Hi FBH, 

your measurments sounds fine.  i built a night heron strip

https://www.clcboats.com/scripts/phpThumb/phpThumb.php?src=/images/photos/boats/nightheron/nIghtheron--howard-spira.jpg&w=580

number 4 in the picture directory ..A Night Heron Kayak built by Howard S. of Bethesda, MD

the forms will be several inchest short of 18 feet and the forms/spacing/strongback are designed that way with an understanding that when you put a 1/4 skin on it and build out the stem and stern....you will basically gain it back.  will the boat end up exactly 18 feet?  it will be close....but the most important thing is to set up the strong back and forms per the instructions given.  i went to the garage just now and pulled out my old forms/stem and stern and measured them and added to the 168 and i get 17.8 feet which seems pretty close to where you are at.

hope that helps...i get a lot of great comments on this boat and it is a joy to paddle.  happy to share ideas on outfitting.....fwiw...i would definitely add a retractable skeg which is very easy to do before the boat is totally built.

h

   Thanks for your response. I have been sorting out the reference lines on the strong back today. At 12 inches apart with the half inch forms set on the same side of them ie towards the centre the gaps must be 11.5 inches. This contradicts what is on a supplementary sheet that came with the kit that states 11 inches between forms. I spent considerable time measuring and re measuring and everything only works out with the 11.5 inch gaps. Am I going mad ? I have a long way to go!

      Thanks for your response. I have been sorting out the reference lines on the strong back today. At 12 inches apart with the half inch forms set on the same side of them ie towards the centre the gaps must be 11.5 inches. This contradicts what is on a supplementary sheet that came with the kit that states 11 inches between forms. I spent considerable time measuring and re measuring and everything only works out with the 11.5 inch gaps. Am I going mad ? I have a long way to go! Forum novice too!

just a little mad....

the strong back is exactly 14 feet (168 inches).  you should pencil mark it every 12 inches to break it into 14 equal segments.

from the front of the boat working backwards you will place a form flush with the beginning of the strong back and each subsequent 12 inch mark giving you 11.5 inches in between.    you will do the same procedure from the back.

between forms labeled 10 and 11 (which is the widest part of the boat) forms whose face is aligned with the front of the strongback/lines will come up against withe the forms aligned with the back face of the strong-back so that there is only 11 inches between them.

i would note, that this can be pretty confusing because on the line drawings, they have a bulkhead #1 (the very little section close to the bow of the boat) which was so small that they did not actually cut it out but simply put a reference line on the stem cutout.   So the cut out bulkhead # 2, in the drawing, is labled #3....and they started from there.

anyway.....i am hoping with this explanation....you can now sort it out.....please make sure to confirm the location on your kit/forms where the front-aligned forms meet the back-aligned forms becuase, like i said, when i built the kit, the form numbers on the forms they sent me were one off the line drawings number.

h

just to clarify one of the paragraphs above. 

from the front of the boat working backwards you will place a form flush with the beginning of the strong back and each subsequent 12 inch mark giving you 11.5 inches in between.    you will do the same procedure from the back. (from the back of the boat, the back side of the form will be flush with the end of the strong-back....and each subsequent mark giving you 11.5 inches in between)

so the spacing between is 11.5 inches in-between for every location except between forms 10 and 11 where the spacing is only 11 inches.