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Old 05-03-2013, 02:07 PM   #1
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Rolling Theory

Do you agree that an unballasted cylinder, w/o a keel, has no reason to roll (rotate) laying to a seaway?

Does it follow that a ballasted, round bottom vessel would be no more inclined (no pun) to roll than a flat bottom vessel.
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Old 05-03-2013, 02:33 PM   #2
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Sure.
As long as the ballast stayed put.

Seriously the CG would remain the same w a sea under one side wo the ballast but w the ballast a sea under the port side would put the center of gravity to stbd of the center of lift which would be on the port side.

So interestingly the unballasted hull would be more stable than the ballasted BUT ........ if the seas didn't get rough the unballasted hull could and would be "up side down" at times so looking at it like that the ballasted hull would be more stable.

Initially the round bottomed vessel would be inclined to roll more w or w/o ballast. But time and inertia eventually may cause the round bottomed vessel to roll more. A heavy round bottomed fishing boat will roll (but slowly) more than the rectangular vessel but the rectangular vessel will be much easier to capsize. I think I've got that right. Interesting and fun to think about.
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Old 05-03-2013, 03:42 PM   #3
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Quote:
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Sure.
Interesting and fun to think about.
Guess this discussion is just for Willards...
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Old 05-03-2013, 04:17 PM   #4
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And submarines. Mine rolled like a pig, as evident by watchstanders wearing buckets around their neck during periscope depth operations "up north".
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Old 05-03-2013, 04:33 PM   #5
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A wonderfully complex subject.......

Anything will roll given a wave (or rather a series of waves) of the right height, speed, and distance between crests. It's the damping (slowing and stopping rolling) that's important. A round bottom hull generally has poor roll damping, and thus will just keep on rolling. A flat bottomed hull has excellent damping, so good it can throw things in a seaway.........
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Old 05-03-2013, 06:48 PM   #6
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Yup ... some fish boats will roll 10 degrees just stepping on the gunn'l. But it takes two seconds or more.

TAD,
Here's "Flat Top Is" just no of POW Is in Alaska.
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Old 05-03-2013, 09:58 PM   #7
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Initially the round bottomed vessel would be inclined to roll more w or w/o ballast. .
Not talking vessel here. A piece of piling, nothing sticking out , half awash, CG right in the center, lays broadside to a sea. Why would it want to roll?
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Old 05-03-2013, 10:35 PM   #8
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Brooksie,
Lot of variables like water washing over the top of the piling would cause it to rotate not to mention wind. The waves will pass under it and over it. Current could get into the act and for all I know the Cornelius Effect may play a part.

I'm confused. I thought we were talking about a ballasted round bottomed vessel.

What connection to the conversation does Willard Boats play? Just that they are considered to be round? There's actually a small part of the bottom on each side that is flat on a W30. Lots of stability and stiffness comes from that. Some fishing boats I've seen are almost a half a cylinder. FAR more round bottomed than a W30.
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Old 05-03-2013, 10:38 PM   #9
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Just a guess...since gravity would be pulling it straight down, but the wave face is on an angle, the log/piling would slide down the face of the wave and surface friction of the logs leading edge would cause it to roll.
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Old 05-03-2013, 10:43 PM   #10
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Waves don't just act on the surface. We used to get rolled in beam seas at 400' depth in the Gulf of Alaska.
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Old 05-03-2013, 11:45 PM   #11
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Brooksie,

I'm confused. I thought we were talking about a ballasted round bottomed vessel.

.
Well, we are talking vessels in the end.
But first I wanted to ask if there is any reason for a cylinder w/ CG in center to roll. Then a cylinder, same same, but with CG on one side , would it roll? Then same with a keel (or conning tower), would it roll?
Just a thought experiment, that's all.
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Old 05-04-2013, 06:49 AM   #12
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Staying in the purely theoretical, I'd think one factor would be that the individual water molecules don't just move up and down when a wave passes, they move in a circular motion. I wonder if that would impart some rotational motion to the hypothetical cylinder.
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Old 05-04-2013, 09:38 AM   #13
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CaptTom,
Yes I think it would but it would depend greatly on how large/small the waves were.

Brooksie,
Of course if the CG was off center it would roll .... once and dampen out.
Do you want to say something about your W36 and it's roll characteristics?
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Old 05-04-2013, 10:15 AM   #14
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Im a sucker for mind games. - I'll have an uneducated guess.

The cylinder would continually rotate, just due to the friction of water movement across the bottom. The roll direction would usually be "downhill" but this would also be affected by current.

The ballasted cylinder would only roll very slightly as the force of the ballast should vastly overpower the slight bit of friction from water movement. The most stable of the three (if enough ballast).

The keeled cylinder would roll more than the round ballasted cylinder due to the water movement force having the extra leverage on the appendage, although it would be the slowest roll.
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Old 05-04-2013, 10:35 AM   #15
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Im a sucker for mind games. - I'll have an uneducated guess.

The cylinder would continually rotate, just due to the friction of water movement across the bottom. The roll direction would usually be "downhill" but this would also be affected by current.

The ballasted cylinder would only roll very slightly as the force of the ballast should vastly overpower the slight bit of friction from water movement. The most stable of the three (if enough ballast).

The keeled cylinder would roll more than the round ballasted cylinder due to the water movement force having the extra leverage on the appendage, although it would be the slowest roll.
That's my guess too. In the drawing I was trying to show that a keel-less ballasted cylinder could be more stable than a keel-less ballasted cube.
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Old 05-04-2013, 11:23 AM   #16
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Brooksie wrote,

"That's my guess too. In the drawing I was trying to show that a keel-less ballasted cylinder could be more stable than a keel-less ballasted cube."

Certainly not initially.
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Old 05-04-2013, 11:52 AM   #17
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Yes. Initial stability (tenderness) is quite apart from actual stability.
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Old 05-04-2013, 02:32 PM   #18
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Quote:
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Why would it want to roll?
I have seen low floating hemlock logs drifting in a sea, they just roll and roll, usually fairly fast and not stopping.

The amount they roll and the ease thereof is dependent on how low they float if VCG is in the middle. If they float high (dry cedar) then CB (center of Buoyancy)will be well below G and as the log heels there will be a positive righting force. If the log floats low and CB is close to or above G, there will be little, no, or negative righting force, allowing the log to roll continuously........
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