Tail rocker and pre tensioned stringer a question of energy return.

Bit of a long set up but I think interesting engineering idea in pre tensioning a blank to flatten tail rocker and create energy return.   Also  how much is too much tail rocker?

So I finished shaping my blank today. some strange issues arose. As a proponent of increased tail rocker I would like some input. 

The board is is a 6’9" Mini longboard with a good high performance squash tail. 20.5 inches wide before glass. I am attempting to gain greater control over my foiling abilities and…while my magic thickness number seems to generally be 2.625 thick…I have brought this one down to 2.5 thick before glassing. PLUS, I have added just over 1/4" of concave through the guts of the board and blending slightly through the tail only flattening out at the last few inches. This brings the effective center thickness to 2.25" .

The rails however are boxy but nice thickness closer to 2.5. simply meaning I have pushed the foam out towards the rails a bit. Now…the nose rocker is at 3.5" Good. I like this. The continuous curve along the rail looks good. It’s got more curve than I normally put in a board…but it’s clean and arcs out nicely. The TAIL ROCKER. Along the path of the stringer…it terminates at 2.5" maybe a touch more but this will end up at 2.5" after glassing and resin bead. I think 2.5" is good for a board that is 6’9" long and has a dropped nose rocker. My concern is this. I placed the board on the ground. Where the edges of the rails are what touched the ground at approx. center mass of the board. (remember deep concave) The tail (not on the rack flipped on its deck belly up) reaches up to 3.25 inches. So the question is how bad did I screw this thing? I’ve been watching Maurice Cole’s videos and his tail rocker due to deep concave is fairly slight down the stringer line. I would say about 1/4" as the water shoots through the center of the board and out the tail. The rail carries all of the curve…and thus the tail rocker is or possibly should be measured from the rail rather than the stringer.

Mine…at 2.5 at stringer and 3.5 from the apex of the curve of the rail is kind of freaking me out.

Additionally I came up with an Idea. Possibly an interesting experiment into physics and engineering. Pretension the stringer to carry a biased flex towards flatting out. Here is what I mean to do:

The board when bottom up on the rack flexes fairly easily in the middle. ( the stringer being the “I” beam that carries load. it’s a 1/8" plywood stringer EPS blank) It’s thinner at the middle than my usual boards. by 3/8". Now…I was pressing down over and over again in mild concern with the amount of flex the board seemed to have. Obviously this could be good or bad…I don’t know. But I do know the 3.25" of tail kick has me worried as to how crazy loose the board might be…hopefully counteracted by the deeper concave speed. (Who knows??) But what if I were to use the fiberglass and epoxy resin the flatten out the board and create an energy return system by weighting the board during the glassing process? I have a 10’ 2x4 that weighs about 12 lbs. or so. I glass the boards bottom. one layer 6.0z. While its still fairly wet. I take a sheet of plastic and lay it along the stringer. (Epoxy resin does not stick to plastic) I then take the 12 or so lbs 2x4 and gently place it on the stringer. It’s weight flattens out the board a bit. The tail…arcs in the reverse direction flattening out a bit. approx 1/2". The arc in the rail line flattens a bit as well. The resin kicks and the board is stuck in pretension camber. I flip it and free lap the top with two layers 6 oz + 4oz. Theoretically the board is now tight like a guitar string. As I load a bottom turn with my weight the board flexes into an greater arc than it’s natural shape. (this is typical…but it’s tight because of the preloaded tension) And because the board has been trapped in a position that is flatter than it’s normal arc…It snaps to a flatter position more with more spring and quickness than even a normal epoxy board has. (Epoxy is already more “lively” than Poly)

So my questions are:

Still too much tail rocker for a 6’9"

And your thoughts on the energy return system that may arise in an effort to flatten the tail rocker. ?

Or is the answer?.. Leave it alone…glass as normal…2.5" of tail rocker along a dropped concave is fine and 3.25 off the rail for a tail rocker is fine too? Wont sacrifice too much speed…don’t worry about it. 



**Ah, why not surf it and adjust tail rocker accordingly and let us know?**

**When looking @ your tail rocker use the rail line, as concaves differ. Only use Poly here but as for pretension and reflex or (rebound) been using Vector net to reinforce parabolic sticked jobs to some degree of success.**

**Aloha! **


It sounds like two and a half inches of tail rocker? Yes no? To me, that would be my limit. Atleast for a small to medium size wave board. Also, if you put a deep concave in theory it Would counteract the rocker sum…figure in you now got two and a quarter inch of tail rocker now.Leave it be! I AGREE SURF IT!

Sir - I think ya had slight typo error…cuz if you did shave in three and a quarter inches of tail . Rocker it would prob do a flip on ur bottom turn!:smiley: :open_mouth:

Ps: id glass it, and hopefully it works well. If not, get another blank and learn from it… question: how did it get soo deep? Did you just get carried away planing? It happens. Fyi on my fourth board I prob could pull my board’s nose and break it off np…it works, but not ideal for gutless waves. I took her out at Asilomar beach and surf way out in Monterey bay and it moves quik enough…but my tail felt a bit corky, and I absolutely hate boxy rails now!

Hello! You won’t get an accurate reading put n your board on ground to figure out how much rocker it has. What you do is lay it belly up on shaping stands, or saw horses and you push with just finger pressure your stick on center or apex…then measure how much rocker at tips. So you need a rular there etc…I’m new to it, but I’ve seen Shapers using a straight edge for yrs. Ideally, you want it long enough for length of board, maybe a half inch thik alluminum window track, or piece of hard wood…on my first board I didn’t have one handy…but I was not altering rocker much. Infact, I left nose flip alone, and just cut a tad more tail kick…I checked with caliper s my thickness a foot up from tail, checked the sides at rail to verify they were even…it felt even within my hands…a little squeeze! Gently squeeze again, lay a level across to even it out…goood, ok here, ok there…ok, I hope it works!:smiley:

I think even if you had three inches of tail rocker it probably will be ok - being the board kinda looks like a scaled down long board…Im think n if you put a big raked single fin it might give more drive but im not sure…it looks ok. Dont overthink it. All of us new garage shaper do that!?Jk

Right…3.25 is fine for a longboard but a 6’9". Should be about 2.5.  And it is at the stringer using the lOng stick method.  It’s along the rail that things got a bit arced out.   It’s  all an issue of water displacement and Apex of an arc. The nose is at 3.5. Where a normal board is at 5 or so. So the added tail arc might work.  Bottom line is this.  I chopped the tail by 6".  It’s now a 6’3 Deepened the concave even more.  It’s now at 3/8" of concave. Dropped an  Inset double at the rear fin and swallowed the tail.  It looks like a longboard/fish.  The board looks good…funky…but good.  If these deep concaves and flattened centerline work the way cole/proctor/tomo have shown. The board should be surprisingly fast and loose.  Here’s hoping.  And thanks for the input. Vector net and weight system were good ideas. It just wasn’t pulling the tail kick down enough. 


…just shape it how you want it , and don’t worry about what cole/proctor/tomo are doing.

No to the Pretensioned stringer idea

It would be stiffer, but it would also be closer to its ultimate breaking point, so it would snap sooner when stressed.  And center of mass is not where you measure rocker from.

I’ve tried the pretensioned stringer, although not using the method you described (I bent the stringer rather than the whole board)… try it, you may be surprised.    I like the way mine surfed… and it never snapped, but then I like to build strong boards.


Thanks Kayu

I always end up shaping it the way I want to.  I’m at a point that I have figured out how to make a fairly good board. I’m testing theory’s. And using examples or precedent to help me understand what it is that I am doing.  My mistakes often lead to surprising results. Then I research here to figure it out.  My boards don’t look like cole’s or proctor’s or Tomo’s…but I can play with contouring in the same sandbox.  I think the other people here have researched some of these shapers as well…so they can help me to think about my ideas.  Ultimately I make my own mistakes and go my own way…but,  there is always some real insight to be garnered here.  And from shapers who have made a living at shaping. They throw more money at R&D pushing out more boards.  It all works to some degree or another.  It’s about finding the ability to create a magic formula on a consistant level.  Depending so many veriables that it seems endless…And fun.  


I don’t think it would snap.  Though in theory your idea of an over stressed stringer seems correct.  The camber would be stressed in an un-natural state.  Creating tension at the top of the board and compression at the bottom of the stringer.   As I would load into a bottom turn at speed my weight would press upon the Deck of the board.  Increasing my load to something more that my own body mass.  However…At the apex of the arc as my load is greatest the stringer would flex into it’s natural state of arc.  Thereby releasing the pretention that has been placed upon it.  As I unload my weight towards the top of a turn or when riding neutral the flex would rebound…back to a more tense or flexed state.   

The Idea that the board might snap I believe would be more appropriate if the tension placed into the board was to increase the rocker rather than what I want to do…which is to flatten the board out a bit.  

Thank you though.  This helped me think about it a bit more.


also… Where do you measure rocker from?

I generally measure from the stringer.  But I have been trying to figure out how to measure from the rail.  The issue being that if I keep digging deeper and deeper concaves to create a tunnel for the water to push through then the stringer is very flat by comparison to the rail.  Also…obviously we don’t turn off the stringer …we turn off the rail.  It would make more sense to measure off the rail.  Not as easy to measure using a long straight stick.  

How about a picture??  You’ve peaked our interest.


I’m fairly new to measuring rocker, but you measure from apex + you balance a long rocker stick at center or apex, then take a rular or a square at tip ends to measure the
Rocker. Oh, and you would push stick down on apex with only light finger pressure.

AviG- I’ve seen a shaper just lay it (stick) on apex/center, and stick balancing by itself - then he took rula or a square to measure under tip and board to get #…


**The old brick on the blank trick from the bygone stringer less period. Haven’t thought of loading up the parabolic job. Thanks!!!**

yes of course.  I have always measured rocker this way.  The stick rests on the stringer and you measure the ends.  The issue is that what I am trying to do is deepen the concave at the center of the board.  The rail is what is engaged during the turn.  The rail is a fin at a certain point in the arc of a turn.  The deeper the rail through the arc the sharper the turn…especially if its lifted in the tail.  the issue is that too much tail rocker creates suction.  water will not release off it quick enough. It’s called surface tension.  Additionally to find the mid point I may have be standing on the tail more than I want to to get the board to find it’s sweet spot.  Traditionally you stomp the tail to wash speed off…Not to generate speed.  Unless your pumping…(whole other topic)  So the deep concave flattens the arc in the center of the board.  The rail…the rail…the rail.   There lies the question of rocker.  Too much rail rocker…welll…It. might just be too much.  

Maybe you should just try your ideas… if we don’t think for ourselves we will all be riding five finners…remember people used to think the world was flat…

As for your concave I don’t think it’s advisable to go very deep…but kicked tail like you got the flow of water should exit at kick… if you are concerned - why not let concave carry all the way through! Ryan Burch did it on his-Pink Fish ! Utube it.