hmmm..try again. Heres some actual data for this physics question.

i weigh 215...i measure full boat length at 201 ".
waterline length = 181"
my center of gravity when sitting in the boat is 118" back from the bow tip
or 106" back from the waterline at the bow tip.

How anyone could answer that without knowing the volume of the bow and stern is beyond me but i am just looking for a ballpark answer. I'm guessing scoot forward 1 foot. I added 1/4" to all dimensions of a Petrel.

I have a practical solution now.
i have a canvas strap i can tie between two posts....i can have someone hold the bow while i get in the kayak while it's balanced on the strap and get balanced so he just using fingertips to keep me on the strap...neutral.

from there he can stand on scale and let me know when he's 15'lbs heavier as i scoot forward and mark the spot. Then i can scoot back to neutral and put 15lbs weight back in the bow tip and see if he comes out at 15 on the scale as validation..

: How about 1/10 the current distance forward at 10X the weight, etc. Of course
: this only works if you weigh 150 lbs.

: Reg