From: "735769" <735769@ican.net>
To: <baidarka@lists.intelenet.net>
Subject: Re: baidarka Re:fish-form/swede-form
Date: Wed, 15 Sep 1999 07:18:52 -0400
-----Original Message-----
From: James Mitchell <mariner@seanet.com>
To: baidarka@lists.intelenet.net <baidarka@lists.intelenet.net>
Date: Wednesday, September 15, 1999 1:09 AM
Subject: Re: baidarka Re:fish-form/swede-form
>Awright, was it Wolfgang who was talking about the CB of the fish form hull
>vs. swede form hulls?
Mostly me.
Hope this helps. The CG shifts with a shift in weight and the afloat CB
shifts accordingly. That is why one cannot rely upon the shape of the
waterline to determine whether a boat has its CB forward or aft of the
midpoint (note that the midpoint refers to the midpoint of the waterline not
the overall length). Often the trim of the boat as drawn has no relationship
to the trim afloat since the person taking off the lines might not have had
the opportunity to put the boat in the water.
In determining the flotation of a boat one must either put it in the water
or do the analysis of the boat based upon the weights of the boat and the
position of the cockpit. This complicates matters slightly. One doesn't
really know if the paddler sat with his back against the coaming or more
towards the middle of the cockpit. In my studies I used a point 10" forward
of he cockpit back to establish the CG of the paddler. I also assumed that
CG of the boat matched the center of areas. All regrettable assumptions.
>Been thinking about that when I shoulda been thinking
>about building airplanes, and some things made sense. If the boat is going
>to trim, the CB is the CB. You can't arbitrarily move the maximum beam
>around that hull without throwing the trim off, you have to move the
paddler
>too. Ergo... would not a "fish form" hull have to have the paddler set
>further to the bow, and a swede form further aft? I would be interested in
>relating the waterline plan form of the hull to the position of the
>paddler's spine, as a percentage of LWL. I think you will find the "fish
>form" designs position the paddler at about 50% LWL, with the "swede form"
>hulls set at 60-65% LWL.
The range of CB's for the kayaks I have studied vary from 48% to 56%. This
does not surprise me as this range with the exception of the 56% boat) fits
common practice for ship and yacht design. Much further aft than 55% tends
to cause flow separation aft (usually resultingt in control problems) and
much further forward seems to increase pitching. I would expect native
builders to have discovered any drawbacks to extreme locations for the CB.
>
>I guess I will have an interesting test of this theory coming up this
>winter. The Cape Espenberg hull that I am planning to build sets the
>paddler forward, at about 50% LWL. Haven't scaled that exactly yet, but
>that is what my observations tell me. In so doing, the hull takes on a
>symmetrical shape that seems to be close to fish form when viewed in
person.
I would excersize caution in this. If you shift the cockpit from its
original location you may do more harm than good. On the other hand you may
do a lot of good. :-) Keep in mind that you must look at the entire volume
of the boat not just its waterline shape.
(SNIP)
Cheers,
John Winters
Redwing Designs
Web site address, http://home.ican.net/~735769
.