Bulkhead Connection - Cored Construction

[SeaJay]

Greetings forum members,

At last my project is underway.

( http://www.boatdesign.net/forums/showthread.php?t=19034 )

After waiting almost an entire year for reasonable transport, my hull was delivered earlier this summer. I’ve got the cradle/hull leveled and with the help of a 5-beam and cross beam laser, I’ve been able to establish the offsets of the hull. I’m entering them into the computer and about to start working on the revised accommodation layout.

While this is taking place, I’d like to mine the collective wisdom of this forum regarding a construction detail. I’ve settled on Coosa Board (Bluewater 26) as the material for the deck, cabin and bulkhead structures. The composite hull has a 1” Core-Cell core. My question has to do with the bulkhead to hull connection.

Dave Gerr’s Elements of Boat Strength, calls for a trapezoidal foam spacer between the edge of the bulkhead and the hull of a FRP vessel. The foam is to spread the loads and eliminate hard spots. Disregarding for a moment the spacer’s additional function of providing a 45 degree transition from the hull to bulkhead for the tabbing material, does the hull still need to be cushioned from the hard edge of the bulkhead in cored construction? Since there is already 1” of foam and an inner laminated skin between the bulkhead and outer skin, I’m wondering if another inch or so of foam is required?

Regards,
SeaJay

 

[westsail42]

SeaJay,

While pondering the same question a couple years ago, I asked the local marine construction yard guys about it. They said:

Primary reason for foam between bulkheads and the hull is to prevent "hardpoints" on the hull which is a concern for NEW hulls just coming out of the mold as the resin is still curing for weeks after layup. For a hull that is post-cured, at least a couple years, it should not be necessary. If the hull is solid and particularly thin then maybe. Coosa will give a little bit under compression (not as much as Airex), and, the closer you can cut it to the hull, the better of course.

Eitherway, it is a good idea to have a 45 degree fillet along the edge.

-rob

 

[Sea Jay]

Rob,

Yes, I plan to use a fillet, but I got to thinking about a foam spacer between the bulkhead and the foam-core hull, and it seems to be redundant.

Regards,

Doug

 

[fcfc]

I am also investigating the same subject. Ie sizing a bulkhead / hull joint in a cored construction. (what filler density/strength, radius, tape width and weigth etc ...)

I have found about nothing except a vague formula for fillet radius, given bulkhead thickness assumed plywood.
Also, if using a trapezoidal pad instead of fillets, the pad heigth must be at least the bulkhead thickness, and foam density at least the bulkhead core density.
That very very broad, and nearly unusable.
See http://www.marinecomposites.com/Chapter_3K.htm , in the PDF file, chapter bulkhead attachement.

Even the book "Principle of Yacht Design", which claims for quantitive, not qualitative analysis is short on bulkhead / hull joint (or even deck hull joint).

Seems that University of Southampton have done a bit of research on cored tee joints, http://www.wumtia.soton.ac.uk/composites.html, but I do not know what is available, understandable or even merely usable by homebuilders (or wanabee ...)

Now, for spacer between hull and bulkhead, what I assume is the bulkhead will move (flex ) against the hull, and the bulkhead edge must not destroy the integrity of the inner skin. If the inner skin is sheared or punctured, sandwich stiffness effect is lost.

 

[westsail42]

Some data from the local yard rats:

They rarely see PRODUCTION boats use spacers, let alone foam fillets on bulkhead joints. It seems as though fillets are somewhat of a luxury, spacers even moreso.

 

[fcfc]

PRODUCTION boats are very unfrequently in Corecell/kevlar/glass cored construction.

Plain fiberglass boats have much less problem of delaminating / destroying the inner skin.

 

[westsail42]

foam cored glass is not uncommon around here. Kevlar we dont see much.

But yes, I see the importance of spacers with this construction. SeaJay, the laminate schedule on the inner skin would help the decision if you know it.

 

[TeddyDiver]

They rarely see PRODUCTION boats use spacers, let alone foam fillets on bulkhead joints. It seems as though fillets are somewhat of a luxury, spacers even moreso.

When the bulkheads are "weak" enough they bend before breaking the hull laminate. Strong bulkheads are more a problem if they prevent the (inner) hull laminate from bending and thus rips it apart from the core..

 

[Sea Jay]

Here is a description of the laminate/core:

The construction is of the ATC bead and cove method using 1 inch thick A500 (5 lb) polyurethane foam. The inner and outer skins are three layers of 13 oz. diagonally applied kevlar/glass hybrid with Derakane vinylester resin.

On page 47 of Elements of Boat Strength, Gerr also disucsses a "backing strip" to be laid where the bulkhead contacts the bulkhead. The strip is to be the weight of the tabbing and 8 times the thickness of the solid plywood bulkhead. The call-out for the foam spacers relates to a U.S. Coast Guard requirement for passenger-carrying vessels.

I'm thinking that the backing strip is the right approach and that the foam is a bit of overkill. However, my concern is not so much for material savings as it is for reducing the time for fitting the bulkheads. It actually might be just as easy to prefabricate the trapazoidal shapes (incorporating the fillets) and kill two birds with one stone.

 

[fcfc]

Have you found more information for joints ?

 

[mongo75]

How about some pics Doug??

 

[Sea Jay]

fcfc - No new data on the joints. My plan is to cut the bulkheads about 1+" shy of the hull and position them on small 1+" foam spacers. I'll experiment with the easiest way to form the required angle between the hull and bulkhead. Right now I'm leaning towards a thickened epoxy paste. This method would not require precise fitting and would accomodate any irregularity between the hull and bulkheads.

I don't have a lot of good photos, but will attach one that gives you some idea of the state of affairs...a clean slate!

Best Regards,

 

[fcfc]

I am looking for information about methacrylate adhesives. Such
http://www.scottbader.com/pub.nsf/A...mer_Brochures/$FILE/UK_CRESTOMER_BROCHURE.pdf
http://www.scottbader.com/pub.nsf/A...er_Brochures/$FILE/Bulkhead_Bonding_Mar08.pdf
or similar plexus itw products.

They seem much more interesting than epoxy. They have a tremendous elongation at break, really much much better than thickened epoxy. I do not know price or availability for homebuilders.

The other thing I saw is molded bulkheads with a integral single side flange. The mold may be male of female. And the flange is made at the same time the bulkhead is made (infused cored bulkhead). The bulkhead is simply bonded (with epoxy bond) to the hull by its single flange. It is just a thin bond between flange and hull, no a big epoxy fillet.

 

[Sea Jay]

fcfc,

The Scott Bader info is very interesting. Their method is similar to what I had proposed. I've written to locate a local distributor so that I can check prices and confirm construction details.

Do you have a link to the detail of the intregal flange?

Thanks for the input, I'll report back if I find anything to add.

Regards,

SeaJay

 

[Man Overboard]

I remember reading a good article on this subject; I think it was in Pro. Boat There are a couple of articles in issue 39:

Taping and Tabbing -This reprise of an important IBEX panel focuses on critical FRP construction procedures that are often overlooked.

Secondary Bonding Revisited Details- the differences between primary and secondary bonds.

I can not find issue 39, Maybe someone can look it up, If not I can look a little harder for that issue to see if either of those articles is the one that I am thinking of.

Here is an article that discusses bonding issues in boat construction. It is some what general in nature. Note: you have to go through an extensive registration process to access the article.

Bonding composite materials
Part 5: Bonding in naval construction, Boat building
http://www.specialchem4adhesives.com/resources/articles/article.aspx?id=1929#bulk