Re: [Paddlewise] Radius of Gyration

Michael Daly wrote:

> Elaine Harmon wrote:
>
> > On Sun, 25 Jul 1999, Matt Broze wrote:
> >
> > > The radius of gyration is the distance k from the axis of rotation at which
> >
> > OK, which axis, the vertical(yaw), long axis of the boat(roll), or beam
> > axis(pitch). (Hope I got those right, Pat Moss will be at me if not....) e
>
> Each has its own radius of gyration.   For a long, skinny kayak, the
> pitch and yaw values will be similar in value.  The roll radius of gyration
> will be small.

Further to my own message:

Actually, as far as the similarity in value of the pitch and yaw radii
of gyration, this is true if you consider the kayak without a paddler in it.
With a paddler, the pitch gyradius will be greater than the yaw.
This is because the paddler sticks up from the kayak.  The exact
value would be difficult to agree on, since one assumes a rigid object.
Paddlers have a nasty habit of keeping their bodies vertical while
the kayak gyrates with the waves.  Thus their legs would be relatively
static WRT the kayak, but their torso would be quasi-independent.
The paddle would affect the yaw gyradius as well, depending on the
length, weight, position held etc.

On a big ship, you can ignore the "live" load somewhat, as it is not as
dynamic as a paddler. That is, compared to the size and mass of the
overall vessel, the live contents don't move around much.  Paddlers
move a lot and would complicate the analysis of boat motion and
interaction with waves considerably.

Here's a slightly counter intuitive fact, variations of which are used to
trip up engineering students in exams:  The roll radius of gyration
is less in a fully loaded kayak than in an empty one, even though the
angular inertia is greater.

Mike (whose only experience in naval architecture is reading
Marchaj and doing dynamic analyses of ice breaker hulls)

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