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PostPosted: Sun Jan 06, 2013 3:30 am 
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Hey guys,

A wanted some input for a general consensus about the theories of Primary and the so called secondary stability.

A yak is a yak, it is not a boat. If I was to go out in a cyclonic gust i'd choose a large boat or a sub marine lol.

Having had a trial of the rev, ob and buying the pa12 my logic tells me that the advantages of primary stability (ie being able to navigate around the yak without being capsized) much more of a plus than some far fetched theory about a log going up and down in rough seas compared to a plank of wood which a minority here preach.

On further research I have read articles by some very learned scientists that actually question the existence of Secondary stability,, what are people's thoughts?

Ben


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PostPosted: Sun Jan 06, 2013 6:07 am 
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Well, without any scientific explanation, I am rather clear over the fact that there is both primary and secondary stability.

For me the primary stability, if somewhat low, is something I have to get used to. That is (for me) how easy it is to affect the angle sideways by changing position or by moving my package. If there is an angle where its getting harder to affect the angle, as some kind of threshold, then I have found the angle where the secondary stability starts.

If you pass that angle, the hull would capzie.

I don´t think its so important to know these things with logic thinking. Its more important that my body is learning these facts to the back of my bones.

The same goes for the theory of handling a bicycle.
Logic thinking does not help much.

I have an Adventure kayak and that is probably the Hobie kayak with lowest primary stability. It will very easy respond to my movements of position. But the secondary stability is very good and perhaps there is other Hobie kayaks with lower secondary stability due to height of sitting position or other facts. There is also a very distinct threshold where the secondary stability begins in the Adventure kayak.

br thomas


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PostPosted: Sun Jan 06, 2013 7:12 am 
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I have no idea about the science behind this stuff. All I know is what I feel under me. In calm water I feel like my big wide Outback is more stable than my skinny little Revo. But in big wind and chop I feel more safe in the Revo. In those conditions the OB seems to roll heavily while the Revo just rides up and down under me. The Revo feels easier for me to control in rough conditions.

FWIW I was able to find this on the net-

Primary and secondary stability
Primary (sometimes called initial) stability describes how much a boat tips, or rocks back and forth when displaced from level by water movement or paddler weight shifts. Being based on the paddler's movement, it is mostly a subjective notion. Secondary (final) stability describes how readily a boat capsizes. Primary stability is often a big concern to a beginner, while secondary stability matters both to beginners and experienced travelers, as it is a physical attribute of their boat. Both primary stability and secondary stability increase as the boat's volume (i.e. buoyancy) is distributed away from its center line. For example, catamarans offer both high primary stability and secondary stability, to a point where they can be erroneously viewed as being unsinkable.


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PostPosted: Sun Jan 06, 2013 9:55 pm 
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I don't understand what is meant by "primary" and "secondary" stability. STABILITY is a function of center of bouyancy, center of gravity, metacentric height and righting arm. These are related to hull shape, volume and displacement.

Shifting of the load within the hull affects stability because it affects the righting arm. The center of gravity, relative to the center of bouyany, determines the righting arm as the hull tilts. When the metacentric height becomes zeor or negative (i.e., center of gravity at or above the center of bouyancy with insufficent righting arm) the hull will capsize.

"Primary" and "secondary" may be intended to mean the stabiliy condition when the hull is loaded or unloaded; nevethelelss, stability is a singular function of the physical relationships of the foregoing factors.

A catamaran is stable due to the large righting arm; larger sailboats are stable due to the metacentric height (i.e., low center of gravity, hence larger metacentric height and righting arm when the hull tilts); large displacement hulls are stable due to the weight/volume/displacement values of the structure.

However, all hull shapes have a critical range of stabiltiy, at which point, when the metacentric height becomes inverted, the hull will capsize, with or without shifting any interior load; however, such a shift may expedite the result.

Wide, shallow hulls are more stable than narrow shallow hulls; deep hulls are more stable than shallow hulls. Wide, shallow hulls may bounce around more, but they won't readily capsize; neither will deep hulls. However, the load in the hull can have an effect on the righting arm as the hull tilts. This can cause the hull to capzise due to the relationship of the center of gravity, center of bouyancy, metacentric height and righting arm and "primary" or "secondary" has little significance.

Simply put, a floating raft is more stable than a kayak but, with a sufficiently high center of gravity, even a raft will capsize. Actually, allmost anything is more stable than a kayak except, maybe, a log! In a kayak, the center of gravity is actually above the center of bouyancy which is why great care is required to prevent tipping over; sidekicks provide a larger righting arm, hence, greater stability. This is why anchoring a kayak is dangerous; it becomes unstable due to the force of the anchor rode tilting the kayak which affects the relationship between the center of bouyancy, the center of gravity and the righting arm.

Just be careful and don't lean over too far!


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PostPosted: Sun Jan 06, 2013 10:34 pm 
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I agree (censored),

You mentioned how unstable a log is, and you would be correct. Ther crazy thing is there is a frequent poster here who thinks the most stable thing in rough oceanic water is a log! LOL


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PostPosted: Mon Jan 07, 2013 1:04 am 
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Fish, what you describe is what would be called secondary stability or absolute stability -- the roll angle where the boat capsizes. What is commonly described as "Primary stability" is entirely different -- it is how easily the boat tilts from its upright position or how "twitchy" the boat is.

Low primary stability is often unnerving to new kayak users who may mistakenly think it makes the boat unstable. Experienced users usually prefer it though because in rough weather or boat wakes, the user controls the tilt of the boat against a beam swell (so the boat bobs rather than rolls). In a comparable boat with a higher initial stability, the user has less input so the boat rolls more in response to beam swells and is more likely to toss the user out and subsequently capsize in the process, regardless of the boat's secondary stability.

This is an important distinction and relates significantly to any boat's handling characteristics, especially seaworthiness. In some circumstances, a "log" can be better than a "raft". 8)


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PostPosted: Mon Jan 07, 2013 2:17 am 
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...


Last edited by jervis_bay84 on Mon Jan 07, 2013 5:22 pm, edited 1 time in total.

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PostPosted: Mon Jan 07, 2013 3:18 am 
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Fish It wrote:
I don't understand what is meant by "primary" and "secondary" stability. STABILITY is a function of center of bouyancy, center of gravity, metacentric height and righting arm. These are related to hull shape, volume and displacement.

This is correct. But keep in mind that the actual hull shape below water line is changing while tilting the hull. That means you can find a "treshold" where the righting arm is actually getting larger and the stability increases caused by hull shape (below water line) getting wider when you tilt it.

Roadrunner wrote:
Fish, what you describe is what would be called secondary stability or absolute stability -- the roll angle where the boat capsizes. What is commonly described as "Primary stability" is entirely different -- it is how easily the boat tilts from its upright position or how "twitchy" the boat is.

This is what I meant in my earlier post. In an Hobie Adventure there is distinct difference beetween the primary and the secondary stability. I think the same goes for all Hobie kayaks.

But if you take an kayak made for serious racing there will be no practical difference for a no-pro kayaker. He have to be very ACTIVE, and help with paddel to not capzise. Primary and secondary stability is very close and very low, if any.

There is no problem to ride a bike as long as you move and you have to have very good balance and be active to balance an bike not moving.

And I still think there is no need or even missleading to make to much theory thinking about kayak stability.

As Roadrunner said: "Experienced users usually prefer it though because in rough weather or boat wakes, the user controls the tilt of the boat against a beam swell"
My comment: This goes for normal sea kayaks. Hobie kayak hulls are to stable to actually control that way.

This is an Struer Hunter from the sixties. Nice video showing serious racing kayak and handling by a no-pro, but experienced paddler. This is nothing like a Hobie kayak!

http://www.youtube.com/watch?v=CWy6Fhr83ww

best regards
thomas


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PostPosted: Mon Jan 07, 2013 5:58 am 
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The reference about how a log rides up and down and a flat board rocks from side to side in rollers and waves is just an image reference to help people visualize how the object under you can or cannot be manipulated to keep yourself vertical. In rough water you can ride a log and keep yourself vertical but at some point the flat board prevents you from doing that as you cannot fight it's wish to move with the water surface. And it is exactly what I feel when I compare my wide Outback to my narrow Revo. Roadrunner and some others seem to get the analogy.... "In some circumstances, a "log" can be better than a "raft"."

You did not start this topic to learn anything you started it and have kept it up to take a jab at another poster who has done a lot here to help others. You have not even been here for a month and are already doing all you can to cause a problem with somebody that has been here for many years. I can already guess you will go after others in the same way in future. This sort of behavior is not helpful to any forum.


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PostPosted: Mon Jan 07, 2013 11:03 am 
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Here is what I think is meant my primary and secondary stability.

Primary stability is the stability (restoring force) of the hull when sitting flat on the water. An outback is and feels very stable.

Secondary stability is the stability (restoring force) of the hull when it is already leaning. In this case the shape of the hull under the water may be different from the primary stability conditions.

Thus, they are both subjective measures of the restorative force of the hull in different conditions. In some hull shapes you very quickly go from a strong restorative force to none (tipping) over a very small range of angle. I believe this is what folks refer to as poor secondary stability. You feel really safe until disaster hits. A perfectly rectangular hull shape would be a good example. A rounder hull feels "tippy" right off the bat but has a smoother transition from feeling the tippiness until you do actually tip over. You feel the tippiness gradually increase. I believe this is what folks describe as a hull with good secondary stability.

Here is an example of how they behave. If we are sitting parallel to a set of waves (waves coming from our right to our left). A hull with good "primary" stability will lift the hull on the leading edge side of the wave (good restorative force) and cause the kayak to lean to the side. If it leans too much you tip over. Here the good restorative force works against you. It is keeping you flay against the wave front which is leaning with respect to gravity. A hull with good "secondary" stability will rotate underneath you keeping you more upright since there is less restorative force and you may be less likely to tip since the kayak is not trying to tip you over. The less stable hull can't generate the force to lean the kayak.

Just my 2 cents.

Edit: I think the proper way to think of this is that primary stability is the initial or basic stability (restorative force). Secondary stability refers to the maximum rate of change of stability (restorative) with respect to hull rotation. Thus going from a strong restorative force to a weak one over a small change in hull angle is a case of good "primary" but poor "secondary".

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A ship in harbor is safe -- but that is not what ships are built for.
--John A. Shedd, Salt from My Attic, 1928


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PostPosted: Mon Jan 07, 2013 1:55 pm 
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I guess the terminology is peculiar to small boats/kayaks. In my experience in Navy ships, no distinction was made as to "primary" or "secondary" in determining the stability of a vessel. "Quick" and "tender" refer to the "twitchiness" factor, but stability is determined by the relationships of CB, CG, MH and RA; these in turn are determined by hull shape, volume and displacement.

I gather the distinction in terms is intended to provide conversational descriptors of kayak behavior/response in various conditions.

Found this in Wikipedia in a discussion of kayaks:

"Primary and secondary stability Primary (sometimes called initial) stability describes how much a boat tips, or rocks back and forth when displaced from level by water movement or paddler weight shifts. Being based on the paddler's movement, it is mostly a subjective notion. Secondary (final) stability describes how readily a boat capsizes. Primary stability is often a big concern to a beginner, while secondary stability matters both to beginners and experienced travelers, as it is a physical attribute of their boat. Both primary stability and secondary stability increase as the boat's volume (i.e. buoyancy) is distributed away from its center line. For example, catamarans offer both high primary stability and secondary stability, to a point where they can be erroneously viewed as being unsinkable."

On review, I just discovered that this is what RHALE quoted from the same source. Regret the repetition, but it seems to answer the question...


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