bulkhead placement?

I'm planning a 20 foot plywood stitch and glue pontoon boat. Something along the lines of this, but longer:

http://www.jemwatercraft.com/pontoon.php

My question is this: When I lengthen the design, how far apart should I place the bulkheads? Is there a rule / formula / guideline for this, or do I build it and find out if it breaks apart?

Left to my own devices, I'd add more bulkheads so that the distance between them is the same as in the original plans.

Any suggestions will be appreciated.

Tom

 

By changing the length, will you change the thickness of the ply? Those bulkheads are spaced to shape the pontoons as much as for strength. Thickness will matter.
Looks like you could strech the middle by doing exactly what you would do left to yourself and follow their spacing, while shaping the ends as they have designed it.
You are making it longer because??? 'toons are very load sensitive. If adding a cabin or other substanchel weight, you may want to increase all dimentions.
Also, the amount of displacemrnt to material used in 'toons is more than a single hull design. Add the total sq. ft. of ply in those 'toons VS their displasment. Now build a single hull from that same material & check that disp. figure. Wierd huh?

 

I don't want to change the load much, I just want a little more space to fish. I figure the boat will draft shallower, but that's not so bad.

I was hoping to still use 1/4 inch ply as specified. I already bought it. :^)

Thanks for pointing out the material used versus displacement. I'll think that over.

Thanks! Tom

 

It looks as if the 2 sections on either side of the center frame are 4'. Then a smaller frame and 2' to a point.
If you add 8' more then I would add it by building 1 more frame the same size as the middle one. add 4' more on either side of the new frame. This gives 4 four foot spaces, leaves the 2 two foot spaces as they are.
So... instead of a taper starting at midpoint, you will have an 8' straight section and then the taper starts. I see no harm in that. IF... you are a "belt AND suspenders" type person, then I would notch the center of all sides of all frames and inset a 1 X 4 stringer for a horizontal rib on all surfaces. The very ends of these need not go to the very ends of the 'toons.

 

Sorry. I can't figure how to edit. It should read, "build 2 more frames".

 

belt and suspenders

"Overbuild" is my middle name.

Houw could it miss?

Thanks for your help.

Tom