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 Post subject: Stability
PostPosted: Wed Dec 19, 2012 3:57 pm 
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Joined: Wed Oct 19, 2011 10:50 am
Posts: 65
Location: South Florida
I noticed in a thread for a new member that there was an exchange of information on stability for kayaks. For anyone interested, Sea Kayaker Magazine publishes a graph for stability for every kayak that is tested. The model assumes calm water and no movement, i.e. stasis. The graph makes sense to me for initial stability, but I am not clear on the righting moment (aka secondary stability) effect. Essentially for every tested kayak, Sea Kayaker includes a stability graph. There are 4 parts to it: 150 lbs. and 200 lbs. kayaker respectively, with no load in the boat; and 150 lbs. and 200 lbs. kayakers each with 100 lbs. of cargo. The steeper the curve the more stable the boat. In every case to the best of my recollection the 150 lbs. kayaker with 100 lbs. load is the most stable configuration, the 200 lbs. kayaker with 100 lbs. load is second most stable (i.e. having a static load inside the hull of the vessel adds stability). The least stable configuration is the 200 lbs. kayaker with no additional load. Assuming that the lower the center of gravity of the vessel and paddler the more stable the combination would be, this makes sense. What is not crystal clear to me, however, is how the righting moment is affected. It would seem that the lighter weight paddler with cargo would display the greatest initial stability and perhaps the greatest righting moment as well. Is that right?

Thanks,

Bill


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 Post subject: Re: Stability
PostPosted: Wed Dec 19, 2012 6:23 pm 
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Joined: Thu Apr 14, 2005 12:34 pm
Posts: 1727
Location: Southeast Michigan
Imagine a typical kayak hull cross section. It's basically a section of a circle , something less than half. As you submerge a circular cross section, the beam in the water increases. The deeper the hull sits, the more broader the beam in the water, and hence the highest initial stability. As the boat heels, more of the hull is in the water, and the center of buoyancy moves in the direction of the heel- up to a point. That's secondary stability.

The cargo ideally sits below the center of buoyancy, and so as the boat heels, it moves in the opposite direction of the heel, providing a counterbalancing effect.

The paddler's center of mass is higher than the center of buoyancy. As the boat heels, the center of mass of the paddler moves in the direction of the heel, but being at the end of a long art (the paddler's body) it moves more rapidly than does the mass lower down. When enough of that mass is beyond the center of buoyancy, the boat tips.

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 Post subject: Re: Stability
PostPosted: Thu Dec 20, 2012 11:11 am 
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Joined: Wed Oct 19, 2011 10:50 am
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Location: South Florida
Hi Mike,

Thanks for your explanation, although we are both still not completely clear in our communication methinks. If our kayaks become more stable as the load goes up, as you stated in your first paragraph, then the heavier paddler would be more stable than the lighter paddler, which is not the case. Take a look at any of the graphs in Sea Kayaker Magazine tests and you will see. I think your thought about the positional relationship between center of buoyancy and center of gravity is correct and probably explains it, but I am still a bit muddled as to how this all happens.

With respect to secondary stability your explanation makes sense. If we are sitting in our kayak and encounter a wave broadside, the ability of the side of the boat to rise with the wave is another way to look at it. Greater secondary stability manifests itself as the side of the boat that first encounters the wave tending to float up with the rising water. A boat with less secondary stability tends to dig into the side of the wave more. Or put another way the boat with less secondary stability has less righting moment. I am not sure which is best in rough water. I have both kinds of kayaks and can't decide. What are your thoughts?

Bill


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 Post subject: Re: Stability
PostPosted: Sat Dec 22, 2012 10:59 am 
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Joined: Thu Apr 14, 2005 12:34 pm
Posts: 1727
Location: Southeast Michigan
I've noticed that in the SK graphs. As the heavier paddler carrier more weight above the CG, that will also reduce primary stability- make the boat more tender, as it were. This had to be balanced against the bull riding deeper, which increases primary stability. I suspect that as the boat reaches its waterline, and the hull flares less, adding weight won't significantly increase the beam of the hull.

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