G’day daddio,
LOL! Putting you in a pleasant light is easy - not making myself look like daft is the hard part 
But seriously - I’d certainly never intentionally make anyone look bad. And I extend an upfront sincere apology to anyone I may offend or accidentally belittle.
Do you have any particular references to harmonic oscillators and dampeners, etc that you think would be a good place to start? Preferably some soft copy starts and maybe a hardcopy or two.
Thanks in advance!
-doug
Hey doc!
If I may -
Please do!
Among the nice things about flex in a planing hull is that not only can it assume a more useful curvature in a turn, but inherently
have the ability to assume a rocker that’s paralell to the water surface and thus minimising drag. Little or no distortion of the
path of the water flowing along the bottom. This goes some way towards describing how low pressure mats ( a’ la’ Solomonsen)
have such nice speed characteristics: they skim on top of the water surface by conforming to it.
Hadn’t thought of it that way - interesting!
Using a planing surface without flexibility, especially on a curved water surface ( a wave) , finding a position where the curvature
of the surface coincides with the curvature of the planing surface is more or less a happy accident ( give or take operator skill) .
This position does not necessarily coincide with the fastest path along that surface. Moreover, compromises made in the rigid
planing surface to facilitate turning, etc, compromise the ability of that surface to work in those parts of the wave which can
provide the greatest speed.
Got it… I think 
A mat flexes separately all over and doesn’t recover/return - countouring to match the water contours. A surfboard may flex it’s entire length - tho more in some areas and less in others. A boat (displacement hull) doesn’t flex at all (and as a result doesn’t plane).
So how much flex (solid stand-up board) is too much? And where is flex most critical?
Now, ‘pumping’ a board may have some utility in small, inconsequential ‘contest’ waves, where the power available from the wave
itself is so little that any added power would be of use, but that’s kinda irrelevant
It is for me - I like glide 
-doug
Hi “Other” George,
If what you are saying is true would a flex-spoon ride faster than non-flexing designs in choppy water? Tho there’d be a limit on the size of the bumps it can absorb and return. And do they?
-doug
Mark,
I’m analyzing and explaining what I have up to now only experienced.
Just pulling up on one corner of the front does not guarantee the cross corner will go down. Now what are you making a body board knee board or surfboard? If pushing down on a rear corner you want the rail of the same side to hold down not rise up. If you are connected to the cross corner with some sort of rigid mechanical device like a perimeter stringer or internal frame it will pull up the cross corner as you drive down the rear corner and drive the inside front rail down and help turn. So whether you are pulling up on the front or pushing down on the rear you want a connection that will result in controlling the opposite corner. That is where turn control is.
With the 4-point stance while pulling up and twisting the left front corner I can lean to the right(into the turn) and sit back on my heels(pressure on right tail). Many options and combinations. So I am the mechanical device in charge of turn control. And the weight-shifting and applying pressure can be done with subtle, tiny movements. Part of the brilliance of Greenough’s design is that you always kneel in the same spot. From that “sweet spot” you can control everything.
dougirwin13 If what you are saying is true would a flex-spoon ride faster than non-flexing designs in choppy water? Tho there’d be a limit on the size of the bumps it can absorb and return. And do they?
Don’t know about the speed, think control. Bumps are no fun, flex handles them a little better than rigid. Control again.
flex is twist and bend
flex is the the dream of fling after a huge bottom turn on a lined up wave.
i used one of MTB’s female molds to start a spoon. you can see the whole project step by step on Flexspoon’s website. (MTB i’d like to return that mold soon. i’ll call you in the next few days and bring the beast i have created too.)
i am looking for practical input for subsequent laminations…
should they cover the entire surface of the spoon dish? should the laminations be focused toward the tail? what shapes should the lams be? roundish, triangular?
i am a preschooler, what can i say. it is hard for me to comprehend MTB’s mathmatical explainations. i comprehend by touching and flexing things with my hands—it’s tactile–it’s visualizing that flex while imagining waveriding situations. instinctive mechanics based on a picture in the mind.
any suggestions would be killer! also scheduals for attaching the fin would be cool to.
Quote
dougirwin13 If what you are saying is true would a flex-spoon ride faster than non-flexing designs in choppy water? Tho there’d be a limit on the size of the bumps it can absorb and return. And do they?
Don’t know about the speed, think control. Bumps are no fun, flex handles them a little better than rigid. Control again.
Sorry - wasn’t trying to suggest that flex = speed… We just worked through that
I said earlier in this thread that I am thinking of flex as important in responsiveness and control (at the moment, anyway :). Not speed. I guess I was unclear with my question. What I was trying to say was that your theory is somewhat supported if flexspoons travel faster than other designs over a corregated surface (the bumps would have less effect, not no effect). If, as you say, flex handles it better than non-flex I think that still supports your idea to some degree.
And no - bumps aren’t any fun. Don’t know anyone who prefers chops over glass
Man… You guys are gonna have me making kneeboards the way things are going!
-doug
markgnome: i am looking for practical input for subsequent laminations…
should they cover the entire surface of the spoon dish? should the laminations be focused toward the tail? what shapes should the lams be? roundish, triangular?
markgnome has been documenting his spoon building here:
http://flexspoon.com/forum/viewtopic.php?t=17
You can see a little of the glassing layers/pattern here:
http://www.swaylocks.com/resources/detail_page.cgi?ID=445
At least a couple Swaylockers will be looking at that board close up tomorrow night.
I’ll ask them to analyze it from that perspective. That should cover the fin also.
markgnome: i am a preschooler, what can i say. it is hard for me to comprehend MTB’s mathmatical explainations. i comprehend by touching and flexing things with my hands—it’s tactile–it’s visualizing that flex while imagining waveriding situations. instinctive mechanics based on a picture in the mind.
Just how I feel. All I’ve ever done is ride them instinctively - they are the right vehicles for me and have never required thinking. The whole reason I am here is to learn about the boards I’ve been riding so I can make better ones.
dougirwin13 :
Since I’ve never ridden non-flex(except windsurfing) I really don’t know. I do know that the spoons I have ridden have always liked a textured surface over a glassy surface. They literaly fly over the surface and chatter like skis. You can feel the texture through the bottom.
Man… You guys are gonna have me making kneeboards the way things are going!
Greenough analyzed everything and came up with a no-compromise performance is everything wave-riding vehicle. Any thoughts about paddling for instance were not even part of the formula - performance only.
“And no - bumps aren’t any fun.”
On the right vehicle, twisted, gnarly ledges and bumps are great fun. They can even be used to go faster. Efficient,satisfying performance in challenging conditions is not achieved by bouncing over and/or plowing through water.
“Don’t know anyone who prefers chops over glass”
Many of the world’s most experienced mat surfers, including George Greenough. They’ve learned things about waves that surfers on hard equipment can’t see, or even feel. And arguably, the feeling is what wave riding is all about…
Totally agree with Dale.
When you accumulate data from guys riding uninspired gear you might get these notions that bumps are bad or glass is best.
But, if gear is designed to handle bumps, new areas of fun can be explored.
Just look at what suspensions have done for mountain biking.
Increased control, allowed for more speed, and greater excitement. m
Of course that is for one type of riding.
Street bikes like Lance’s are super light and super rigid. And Freestyle BMX is strong and rigid.
I refer back to what I said about different gear for different conditions. m
excellent thread gents
so is MTB a hit and run artist? did i really read his lengthy mathematical post or just gloss over it?
so…what is FLEX?
technically, FLEX is an inadequate term as it relates to mechanical objects…
so to clarify…
undampened flex with strong return is a spring
undampened flex with weak or no return is compliance
flex with a combination of both is a dampened spring (classic spring/mass/damper mechanical vibrations model)
when you look at modern shortboard surfing from the 70’s vs today, one of the key differences is that lighter equipment allows the surfer to use his legs more effectively as a spring (the pump)…
so legs are the spring, the board is the mass, the water under the board is the damper…
the lower the mass (lighter board) and the lower the wetted area (smaller shortboards), you have reduced the mass and damper and thus faster stronger spring oscillation is facilitated (faster springier leg pumps - classic cause and effect of the spring/mass/damper model)…combine that with todays dampened springed poopees and you can see the effect in action…
as far as velos kneeboards…from the sound of it they benefit more from compliance than a standup surfboard…the feeling on intimacy with the wave must be epic…
Hey doc!
Hey, Doug
If I may -
Please do! As I blunder along once again
Quote:Among the nice things about flex in a planing hull is that not only can it assume a more useful curvature in a turn, but inherently
have the ability to assume a rocker that’s paralell to the water surface and thus minimising drag. Little or no distortion of the
path of the water flowing along the bottom. This goes some way towards describing how low pressure mats ( a’ la’ Solomonsen)
have such nice speed characteristics: they skim on top of the water surface by conforming to it.
Hadn’t thought of it that way - interesting!
Using a planing surface without flexibility, especially on a curved water surface ( a wave) , finding a position where the curvature
of the surface coincides with the curvature of the planing surface is more or less a happy accident ( give or take operator skill) .
This position does not necessarily coincide with the fastest path along that surface. Moreover, compromises made in the rigid
planing surface to facilitate turning, etc, compromise the ability of that surface to work in those parts of the wave which can
provide the greatest speed.
Got it… I think
A mat flexes separately all over and doesn’t recover/return - countouring to match the water contours. A surfboard may flex it’s entire length - tho more in some areas and less in others. A boat (displacement hull) doesn’t flex at all (and as a result doesn’t plane).
Well, I may have oversimplified that explanation or otherwise dropped the ball…
Lets put it another way:
A kneeboard with built-in flex, a mat or a bodyboard that follows Morey’s original concepts* flexes or bends to conform to the surface it’s on, turns as a separate issue, such that drag is less and speed is greater for a given position on a wave, slope of the wave, etc.
As an aside, Morey had a very good idea with the original flexible boogie boards. Which his successors have definitely missed the boat on, pardon the pun. They have added stiffeners so that for all intents it may as well be a plywood paipo, 1" plywood at that.
A surfboard flexes, yes, but with a very few exceptions, that’s not intentional, though something could be said for step-decks and the like.
A boat or ship hull generally shouldn’t flex, though I have seen quite a few that did. Displacement hulls ( those which move within a speed range specified by 0>(1.34*(square root of the water line length in feet)) mile per hour) don’t plane on top of the water, so ( contrary to the frequent misnaming of surfboards) they are not real relevant.
Planing hulls, a class that includes all surf craft, ride on top of the water. Those with relatively rigid hulls ( planing hull boats, surfboards in general ) have bottom shapes which either conform to the waterflow or don’t. Surfboards that don’t are slow, boats that don’t have trim tabs.
[=1]
Quote:[ 2]So how much flex (solid stand-up board) is too much? And where is flex most critical? [/]
[/]
Not my field, but… I would say that flex, for speed purposes, is useful along any part of the planing hull that is in contact with the water.
How much is too much? Well, herein lies the fun part.
On a surfboard, you got one , basicly, point of contact. Where your feet are. Relatively close together.So it’s gonna bend around that point, where your weight is, and at the same time you got pressure against the bottom all over. Too much curve, ya wind up pushing water, etc. Kinda like standing on a thin board or a piece of sheet metal between two sawhorses. This is why the BZ and Doyle-type soft boards have fiberglass structural members in 'em.
A kneeboard or one of the other happy alternatives, well, weight is distributed more widely and at more points of contact, such that a more flexible planing hull can work without bending excessively.
So, the answer to your question, how much flex is a good thing in a surfboard, especially a gliding-type longboard? ‘not much’ is about my best answer.
Quote:Now, ‘pumping’ a board may have some utility in small, inconsequential ‘contest’ waves, where the power available from the wave
itself is so little that any added power would be of use, but that’s kinda irrelevant
It is for me - I like glide
-doug
Ah- well, in the words of Huey Lewis; " I like things that go fast"
hope that’s of some use
doc…
To further clarify…
A mat flexes separately all over and DOES recover/return - countouring to match the water contours. Otherwise it would have no drive, no ability to project itself through long turns. A surf mat can be momentarily tightened/stiffened up, much like like a muscle.
Altering a mat’s internal pressure (while surfing) by means of rider grip and body posture directly influences twist, flex and the resistance/ compliance of it’s contact surfaces.
A surf mat has active flex… top and bottom surfaces respond independently to both rider and wave… literally an air suspension system between human and wave.
The supple, flexural characteristics of a surf mat also allow it’s dimensional shape to change: an 18" wide mat can instantly become 25" (and vice-versa) when extra planing area is needed.
The active areas of its rails can be thick and full, or thin and pinched. Changes in length, profile thickness, surface contours, and buoyancy distribution are all related and influenced at the same time.
With something as small, light and fast as a surf mat, balanced handling is critical to wide range performance: speed is a function of control, and control is a function
of flexibility.
“And no - bumps aren’t any fun.”
My quote is actually more in reference to my personal experiences. I’ve broken one spoon(snapped the rail) 40+ times. This is usually the result of taking off late and freefalling or bumpy, lumpy conditions that cause the board(and rider) to get air and then land with a “snap”. Because of this experience “lumpy and bumpy” have been ruled undesirable. This is part of the reason for my search to learn about new materials and construction. Not only do I want higher performance but also strength and durability.
On the right vehicle, twisted, gnarly ledges and bumps are great fun.
Yes I totally agree with Dale. And this is where the different types of flex come in. The compliant flex rules in those conditions while flex/spring can leave you out of control. The suspension has to be tuned for the conditions. Haven’t quite figured out how to have a “tuneable” suspension but it is on the list. adjustable torsien bars?
On a “pit stop” I can envision softening the springs and adjusting the shocks when it is big and lumpy. Better yet have the adjustments on the board. Turn the knob to the right to stiffen/left to soften. This would also be an adjustment for rider weight(stiffer for heavier rider) and wave conditions(stiffer for larger waves). One vehicle that handles many conditions.
With Dale’s vehicles you just add or remove air - active suspension adjustable on a macro level through inflation and on another level by rider manipulation.
as far as velos kneeboards…from the sound of it they benefit more from compliance than a standup surfboard…the feeling on intimacy with the wave must be epic…
Yes its the “feeling” with both flexspoons and mats. The vehicle becomes an extension of you allowing you to experience the wave thru it. Almost like bodysurfing with additional sensors and control surfaces.
excellent thread gents
so is MTB a hit and run artist? did i really read his lengthy mathematical post or just gloss over it?
so…what is FLEX?
technically, FLEX is an inadequate term as it relates to mechanical objects…
so to clarify…
undampened flex with strong return is a spring
undampened flex with weak or no return is compliance
flex with a combination of both is a dampened spring (classic spring/mass/damper mechanical vibrations model)
when you look at modern shortboard surfing from the 70’s vs today, one of the key differences is that lighter equipment allows the surfer to use his legs more effectively as a spring (the pump)…
so legs are the spring, the board is the mass, the water under the board is the damper…
the lower the mass (lighter board) and the lower the wetted area (smaller shortboards), you have reduced the mass and damper and thus faster stronger spring oscillation is facilitated (faster springier leg pumps - classic cause and effect of the spring/mass/damper model)…combine that with todays dampened springed poopees and you can see the effect in action…
as far as velos kneeboards…from the sound of it they benefit more from compliance than a standup surfboard…the feeling on intimacy with the wave must be epic…
mtb here, on the rebound…(sorry for the delay in responding, but I also have other things to do which place demands on my time)
Please keep in mind that my post was addressing only the commonly expressed concept that energy stored in the (presumably longitudinal) flexing of a surfboard might result in an increase in speed due to the momentum imparted to the water as the energy associated with the flexing of the board (the spring in the system ) returns to its equilibrium position.
To summarize and/or restate the points I tried to make in my previous post…
What one has is not an energy conserving system, but rather a highly damped system (as indicated by the power required to drive the board at representative speeds–e.g. approx 4.3 hp at 19.5 mph). Hence to maintain a speed, the craft must constantly be resupplied with new energy. Rates of energy input (and dissipation) constitute (and are defined as) power. When the propulsive power matches the rate of dissipation of energy (power loss), the craft travels at a constant rate of speed. To go faster either:
More power must be supplied to the craft–e.g. riding a faster moving (e.g. typically bigger) wave, or riding higher on the wave face (to increase the wave face slope angle), or…
The power dissipated must be reduced (e.g. by riding a hydrodynamically more efficient craft), or…
Additional energy must be supplied to the system (on a continuing basis).
The primary source of propulsive power is the force of gravity acting in concert with the sloping face of the wave. The only possible additional source of energy (apart from an external input…such as being pulled by a towline to a PWC) is the potential energy stored in the rider (e.g. via ATP) and subsequently expressed by extensions and contractions of various elements of his body (e.g. arms, legs, etc.). In the present case, the rider’s legs pumping up and down would be analogous to pistons extending and contracting to transfer energy to the spring (surfboard).
The point of the earlier post was to demonstrate that the energy stored in a (longitudinally) flexed surfboard, and the rate at which that energy can be supplied via flexing the board (i.e. the power associated with the flexing board) is small compared the rate of power dissipation due to drag. Hence the increase in speed resulting from this additional power input is small–even if the efficiency of the transfer mechanism is 100 percent.
[An aside…
As regards the comment that a 4 inch flex is not representative of the longitudinal flex (flex rocker?) in a shortboard, that is true. But as is evident from the flexural equations in that post, the stiffer the board is (for a given surfer’s weight), the less energy that can be stored in the flex of the surfboard (i.e. the “spring”). Hence “pumping” a stiffer board up and down to transfer energy to the flexing board will, all other things being equal, be even less of a factor. Hence by choosing what is clearly an exaggeration in the flex, I could demonstrate that even with that favorable (but unrepresentative) quality of the board, the energy transfer was too small to make a significant difference. While it is also true that with a smaller degree of flex the rider may be able to complete a flex cycle more frequently than for a greater flex, that certainly isn’t what appears to be the case when observing real rides with pumping. ]
Therefore, if flex (and the rider’s use of flex) is to significantly increase the speed of a board, that increase must result from:
(1) allowing the board to continue to operate efficiently at positions on the wave face with a greater wave face slope (as to where this occurs on the face of the wave obviously depends on the shape of the wave as it breaks), or…
(2) the flex must be used to improve the hydrodynamic efficiency of the hull (as I alluded to in my earlier comments about “natural rocker”), or…
(3) both (1) and (2).
As I noted above, my earlier post addressed only whether energy stored in the flexing of a board could substantially increase speed–and my conclusion was that it could not. Note, however, that I did not address in any detail the potential role of flex with regard to items (1) and (2) (immediately above). Not surprising, since the question I addressed could be answered with some simple and pretty defensible calclulations and assumptions combined with basic laws of physics, whereas (2) and (3) get into the nitty-gritty of the specific hydrodynamics of the interaction between the wetted area of the board and the wave, plus the details of the compound-curved, 3-d flow field within face of the wave in the immediate vicinity of the board. Both are very difficult problems to solve–even with the current state-of-the-art computational fluid dynamics programs and modern computers.
However, it would seem conceivable, and perhaps even highly likely, that the energy (and power) that can be produced by the rider, combined with an appropriately designed craft, could collectively be sufficient to dynamically modify the shape of a (planing) hull so as to expand the scope of conditions over which a hull will maintain some degree of hydrodynamic efficiency. And by so doing, expand the region of the wave face over which the hull will operate efficiently–in particular, riding where the slope of the wave face is increased. Either in a “static” fashion (as in maintaining a constant position), or through some near optimum dynamic path (e.g. a series of “skating/pumping” motions that traverses some vertical region of the wave face in the vicinity of the craft) so as to produce a substantial increase in propulsive power while minimizing the loss of hydrodynamic efficiency.
Similar considerations can also be applied to minimizing the speed lost while executing turning maneuvers. It is this consideration, rather than pure speed, that was one of the primary motivations for my ongoing experiments with my hydrofoil paipo (“HYPO”) board. Also, as has been commented on by several in this thread, flex can also considerably alter the “feel” of a board board, thus allowing the builder/rider to find the combination that he perceives to be the best for his style and desires.
My present impression is that in general it is much easier to make a board “feel” different than it is to substantially alter its speed performance (although the performance may be perceived to be substantially different). While no one who has ridden my flex kneeboards has described them as “slow”, for me it is largely this “feel” factor–especially during maneuvering–that has driven the design of them.
mtb
Okay.
Brief recap.
I said:
The speed is in the top 1/4 of the wave. etc.
Next is what Kelly said yesterday in Japan:
“Here you have to generate speed,” Slater continued. “At J-Bay everyone is going to get waves that they can surf really well on. Here it’s usually the smaller guys who can generate the speed and stay in the pocket. You have to have different boards and have a different approach. You have to turn differently and you really have to pay attention to what the wave is going to let you do.”
Finally what MTB said:
However, it would seem conceivable, and perhaps even highly likely, that the energy (and power) that can be produced by the rider, combined with an appropriately designed craft, could collectively be sufficient to dynamically modify the shape of a (planing) hull so as to expand the scope of conditions over which a hull will maintain some degree of hydrodynamic efficiency. And by so doing, expand the region of the wave face over which the hull will operate efficiently–in particular, riding where the slope of the wave face is increased. Either in a “static” fashion (as in maintaining a constant position), or through some near optimum dynamic path (e.g. a series of “skating/pumping” motions that traverses some vertical region of the wave face in the vicinity of the craft) so as to produce a substantial increase in propulsive power while minimizing the loss of hydrodynamic efficiency.
MTB:
My understanding of your previous post is as follows. flex built into a wavriding craft does not in and of itself increase the speed potential of that vehicle. any excess speed may be produced by the riders input-- (1.)his or her ability to extend or contract various parts of the body, (2.)while riding the optimum “line” on the wave… ummm… where the slope angle of the wave face is ever increasing or some other line associated with the pumping of the craft…
the “dream of fling” out of a huge bottom turn(based on any stored energy in the flex) is not really producing any excess speed? the “sensation of fling”(in terms of speed increase) is only measured in the perception of the the rider and the feel of the craft?
i’m gonna go surfing and think about this a little more…
MTB - My present impression is that in general it is much easier to make a board “feel” different than it is to substantially alter its speed performance (although the performance may be perceived to be substantially different). While no one who has ridden my flex kneeboards has described them as “slow”, for me it is largely this “feel” factor–especially during maneuvering–that has driven the design of them.
Feel, feeling, “feel” factor. For me this is the key part of what MTB is saying. Interesting how it keeps coming up from those who have actually ridden “flex” kneeboards or mats.
Kelly - “Here you have to generate speed,” Slater continued. “At J-Bay everyone is going to get waves that they can surf really well on. Here it’s usually the smaller guys who can generate the speed and stay in the pocket. You have to have different boards and have a different approach. You have to turn differently and you really have to pay attention to what the wave is going to let you do.”
Flex gives much more feedback from the wave so you are much more aware of what the wave will let you do.
MarkSpindler - The speed is in the top 1/4 of the wave. etc.
In my experience flexspoons have always “wanted to” and had the ablility to ride higher in the wave. This is one of the reasons a down-the-line point break wave is desirable. Flexspoons give you the ability to stay high in the “power pocket”, to hang on the vertical face. And it’s on the vertical face that the flex/spring(pumping) action works the best. Also not much drag when you are almost falling.
Mark first posted: ---------------------- For this I would go back to observation step. It appears to me that Slater at Chops a few months ago got his perfect tens by riding as high on the face as he humanly could. From that information as as 1st step I was wondering if we could go back and look at the wave face angle and try differnt numbers. And see if it changes the form or direction the analysis takes. See I base this on the fact that a wave face has different places with different angles. Those angles range from flat in front to 90 degrees to past 90 as the wave forms a jet. I am presuming that the pump is to get to that part of the wave that is most verticle because that is “where the speed is”. And the the pump is not necessarirly a flex power issue. Or if there is some springbord effect from the board what suggestions would you have to how muuch spring?
Pros tell me that whatever speed a rider can obtain from a wave is in the top 1/2 for some waves, the top 1/3 for others, and the top 1/4 on the slopeiest waves like in CFL. Understnding this small minute bit of info helps a top surfer determine how and what gear he will ride on any given day. It is perhaps the information that separates the top surfers from the rest. But I digress.
How, if you could, would you incorporate this information into your thinking. Thanks take you time. ----------------------------------------------------------------------------------------- And subsequently followed this up with: Okay.
Brief recap.
I said:
The speed is in the top 1/4 of the wave. etc.
Next is what Kelly said yesterday in Japan:
“Here you have to generate speed,” Slater continued. “At J-Bay everyone is going to get waves that they can surf really well on. Here it’s usually the smaller guys who can generate the speed and stay in the pocket. You have to have different boards and have a different approach. You have to turn differently and you really have to pay attention to what the wave is going to let you do.”
Finally what MTB said:
However, it would seem conceivable, and perhaps even highly likely, that the energy (and power) that can be produced by the rider, combined with an appropriately designed craft, could collectively be sufficient to dynamically modify the shape of a (planing) hull so as to expand the scope of conditions over which a hull will maintain some degree of hydrodynamic efficiency. And by so doing, expand the region of the wave face over which the hull will operate efficiently–in particular, riding where the slope of the wave face is increased. Either in a “static” fashion (as in maintaining a constant position), or through some near optimum dynamic path (e.g. a series of “skating/pumping” motions that traverses some vertical region of the wave face in the vicinity of the craft) so as to produce a substantial increase in propulsive power while minimizing the loss of hydrodynamic efficiency.
Oops. When I read that I noticed that I should have said “…modify the shape of a flexing (planing) hull so as…”
Thanks for the comments! I certainly don’t disagree with the quote from Kelly–nor with most of what else you said.
As far as Kelly’s 10’s, since I don’t know the details by which a wave is scored, I can’t assess if the high marks were given because of the speed he may have achieved (and by doing so, perhaps was able to make the wave)…or because he was taking extra risk by riding the wave in that manner.
From a purely hydrodynamical point of view, in general, the problem with riding too high on steep waves is that the wetted area of the bottom of the board (not counting the area wetted by spray) becomes a long, slender “sliver”. This reduces the aspect ratio of the wetted area, and thus reduces the magnitude of the lift coefficient (almost proportional to the aspect ratio). To make up for this, the angle-of-attack that the board makes with respect to the local surface of the water must increase in order to support the weight of the rider. This increased angle-of-attack increases the induced drag. At very high speeds, the induced drag (of the board) will be small compared with the parasitic (skin friction) and form drag (and the induced drag of the fin(s)), so the effect may be reduced. Conversely, at low speeds, the induced drag dominates, so this can become important. Hence I would expect that the optimal place on the face of the wave would also depend on the wave size (via its onshore speed at the point of breaking and the resultant change in speed over the water) as well as the details of the flows in the face. There are also some effects on drag due to the change in flow velocity between the forward wetted surface of the board and in the area of the fins that are difficult to assess.
Where its not quite clear to me what your saying is when you reported that pro riders indicate that–depending on the kind of the waves–the maximum speed that can be achieved can vary from the top 1/2 to top 1/4 of the wave. The later (1/4) being for the “slopeiest” waves “…like in CFL.” Part of my problem is that I have to confess that I don’t know what CFL stands for (central Florida?). Also I don’t quite understand the term “slopeiest” since any barrel, or even waves in which the crest is beginning to pitch out, has a slope of infinity wherever the face becomes vertical.
In the real world, I suspect that a lot of the positioning on the face of a wave may be related to factors other than maximizing speed. For example, there are certainly also simple physical constraints on where one can ride in a barrel–especially as the dimensions of the wave become comparable with the vertical height of the rider (a bodyboarder, mat rider, or bodysurfer having the greatest number of positioning options as long as he can stay on the face of the wave). My impression (I will have to check though) is that virtually all the rides in the video “Solid” are with the rider (or, more correctly, the rider’s board) positioned below (and perhaps well below) the midpoint on the face of the wave (one of the rides on the experimental board momentarily being a possible exception). I have no idea if this is determined by the hydrodynamics of the board/wave combination, or simply reflects some consideration for self preservation. When I get the time, perhaps I will view the rides in this video in step motion to see how well it might agree, or disagree, with my earlier comment that the photographs that I have examined seem to indicate that when somebody is locked into a barrel, the slope of the wave face in the region of the board tends to be around 45 degrees.
MTB:
……the “dream of fling” out of a huge bottom turn(based on any stored energy in the flex) is not really producing any excess speed? the “sensation of fling”(in terms of speed increase) is only measured in the perception of the the rider and the feel of the craft?
i’m gonna go surfing and think about this a little more…
Yup…that’s pretty much it. In the absence of other factors, the execution of a (carving) turn, such as a bottom turn, should increase the drag. During the turn, the increased “load” the craft must support as a result of the centripetal acceleration (“centrifugal force”) during the turn, plus the additional centrifugal force associated with changing from a net downward direction to a net upward direction (like being at the bottom of a roller coaster dip) means that the angle-of-attack must increase to support this additional load. Also the banking of the board typically reduces the aspect ratio of the wetted area (and the magnitude of the wetted area unless the wetted length increases sufficiently to offset the reduced width), causing a further increase in the angle-of-attack in order to support the craft and rider. Since the induced drag is proportional to the square of the angle-of-attack, this results in an increase in drag and consequently a slowing down of the craft as soon as the propulsive force (gravity and wave slope) becomes less than the drag force.
Of course, there are other things going on at the same time to complicate matters. For example, if the wetted area is diminished during the turn, the parasitic drag will be reduced (same for the induced drag of any fins that are now out of the water). Thus conceiveably the total drag could be reduced. However, for most situations I don’t think this will be the case.
It may be possible to test this hypothesis with a recording water speedometer by rapidly recording (say every 1/4 to second) as the instrumented craft drops in on a wave and executes a bottom turn, and then examining the record to see if the speed coming out of the turn is greater, or less than at the beginning of the turn. Ideally one would want to simultaneously video the craft while looking directly at the wave face so that the before/after measurements would be compared when the craft is at the same (vertical) position on the face of the wave.
I’ve got a water speedometer–anyone got a compact video camera in a waterproof housing?
mtb