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Volume distribution
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windward1



Joined: 18 Jun 2000
Posts: 1155

PostPosted: Mon Sep 16, 2019 10:29 pm    Post subject: Reply with quote

Where the bouyancy is and its distance from the centerline is what affects the righting moment. The righting moment is the force generated to right the tilted floating item by its distance away from the balance point. For symmetrical boards we can generalize this around the centerline.

So, if you look at a cross-section of the board and take it at an angle away from horizontal and know how much is underwater at your weight, there will be a portion that is in the water and its buoyancy will be creating a righting moment. The larger this area and then the center of it being further from the centerline, the more the righting moment. So wide rails help tremendously towards this end.

Also this happens all along the length of the board. So taking sections fore and aft of amidships, tells more of the story. Yes, straight rails where fore and aft the buoyancy is generated further from the centerline the more the righting moment there.

Of course there are an infinite number of sections and angles of tilt, so one works by usually dividing the board into ten sections and chooses a few representative angles. Then integrating.

And actually there is an easier method with weights, which one can do with the actual boards in hand and in the water, but I will have to refresh my memory of that procedure.

Once the board is fully under water one has a different situation, altogether.
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grantmac017



Joined: 04 Aug 2016
Posts: 830

PostPosted: Tue Sep 17, 2019 10:31 am    Post subject: Reply with quote

cgoudie1 wrote:
Go with 120 ltrs. ;*)

-Craig


Unfortunately the biggest they make is 108L.

Windward1: excellent information. Do the areas not under water (ie:the nose) contribute anything until they are pressed into the water?

Thanks,
Grant
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windward1



Joined: 18 Jun 2000
Posts: 1155

PostPosted: Tue Sep 17, 2019 12:13 pm    Post subject: Reply with quote

Hi grantmac017,

No. The Buoyancy only comes from the portion below the waterline. Anything above it contributes nothing to buoyancy.

A correction I should make regarding my first post. The Righting Moment is actually the force generated by the Buoyancy X the distance from the Center of Gravity. Of course on a board alone, this would be somewhere along the centerline. In the action of the windsurfer uphauling, one has to take the rider and rig into account in determining the Center of Gravity. The situation is unstable anytime the Center of Gravity is outboard of the Center of Buoyancy. That happens a lot with a big rider on a small board. Wink

W1
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isobars



Joined: 12 Dec 1999
Posts: 19262

PostPosted: Tue Sep 17, 2019 6:29 pm    Post subject: Reply with quote

Splitting volume distribution hairs makes marginal difference when there are so many additional factors, such as:
Are there any gusts to work with?
Has the wind quit, or is this simply a temporary lull?
Is chop a factor?
Are there clean swells or waves to aid in waterstarting?
Which performance feature is more important ... uphauling or sailing?
And perhaps the most important factor: Why would you uphaul, period? There are some valid reasons, but I don't consider "So I can stand there and do the isometric hula until the wind resumes" a valid one. Other than 2 or 3 times on a foil board, I haven't been forced to uphaul in several decades.

If it's all about safety when the wind quits at 9:15 PM under the Golden Gate Bridge, screw hair-splitting and Go Big. If it's about simply getting back to rig a bigger sail or board, performance under sail looms large, as there are other ways to minimize the threat of dying winds.
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grantmac017



Joined: 04 Aug 2016
Posts: 830

PostPosted: Tue Sep 17, 2019 9:51 pm    Post subject: Reply with quote

We sail in very different places. Comparatively the Gorge isn't as difficult or dangerous so long as there isn't a barge coming (but we have whale watching boats doing +30kts and far more marine traffic). Also our currents are both striong and adverse half the time, which is totally unlike the Gorge.

I uphaul frequently when foiling and occasionally when using a fin. At those times I don't have the luxury of being helped upwind while waiting on a gust; it's either uphaul or swim, hard.
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isobars



Joined: 12 Dec 1999
Posts: 19262

PostPosted: Tue Sep 17, 2019 10:14 pm    Post subject: Reply with quote

For at least the umpteenth time:
There is no perceptible current where I sail in the Gorge.
When/where there are currents, they can usually be used to our advantage, as you suggest. (Unfortunately, I've found no advantage to the relentless downwind Oregon coast current.)
I have sailed over much of the US and in some other countries.
I sometimes sail east (with the current) winds.
As best as I can tell, the thread isn't about, and surely not confined to, foiling.
My factors apply just about anywhere.

Maybe above all, it sounds like VOLUME is your best friend, with its distribution a distant third or fourth. I keep casually looking for a wave board >125 liters, but they're hard to find. Maybe a Kona is a possibility; Ingerbritsen sure makes them look good.
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manuel



Joined: 08 Oct 2007
Posts: 957

PostPosted: Wed Sep 18, 2019 1:27 am    Post subject: Reply with quote

Although no longer sailing sinkers, I found it easier to uphaul a sunk board than a floating but super tipsy board. Boards tend to become more stable once sunk starting ankle deep.

Volume distribution does play a role with even sinkage or float. My formula board was a bit more technical to slog even though it was old school long. Its volume was mostly in the back and not much on the front which was stretched out thin.

My freewave 77 is easier to tack and slog than my 85 wave board. The former is both longer, wider with a more evenly distributed volume. My 87L sideon board is incredibly sloggable and feels much bigger. Maneuverability isn't affected but it does feel a bit bigger in the air.

Larger folks (or freestylers) may want more volume out back while thin sideoff wave riders may need volume to be pushed forward more around the mast foot.

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grantmac017



Joined: 04 Aug 2016
Posts: 830

PostPosted: Wed Sep 18, 2019 12:56 pm    Post subject: Reply with quote

Any board I can uphaul I will foil with.

I've sailed both ends of the Gorge, it's nothing like here in terms of technical challenge having far more consistent and favorable conditions which is why it's a destination while Victoria isn't. So I won't debate Gorge-only technique and equipment.

That all out of the way:
I'm discussing how two boards of very similar volume can have different stability in opposition to what you'd expect given their width.

Manue, would you save the 85L wave board has its volume more centered under the mast track?
The more stable of my two boards definitely has more in the back and less in the nose. But as you say it feels technical to slog and doing so in the harness can sink the nose easily in light wind.
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dllee



Joined: 03 Jul 2009
Posts: 4593
Location: East Bay

PostPosted: Wed Sep 18, 2019 2:03 pm    Post subject: Reply with quote

Lots factors.
Dome or flat deck.
Volume of rails.
Width.
Length, for nose or tail sinking.
Weight, inertia.
Bottom shape, concave stable, belly bottom least.
Fin or mast length.
Size of sail.
Your onboarding weight vs balance.
Water state..flat vs choppy.
What did I miss?
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manuel



Joined: 08 Oct 2007
Posts: 957

PostPosted: Wed Sep 18, 2019 3:00 pm    Post subject: Reply with quote

My 85L wave board has a domed deck so there's still some volume out back but more concentrated in the middle. With thinner rails it can dig into the water and turn well controlling bouncing yet the tail resists sinking when slogging.
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