Transverse/Primary Stiffeners

Hi All,

I'm working on a 42 foot custom designed Wood/epoxy composite ketch. Attached is an image to illustrate what I am referring to. I've worked out the scantlings for the hull skin and longitudinal stiffeners using the ISO standard.

As it stands right now:
- the longitudinals are 2"x2" mahogany stringers, with a 10"x5" keel and a stem that is 5" deep at its thinest.
- the hull skin is half inch thick white pine strip planking and will have 2 or 3 layers of 1/8" mahogany veneer, for a total thickness of 3/4" ~ 7/8".

Not pictured in the attached image is any of the floors which will run along from the second bulkhead (collision bulkhead) all the way to the stern.

The stringers are positioned like so:
- one at the waterline
- one halfway between the sheer line and the water line
- one roughly halfway between the keel and the waterline.

Bulkheads are 3/4" marine grade ply, with first, second and last ones being collision bulkheads (fully sealed) and nice big cheek plates holding them there.

The scantlings I've worked out indicate that so long as I keep the longest span of open panel under 2 meters, then my stringers and hull thickness is well within norms. (required stringer thickness is in average 1 1/2" and 2" at max, required hull thickness is in average 5/8" and 3/4" at max)


This means that I need to add the indicated transverse stiffeners (blue). They would be "floating", ie not touch the actual hull skin but rather only connect at the stringer intersections.

So the question is: given that I don't want to have those members be bulkheads, what should I be making them out of (thickness, material, SM) so that they can be considered strong enough to be primary stiffeners?


Thanks for your time, let me know if you need additional information.

 

is there a particularly reason for making this transversal stiffeners no bulkheads?

look at this dix design... almost identical building method and he used bulkheads throughout....
http://www.dixdesign.com/didi40cr.htm
http://www.dixdesign.com/didi38build1.htm

the second of your stiffeners will most probably take the chainplates so its dimensions will primarely be dependent on the loads from the rig...

 

Thanks for those.

It makes sense and I Think I can integrate that into the design.