Which is more stable in the water ?

Which is more stable in the water ? 15 years 3 months ago #28905

WilliH2O Topic Author Offline Elite Member Posts: 105 Thank you received: 0	Which is more stable in the water and explain why ? a.) a perfectly concentric round long straight log b.) an exact square sheet of plywood
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Re:Which is more stable in the water ? 15 years 3 months ago #28907

AdrianTregoning Offline Moderator Posts: 2212 Thank you received: 8	Depends on what your definition of 'stable' is...
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Re:Which is more stable in the water ? 15 years 3 months ago #28921

WilliH2O Topic Author Offline Elite Member Posts: 105 Thank you received: 0	How about the standard geometrical mathematical version - a cylinder vs a plane My guess is that most on this forum had some resemblance of high school geometry ?
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Re:Which is more stable in the water ? 15 years 3 months ago #28923

skirnir
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I hate to be difficult but I think some dimensions are important.

Assuming the plane and the cylinder have the same mass, but the plane is as thin as one likes then the plane will be more stable since the width of the plane would scale with the thickness to achieve the same mass. One might end up with an infinitely thin and infinitely wide plane.

Another scenario would be the case that the plane became infinitely thin and infinitely high. Making it very unstable.

Assuming the plane and the cylinder have the same width but different masses the inertia of the cylinder might become quite important.

Also depending on the geometry the surface tension of the water might start playing a big role.

So all in all I think this problem needs to be more specific. Or I may have over analyzed it a bit

I am leaning towards the plane being more stable

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Re:Which is more stable in the water ? 15 years 3 months ago #28924

WilliH2O
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It is indeed a thinking riddle applied directly to kayaking
- it's all a matter of the 3 axis also known as pitch, yaw, roll

The log will tend to roll easily and the plywood won't.
- but the plywood will rotate on it's vertical axis and dip quickly

Many define stability as a function of the height to base ratio.
A lower ratio yields more stability than a higher ratio

Floating logs rotate around their length,
almost exactly like a kayak would.

The reality of it all is that stability is NOT simple.
Attempting to deal in 3 axis of forces simultaneously makes it rough
Paddling with waves involves compound force systems.

Explaining stability to a novice without involving a lot of physics,
jargon, and countless diagrams is not the easiest thing to pull off :unsure:

Yet almost every newbie asks about "stability"

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Re:Which is more stable in the water ? 15 years 3 months ago #28925

skirnir Offline Elite Member Posts: 162 Thank you received: 0	Thats why I only teach kayaking to physics and engineering students Adrian I guess you value you degree now
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Re:Which is more stable in the water ? 15 years 3 months ago #28927

AdrianTregoning
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Take an object floating in water (not moving) it will (well, hopefully) be in a position of equilibrium. ie. ‘stable’ if you wish to call it that. How stable is it really and how can we calculate that? Well, one way to find out is to ‘disturb’ the object in question a slight amount mathematically and see whether it develops a restoring moment which will make it return to its original position. A moment can also be seen as a rotating/twisting force. Like what you probably know as torque. Moment is just a different engineering term for it which is more accurate. Sorry about that but it’s the language we use. Put another way: a moment is a force multiplied by a distance. If it doesn’t return, it is not stable. Simple. Right? Yes!

How to calculate: (maybe not that simple but read on below)
F_B=ρ×g×displaced volume = (buoyancy force)
The objects centre of mass G and centre of buoyancy B are also calculated. (read up on this if you wish)
Then the object is tilted a small angle ∆θ and a new waterline established for the body to float at this angle. The new position of B’ (new centre of buoyancy) is calculated. A vertical line drawn upward from B’ intersects the line of symmetry at a point M, called the metacentre, which is independent of ∆θ for small angles.
If point M is above G, a restoring moment is present and original position is stable. In other words, the metacentric height MG is positive. BUT, if M is below G (negative MG) the body is unstable and will overturn if disturbed. Stability increases with increasing MG.

Probably won’t make a lot of sense to you and I don’t blame you but this is the fundamentals of stability. If someone wants to understand it, then I recommend two good books which I have. Fluid Mechanics by Frank E. White and Mechanics of Fluids by Irving H. Shames. Two excellent books which most mechanical engineering students would have. So ask a friend.

Take your sheet of plywood and let’s make it a little thicker. So now it can be a rectangular piece instead of a flat sheet. Let’s not talk about planes or surface tension in water as that’s very different to comparing a round log. Ok? :-)

And also not very practical in terms of any vessel design.

Through the steps above, one can calculate the stability as a relation. Let’s say the rectangle is floating along nicely and the distance from the waterline to the bottom (the part under water) is H. And the width is divided into two equal parts which we’ll call L. So the width is effectively 2L and the height is H plus whatever is sticking up out of the water and is not important for this quick discussion. Through those steps (not that many actually), one can find that the rectangle can only be stable if the following condition is satisfied:
L>1.225H
SO, the wider the rectangle relative to its draft (draft is the part of a boat or our block in this case, that is under water), the more stable it is. It’s so obvious, but now you know a cool little formula.

Think about that log. How stable is it? Not very if you examine it through its round cross section. I’m now I haven’t even begun to examine the other cross sections at all. That’s another story….and I’m no expert.

I recommend another excellent book which I bought last year. Understanding Boat Design by Ted Brewer. You don’t need to be an engineer to understand it and it is actually quite interesting if boats and yachts interest you as they do me. To be able to calculate stuff like this for boats requires specific software and cannot be explained simply or even calculate on the back of a cigarette packet. Stuff like the length, beam, beam WL, displacement, centre of buoyancy, centre of floatation, chines, the amount, position and size thereof, wetted surface area and deadrise angle all have effects on your hull. Most people design kayaks from known recipes that work, same goes for boats. Of course there are variations to the theme, hundreds if not thousands of them. It’s quite mind boggling.

The simplest answer: Its cross sectional shape determines its stability. Hence a creek boat with a round hull is easier to roll than a playboat with a box shaped hull.

Hope that helps! I always knew what I studied would not go to waste. LOL :blush:

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Re:Which is more stable in the water ? 15 years 3 months ago #28932

WilliH2O Topic Author Offline Elite Member Posts: 105 Thank you received: 0	Sea kayakers have been known to suction cup a sail to their hull and perhaps the skeg is a sort of keel - - - - Now all we have to discuss is the thermodynamics of sweat vapor within various fabrics of dry suit manufacturers......... Attachments:
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Re:Which is more stable in the water ? 15 years 3 months ago #28943

jjdebird Offline Junior Member Posts: 17 Thank you received: 0	The log, anything below the water line becomes a "keel", keeping the log on some course, the key will be grip. The flat plain will have no direction or allow any control. If stranded with no other choices, paddle the log out and make a sign with the flat plain.
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Which is more stable in the water ? 15 years 3 months ago #28905

Re:Which is more stable in the water ? 15 years 3 months ago #28907

Re:Which is more stable in the water ? 15 years 3 months ago #28921

Re:Which is more stable in the water ? 15 years 3 months ago #28923

Re:Which is more stable in the water ? 15 years 3 months ago #28924

Re:Which is more stable in the water ? 15 years 3 months ago #28925

Re:Which is more stable in the water ? 15 years 3 months ago #28927

Re:Which is more stable in the water ? 15 years 3 months ago #28932

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Re:Which is more stable in the water ? 15 years 3 months ago #28943