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So here are some pictures of my port and starboard bulkheads, forward of the engines. If you notice, there is a lot of load on the edges of profile on topsides were the pilasters rest on the top of. My question is, I'm not sure if I have the confidence of "foam core material" (Coosa Board) to support these downward forces. If Coosa has been used here in past with success, I may turn to that material, or just fully glass the existing bulkheads? Any thoughts/ experiences greatly appreciated.
Thanks guys.
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Let me add I did ask about Coosa before but that was rear of engines were it is only deck support, I'm just not sure of compressive strength of Coosa?

Rocky, I used okume plywood covered with 1708 biaxle coated with epoxy, both sides. The engines are trying to exit through the front of the boat...to offset that thrust and torque I used two layers of 1/2" with 1708 between and on both sides for the bulkhead in front of the engines. I then sistered the stringers going forward with fir and covered that with 1708 and roven.. in effect the thrust is being spread through two bulkheads and two sets of sistered stringers..the next (or main bulkhead in a Bahia mar) I used one layer of 3/4 and another layer of 1/2 okume and again 1708 both sides and in between. Heck if Bertram used 3/4 painted ply and it lasted 44 years the ply epoxy should do for another 44..

As a student of that profesor that teaches out of LLano Tx, I went with coosa, 3/4 epoxied to 1/2 , I wanted to recess the windows and wanted the strength

Holy crap Bob, I like the overkill idea but wow! Needless to say that aint goin anywere. Marlin I do also see what you are saying, but maybe you can explain in a little more detail how the Coosa is so strong. All I know is if I take a piece of 1/2" Bluewater26Coosa I CAN break it with my hands, 6"x6" piece. I take a 1/2" marine ply same dimension and nope, I can't break that. Hope I'm not starting a building material argument, I just want to be sure I'm going through the time/labor with the best material.
Thanks for sharing those pics Bob keeps me motivated..

Rocky,

You're break test was done on the wrong plane... Try the strength test using, instead of the 1/2" cross section, the 6" longitudinal direction.

Coosa type board is quite rigid on it's own, but like all composite cores, becomes exceptionally stronger when captured between two fiberglass faces. (See: Basic Composite Structure and Benefits of Core Materials)

Unlike a load bearing wall in a wood framed house, the structures being supported on a 31 Bertram are very light and have been structurally designed to carry much of their own weight with rather minimal additional support. I wouldn't install a Coosa panel without facing both sides without glassing them up, or at least epoxy laminating the face(s) with hard laminate. If for no other reason, the ability to easily paint the surface.

With regards to the carrying weight under the gunnel, the maximum live load directly above the bulkhead probably won't exceed 200 lbs per square inch. In the pilaster sections, the load from the bridge is being distributed over a larger area, and in the case of the Flybridge Cruiser & Express cruiser, the load is also be distributed across the full aft bulkhead.

Even at only 1/2" and without fiberglass faces, once a Coosa bulkhead is tabbed in, there would be adequate structural support.

All of my aft bulkheads have been laminated plywood. Two 1/2" panels epoxied together with heavy weight hard laminate epoxied to the resulting faces. While the same could be done with Coosa board, saving a lot of weight and being totally rot proof, I would probably continue to use my plywood technique...

Remember, with whatever materials you decide on for the new bulkhead, all intersetions with the hull sides and gunnels must be done in a manner that will not create hard spots that will lead to structural fatigue.

Br,

Patrick

Very helpfull Capt., thanks. I do have on either side of hull a good 1/4" gap between bulkhead and hull, just not at the top of bulkhead were it is not tabbed by Bertram but gunnel resting on top edge of bulkhead.I've read the basic composite structure tips, and sounds like Coosa would have to be laminated minimum two 1/2" pieces together, and minimum epoxied the outsides for strength/painting. I would do the same with ply so, no real labor time benefits for Coosa it seems. Would you tab in the top of bulkhead under gunnel were bertram did not, and not have the top edge of bulkhead touching under gunnel? (To retain that gap all the way around bulkhead).

gunnel resting on top edge of bulkhead

Tabbed or otherwise, that contact is a hard spot. On the boat I just shipped off, both of those contact spots had caused the gunnels to crack completely through. Every time the boat slams down in a sea that bulkhead contact acted like a chisel.

Relieve a solid 1/2" of space there and epoxy a broad piece of material, (Coosa Board, plywood, PVC foam Board, etc.), to under side of the gunnel to spread the load and protect the fiberglass. Don't tab it, just let it ride on the top of the bulkhead. I used H80 Divinycell on the Hancock's boat.

O.K. then thanks Capt., I'll spread the load under gunnel, no tab. Time to get down to it. I'll take progression pics along the way, you could check my work as I go!

If I were to use plywood I would sandwich cloth between the sheets and cover both outside surfaces with cloth, now you have to either fill and fair or laminate formica, after u have cut these panals to shape, u have to waterproof these cut edges with epoxy and then tab in, seems like a lot of work and expense. I can not break the heavier density coosa, I can get it to start to splinter and see the shards of cloth start to protrude when I jump on a 4" x18" piece proped up on both ends, tough stuff, I think I purchased 16 sheets of coosa , 1/4 ,1/2 and 3/4 to liten up the boat in addition to the more contemporary boat building methods I have seen at the custom boat builders in southeast florida, capt pat once told me if he ever did another it would be coosa composite

Just for shts and giggles I took some Coosa, and some marine ply to work. I then put the Coosa in a press but longitudinally (on end) and applied force downward. The Coosa actually took some force and finally just kind of splintered (Marlin), then slowly gave way to the press. Then what I thought would absolutely kill the Coosa in this test to my surprise WOW, the ply just cracked/splintered and broke very early. I could not get any pressure readings on the press as both pieces didn't acheive even 500 lbs, but i could feel more pressure applied from press handle with the Coosa. This was no lamination on either pieces, just raw material. That surprised the heck out of me, didn't expect that!

rocky my cockpit has fish kill boxes on both sides i had to show non believers at greenport . 3/4" coosa with just gel cote to filled the poures surface on all sides . they are 20" wide and about 5 and half feet long .you could jump up and down on them (and we do) they look like the day i put them in 3 years ago. the best quality feature is they will never absorb a drop of water.

Thanks Bob, I need to jump into the 21st century and realize the benefits of modern day materials. There is something to be said about not absorbing water at the same time being strong, here's more proof. The plywood just kind of exploded, and the Coosa deflected first, then finally broke the fibers and sheared away. Whish I had a pressure reading but could feel with the press handle more psi with the Coosa. WOW.
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