Re: [Paddlewise] rack spacing & 'reshaping' plastic hulls

On 24 Jun 2003 at 11:07, Keith Wrage wrote:

> Someone's comment in the bow-stern tie-down exchange got me to
> wondering if it is better to space the rack bars apart further - or
> closer together?  Say on a 16 ft boat - should it be support 4 ft from
> either end - leaving 8' in the middle?  Or under the main section of
> the hull - rather than the highly tapered sections?

A uniformly distributed load would best be supported under the 
conditions you describe - the outer quarter points.  However, since a 
kayak is skinny at the ends and fat in the middle, you have to move 
the supports in towards the middle to compensate.  How much could be 
determined thru a number of creative means, but play it by ear.  
Basically, you want the weight of the overhanging ends to equal the 
weight of the bit between the bars.  If you took a Star Trek phasar 
and cut through the kayak at the exact centre between the bars, the 
two pieces would be perfectly balanced on each bar.

If you put tension in the tie downs at the ends, this could allow for 
greater spacing since it would be like adding weight to the ends of 
the kayak.

> How does the recommendation change for a plastic boat - exposed to 
> sunlight/heat while loaded....it a wide spacing more or less apt to 
> allow the hull to deform in the sun?  Seems that a wide spacing 
> might be more prone to lengthwise deformation (adding 'rocker') 
> while a close spacing would more quickly cause 'boops' in the
> flatter, less rigid section of the hull around the cockpit - from
>  the saddles or foam blocks.

One compromise would be to locate the bars near the bulkheads, since 
the bulkhead provides considerable support to keep the hull in shape. 
 This would avoid local bumps in the plastic at the possible expense 
of a rocker change.  If you don't do this, the closer to the 
bulkheads you do get, the better.

It sounds like you have a good, intuitive understanding of what's 
going on, so just fiddle till you feel comfortable.

Mike

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