Board Volume- upper limit.

Good morning all,
Probably not a simple question to answer due to many variables including bottom shape, rider weight/board volume (a board with +10 lt volume for a 75kg rider will have a different effect compared to a board with +10 lt volume for a 100kg rider)

Is there an upper limit to board volume vs body weight on the longer skinner shaped boards?

If or when does a board lose some side to side stability because it is sitting higher in the water due to excessive buoyancy?

Just thinking about getting a new light wind wing board and I don't want to make the mistake of going to big in volume.

I think this is a really interesting question. From a purely hydrostatics perspective, the upper limit is well over +100% of body weight. Hydrostatic stability is proportional to the 4th moment of area of the waterplane (the area defined be the intersection of the water surface and the board), around the longitudinal axis of the board. In other words, the total size and especially the width of the waterplane matters tremendously for stability, so the moment a good size chunk of the tail or especially the rail goes underwater, stability drops dramatically. Having enough board (volume) above the static waterline is essential to be able to handle some trim angle and especially some roll angle before stability starts to diminish significantly.

Now, boards are not ships (the metacentric height is pretty much always negative on a board, meaning the board-rider system is dynamically unstable) and so the overall dynamic situation is quite different. High stability from a large reserve volume gives the waves more leverage to perturb the board (in combination with low damping this is often described as corky), the increase in center of gravity is much more significant dynamically than statically, and the system dynamics are faster with a more stable (stiffer, in ship terms) platform, demanding the rider respond faster to maintain equilibrium. Damping is a factor that is very relevant to dynamic stability, and here the hard chine shapes like the Kalama E3 have a clear advantage over the very rounded shape like the KT Dragonfly. Whether the stability gains outweigh the low speed drag penalty of hard chines is very rider skill dependent, I think.

The conflicting between static and dynamic stability very strongly suggest there is an optimum overall stability point, for a given rider weight and height. I think the height dependency is often overlooked and is what makes it so hard to clearly identify this optimum point from trial and error.

Conpletely agree wing and SUP are different. Wing I also prefer -10L on my long narrow board. I find it easier to waterbug start in big conditions when the board is very low in the water, the mostly sunk board has way more damping and less leverage for the sea state to perturb things, and as hilly says you have the wing to help stabilize everything so the fact that hydrostatic stability is very nearly zero is not much of an issue. Could probably continue to improve the ease of the waterbug down to -20L or more without making the launch much harder, but my personal constraint is that I have to be able to sail at least on a beam reach in low rider mode so -10 is about as low as I can go with current gear design.

In general, for the long narrow SUP boards, I suspect a lot of folks (at least heavier folks) tend to ride a volume that is below the optimum stability point discussed above; the SUP getup process depends much more heavily on hydrostatic stability than winging does. I don't sup foil yet and I've only got to try a quick paddle in pretty calm conditions on a couple of of newer SUP designs, so have to defer to more experienced folks here. For me personally, based on limited experience so far, absolute minimum for my first board will be +25% body weight (so roughly 125L), and if the overall dimensions can be kept somewhat manageable, I'd really like to experiment with something that is more like +40%, but still very narrow.

He means 1.5x your body weight as an upper limit. From the casey catchup video below they discuss the range. In effect he thinks you should stick to a (volume/weight) ratio of between 1.3-1.5 (ie multiply your weight by 1.3 to 1.5, or weight+30% to weight+50%)

See below at 55:22

?

This pretty closely matches to my matrix for weight on the pp forum - forum.progressionproject.com/t/help-with-downwind-board-dimensions/3225/26?u=matt

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CJ2478 said..

PKenny said..
Good morning all,
Probably not a simple question to answer due to many variables including bottom shape, rider weight/board volume (a board with +10 lt volume for a 75kg rider will have a different effect compared to a board with +10 lt volume for a 100kg rider)

Is there an upper limit to board volume vs body weight on the longer skinner shaped boards?

If or when does a board lose some side to side stability because it is sitting higher in the water due to excessive buoyancy?

Just thinking about getting a new light wind wing board and I don't want to make the mistake of going to big in volume.

I think you want to look at percentages of your bodyweight, because +10L for a 80kg person is not the same as +10L for a 110kg person.

I am 75kg and have a 7 11 x 19 119L DW board (~150% my bodyweight in Litres). Dont know many people my weight who are running bigger boards. I would say the only downside is that as the board becomes bigger pumping becomes harder, but it isn't prohibitively hard for me to pump this board around, so if there is a limit this isnt it (for me atleast).

Good point , percentages make much more sense, I'm 80 kg and using a 7'3 x23 128lt board and it doesn't feel too corky (160% my body weight in litres ). I'd be interested to see how much more I could go before it did feel too corky and unstable.

If a large volume boards feels corky and unstable due to excess volume, the design is the problem - not the volume. The only time less volume can feel less unstable is when part of deck is submerged and locking in water over the deck of the board. This reduces roll as water is heavier than air. This effect is marginal though, as a partially submarined board will be less stable than a higher volume board of same design. This is because the submarined board will have less vertical rail, length, width etc than the higher volumed board with same design - all the things that give stability.

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PKenny said..
Good morning all,
Probably not a simple question to answer due to many variables including bottom shape, rider weight/board volume (a board with +10 lt volume for a 75kg rider will have a different effect compared to a board with +10 lt volume for a 100kg rider)

Is there an upper limit to board volume vs body weight on the longer skinner shaped boards?

If or when does a board lose some side to side stability because it is sitting higher in the water due to excessive buoyancy?

Just thinking about getting a new light wind wing board and I don't want to make the mistake of going to big in volume.

G'day PKenny
I'm 95kg, Heavyweight. I made me a DW Foilboard when I got back from Hawaii last August - Anybody can make a Foilboard all you need is a Computer to design your board AND a Shaping Machine. Remember I'm a Beginner KOOK when it comes to learning how to DW foil. (Yes, I can read the ocean as I've been Downwind Paddling since May 2003 learning on a 18ft Prone Paddleboard then defecting to 18ft SUP, DW )

So looking forward to Maui this year for a 10 week Maliko Run to workout DW foiling."Can't Wait Already"

My DW foilboard is 9ft x 21''inches @118 litres, it paddles very fast in a straight line ( I used the same nose as my 18ft SUP, the V3 nose ) my Foilboard is very stable and sit about the water, for my weight at 95kg.

four things that are going to help you DW foil:-
1. a Good DW foilboard under your feet
2. Ability
3. Good Core Strength
4. Very Good Cardio

just made me another DW foilboard, I want to go narrower with less Litres like, 9ft x 19'' inch @ 112litres it's getting glassed this week.

I suspect downwind board stability has as much or more to do with thickness than width. I have a 125L 8'2" x 20" x 6.8" thick and just recently got a 129L 8'0" x 18" x 6.9" thick board and the stability is pretty much identical. @beasho and @downwinder are on it, I think the next round of designs you will see coming out will be thinner and lower volume per length. My next design is at the factory now and will be 9'2 x 18.5 and 5.5" thick 120L (I'm 98kg in my wetsuit so my volume fraction is 1.22)

I love the Deck Grip that Armie has come up with "GREAT IDEA"

I always enjoy hanging out with Armie Armstrong when ever I'm on Maui

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Yer I found the same. The centre bump/ridge on boards gives me a sore foot. Flat so best or sunken concave is fine but the bump fired up my foot arch after time

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tightlines said..

Scotty Mac said..
Yer I found the same. The centre bump/ridge on boards gives me a sore foot. Flat so best or sunken concave is fine but the bump fired up my foot arch after time

Absolutely agree, I hate the centre bumps in deck grips.
Not sure about the volume, my last 2 boards have been 7 x 23 and 8' x 20 and both 120ltrs which feels about right for me at 90kg.
I think my next one will be around 8'6 x 18 but still pretty much 120ltr.

Very similar experience but always around 110L for 80 kg. Now I have 7,7x20 and 8,8x18. I will not want more volume because of the extra weight. The extra volume is for the Pop-up which is just a few seconds during the run, for the rest the lighter the better.