Re: [Paddlewise] Rollability Hypothosis

>Nick wrote;
>
>> I have been having some discussions about kayak "rollability". The common
>> idea is a kayak will be hard to roll if it is stable upside-down. This
>> makes sense initially, but then again most people can get most of the way
>> up in a roll. The hard part is getting all the way. If inverted stability
>> was the most important thing, getting from upside-down to the point where
>> you can breath would be the hardest part.
>>
>> I thought that maybe the slope of the stability curve at 90 and 270
>degrees
>> would be a better indication of how easy a boat is to roll. A lower slope
>> indicating and easier boat to roll, i.e. the effort required to change the
>> angle fo the boat would be less.
>>
>John W wrote:
>
>A few thoughts:
>
>Lower stability when upside should make rolling easier particularly when you
>factor in the righting moment of the life jacket/paddler. I would suspect
>that the deck shape along the sheer impacts as well since the boat rolls
>rather rapidly. A rounded sheer such as Nick uses on some of his boats
>should roll more easily than a more angular sheer (such as I  use on some of
>my boats). The narrower the boat the easier it seems to roll in both
>directions - up and down.
>
>One could calculate the rolling resistance of the shape and merge that with
>the upside down stability. Seems like a lot of work when taking a boat out
>and rolling it tells whether you find it easy or difficult to roll. Of
>course, none of this applies to owners of wide kayaks who consider rolling
>something "other" people do and people like myself who has done it and can't
>imagine why anyone would want to do it more than once. :-)

Well that seems to leave an awful lot up to unreliable human judgement.
Next you'll be saying people should just paddle a boat to determine how
efficient it is. :)  I won't try to convince you of the entertainment value
of being able to roll. I agree with you to a large degree, but there is
some value to being able to get an idea about rolling performance from
looking at the stability curve. For one it is less open to different
interpretations than descriptive characterizations of sheer shape.

I agree that the shape of the sheer is effecting the easy of rolling, but
how do you describe a sheer shape such that it will consistently
characterize what is pertinent to ease of rolling. My hypothesis is that
the slope of the stability curve supplies the information required in one
easy-to-interpret format.

The rise of the bow may also effect rollability, but some bows have a lot
of volume while others don't. Again, you have the trouble of interpreting
the data accurately.

Wide flat boats with high inverted stability will be hard to roll. The
slope of the curve will reflect this because the slope will increase to
account for the large change from inverted to upright. But two boats with
roughly equivalent inverted stability could have quite different rolling
characteristics (due to different sheershapes or other factors) and I
suspect the slope of the curve would reflect the differences.

Nick




Nick Schade
Guillemot Kayaks
824 Thompson St, Suite I
Glastonbury, CT 06033
(860) 659-8847

Schade_at_guillemot-kayaks.com
http://www.guillemot-kayaks.com/

>>>>"It's not just Art, It's a Craft!"<<<<


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