The relevance of volume in kite surfboards

[Yann]

There is a lot of talk about volume in surfboards and since tungsten also mentioned the importance of volume (great post btw) in kite surfboards I thought I'll start a thread about it. What I don't understand is how the volume can affect how a board rides when you're already riding on top of the surface with just a tiny bit of the board being below the water level and acting as an upward force. I understand that there is a huge difference in how a board paddles when surfing without a kite and being a surfer myself I'm very aware of this. I have boards that I cannot paddle with a wetsuit but worked fine in boardies and the low amount of salt in the water here also gives a little less flotation.

There is no correlation between rails and volume. A board can have a doomed deck (a lot of volume) or a concave deck (less volume) and still have the same rails.
There is no correlation between plane and volume. A board can be x wide and y long ( x * y = plane ) and still have varying volume (z) depending on the thickness.

What I'm saying is that when a board is paddled almost the whole board is submerged. If a board for instance has 36 liters of volume it's upward force is almost 36kg (minus the weight of the board minus the board that is above the surface). These almost 36 kg is making the surfer 36 kilos lighter pushing the surfer 36 liters above the surface making it a lot easier to paddle.

When surfing (riding), the board is skimming on top of the surface and since a board is heavier than air and more than half the board is on top of the surface it's only acting as a downward force together with the surfers body weight.

I'm no physicist and I'm not sure if a got it all wrong but can someone please explain why volume is important on kite specific surfboards (or skimboards for that matter) because I'm not buying it.

[sundogchicky]

i dont chime in on these ones usually as lonny todd and yann etc. are sooo tech . and i usually sound as dumb as i am. but i slept in a holidayinn express last night so here goes.
i like volume and sometimes i dont.
i like volume in a board when the wind is gusty as it floats not so much planes going almost zero mph or ks an hour in a lull or mistake.
volume is total, yes short fat, long skinny thick, wide . most girls say yes it is volume that matters.
my really fat friend rides a 6 foot one that must be 3 and a half inches thick, it is a normal six one but thick. it surfs really nice as i have borrowed it on no wind days.
some guys round here have 6 8s and 6 1 or 6 3s and swap depending on wind. they like the fact that they arent sinking in between gusts or kite flying mistakes on their 6 8s.
when the kite is turned off and its just you and the waves more volume will add a surfier feel, allowing complete stall to one pump accelleration.

more volume will allow a slower riding speed and cruisy feel getting bouncy at higher speeds which many of the older less in shape wave riders prefer.
but always with a sleeker less voluume model tucked away for more control in ideal and stronger wind speeds.
my personal preference is the board i surfed on that morning unless it was double overhead and i was on my seven footer then id stick with my 6 1.
kite specific meaning that the kite is always pulling ,no gusts, always strong enough, perfect, no mistakes, right size, then no it woudnt matter, ride a mcdonalds lunch tray.

[patrickrynne]

interesting thread here

first i see what you are trying to say, but don't totally agree with it.

"There is no correlation between rails and volume. A board can have a doomed deck (a lot of volume) or a concave deck (less volume) and still have the same rails.
There is no correlation between plane and volume. A board can be x wide and y long ( x * y = plane ) and still have varying volume (z) depending on the thickness."

the major flaw here is that when you are thinking about "volume of the board" you need to be thinking about the submerged volume that is actually providing you with positive buoyancy. So you are correct in saying that you could have two boards, one with a domed deck, and one with a concave deck with all other things equal. This does not imply there is no correlation between rails and volume. When you go from domed deck to concave deck, you are reducing the volume of the board, but does that change the buoyancy response of the board? No, it simply changes the weight of the board. The board with less volume will weight slightly less, so the buoyancy required to float the board using Archimedes principle is less. The problem doesn't really change.

If you changed the volume of the SUBMERGED part of the board, then things would change much more dramatically. Thus why the art of shaping bottom count ours is so challenging.

my view on volume has to do with dynamic performance (similar to what sundigchicky is saying).

Yes, once the board gets planning, the dominant force keeping you from sinking is the upward lift generated by the board (mostly due to redirection of water downward, to the sides etc...) But when you screw up or slow down, this force can drop suddenly and potentially become of the same order of magnitude as the buoyancy force of the board. This is when higher volume helps you, it makes the board more user friendly. Transitioning through lights spots, keeping you planning when you botch your kite flying or get too overzealous on a cutback, etc. etc.

If i were to design a board that was only meant to plane fast and efficiently, I would probably trim off as much extra material as possible to keep the board as light as possible. If I wanted to design a board that could

1.) Go upwind
2.) Have good pop
3.) Wide waves
4.) Turn
5.) Carve

then the problem becomes a lot more challening. No single variable can be isolated as "THE ONE". There are tradeoffs for each parameter. Planning area, rail shape, concave, fin setup, tail arrangement, volume distribution....they all play a role

thats why shaping is an art....we try to quantify it in science, but the problem is too dynamic....too many variables.

-patrick

[Yann]

I understand what you're saying and I agree in most part except when you say you screw up or stall the board. You need to submerge the whole board for that last centimeter of volume on the thickness to start to kick in. At that time i think it's to late to recover and you're probably at the backside of the wave anyway. As for myself I just bend my knees and pull in the bar to take some kilos of me. IMO, true depowered kiting is an utopia anyway. I'm doing my best to get there but some line tension is unavoidable with todays gear. The waves up here are all wind driven and powerless so that might add to it.

The plane also plays a bigger role when going into stall. Imagine having two boards with the exact same volume but one is thick and narrow and the other one is wide and thin. I think the wide and thin is going to stay above the surface longer than the narrow and thick when you stand on it. When I think of it my 6'8" minigun is hell of a lot easier to duck dive than my fishes.I don't know the volume of those but they should be about the same. It's the plane of the nose on the fishes that are making them hard to duck dive. I sometimes tilt the board to one side to make them easier to submerge. Same volume but less effective plane working against the direction I'm pushing the board through the water.

I'm not saying that volume doesn't play a role. Every design feature plays a role and it's all very complex like you said. I'm just thinking that the role of the volume is very exaggerated. I see so many kiters wanting to get into strapless riding and they want to buy a board that is 6'6" and the rocker of a banana because they think that the extra volume a longer board gives is going to help them ride depowered. When they say they want a board that is at least 6'4 with a lot of volume they should be saying they want a flat rocker and a big plane (like a modern fish). The width, rocker, wide point, rails, tail, every other measurement I can think of plays a bigger role than the volume when kiting but people focus on the length because they think they want volume.

Have a great weekend! Forecast says 24-42 knots here for both saturday and sunday!!!

[sundogchicky]

i should known better than to comment on anything with the word relevance. i too stupit to understand this, im goin kiteskimmin

[patrickrynne]

That is an interesting to think about, duck diving a long thin semi-gun vs. a short stubby fish. Assume both boards have the same total volume.

It seems to make sense that the fish is harder since it will have more volume in the nose. What I am not sure about is whether a typical pin tail (6'8" ish?) actually does have the same volume as a fish (say a 5'8")

Lets see...a quick google and I get

6'8" Semi Gun (6'8" 18.25" wide 2.25" thick) - This is a channel islands semi gun

6.0' Fish (6' 21.75" wide 2.5" thick) - This is a larry mabile twinzer (i own a 5'7" and its amazing)

If we look at just the volume of the inscribing rectangle for each board (that is, if you multiply the length X width X thickness) we get a approximate value for the volume of the board. Before we do it, we know that the actual volume is less. For the semi gun, it is probably a lot less since the noses of these boards are so thin, while the actual volume of the fish is probably closer to this estimate.

Semi Gun - 3285 cubic inches
Fish - 3915 cubic inches

so despite being almost a foot shorter, the fish has 20% more volume than the semi gun. In reality, this is probably more like 30%.

I think what you are saying is right though, because my favorite kite board definitely has less volume than my favorite surfboard. Lets look at the numbers.

Favorite surfboard - 5'7" 21.0" 2.5" = 3517.5 cubic inches =130 lbs of buoyancy
Favorite kite surfboard - 5'6" 19" 1.875" = 2351.25 cubic inches = 86 lbs of buoyancy

so there you go. Volume is officially overrated. You don't need a high volume board, but you do need to have volume in the right places. Like yann says, width is important, length is important, thickness seems to be where you can trim the fat.

cheers

-patrick

[sundogchicky]

i hate those dumb people who buy over rockered banana boards
wait , isnt there some kiting condition or approach that might be perfectly served by this offensive fruit.
maybe that slab down the street only the spongers play with.

im not sure who out there is buying or selling overrockered bananas but they may be too dumb to even understand this post so your gunna hafta explain it like we are stoned third graders.

"so there you go. Volume is officially overrated. You don't need a high volume board, but you do need to have volume in the right places. Like yann says, width is important, length is important, thickness seems to be where you can trim the fat"

so do i just pull the glass off and sand away?

my girlfriend disagrees. she likes long "thick" overrockered bananas.

my fat friends 6 foot 1 is sooo sweet, a really nice balance and quite complimentry to his beer belly. he throws buckets strapless.

my other friends who ride their 6 8 merrick flyers also prefer that board for surfing and are exempt from this discussion due to their duel usage/ single storage vehicle space and the fact that, that model isnt a banana either.

thank you i actually did learn something from this, but im not quite sure what it is.......

loseyouroverockeredbananaboard.com

[patrickrynne]

hahaha, I am thinking the same thing. There are things to learn here, but I am not really sure what they are either.

with respect to board thickness, this seems to be where the biggest differences are between perfect surfboards, and perfect "kite assisted" surfboards. By biggest difference, I mean the biggest percentage difference. That doesn't mean that you should sand your board thinner....it just means that the statement "high volume surfboards are better to kite on than low volume boards" is not true.

post your dimensions of you favorite boards, and lets see if there is a pattern.

Always keep in mind that certain boards are better for certain conditions. So here in south florida, 99% of the time the waves are mushy, and the wind is medium to light (15-20 in the winter months). So the boards I surf and kitesurf on are meant for those conditions. With respect to rocker, that means a flatter board that has less drag in both cases.

A board with more rocker (more curve) will "skim" over the water less than a flat board, creating more drag. The windier it gets and the better the waves, the less important this drag loss becomes because you have more power to play with. The more rocker the board has, the easier it is to turn, pivot, and maneuver. So at pipeline (or when its blowing 30 knots), you would have more than enough power to overcome the drag associated with more rocker. And while you are making a critical bottom turn to avoid getting jack-hammered on the reef, you'll probably appreciate that extra maneuverability.

but here in florida, i am more concerned with actually catching the wave in the first place or staying on a plane in 12 knots than I am at making a really sharp turn at a critical moment.

so, depending on where you are and what you ride in, an over-rockered banana might be a stupid or brilliant stick to have.

In general though, I think the surf industry historically (especially late 90's and early 2000's) has flooded the market with boards that just don't make sense for the average conditions. In florida its ridiculous....walking to the beach and seeing an army of groms with the latest Channel Islands Pringle Chip. Imagine if they were on a fish! I wouldn't ever get a wave. So maybe its for the best.

things are better these days, the entire "retro movement" has opened peoples minds.

for us that can only afford a couple boards, figure out the dominant conditions in your area and go talk to a local shaper. To make it kite friendly and surfable, add a bit more volume in the nose, make it slightly thinner, and slightly shorter, but nothing dramatic and you'll have a 1 board quiver for most days whether your paddling or kiting.

at least thats what i think....

-patrick