NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Ships as dancers
From: John Huth
Date: 2010 Mar 17, 06:32 -0400
From: John Huth
Date: 2010 Mar 17, 06:32 -0400
What's parametric rolling?
I always thought this was an example of the "tennis racket" theorem. Objects have three principle moments of inertia. Rotations about the axes with the largest and smallest moments of inertia are stable, but rotations about the intermediate axis is unstable. For a kayak, the pitch and yaw moments are nearly the same (degenerate), so I figured that it was easy to convert some movement that was a combination of pitch and yaw into roll.
On Wed, Mar 17, 2010 at 1:27 AM, captkev-at-telstra.com <captkev@telstra.com> wrote:
..." I've always been fascinated how in my kayak, a bit of yaw when surfing a wave very rapidly gets converted into a roll."Could this be the resonance phenomenon of 'parametric rolling' ?
With the yawing a time varying change in the kayak's transverse stability could occur. Certain encounter periods of following or quarterly seas coupled with crafts natural half roll period can cause parametric rolling in large ships, fishing trawlers and yachts, so why not in kayaks?'Seaworthiness, the forgotten factor' by C.A.Marchay has a chapter on this type of rolling in yachts. The IMO(International Maritime Organisation)has a circular MSC 707 which covers wave and ship conditions that can lead to parametric rolling.
Regards,
Kevin
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