SHAPE - what makes a fast kite surf board

[tungsten]

For the use with kites, most of the time we want fast kite surf boards. Unlike paddle surfers, who optimize their boards for the shape of the pocket of the wave they are made for, we want boards which go early, plane easily, for use with a smaller kite and in lighter winds. If you're lucky enough to live where it blows 24kn every day of the year, you can concentrate on other things in your board. Most of us are infested with light airs though.

Flat rockered boards usually get going early and are fast: Alaia, Skim, Door. Some say, well, concave works because you have less rocker along the center line. This is partly true: less rocker usually makes for a faster board. But it's not completely true.

If you make two boards, equal outline, equal flat-ish rocker along the center line (stringer). One board has no concave. The other board, with the same rocker line along the stringer, has got concave, which means it has more rocker along the rails. Which one is faster? If you shape it right, it's the board with the concave. Not the flat board. I tried it.

Then - there are exceptions from the flat rocker rule. Fast boards with lots of rocker. Like the OR Mako for example.

And there are boards with single concave, which are complete dogs. I won't point any fingers here.

Now - how does that all make sense?

Here's how: the trick is a single concave, WELL SHAPED IN RELATION TO THE ROCKER. It's the saddle shape. A fast shape is when you achieve a saddle shape with a straight line PARALLEL TO THE WATERLINE.


Let me explain:

It's not the line along the stringer - the rocker - which has to be straight. It is the waterline, in the board position where we usually want to go fast: upwind. Going upwind, our board is pointing slightly higher than our course through the water. The red line in this kid's pic (stolen from the web, thanks Drew!) is the water surface line. This is the line we are talking about. This is the one which has to be straight, to make for a fast board.

Drew.jpg (150.14 KiB) Viewed 1469936 times

Now how would we achieve this in a surf board? Here are some pics from the shaping process.
-The rocker is not straight, it's a curve.
-Concave is also a curve.
-But: rocker + concave form a saddle; if you shape it right, this allows for a straight diagonal line.
That's just mathematics

I always check the bottom shape with a long ruler when shaping. Look a those pics:
-Rocker is curved;
-board has concave;
-resulting diagonal line is straight.



ROCKER CURVE:

DSC07448.jpg (65.97 KiB) Viewed 1469935 times

CONCAVE all along the board:

DSC07432.jpg (52.54 KiB) Viewed 1469935 times

DSC07435.jpg (58.86 KiB) Viewed 1469935 times

DSC07436.jpg (57.58 KiB) Viewed 1469936 times

DSC07437.jpg (68.59 KiB) Viewed 1469936 times

THE RESULTING DIAGONAL LINE: STRAIGHT (not perfectly, but almost; this is not the final shape. final shape is perfectly straight)

DSC07447.jpg (68.84 KiB) Viewed 1469936 times

This is the one most important thing I learnt about shaping a kite board.

There are other things like the outside curve, sharp edges, fin toe in, which also play a role. But first comes, by far and large, this little detail of the bottom shape. Second (for a surf board) would be fin toe in.


Now I've spilled the beans for you


cheers, tungsten

[spewing]

Thanks for that. I had never thought about the bottom contours of a board like that.
I had to read through it a couple of times to get my head around it. Now off to put a straight edge over some of my boards.

[tungsten]

Yeah spewing, you are a faster learner than me, it took me a year and many hours of studying yacht design literature to finally understand it. The biggest part for me was to understand how a board moves through the water, how it's angled vertically and horizontally, relative to the course. Once you see that, the rest is obvious.

Paddle surf boards are designed by the same principles btw. (if done right). A board sitting nicely in the pocket is mainly because the rocker reflects the curvature of the pocket. On the wave, the waterline is a curve, so the board has to be curved. Paddle surf boards are ridden with less vertical angle though (no kite force to push against), so the equation is slightly different.

Now the art is to anticipate where exactly the waterline - the "diagonal" - will be. This depends on many factors - volume distribution, rider weight, riding style (more back foot or more front foot, flat out using the fins or using the rail) -, but, as a rule of thumb, it is roughly the line from the upwind rail where the rocker starts to get flat-ish, maybe 40cm in from the tip, to the downwind rail, where the front fin is. The rule of thumb is good enough to achieve at least 80% of an optimal result (the pareto priciple). If you know more about the rider and the board specs, you get closer.

This does not only apply to concave boards or surf boards; concave surf boards are only one example I chose to explain the principle. It is less obvious in a surf board ("how the heck would you get a straight waterline in a rockered board??") than in, let's say, an alaia or a skim, which have hardly any rocker. Door style boards i.e. achieve this by using a waisted outline. Rocker + waisted outline = straight waterline, if done right.

You can think of many geometric shapes which allow for straight diagonal line. Many won't make sense in a board though. Just food for thought. Check the boards you have with respect to this aspect. Which are the fast ones? Which are the dogs?

Now if you have a board which fits the bill as above, goes off easily and accelerates, but once reaching moderate speed, won't accelerate any more, no matter how hard you push it, then you have a fin problem. Try less toe in, smaller fins, less rake - rake together with toe in is an efficient drift anchor.

cheers, tungsten

[jjtango]

I'm not a shaper... can only do some rudamentary repairs on my stuff.
However, I will say that this post blew my mind.

salute to the SMART GUY.
Brilliant.

[tungsten]

And while we're at it, I put the ruler on my 150x34 Ocean Rodeo Mako. This super fast puppy is the TT with most concave and rocker I've seen, ever. Now guess what:

ROCKER. Lots of it. Look at the AA batteries I put at the end of the board to give you an idea.

Rocker.jpg (106.04 KiB) Viewed 1467693 times

CONCAVE. 1.9cm / 3/4". Massive.

conc.jpg (77.6 KiB) Viewed 1467693 times

And the DIAGONAL line is STRAIGHT.

diag.jpg (73.77 KiB) Viewed 1467693 times

[John L]

Tungsten - Having read your build threads and this thread I would really like to know ...

What is now your favorite finless or small finned board for small/mushy surf ... in the shallows? Your Kipuna? It need not have a name. A description is what I really want.

I am 160#, and once the Mako 150, ridden strapped TT, was my favorite surf board. Then I started liking regular surf boards more, settled on a 5' 10" Surftech Rat Boy minus the back fin, and sold the Mako. Then my old injured knee started to give me trouble due to the pounding it took sometimes on the surfboard. Now I have built something of a Mako ripoff. I am not proud of its looks. I just hope it will work and, being smaller and lighter, be easier on my knee. If you wish you can read about it starting on page 15, and see pictures, at- http://www.swaylocks.com/forums/xpsglas ... on?page=14

But I am thinking about what might be better. Since you have already been the factory-and-home-made Mako route, and seem to be always thinking and trying stuff, I would not be surprised if you have something for me.

Cheers -

[tungsten]

Hi John,

first of all, making and riding ugly boards shows character

Here's my take on your board issue. Starting with the boards you mention.

MAKO:
On the up side, the Mako has this flying carpet feeling which allows you to speed over chop like it doesn't exist. That is the effect of a huge concave ridden flat-ish (not angled, like upwind). It also allows you to lay down knee buckling carves.
On the downside, there's a little bit of twist in the knees b/c the board is so slim, which can be hard on the meniscus. And the Mako has a relatively small surface, especially in the tail, so you want to keep speed up. Not ideal for getting through lulls.

RAT BOY:
On the up side, a short board shape like the Surftech Rat Boy has enough surface in the tail, and an outline good for carving.
On the down side, this board is quite big for your tiny 160lb. It has a lot of volume, which makes it bounce on chop and rattles your knees. Probably good for surfing, but a handful used with a kite. Basically you're moving more board with your legs than you need to - my ideal kite surf board at 200lb is half of the volume of the Rat Boy, sized 5'8 x19''. Good enough down to 10kn of wind.

CONCLUSION:
My take is, instead of putting a lot of volume on a board to carry you through the lulls, I make a fast board. Volume is slow, you need to overcome lots of resistance to get up gliding. A fast shape without much volume starts gliding immediately.

And it works.

So I came up with the combination of a
-surf board outline, which I borrowed from the Kipuna
-thin board construction - low volume (11 liters) using the redecessed deck - which I also borrowed from the Kipuna
-huge concave, which I borrowed from the Mako
-and the saddle pattern in which concave and rocker combine, which this thread is about

YOUR BOARD
For what you mention (slop, shore break, 160lb) a board like the first Kipuna I made would work really well. It is a carving machine, flies over chop like the Mako, and it's very little board to deal with due to minimum volume. Sized 5'4 to 5'8, depending on the wind you're usually dealing with. If you want the board to work with ease in 30kn, 5'4. If you want a real low wind board which works well to 25kn, 5'8. Get cheap plastic fins (FCS 3) and cut them down, or get some kite specific surf fins (10cm high). Those are good enough for my 200lb, you don't need much fin with 160lb.

And / or get yourself a skim. I say and / or, because I use both, but for different purposes.
Skim has limited upper end, it sure is ridable with lots of wind, but the sea which comes together with lots of wind is just too much for skimming to be fun. I use the skim when there's just ankle snappers, or when the wind is so low I have to walk the walk anyways. Then the skim is so much fun, doing down winders close to shore, riding up and down the beach (no fins ), and generally fooling around. Soft summer breeze.

Thanks to Lonny we have the correct specs for making you skim - I used those, and they are spot on. And if you want that Kipuna board, I'll send you the .brd file.


My preferred boards are the Kipuna style board for the normal days up to 30kn, the skim for the days described above, and I always have a third board in the car, the Mako 150x34. This is for 30kn+, when the sea gets really ugly and riding strapless means losing the board.


hope that helps,

cheers tungsten

[John L]

Tungsten - Very helpful answer! Your analysis of the problems my old 150x34 Mako and Rat Boy presented seems right on. The board I just built is 3" wider than that Mako and lighter. Compared to the Rat Boy it is shorter and lower volume and of course has much concave. It is in the direction of what you suggest. If it works well for me in the surf in late April and May, then fine. After that month I will also have had the experience of a number of other boards - as many as I can.

After that, if I feel like building another board it surely will include the saddle-like combination of rocker and cancave, for speed, which is the subject of this thread. Also low volume, etc, as you suggest.

Since your Kipuna is still your gold standard for the kind of surf we talked about, I might then take up your offer of its .brd file. I would PM you. Paulonia is not available locally and is expensive to order and have shipped, so I might want to experiment with completely different materials requiring a different design. The design principles you gave me, however, are gold. Thank you so much!

John L

[tungsten]

There are actually other boards which come to my mind, which might fit your bill. I rode that kind of boards for years and it's sweet from flats to head high and in mush. They are fast as well. I just like the feel of a bit more rocker better, so I went on to the current Kipuna style board as described.

They have a flat, skim-like rocker, with a surf board outline, quite small (5'4'' x 19.5''), and TT construction. You can ride them strapped or strapless, finned or finless, although at the time, I rode them always strapped. That was back in the day before losing the straps.

Boards are i.e.
-DECAY Scimitar
-Flysurfer Flywave
-Spleene Zone

Check those for inspiration

cheers, tungsten

[John L]

My wife rode the Spleene Zone once. She liked it. The other two we are unfamiliar with, though now I have Googled them. I see the Flywave has that unusual lip under the ends like the Hammer and maybe a few others. That is a feature nobody uses any more? Thanks for more food for thought.

I didn't want to build any more until after May when I have had a chance to compare everything we have plus whatever I can borrow. But then I ran into a video that changed my mind. I bought some 1" thick red cedar boards at the local hardware store today and they are already Gorilla glued side on. I have PMd Mr. TH to try to get some specifics about the tail's importance, flex, etc....



Much fun,
John L

[John L]

Top Hat didn't reply yet, so I went ahead and built the board. I put as much rocker as I could carve into the 7/8" starting blank - not much! Some but not much concave, too. There are foot strap inserts for those of us not confident strapless in the surf. 4oz glass all around to minimize possibility of warping and splitting.

With so little rocker and concave it was easy to not violate the fast-board-bottom rule.

It just remains to be seen if this and my other board have riding merit.

[gnarf]

IMG_1203.jpg (11.76 KiB) Viewed 1450256 times

5'4 , 18 3/8 , 1 3/4 round pin, flattish rocker , single concave , set up as quad, the board sizzles upwind , turns nice and doesnt slide out ! made out of an old matt willis surfboard! havent tried in surf or the thruster set up yet.
i was wondering if the flex of the board would affect the speed and the ride? stiffer = faster? would like to get more tech on the next one.

[John L]

The thin red cedar board I posted about above was so low volume and dense that when I fell off I could hardly get back on. The slightest touch of a foot on the deck or rail would sink it and send it moving away, its mass tending to keep it going. When up on plane the board was blindingly fast and (because of the pronounced swallow tail?) really held in on wave faces and during turns. And it may have had a good combination of the little rocker and concave. Still I considered it a failure, after I got to ride a new type Mako, a 150X40, which surfed so well I sold the wood board and, with additional use of a credit card, bought the Mako.

This Mako 150X40 is more flexible than my old Mako 150X34 and I would say performs better in every way. The wider new Mako is as easy or easier on my knees than the narrower old one. Especially it is better at getting through lulls in kite pull and so more satisfying for surfing. I use it in twin tip fin configuration and foot placement and it shines that way.

It is not a DIY board, so to some extent I am diluting this thread, and for that I appologize.

My old traditional plywood skim was also still fun when the surf was small.

[valsoansalosn]

For those of us who are unsure about going strapless in the surf, there are foot strap inserts. 4oz glass is used everywhere to reduce the chance of warping and splitting.

https://www.youtube.com/@222alohaphotobooths4/about

[Annasmith]

The other board, with the same rocker line along the stringer, has got concave, which means it has more rocker along the rails.photo booth Dallas TX
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