Composite Bulkhead - Lamination Schedule

I’m still trying to wrestle with the relative merits of Coosa Board vs. 5 lb Core-Cell for the bulkheads on my 46’ motorsailer . In order to make a reasonable comparison I need to work out a laminate schedule for cored bulkheads. Gerr’s “Elements of Boat Strength” gives formulas for bulkhead thickness, laminate schedule for tabbing and laminate runoff length, but I cannot find a formula for lamination of the bulkheads themselves. My “Gerr’s Scantling Number” is 2.68. What sorts of bulkhead laminations are typically seen on 5 lb. core? Suggestions?

 

Look at formula 5-6, at the end.

 

As I read Formula 5-6 it applies to the backing strip and tabbing, but not the bulkheads themselves. Does this mean that you can only extend reinforcing up 6 inches or so and leave the rest of the bulkhead as unprotected foam? That doesn't make sense to me, especially in light of the fact that we would typically see bulkheads in the range of 1/2" to 3/4" if constructed from plywood.

 

Sorry, formula 5-5 p. 46. Why should bulkheads be very different compared to top sides of the hull? I would use glass weigth on the bulkhead the same as inside glass weight of the top side plates.

But I am not Dave Gerr...

Forum has a username dgerr, private post?

 

Look www.barracudatec.com.br/pdf/RE_Paper.pdf at page 10.

ISO rule give a minimum plywood thickness, then give rules to switch from plywood to sandwich. Beware 2006 is a preliminary version. Final is 2008 and things may have changed.
sigma,E, and fiber thickness are defined in the ISO rule, alas not free.

Note this is only a partial answer.

The full answer is the bulkhead is also seen as a transversal stiffener. I do it from memory, but the bulkhead should be considered as a stiffener of heigh 20 * thickness. So there is a minimum Section Modulus for the stiffener. And there is a minimal shear the joint between hull and bulkhead/stiffener should hold.

 

fcfc,

Perfect, this is what I was looking for. I actually remember seeing this article before but had forgotten about it.

Thanks!

Doug

 

Terholalme,

At your suggestion I contacted Dave Gerr and he was kind enough to provide some recent updates to his book, Elements of Boat Strength. The updates included a section on Cored Bulkheads (pages 45-47) I quote:

"Some people inquire about the laminate for cored bulkheads in Formula 5-5. This formula is intended for use only with commercially available cored panels, such as Baltek's Decolite. These panels come in standard thickness up to 1 inch (25.4 cm), and are faced with a laminate of 0/90 1208 on both sides. This is the laminate intended for this formula and the maximum cored-bulkhead thickness covered in this formula is 1 inch (25.4cm). If thicker is required, use solid-ply bulkheads."

end quote.

This really clarifies my question and seems to corresond well with the calculations found in the Rescue Boat paper. Clearly there can be a signficant weight savings using composite bulkheads.

 

Fine, case solved.