New Bulkhead on Thermoform

-- Last Updated: Jun-08-09 4:42 PM EST --

I'm about to add a new bulkhead where there previously was none on a thermoformed kayak (Perception's Airalite material). Mainly to limit the amount of water it holds when swamped but also to allow me to completely remove my current center footrest setup that adds some bulk and can use a little improvement in ergonomics.

I have a a choice of materials at hand: similar plastic material as that of the boat, or marine plywood + fiberglass + epoxy.

The questions: since it needs to be strong (I would use it as a foot rest to push against while paddling), would I be better off with the plywood/fiberglass combo?

Would you glass it in to the hull with epoxy or would you suggest other adhesive?

Also, since the kayak does not have and most likely will not have a front hatch cover (no room for anything in the front behind where the bulkhead would be as there is a foam block in there for rigidity), would you recommend installing a drain plug on the deck or on the bulkhead?

How do I ensure good adhesion on the rear end (the "front" of the bulkhead towards the bow) as I will have no access there once the bulkhead is in place? Would a fiberglass tape on the inside of the cockpit where I do have access be enough or is there some trick to put glue/epoxy behind the bulkhead so that it is glued/sealed on both sides?

Lastly, would you recommend glueing some backing strips against the hull before I install the bulk head? E.g. a strip of 1/4 inch thickness forming a circle just behind where the bulkhead would be, against which the bulkhead would fit so that when pushed against it will not slide forward even if not glued? If the area becomes too rigid would that create some problems for the somewhat flexible Airalite construction leading to potential issues?


PS. Forgot to mention, I already have an outline made out of cardboard with the help of a wire hanger to "lift" the shape of the hull/deck where I want the bulkhead - thanks for the tip on these forums previously given to someone else...

Yipes! This is complicated. Salty?
If you want to use marine plywood, to save weight, use Kevlar or polyester facings rather than glass. Make sure the edges of the plywood are completely protected by epoxy. For lightness, the plywood should be thin.

For attachment to the hull, I recommend West G-flex epoxy. It sets slow, so you will have some time to mess around getting the bulkhead in place. It should stick well to thermoformed plastic, provided the surface is sanded and cleaned, and possibly flamed. See West technical info for help.

Your idea about applying some cloth to the inside hull to which the bulkhead would later be glued, is interesting. Again, of the adhesives I know, G-flex is the best, even though it is thicker than regular West 105/205 resin and takes more time to wet out cloth. However I am not sure whether it would be worth the extra work.

You refer to the problem of getting at the bulkhead- hull interface on the bow side of the bulkhead. Is there no hatch up there through which you could work?

My final question is, are you sure this is worth it, or can you solve all these issues in an easier way?

one concern
I don’t think you should ever wall off bow or stern with no access port or hatch. You just might get water in there, against all logic. Or, you might sustain hull damage that you’d like to inspect from the inside. It’ seems a 5 or 6-inch screw inspection port, at the very least, would be advisable. 'Course, then you could work the structure/seal of your bulkhead from the “back” side.


– Last Updated: Jun-08-09 7:41 PM EST –

I'd be mighty wary about installing a rigid bulkhead in a flexible hull. Do you have to use this bulkhead as a foot rest? Also is it strongly tapered so that it can back out easily but not push in easily?

My suggestion would be to make a two pieced flange out of minicell about 3" thick, two 1 1/2" thick pieces about 2"-3" tall. The plywood with a hole big enough to put a Beckson hatch into would fit about 1" in from the edges of the hull. So you'd have one minicell flange glued in the hull, then you'll glue the bulkhead with it's hole cut out onto the completely glued flage and check to see the glue inside was well applied with no gaps. Then you'd apply the other minicell flange. I'll leave it up to you as to the type of sealant/adhesive. Maybe 3m5200 would work.

That way any flexing in the hull won't crack out the totally rigid bulkhead.

If this is a rec. kayak it's kind of irrelevant as the amount of free water is huge compared to the flotation and putting in float bags with a lacing of lines to secure it would be a lot easier.

Some answers
Hmmm… Solving the easy way you say? Meaning a new kayak? Unfortunately, that’d be the hardest way right now :wink:

The kayak is the Perception Sonoma 13.5. It tapers down going forward from the bulkhead area.

I guess I do not need to have a strong bulkhead there to step on as I already have a center board installed at the foot peg rails. But, since I am doing a bulkhead I thought I might as well use it as a foot rest and remove the other contraption I have as a centerboard right now.

I could I suppose instead just use thin split flotation bags to avoid flooding and keep my centerboard for my feet.

And no, there is no front hatch opening and the way the deck is shaped there is no room to install even a 4" one on the deck. I too thought of installing an access port (4" screw-in hatch) but it will only be for inspections - I can’t reasonably store almost anything in the front and I do not want to - the boat is already balanced a little too nose heavy as it is with me in it. But an access hatch will make it easier to work…

May be I should do a 2-piece bulkhead. One like a 2-3" wide rib that would be well secured from both sides and then a center one overlapping it and screwed on to it. Sort of a semi-permanent hatch cover over most of the opening.

The adhesive used to seal the seam b/w hull and deck is actually rock hard. So the more I think about it the less I think there will be an issue with the bulkhead being stiffer than the hull.

From a safety prospective - the cockpit probably holds just as much water behind the seat as it does infront of where this bulkhead would sit and I’ve been thinking of putting a second rear bulkhead to form a sealed day hatch compartment. I have already put a day hatch but it is only to access the cockpit when seated with the skirt on. I’ve been resisting the idea only because my bilge pump would not fit there if I put a second rear bulkhead, but perhaps I should do it anyway…

Thanks for the thoughts so far…

ok, now we’re getting somewhere
Sonoma 13. Not a rec. kayak. My suggestion is to throw out the idea of a hard bulkhead and make one out of 3" minicell that you can jam past the foot braces and stuff up up against some glued in minicell blocks so it can’t go any far forward with the foot braces keeping it from sliding back. . If you like how it works glue it in some day but if you cut it just right and cut come flexible flanges on the outer edge it’ll keep water out during most rescue situations. You can thread a 3/16 rope through it to the other side and knot it against a large piece of plastic so when you pull it out the rope doesn’t tear the minicell.

I should probably say remove the foot braces, install bulkhead, then reinstall footbraces to hold bulkhead in place.

That’s what I did for a Mariner Express for a couple years as float bags just didn’t keep out enough water.

When I say flange around the out edge it’s just like some production boats where you see that there’s a recess cut by a router about 1/2" in from the edge and about 1" deep. The same is done on the other side and it makes for a better fit.

To get the size right you pick where the aft edge of the 3" bulkhead will sit then get a bunch of 3x5 cards and staple them together folling the outline of the exterior of the hull from top to bottum. That 1/2 contour of the kayak should work bay flipping it over for the other side. It’s probably worth checking on the kayak to see if there’s any assymetry or whether you’ll have to make another half for the other side.

What about this?

– Last Updated: Jun-10-09 3:16 PM EST –

Whoops - this was suggested just one post above, I deleted text, have to leave the sub-thread alive, since some people replied with technical suggestions

Hmmm fun question!
Typically ABS boats are bonded with a methacrylate such as Plexus. Epoxy OK, but in this case I think I’d use some EVA foam bonded in with contact cement and beaded each side with Lexel. Rough up the ABS, and hit it quickly with a torch. Apply contact cement to the bulkhead edges and ABS surface. Place it, clean up smudges and apply a bead of Lexel to each side at bearing surface.

I think think this would provide a reliable and cheap bulkhead that would enhance flotation and be a nice fit / job. Would not create any stress risers either.


Good idea BTW and add a round hatch as well. Bond with methacrylate or epoxy. Lexel is great for sealing under deck fasteners etc.

Trick for cutting: Take some lead or silver wire and wrap it around the outside of the kayak where you want the bulkhead. Twist it tight and gently remove it. now trace the inner edge with a sharpie onto the foam. Average out any slight waves in the line and cut the foam. Test fit and trim as needed.

Salty - re: EVA
Isn’t EVA too soft to use as a reliable push pad for my feet while paddling? I have not handled EVA so I may be mistaken. If it is strong enough, how thik you think it needs to be?

Thanks for the tip on the wire. I think I saw you mention it in another message and that is exactly what I used (soft wire clothes hanger worked fine to outline the outside shape of the kayak).

Thanks for the tip on soft wire. Have been wrestling with how best to shape foam to brace off the bulkhead, but so I can still keep the foot bar in place the Anas.



I think 3" EVA would
Properly glued in, that’s a lot of bearing surface and the contact cement should make a strong bond. You’re feet are right at the bearing surface, so yeah, I think so. I undertand the need to pressure the footbrace, but also know that there’s no need to overdo that, so for low to moderate pressure I think you’re good to go. You can get the foam in a stiffer durometer (sp?) as well.

curious about that one
how does a soft wire retain the correct height/width when removed? Whatever method used get a don’t forget to get a measurment for both sides of the tapered bulkhead. The routered groove(even on the forward side) can go a lot way to making a neat seal without 1/4" gaps from a rough cut job showing up.

expanding foam
There was one project where we had to make a partition, the internal volume surface was a bit convoluted, needless to say measuring each bit was out of question.

We ended up doing the following - used trash bag, blew it up with expanding foam, let it set, removed the plug, cut the plug in half ( perpendicular to impression wall), had very good outline.

Since this was not kayaking project, some adjustments would probably have to be made.

My wire was on the outside

– Last Updated: Jun-10-09 3:57 PM EST –

of the hull/deck and it retains the shape unless you bend it too much, in which case it asumes the new shape -;)

Basically, thru trial and error (e.g. small adjustments) I was able to get a very close approximation of the hull/deck shape with the wire on the outside. The cloth hanger wire I used is stiff enough to keep its shape well when moved about, yet inert enough (e.g. not too springy) to respond to relatively minor bending input from my hands or the pliers I used for the tight turns around the deck/hull joints and some of the ribbed areas on the deck.

Next step is to draw that on a cardboard piece and actually try the fit inside the boat. Have yet to do that, since for this I need to remove my foot brace and I will only do that once fully ready to build/install the bulkhead. The shape of my kayak is unfortunately rather complex with lots of little edges and at least 3 chines on each side, so I expect some tweaking of the cardboard piece before I am comfortable with the shape. Using a foam bulkhead also allows for easy re-shaping and a perfect fit is not 100% needed - some compression is possible and some gaps can be filled with adhesive...

Also having thermoformed vs. rotomolded material will help with the adhesion. I forgot about that…