pump or pumping cycle, the surfer momentarily forces his board downward at least against the direction of the incident flow, which results in an increase in the flow under the board, and hence the force of planing. The same thing happens at the bottom of a turn, or even before. It's the same dynamic phenomena, being applied, fairly creatively, in different ways. [/quote]

 

You got!.  The water acts as sort of a spring that is loaded and unloaded to create or rather transfer energy from one vector to another that is more valuable to the surfer.

 

[quote] I don't know why you think the force developed on fins is upward and out. [/quote]

 

I thought we had agreement on this but I will try to explain again.  The reason why is because of the way the water flow is impacting them.  The water flow is upward, sidward and backward.  Drag will cause a small downward and backward component.  But a surfer will use the other compnents to easily overcome it.  And he will get an upward lift and forward motion.

 

[quote] They can be during particular orientations or maneuvers during surfing, but in general unless you are pulling off some particular maneuver, the toed/canted configuration tends to produce a force which is directed roughly downward and backward. It may be small, it depends on the orientation of the configuration, but its there. You may choose to resolve this force into lift and drag components, but the net force or resultant force, is nevertheless the same. Perhaps you might consider explaining why you think the forces resolve the way you've suggested. Perhaps it's just a matter of language and I've simply misunderstood.

SNIP

kc

[/quote]

I will try again.  Imagine the board laying in a calm pond like Bill T mentioned.  No rider on the board yet.  Gently push it forward.  The fin toe in would not be aligned to the, straight down the board, flow.  Cant wouldn't be much of an issue at this low speed and light weight. 

 

Now run down the beach at full speed and jump onto the board, doing your best to direct the new forces into a straight ahead forward movement of the board.  The water flow is now hugely upward to compensate for the weight on the deck of the board.  The nose is lifted, the tail is sunk.  The water is mostly upward and sideward at first and soon backward as some forward motion is acheived which then quickly begins to slow and stop, increasing the upward and sideward components of the flow again.

 

Now imagine what is happening around the fins.  The force of the water is undeniably upward and sideward backward..... isn't it?  Or am I way off somewhere??

Note that this is also without any "maneuvers".  This is straght forward motion.  Does that explain why I think the flow is not downward and backward as you seem to think.

 

Toe in is aligned to the sideward flow.  Cant presents the inside flat side of the fins to the rising water and captures it to lift the board.  Toe in spills it off to the back of the fin to generate forward movement.

 

That's why I think the the flow is as I have stated.  Does that make sense?  I don't know if I can find a better visual then the one described above, so I hope it works for you. There is probably more I am forgetting.  I gotta get to work!

 

 

68

This brings me to another part of the interactions that I have never mentioned here nor seen discussed...But the water force hugely builds in this corner or wedge between fin and bottom.  The surfer feels this as only the fin or the rail biting.  Which is partially true, but it is more like him having no board under him and rather two inverted Vee forms on each foot and him being able to run across the water due the pressure force that builds in each vee. 

 

Well Bill, if you read my post #48 of this thread you will see that I described this concept five days ago. You call it inverted vees, I call it channels.  I really like the additional supporting concepts you wrote too...like you I dont have a lot of time to read and write...busy busy busy...good stuff.     

 

 

69

So sorry crafty, you did indeed beat me to this.  I don’t know how I missed that it your comments as I did view that one of yours… probably skimming rather then reading as carefully as I should have.

The channel you speak of is exactly what I was referring to.  Somewhat equally there is a similar effect from the water being trapped between the fins.  I have often referred to this as like a fire hose blasting up between the fins that if everything is set right creates kind of a swively sort of feeling from the lift generated.  It is fast, loose and yet still very solid and stable.

I like to think of it as “riding the group of fins” more then the hull itself.  Of course, without the hull there would be no upper cap on the fin group but my point is just that the fins properly matched, grouped, toed, and canted are a huge factor in the ride of a board.  Often more so then subtle bottom features that get way more interest.  Surfers have been denied any understanding of the effects of these factors because only recently has there been the possibility to begin experimenting with them.  Heck we couldn’t even move fins fore or aft in the old fin systems.  

Sorry again, I didn’t mean to steal your thunder.  Glad to know someone else if feeling similar things and thinking similarly about them.  Gotta go now, I am off to see Eddie Vedder!

70

my point is just that the fins properly matched, grouped, toed, and canted are a huge factor in the ride of a board.  Often more so then subtle bottom features that get way more interest.

Couldnt agree more! Huger than huge. I'll also add the fins material composition and physical properties in the HUGE category mix.

No worries about the thunder. Just tooting my own horn a bit, cuz no one else is gonna do it ;)

Sweet! Got Eddie in my ipod...tell Eddie crafty says hi... 

 

71

http://www.youtube.com/watch?v=l5Qg_-sdf3k

I think you can see here what I’m saying about the positive effect of the outside fin tip’s drag – the control and feedback it gives. 

Aside from the turns and backside hacks, where I think the outside fin tip of both guys is in the water most of the time, and giving a lot of control and feedback, it (to me) looks like the oscillation Kelly gets on a couple of his bottom turns is from his outside fin tip–maybe it’s from a little bump in the water.  Either way, I suppose you could argue about what would happen if that fin were allowed to pivot and there were no outside tip drag in that turn, but he’s making it all work.

72

 

With all due respect…

 

Just  because people can use the drag of the outside fin tip…

 

Doesn’t mean they would surf worse if that source of drag were gone…

 

And you have NO experience with the system that you are purportedly discussing.

 

 

73

Ha, very good Dave, it’s always funny being handed a nice sandy wad of “due respect!”  It was just that the more I paid attention, the more evidence there is to see that people–Kelly Slater and the rest of the WCT and probably everyone else using thruster config’d fins–are using this tip drag you’re fighting, very productively, using it, that is, with no objections–this fact does relate to the subject at hand.  It’s all related so I illustrated the use of the tip drag in maneuvers, in addition to Kevin’s thing about staying in the pocket.

 

 

74

Aloha Janklow

Can you explain your “tip drag” theory on the outside fin.  

I watched the video and I see a lot of the outside fin and water hitting it but I want to understand what you think is happening there.

I also, think that the spray crossing the board is of some consequence.  It is still water with density and substance.  Granted it is a fairly thin sheet with little behind it other then the energy it holds from motion.  But I agree that the fins can feel it and so can the surfer.  The fins or board don’t care how it was created.  Just that it is there with enough substance to be able to act on it.

 Drag is not what I would jump to calling this interaction but I am curious about your thoughts on why “drag” is what you have concluded it is.  

It would be interesting if one could retract that outside fin suddenly and then know for sure what the “feeling” is its creating. 

75

[quote="$1"]

Couldnt agree more! Huger than huge. I'll also add the fins material composition and physical properties in the HUGE category mix.

.... [/quote]

G'day Crafty, when I swapped a glass rainbow fin for an almost identical shape plastic Future fin on one of my homebuilt silicon valley hybrids I couldn't tell any difference - it was the front toeside driving fin on a thruster too. However playing with fins in different ways the most significant thing was the size of the fin and its front to rear position in the box.

Toe in changes on that same fin made a little difference, but not much (although I was only doing things such as making it point 2" in front of nose tip instead of 8" in front of nose tip). I'm not sure how much that translates into toe in measured using the difference between leading edges and trailing edges method.

When I fitted the Doc Lausch HVP shortboard with the cambered Future Vector fins I think I got a little more drive out of that already very drivy board, but i'm not absolutely sure about that.

replacing the large FCS AM fins in the Flyer2 with something smaller produced a disappointing result - reduction in drive and glide speed.

I haven't played with cant.

76

 

       [/quote]

                     ... playing with fins in different ways the most significant thing was the size of the fin and its front to rear position in the box.

       [/quote]

                                                                                                          MrJ,  

                                                                                                         My compliments, on the above observation.     You are spot-on!  

77

For an explanation of the 'thruster hypothesis' see BillBarnfield's post above, and my interpretation of this hypothesis, if for no other reason than to understand what I mean by shedding flow.

...start simple

Curious as to the direction of shedding flow relative to incident flow, I modeled the impact of this shedding flow on an uncanted toed lateral fin – just the one lateral fin is considered. I used Carswell's experimental data for the RedX X1 L fin (the left asymmetrically foiled fin), see Hydrodynamics of Surfboard Fins, Carswell, D. J. (2007). Here, the incident flow does not have a upward direction, see below. The point was to start simple.

...results, so far

Rather than bore the shit of most of you with a lot of math, I'll get right to the results, see next paragraph. For those interested, and I hope some are, and will want to actually check my model and numbers, please work though the iillustrations below. It's not a very complex model. It's crude, but hopefully not unreasonable.

In a nutshell, for the model here, a net positive contribution to the actual propulsion of the surfboard doesn't start until the direction of the shedding flow is about 50-degree (probably a little less, let's say 45 degree) relative to the incident flow (in this model.) This isn't inconsistent with BillBarnfield's diagram of how he imagines the flow under a surfboard, see figure 3 below.

For shedding flow angles of less than 45-degree the net contribution is negative, that is, it amounts to a drag. But the amounts suggested here for the one fin aren't all that critical; roughly times three,as it would be for the full configuration, is another matter however. The same would be true for the positive contribution, but times two, if the shedding flow was symmetrical.

I've also compared different toe angles (3-degree, 4-degree, and 5-degree) to try and get a feel for how toe might be contributing (in this model.) It would appear that with increasing toe, the net positive contribution, here for toe greater than 4%, to propulsion starts earlier that 50-degree and the net drag is less for angles less than 45-degree. That would tend to contradict, at least my experience with over-toed configurations. Nevertheless this is a very very simple model, and such a finding neither proves nor disproves anything.

I guess the next step would be to try and incorporate cant into the picture, and off course the upward component of flow. It would seem at this point, with a little (or, maybe a lot?) stretching of Carswell's data, that the impact of cant could be accounted for, but not today, at least by me it won't.

Then after that, take into account the impact of all three fins (or more?, gee that would be interesting to find out what's going on with quads, etc. ) Which would likely be a real mess, as you've got to start to make assumptions about how symmetric this shedding flow is.

Am I convinced that such a mechanism is in play? Sure, It could be. Do I believe that a 45-degree kind of direction of the shedding flow is possible? Sure, it could be. The picture is hardly complete for me.

By the way, the point here was the **45-degree plus or minus shedding angle** – and, that has impact on more than just fins...bottom contours (particularly the impact of concaves and convexes, and how to take advantage of any further changes in this angle with toe and cant), on how fin gizmos work (like turbo tubes or tunnels, or whatever they are called), how configuration gizmos work (like swivel fins, etc.) and more.

By the way, Carswell's doesn't exhaustively treat gizmos (including flexible fins) but he does touch on them. I recommend his thesis to anyone interested in this kind of modeling.

… believe it or not, for me that was fun. Hopefully, somebody will check all this and confirm the model, or rip me a new one, or even better carry on to cant and beyond.

kc

ps

In the spirit of JohnMellor's thread, I'd like to identify myself as having no stake in any particular industry related product or gizmo. I wish I did, there's money to be made in 'dem gizmos – sit back and wait for the cash to come in, kind of money. - the best kind of money.

pss

By the way, Carswell touches on the net impact of a classic tri-fin configuration, but does not incorporate the thruster hypothesis -i.e. the proposed role of shedding flow. That is, in his model there is no shedding flow, its all incident -i.e. all three fins experiencing the same incident flow. To say the real picture lies somewhere in between the kind of shedding flow assumed in the thruster hypothesis and the straightforward approach taken by Carswell (and maybe others, Marsh, T. J. Development of an Optimized Surfboard Fin? … and if anyone has a copy of this paper, I'd love to read it!), is likely an understatement. Still, Carswell's data is interesting.

 

 

 

78

I never suggested placement isnt a huge factor, I was mostly adding more support to BBs comments about fin setups. In my developments, fin flex is a huge factor. You didnt mention how much flex difference there were between fins and your style and types of waves. A heavier surfer that surfs hard on rail in steep surf loads fins a lot more than a light surfer, surfing horizontally in soft waves. Your mileage may vary. I have noticed in the past that the molded futures fins are much stiffer than molded FCS. But I dont use Futures or FCS anymore, homemade customs only thank you. 

One retrospect that pisses me off is all the years I "wasted" surfing cheap FCS fins, particularly the G5. Truly the "dark years". Partly my fault. Pro surfers that travel extensively would have benefited most from FCS and most those guys used glass ons. I admit, the last several years major advancements in fin products have been made, to our benefit. I also believe discussions on Swaylocks have had an influence. Cheers. 

 

79

Hey Bill B, a toed outside fin is at a negative AOA during turns where it’s still under the surface–the tip’s outline being at a negative AOA gives drag just like dragging your hand or anything else stiff through water at a perpendicular angle to flow does.  The net effect is a bit of braking and control to balance the engaged drive of the inside fin.  I liken it to feathering the brakes on a mountain bike as you rail through a sweeper. I would not like to have that ability completely deleted. The effect and a possible solution were the subject of a project of one of the posters here. The objection was/is that when you’re trying to trim on a thruster, or otherwise maintain speed without pumping your board all the time, the outside fin’s tip drag was/is a handicap.  Brake drag, if you will.

But last time I read them (it’s been a few years), reviews of this pivoting rail fin system, which pivots to reduce this exact drag, which also takes that outside fin out of the arsenal of control surfaces when at neg AOA, were negative. The one that really sticks in my mind went something like “Sometimes it was good–but sometimes that fin would seem to disappear at unexpected moments.” Possible results of which are obvious. I speculate it was good when non-critical pumping or trimming was being performed, and it was much missed during turns/critical maneuvers.

The short video I saw showed a competent-looking Hawaiian rider looking out of control and eating it a time or two (it was short).

Maybe I missed some other reviews and video clips.  I became convinced it’s an inventive solution which adds moving parts to a “problem” nobody else judged a deal-breaker.

 

Of note: Greg Griffin’s fins, which are notoriously stiff, and toed, and fixed, are reviewed with phrases like “controlled flight,” and “I could go anywhere and do anything on the wave.”

I suppose there’s some editorial in here, because I’ve come up with my judgment about the matter, but I tried to make this straight reportage without heat, out of respect for all the members of the forum.

80

 

I think you are projecting from very limited data. I rode this system for years, and recruited a fair number of “local” competition surfers to test. Its performance has been flushed out and I make every attempt to represent it fairly. By far the biggest advantage is that when waves get smaller, you can flatten the board to the wave to speed up. This advantage is HUGE. In bigger waves its advantages are small, and it does require changes in riding style to manage speed. You might, as a highly trained thruster rider, flatten the board a little to slow down in bigger waves. This is a mistake on Trux, because you will speed up. The fins are there to turn only, and not as useful to manage speed. However, you can lower or raise the nose to manage speed, and you can do this over a much broader range of conditions than when the fins are fixed. It takes AT LEAST 10 sessions to start to get a good sense of this. However, when people used them on funboards, they see it on the first session, because on longer boards you already manage speed by raising or lowering the nose.  It is counterintuitive on a thruster.

The increased thrust from not having the outside rail fin is smaller. It is strong enough that every rider notices it the first time they surf the system, but not earth shattering. They usually report a little “extra squirt in the second half of the turn.” The ability to control speed by raising or lowering the nose is the big change on a shortboard - especially the ability to coast more efficiently in waves under head high. You eventually start weighting the rear foot to pump the bottom turn, and then weight the center of the board to speed through the turn, then weight the rear foot again for the next turn. Again, counterintuitive on a short thruster.

I never found it hard to control in bigger waves. But then again, I also bomb downhills on a short skateboard retrofitted with longboard trucks and 76mm red kryptos…I think the analogy between the short skateboard and longskate trucks with SurfTrux on a shortboard is valid. Not for everyone…