Bulkhead replacement

richfriend · Post by **richfriend** » Mon Nov 13, 2006 9:20 pm

I am replacing the interior on my Kells 28 due to extensive water damage. The forward bulkhead was previously ~6" forward of the mast and I am relocating it aft adjacent to the upper shroud chainplate knees and inline with the center of the mast. It appears as though originally, the bulkheads were installed prior to the cabin/deck assembly being added. A new, full size,port bulkhead will not fit through the companionway. It misses by about 3". the starboard side is no problem being somewhat narrower. Would it be better to piece the bulkhead together vertically or horizontally (sp?) ? The bulkheads will likely be 9mm Okoume. The original was 1/2" 3 ply with veneer that has almost totally delaminated.

Jason K · Post by **Jason K** » Mon Nov 13, 2006 10:49 pm

It will be better to piece the bulkhead together vertically. The loads are generally compressive, so it seems to me a vertical seam is better than a horizontal one. Mike (Figment) did a bulkhead replacement on his Triton - I'm sure he'll chime in with better advice than I could give.

Post by **Tim** » Tue Nov 14, 2006 7:38 am

The first question is: can you make the port bulkhead 3" smaller, or is there a specific space in which it must fit? If you're reconfiguring the entire interior, there might be enough leeway in your plans to allow for a slightly smaller bulkhead.

Of course, there may not be either, or the longest dimension--the one that won't quite fit--may need to be a certain length based on the design of the boat.

Assuming you do need to make a bulkhead in more than one piece, you can really use either a horizontal or vertical seam--if you build the seam correctly. Once you secure the pieces together properly, it will for all intents and purposes be a single structure anyway.

However, in general I agree that a vertical seam makes the most sense in most cases, and that's probably what you should plan on. Whether or not it's actually stronger in the end is a question that's beyond my capacity to answer, as it surely requires some sort of engineering and calculation to determine. All things being equal, I would probably choose a vertical seam, but the point is that it may not make any difference in the end, from a strength standpoint. Remember that bulkheads are mostly there to support the shape of the hull, so the actual direction of the stresses imposed thereupon may be more complicated than we think. Supporting the weight of the deck above is a secondary purpose of many bulkheads, but not all.

The proper way to join two pieces of plywood, whatever the application, is to create a scarf joint on the two plywood edges before you install them, and join the plywood at the scarf. 12:1 is the normal ratio for plywood scarfs--12" of width for every 1" of material thickness. It's relatively straightforward to build a jig for a router that will allow you to easily cut these angles, or you can use a plane to cut them by hand if you prefer. Two sections of plywood joined in this manner using epoxy will be just as strong as a single piece. To make your joint truly beefy, you can also rout a shallow dado over the entire seam to allow you to lay in a couple layers of biaxial tape to reinforce the glued scarf joint; the dado allows the tape to remain flush with the surface, rather than stand proud. I don't know if the taping is truly necessary, but I would do it just for a little overkill, particularly on a major structural bulkhead.

cantstopnow · Post by **cantstopnow** » Wed Nov 15, 2006 9:16 am

I would love to see photos of this project. My Kells 28 is in need of similar work. I hope that you can post some photos. Many of the Kells 28s were sold as bare hulls and finished by their owners so the interiors vary somewhat.

Figment · Post by **Figment** » Wed Nov 15, 2006 9:36 am

Making the bulkhead smaller by 3" might best be done on an edge which contacts the hull. 3" of infill will pretty much be swallowed by the hull tabbing anyway. Since it really is mere "infill" at that point, I don't think the orientation of the joint (vertical, horizontal, swirly) will have any impact on strength.

Quetzalsailor · Post by **Quetzalsailor** » Wed Nov 15, 2006 10:26 am

Decent plywood is close enough to isotropic for this purpose, I think. Most bulkhead grain is vertical, a convention, but also a taste.

I'd suggest that the scarphing is overkill and way too difficult to clamp once the pieces were installed and way to precious, delicate to move in and out during construction. A plywood butt block will be fine, particularly if you can find a design excuse for disguising its appearance.

I'd suggest leaving a 3" gap to be bridged by tabbing is about 2 1/2" too much. The tabbing normally laps 3-4" and who would want to see 7"? A 1/2" gap is nice, if it does not show and particularly at the bottom. It would keep the end grain out of water.

I have added new chainplates to the hull of a M27 to redirect the shroud loads from the deck. Building up a total of about 1/2" of 'tabbing' is alot of layers and a mess! Expecting tabbing to do the job of spanning 3" to the plywood bulkhead would require a great deal of thickness.

As for fitting and determining size and shape of the new components, start with cardboard then luan underlayment and make a pattern. Lap and cobble it together until you're satisfied with the fit. Then see what you have to do to get your new bulkhead in and where the joint would have to be.

Summersdawn · Post by **Summersdawn** » Wed Nov 15, 2006 12:58 pm

Instead of using 1 layer of 9mm Okoume, what about 2 layers of 4mm or 5mm Okoume plwood? Each layer could have a joint in a different spot - ie 1 layer have the joint vertical, the 2nd layer the joint could be horizontal. It could all be glued together with thickened epoxy, which should give you a nice solid bulkhead.

Post by **Tim** » Wed Nov 15, 2006 1:49 pm

Using 2 or 3 thinner layers of plywood, with overlapping seams, is a good idea--so good, in fact, that I used this when I built a large bulkhead for the daysailor project. I completely forgot that this is how I proceeded before. This might be easier to install in place than a scarfed joint.

Of course, the multiple piece approach is best if you plan on painting the bulkhead, or otherwise covering it with veneer or other finished surface. Of course you don't want to see the seams. Most interiors wouldn't bet oo forgiving of the appearance of a protruding butt block, but if you have a way to hide it this would work OK.

Daysailor Bulkhead Installation

I wouldn't go so far as to call proper plywood scarfs "precious", but they might be a little challenging to install in the confines of your interior. Still, it's an effective, strong, and smooth way to join sheets of plywood, and is the most elegant and effective way to do so.

richfriend · Post by **richfriend** » Wed Nov 15, 2006 1:52 pm

Thanks for all the help. The original bulkhead had limited contact with the cabin roof. One small spot near the center. It was only tabbed to the hull. With a keel stepped mast, I guess there wasn't much loading on the cabin roof. The suggestion of trimming 3" triggered my aging brain cells out of their stupor and now has me considering placing a tension/compression post next to the mast and run the edge of the bulkhead into that. It wouild be easy enough to rout or dado a slot in the post to accept the bulkhead edge, and if I used the equivalent of a 2 x 4 I can gain enough to avoid splicing the bulkhead at all.