Author Topic: Width vs Length stability ratio?  (Read 8090 times)

Wetstuff

  • Cortez Bank Status
  • *****
  • Posts: 2318
    • View Profile
    • Wetstuff
Re: Width vs Length stability ratio?
« Reply #15 on: May 30, 2016, 07:00:20 AM »
I'll counter one thing T-D said; "A board that has more rocker (nose to tail) will feel more stable than a flatter board."  I have not had a long board (9+) to compare but I had a short, narrow Tomo shape that was an absolute nightmare for me. At 27" wide with a hobbyhorse rocker ..foot placement and 'body mass centering' were too critical to allow me to paddle it even in flatwater.  A better man, Surfercook, makes it sing.

There's a thread about 'neutral buoyancy' in the Seabreeze forum.  A fellow, Colas, has some interesting input. More so because he's not 18yrs old and 140lbs.   I've been fortunate to be able to buy/sell myself toward the 'right board for me'.  'getting there.... 

Jim
Atlantis Mistress .. Blue Planet MultiTasker ..   Atlantis Venom

PonoBill

  • Cortez Bank Status
  • *****
  • Posts: 25864
    • View Profile
Re: Width vs Length stability ratio?
« Reply #16 on: May 30, 2016, 08:41:58 AM »
Everything else goes out the window when a board is short enough. Rocker offers stability only after you have rolled enough for the rocker to start lifting your weight. On a short board that comes too late to save you. A long board with the same rocker curve as a short one lifts the monkey in the hammock proportionally higher.
« Last Edit: May 30, 2016, 09:24:20 AM by PonoBill »
Foote 10'4X34", SIC 17.5 V1 hollow and an EPS one in Hood River. Foote 9'0" x 31", L41 8'8", 18' Speedboard, etc. etc.

Califoilia

  • Axis Demo Rep
  • Teahupoo Status
  • ******
  • Posts: 1510
  • San Clemente
    • View Profile
    • Email
Re: Width vs Length stability ratio?
« Reply #17 on: May 30, 2016, 11:00:35 AM »
Well, every time this topic comes up, I can't help but repost what I think is the best response on in the topic, from a guy who knows a little about shaping a couple SUPs, or two himself....not to mention ride some pretty small/narrow ones himself.

More volume = more stable... (in the traditional sense of course). Add volume and width and its even more stable. Instability comes from a rail diving down, the board sinking and then you loosing balance and falling off. Volume, and volume away from the center line, increases stability.

It takes 8lbs of pressure to sink one gallon of air. Lets assume for this purpose that your board is pure air (no material to make the foam as its close enough for this). If you weigh 200lbs, you need a MINIMUM of 100ltr in your board to have neutral floatation. This is just to float you with the very top of the board right at water level.

Now, stability is not the same as float, but they are related. Stability comes from the ability of the board to "push back" at you when you push down on it. at 100ltr, 50lts is on each side of the stringer. So it takes 100lbs of pressure (weight if you like) to push the right rail into the water. If you're standing centered on the board you're good. As you loose balance (lets say to the right) so you transfer more than 50% of your weight to that rail. Now you have more weight on the rail than the volume is able to push back against, and so it sinks. The more it sinks, the more you loose your balance, and the more weight you transfer onto that already sunk rail, and it sinks more... and more... until you either fall over, OR the "center" of the boards volume passes the 'tilt" point and it flips over (and then you fall).

However, its not that simple. Its not just about how much volume is on each side of the stringer. its also about how FAR it is from the stringer (moment arm), and how much of it is how far from the stringer.

If 70% of my right sides volume is within the first 6" of the board (measured away from the stringer) and only 30% is 15" from the stringer (on a 30" wide board), and I'm standing 10" away from the stringer, then my foot is already outside of the area that has the most foam/floatation. Therefore less pressure (weight) is required to sink the rail.

So... a board that has a lot of rocker, or a lot of outline curve, is going to have less volume out away from the stringer to push back at me than a board with wide ends, low rocker (low rocker engages that volume sooner as its already on the waters surface - rockered boards have a certain amount of the volume lifted out of the water and it only engages after the board starts to lean over). Boards with pinched rails and deck crown are going to have less volume to push back at you.

So... what does this mean?

A 25" wide 9' board that has wide ends and a flat deck with full rails and is 100ltr will be more stable than a 30" wide 9' board with dramatically pulled in ends and pinched rails that has 120ltrs.

But this is overly simplistic... standing height affects stability. For every 1/2" you move up, you need to add on average about 1" per side of width )this is dependent on rocker, width, outline and so on, but its about this) to have the same overall stability, so you get quickly reducing returns on increased volume from increased thickness. Also, as your board gets shorter, so you introduce a new dimension of instability  -end to end sinking which compounds the side to side instability. As you get closer to the "critical" volume for your weight (you and board combined) so reduction in length has an exponential affect on overall stability, and thus moving more volume towards the ends of the board becomes paramount to maintaining that stability.

One solution is to stand closer to the stringer. While this puts you in a position of feeling like you're always "slightly wobbly", its also harder for you to put the kind of pressure on the rails that causes them to sink, as your weight is centered.  Closer feet, kung fu style, will allow you to have much less instability as you're not pushing out on the rails. Pulled in ends, more rocker, pinched rails are all benefits of a tight standing stance as YOU loose YOUR moment-arm of pressure to push on the rails, and thus the board needs less of that moment-arm to push back.

Wide boards, wide ends, flat decks, full rails, low rocker ALL increase stability if you stand with a wide flat stance. The downside is a massive reduction in board performance. As your stance narrows, so you can quickly reduce volume, and more significantly, reduce how far that volume is located away from the central part of the board, and thus shape the board, with a view of more surfing in mind rather than paddling (stability) in mind.

Make sense?

Corran

Narrow stance lets you have a smaller, narrower more progressive shape overall.
The bold is key, and the very last sentence pretty much says it all if you want to go shorter/narrower/smaller.  Sort of a, "It's the indian not the arrow" kind of thing.....IMHO naturally.  Works for me.   8) :)
Me: 6'1"/185...(2) 5'1" Kings Foil/Wing Boards...7'10 Kings DW Board...9'6" Bob Pearson "Laird Noserider"...14' Lahui Kai "Manta"...8'0" WaveStorm if/when the proning urges still hit.

PonoBill

  • Cortez Bank Status
  • *****
  • Posts: 25864
    • View Profile
Re: Width vs Length stability ratio?
« Reply #18 on: May 30, 2016, 04:10:22 PM »
When I start having trouble with balance on a downwinder, the first thing I do is look at where my feet are. Nine times out of ten I've unconsiously moved them out.  I over-compensate, trying to keep my heels on the stringer.
Foote 10'4X34", SIC 17.5 V1 hollow and an EPS one in Hood River. Foote 9'0" x 31", L41 8'8", 18' Speedboard, etc. etc.

supsurf-tw

  • Teahupoo Status
  • ******
  • Posts: 1062
    • View Profile
    • Email
Re: Width vs Length stability ratio?
« Reply #19 on: May 30, 2016, 05:40:13 PM »
If people would concentrate on narrowing up their stance they'd have much more stability across the board (literally) and be able to go down in width and volume
Boards:

 
8-10 x 31 Egg
8-11 X 32 Double wing Fangtail Tom Whitaker
8-6 X 30 1\2  Inbetweener Tom Whitaker
8-4 x 30 Hyper quad Tom Whitaker (wife's now)
8-4 X 31 1\4.  Round (wide) Diamond Tail Quad Tom Whitaker
 9-4 X 30 1\2. Swallow Stinger Quad Tom Whitaker (ex wifes now)
10-0 Brusurf for teach

TallDude

  • Cortez Bank Status
  • *****
  • Posts: 5714
  • Capistrano Beach
    • View Profile
    • Email
Re: Width vs Length stability ratio?
« Reply #20 on: May 30, 2016, 09:17:16 PM »
A couple of years ago I was surfing a Huntington just before one of the SUP pro tour events. Two of the pros from Japan were on these 7' x 24" x potato chip boards. They were sunk almost to there knees, standing with one foot directly in front of the other- inline. It looked like they were balancing on a tight rope. Crazy calm balance.
It's not overhead to me!
8'8" L-41 ST and a whole pile of boards I rarely use.

supthecreek

  • Guest
Re: Width vs Length stability ratio?
« Reply #21 on: May 31, 2016, 05:10:12 AM »
Good conversation!

Sano... thanks for the revisit to Corran's explanation..... well put, for easy understanding.
I copy/pasted it to my notes

I was out teetering on my board yesterday and my back was tired from a few days of taxing it.
My board has a very thin, pointy nose, foiled rails and just about water level at the deck

I moved my feet right to the stringer, in a modified surf stance (one foot back 4" and slightly turned out)
The twitching instantly eased up. My back relaxed.

It feels slightly more exposed.... but it really quiets the movement.

It's important to remember. Thanks all

 


* Recent Posts

SimplePortal 2.3.7 © 2008-2024, SimplePortal