The weight of water in helping reduce volume

[kwhilden]

Zoners...

I'm curious about your opinions about the weight of water in adding stability.  I'm ordering a New Deal from Inifinity, and I'm trying to decide on the optimum volume.

Here's the thing... I think there's a point of maximum instability when the volume keeps the deck is just at the water surface. 

I believe there's a benefit from having a large part of the deck just below the water surface, so that the weight of water adds some balance stability.

A step rail also helps in this regard, because there is extra rail below the surface, with a large moment arm relatively speaking.

Let me know what you think about this.  My plan is to order the board on the thin side.

[PonoBill]

For what kind of paddling? If you're going for speed, as in flatwater, then having water wrap over the rail will add a hell of a lot of drag. Other than that it's not a terrible idea.

[Bean]

If you give Dave honest feedback, he’ll nail the correct volume for you.  Looking forward to seeing how you make out!

I’m 215 and feel pretty stable on my 118 liter production BB machine.  Would not be the case if DB did not recommend and shape a 110 liter V2 for me a couple years back.  I was thinking in the 130 range at the time.

[Califoilia]

If you give Dave honest feedback, he’ll nail the correct volume for you.  Looking forward to seeing how you make out!

This was exactly my first that, and what I was going to mention had Bean not beat me to it.

I've only heard one person complain about the size (is "paddlibilty" a word?) of a board that Dave shaped for them...and that's was really a guy who more or less told Dave what he wanted size wise, and not necessarily what I think Dave would have shaped entirely for what he actually needed.

Btw, great choice on boards...the New Deal is a really cool deal, and all of the guys I know who had Dave shape one for them absolutely love them, and can't say enough about them. Sweet looking boards as well.

[Pierre]

Zoners...

I'm curious about your opinions about the weight of water in adding stability.  I'm ordering a New Deal from Inifinity, and I'm trying to decide on the optimum volume.

Here's the thing... I think there's a point of maximum instability when the volume keeps the deck is just at the water surface. 

I believe there's a benefit from having a large part of the deck just below the water surface, so that the weight of water adds some balance stability.

A step rail also helps in this regard, because there is extra rail below the surface, with a large moment arm relatively speaking.

Let me know what you think about this.  My plan is to order the board on the thin side.

No, stability comes from volume above water surface, especially on sides... stability depends on transverse moment of inertia of waterline/volume, and on the height of your center of gravity. On a small surf SUP board this is achieved by a flat deck and boxy rails , on a race SUP board by lowering deck under your feet and adding volume on sides ABOVE waterline and as far as possible from center.... On a recessed deck water retained inside creates a free surface moment which drastically reduces your stability...

[PonoBill]

Yes and no. Certainly, as a board tips, the volume above the waterline at or near the rail being pushed under creates the majority of righting force. But in addition to the righting forces supplied by displacement, there are also inertial effects, and I suspect this is what K is considering. Problem is that it's a relatively small factor compared to buoyancy.

[stoneaxe]

Yes and no. Certainly, as a board tips, the volume above the waterline at or near the rail being pushed under creates the majority of righting force. But in addition to the righting forces supplied by displacement, there are also inertial effects, and I suspect this is what K is considering. Problem is that it's a relatively small factor compared to buoyancy.

And quickly becomes less of a factor as the board tips and the water sluices off.

[Wetstuff]

Pierre, My experience is the opposite.  One of my first boards was an Allwave 9.2; I thought it must have been too small because it pitched-n-rolled like I was standing in a canoe  ...I traded for a 9.6 AW.  Chi't that was worse!  I was ~190lbs.  Those rails were so thick that any chop put against them made them react instantly.  I am not an acrobat.

Contrast that with the Sunnova Shroom or even the Skate, where the rails remain awash.  I don't buy the logic about water above the waterline acting as a counter-weight.  (What do you figure; about 8oz of water along a rail to counter the lever of a 6' post with reactive, hydraulic braces?   (maybe if you added a 6x24" center fin.))  I experience 'chop washing over the rail' as less reactive.

My 'sense' is that thinner rails, and a lowered CG with adequate width is the better option for my local surf. 

Jim

[kwhilden]

Thanks all for your replies.

First... this is for surfing, and stability in the lineup. 

Second, here's my experience that leads to this question.  With two boards I own.

First board: H2-SUP 8'6 x 29 x 4.5"  125 liters   with extreme step rails
Second board:  Starboard Pro XL 9'0 x 29 x 4.5" 139 liters

For me, the H2-SUP is more stable.  I'm 225lbs, and on this board, the deck is mostly under water except for the nose. There's now volume above the waterline, however there's a decent amount of water above the rails.

On the Starboard, the deck is right at the water surface.  So displacement restoring force is small, because there's very little volume above the waterline. Unfortunately the inertial restoring force is also small, because there's very little weight of water on top of the deck.

On the H2-SUP, there is more inertial restoring force. It just feels more stable to me, while the Starboard is very twitchy.  In the surf zone, the Starboard is affected by every little piece of surface bump, whereas the H2-SUP is more resistant. 

On flat water, the Starboard is much more stable and faster, but this is for surfing...

So on to my new board... I'm trying to decide between going low in volume (125 liters) or going higher in liters and getting a board that can cruise and is super stable for when I'm feeling a little lazy.  Trying to decide from a range between 125 and 145 liters. I would go big, but I'm worried about the board getting super thick and harder to bury a rail.  Other dims are 9'6 x 29.5".

Let you know if you have any thoughts on that dilemma.  Thanks all!

[Califoilia]

At the risk of sounding like a broken record, I'm going to repost this...again.

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.

Anyway, it help me with future board decisions after I read it....YMMV however.

[Bean]

At 225, a natural longboard lenght for you would be closer to 10'.  I would go 10' and narrower, say 27 to 28", in the low 120's.  I would focus more on how the board will perform on the wave as opposed to standing in the lineup.  Keep in mind this is a performance longboard, not a log.  Based upon your current boards, you will probably adapt pretty quick.

Again, I would love to hear Dave B's take on this.

[kwhilden]

To help discussion... 

Here's a screenshot of my 8'6 H2-SUP, in Shape 3D.  I've added in the buoyancy line, at the weight of myself+board.   (I love Shape 3D)

[PonoBill]

There are more factors than just rail thickness that influence stability. Rocker is a pretty big one--weight in the middle of a curved line has to be lifted to tip. Certainly tail shape--pintails shorten the moment arm of the bouyant rails, and so do pointed noses. I have a 12' X 25" board with almost no rocker, straight 50/50 rails and a squash tail and nose. Looking at the rail profile you'd expect decent stability, but the minimal rocker makes it about as stable as a floating log.

Continuous rocker boards have useful stability even when the rails are under water.

[PonoBill]

I like the looks of that board, but I'd want it in 10' X 29".

[kwhilden]

I was wondering if anyone would pick up on this shape. I worked with Ry Harris to add the Greenough edge rails to one of his shapes. He gave me the basic AKU file, and then I went nuts on it in Shape 3D.  The modeling power of Shape3D is great.

This design works amazingly well.  The Greenough edge rail solves a lot of issues that SUPs have. Adds speed and rail-to-rail ability. It's my favorite board so far.

Attached is another image of the design, showing the bottom contour.  A couple of renders on the Greenough edges.