Chesapeake triple bulkhead fit

[Yummyipa]

I’m building the chesapeake triple from plans and am at the stage of placing the bulkheads. I have two main issues. First, at the bow and stern the keel and side panels do not want to come together. The gap at the bow is 1/4” and 1/2” at the stern. If I push moderately hard, I can reduce the gaps by about 50%, but I’m worried about going much harder than that. I’m pretty confident the panel dimensions are spot on.

Second, the bulkhead fit isn’t great. The dimensions should be pretty accurate, but I’m building from PDF plans and CLC only give you a 1” scale mark so depending on which pixel you measure from the total dimensions could skew as much as 1/2” inch (for reference my rear bulkhead is about 21.5 x 11 at max dimensions. With the bulkheads in place, the max beam sits around 30 1/8”, which is encouraging that the bulkhead width isn’t too far off. However, if I calculate the beam at each bulkhead from the image on full kayak on page 1 of the plans, it looks like I’m about 1/2”-5/8” too narrow, so perhaps the issue is the scaling off the bulkheads? Most of the height difference you see between the top of the sheer clamp and the top of the bulkhead can be attributed to the fact that my sheer clamps are currently about 1 3/32” tall, so they will get planed down to fit reasonably well there so long as I have the bulkheads push up snug against the sheer clamp. That of course leaves an even larger gap at the bottom. The keel is stitched pretty loosely right now, so if I push up on the keel line the gaps are reduced from ~3/4” to ~3/8”, so that helps.

So I’m wondering the best course of action. I’m not sure at all what to do about issue 1. For the second issue, it seems like I could either tighten up the keel stitching which should help some and then just let the filleting fill the rest of the gap, or I could cut new bulkheads to be slightly larger. Cutting new bulkheads would be easy to do, but I’m guessing I don’t want the bulkheads pushing out too hard against the side panels?

[David_Dewitt1]

Personally, I would make new bulkheads - especially if the hull panels look fair as you sight down them. I do this by using a piece of plywood or heavy cardboard slightly understized to which I hot glue little pointed pieces of wood every inch or so whose ends fit tight against the hull. Howard or Lazlo will undoubtedly be able to describe the process more elegantly.

[Daniel_Thaler]

I would make sure the hull panels are tight and fair. Then, after they are epoxied together I would focus on the bulkheads. If there are some gaps it’s not a big deal as you can filet the gaps and no one will be the wiser.

Dan

[David_Dewitt1]

I would be impressed if one could do a reasonable job at laying a fillet across a span of 1/2”. Sure you could hack with with tape filling the gap from the back side but to get structural integrity you are going to need a huge fillet. Bite the bullet and cut out new bulkheads that fit properly.

[Yummyipa]

Thanks for the suggestions. I think that I will cut new bulkheads. I put in a spreader stick near each bulkhead to get the width to what I calculated it should be from the plans. which makes the keel to side panel angles closer to what is on the bulkheads. Scaling up the bulkheads by a few percent should be pretty close to spot on. I’m guessing part of the problem is the extra .1” height of sheer clamp means they are stiffer and offering more resistance to stretching to the full beam. Everything still stays fair with that extra half inch. As for the gaps, at the bow and stern, I added more stitches which helped. Bow is now 1/8”, which seems OK for epoxy to handle. Stern is still slightly over 1/4”, which I think is still too much.

[Mark_Nye]

I second Dan’s advice, fix the gaps between the hull panels first, then worry about the bulkheads. You will have to loosen some of the other wires and maybe remove the spreader stick, but you need to close those gaps on the bow and stern before doing anything else. You may also find that moving your sawhorses towards the ends helps.

Regarding the bulkheads, keep in mind that the Chesapeake are older designs so the fit is not as good as newer designs designed using more sophisticated CAD. As a result, fitting the bulkheads is a fiddly process. Exact positioning of the bulkheads is not critical, and I have had good luck moving them a little forward or aft for a better fit. Trim the bulkheads as needed. Be careful not to move the aft BH too far forward or it will interfere with the cockpit. As Dan suggests, it is an easy thing to filet the gaps with no adverse impacts.

[Laszlo_Morocz]

I’m not going to weigh in on how to fix the bulkhead problem, others have thoroughly covered that and there’s nothing more to add. I’m here to comment on fillet size issues.

In S&G boats there are basically 2 types of fillets - glassed and unglassed. Unglassed fillets use the cured thickened epoxy to provide all the resistance to forces that affect the joint. This includes tension (pulling), compression (pushing), torsion (twisting), shear (sliding) and flexing (bending). Cured thickened epoxy is really good at resisting compression. It’s a solid block of hard plastic that doesn’t want to bend much. It’s not as good resisting the other forces mostly because the thickeners are physically very short. Wood flour, cab-o-sil and cell-o-fill are all dust. Tension, torsion and shear are best resisted by long fibers. So to increase resistance to these forces, the fillet has to be bulked up. This spreads the forces over a greater volume and brings the force per unit volume down into the range where thickened epoxy can handle it. That’s the basis for David Dewitt’s statement that a huge fillet will be needed. And huge fillets are expensive, heavy and need different techniques than thin fillets.

Glassed fillets are a different story. With those, the long glass fibers provide the resistance to tension, torsion and shear. The fillet itself only has 2 functions - resisting compression and providing support for the glass. Therefore, it can be a lot smaller and still have the same level of structural integrity that a fat fillet has.

The size of a glassed fillet is determined primarily by the weight and weave of the glass used to cover it. All fiberglass has a minimum bend radius. When bending the glass, that’s the amount of curve required to keep it from folding. A gentle curve smoothly transfers stress along the joint. A sharp fold acts as a stress concentrator, focusing the forces at a particular location and leading to possible failure of the joint. Heavier glass needs a gentler curve, lighter glass can bend more sharply. So the fillet’s size is determined by minimum bend radius of the weight of the glass. For 6-oz glass, that’s about the same as a US 5 cent coin. Cutting a plastic putty knife to the same radius as a nickel makes a tool that will correctly set the maximum fillet size needed for a glassed joint when covered with the sizes, types and weaves of glass that are commonly used for CLC boats. If you want to take the trouble to use differently-curved tools for different glass weights and weaves, you can optimize the fillet sizes, but the return for the level of effort is not worth it to me.

So to attach a bulkhead with an oversized gap using a glass taped fillet:

1. Use a very thick, stiff mix of epoxy and thickener
2. Apply the mix with a shaped putty knife cut to the minimum bend radius
3. If the surface is rough, paint it smooth with a chip brush dipped in unthickened epoxy
4. Pre-saturate the glass with epoxy before applying it to the fillet
5. Apply the wet glass to the uncured fillet (wet-on-wet)
6. Smooth it into place with gloved fingers

This will all result in the minimum-sized fillet needed for full strength of the joint. It will also eliminate the need to sand fillets. Again, this is all done wet-on-wet. There should be minimal time between steps 3 and 4. And it needs to be done on both sides of the bulkhead.

As I said before, I don’t have anything to add about how to fix the original bulkhead issue, but if the choice is to use the original bulkheads and accept the gaps, this is how to get maximum strength for minimum materials and weight.

Laszlo

[David_Dewitt1]

I will add one more thing to Lazlo’s comment. If you are using glass on a fillet and there is a curve (even relatively slight) involved, cutting the fiberglass on the bias (45 degrees to the edge of the fabric) will allow the glass to follow the curve (like around the bulkhead) really easily.