To access the forward bulkhead from the cockpit, put the boat upside down on horses, and sit on a bucket to reach inside.

i have a night heron with foam bulkheads.   but i do have hatches.....

first step for me was to clearly identify where i wanted the bulkhead's cockpit-facing-face to be and marked it with tape.   this way i could clearly know what was the right position, how far in the bulkhead needed to be.

my second step was to cut the bulkhead using as my form, a section/form to bigger side of the line i marked on the previous step.   i then used a rasp to make final adjustments to the bulkhead (make it a bit smaller and angled properly) with a lot of in-and-out test fits.

when it was pretty close to where i wanted it to be.... 1/2 inch left to go...i was able to push it into the final position with a little section of 2X4 wood that i ran along the inside of the hull and used to push/compress the foam while it moved the last 1/2 inch.... 

i then went in with silicone and did calked around the edges (between the hull and the foam bulkhead).    it ended up very neat and waterproof.

now the challenge i think you will have is without a hatch, it is very difficult to get the bulkhead in and out while you are test fitting it.  also, you will never know if you accidently got some water on the other side of the bulkhead.....you would probably just discover it when it starts rotting out your boat.

my view is i will only put a bulkhead if i have an access point.  another option, if you are worried about flotation, is to use a float bag that is shaped to be put in the front of a kayak that does not have a compartment.   if you google "kayak float bag bow" you will see lots of options.

my final comment is when i did the bulkhead i had the boat upright but on a low sawhorse and while not really comfortable, for me this position allowed me to reach my head in comfortable to see what i was doing.  also...kayak foot peg rails  had to be removed during the fittings/installation.

hope this helps

h

Hello Matt - Congratulations on the build.  Post pictures when you can.

On one of my builds, I installed bulkheads after the deckwas on and it was a royal PITA.  For me the best way to work is with the boat upside down on sawhorses and me either sitting or knealing with my head in the cockpit.  A seat that is the right height and foam to kneal on are both key.

As Howard says, it will be much easier if you can use the form closest to the BH location as a pattern to cut the BH.  If yot, it will be a tedious process getting a good fit.

Instead of the tape trick that Howard uses, I glued small wooded blocks to the inside of the hull for the BH to rest against when pushed all the way in.  Not only will this make sure the BH is in the right location, it will also provide some support on the backside.

I share the concern regarding lack of access with a closed compartment.  Water will eventually get in there and you must have some some way to get it out.

My other concern with your plan is that you will only be able to seal one side of the foam BH.  No doubt that you can make it relatively water tight but It may not be strong enough to support the weight of a cockpit full of water.  All of that weight would be pushing the foam away from the sealant which could cause the edges of the foam to tear and open the compartment to water just when you need it to be water tight.  You really need a bead of sealant around the BH on the other (non-cockpit) side to secure the BH in place.  Materials like foam are very poor in tension but good in compression, which is why the bead on the back side is crucial.  

If it were mine, I would install plywood bulkheads with Beckson hatches.  The hatches provide access to dry the compartments and the ply will be much more secure with a filet on only one side, especially with a couple of backing blocks describe above.

just to clarify a point about my installation tht mark brought up, i did use the fact that i had a hatch to put a bead of silicone on the bulkhead's bow-facing face.  a concern i had was water getting trapped between the thick bulkhead foam and the hull....and creating a rot problem.

my impression on the foam i used and how tight i fitted it was that it was actually very solidly wedged in/strong which is why i needed the 2X4 and really needed to push it hard to get into the final position.   but my ability to do all that was based on having a hatch that allowed me to push it back out and adjust it.   if i could only go in one direction, to mark's point, i would probably have to have a much looser fit and rely on the silicone to hold it in place and potentially a block preglued to keep it from going farther in then expected.

h

Mark,

I wouldn't worry about the weight of the water pushing glued foam out of place, especially since you have those blocks of wood you mentioned backing it up. Water pressure is a function of depth, not the amount of water, so assuming a 12-inch deep cockpit the most pressure you'll have is less than 1/2 PSI. at the bottom and 0 at the top.

Even if you turn the boat nose up with a cockpit full of water blocked so that it can't drain, that's still on the order of 5 PSI for a long kayak. The surface area of the foam is something like 180 in sq, so in that impossible worst case the pressure will be something like 900 lbs. But the surface area of the foam that's in contact with the hull is something like 540 sq inches, so if that entire surface area is glued to the hull, each square inch would only be resisting a force of 1.7 lbs rising proportionately as the ratio of the total perimeter area to the area actually glued. That's well within the capabilities of 3M 5200.

Assuming that the boat is not reaching for the sky, you can expect 10% or less of that force on the bulkhead. Keep in mind, too, that the force is trying to push the bulkhead tighter in place.

So whatever other issues there are with foam bulkheads, I don't think you have to worry about water pushing them out of place. Now an unfortunate accidental kick...

Laszlo

   Thank you so much to everyone who took the time to reply. This is wonderful information!

I'm going to investigate the float bags I think. This is my first boat build and I went right to the deep end here, so while my build skills have certainly improved (from their previous 0 : ) ) I'm thinking the bags sound like my most effective option.

Hey Matt,  You said that this was your first build, but you never really said where you will paddle the boat and how experienced of a kayaker you are.  If you are experienced, you probably already know all of this, but if not, there are a couple of considerations regarding floatation bags.  

First, mounting for the bags must be secure enough that they will not come out during a swim event but also allow easy removal for inspection and re-inflation.  Builders who are planning to use floatation bags usually install mounting provisions (often a pully/line system in the bow and stern) prior to deck installation.  I am not sure what I would do with the deck already on, maybe some kind of “cargo net” attached to blocks epoxied to the inside of the hull/deck.    

Secondly, the bags must be sized large enough to provide enough flotation to keep the cockpit lip above water with you sitting in the boat and the cockpit completely flooded.  Otherwise, you will not be able to pump out the cockpit after reboarding.  Rougher conditions require the lip to be higher above the water than flat conditions.

The real disadvantage of using floatation bags (no bulkheads) is the amount of water that enters the boat during a capsize.  Water floods the boat bow to stern, and you end up pumping forever to drain the boat after a reboard.  Also consider that the technique of inverting the boat and raising the bow to drain the cockpit does not really work without a rear bulkhead because the water just goes to the stern.  I speak from experience here.  I have owned one boat without bulkheads (pictured) and will not own another.  Because of the amount of water that would fill that boat, it was a 10 minute process to get back paddling after a swim.  

For Lazlo, my concern is not at all the strength of the 5200, but the tensile strength of the foam itself.  Foam, like many other materials, is much stronger in compression than it is in either tension or shear.  This is easy enough to demonstrate.  First, use your thumb to push on a corner of a block of foam, putting it in compression.  It will deform but will not fail unless extreme force is applied.  Next, pinch the same corner and give it a good tug.  It is comparatively easy to pull off a sliver of the foam.  

As somebody who worked as an engineer for a couple of decades, I appreciate the attempted calculations.  I put the routine static loads on a bulkhead at more like 25-50#, assuming a cockpit full of water with the boat raised to 30 degrees to dump it out.  For calculation purposes, you would need to add a safety margin for any additional dynamic loading.  I put the glued surface area very much smaller than you do.  I do not see any way to apply 5200 on the outer edge of the bulkhead without smearing it all off in the inside of the boat as you push the bulkhead in.  That leaves you with just a filet of 5200 around the circumference of the bulkhead to secure it in place.  This glued surface would be in tension with a flooded cockpit.  The width of filet required would be easy to calculate if you knew the Ultimate Strength (Tension) for the foam.  I tried looking in both the CRC and my grad school materials textbook but could not find a similar material.  Whatever the numbers, a single filet applied to the back side of the foam bulkhead would be more secure because the Ultimate Strength (Compression) is higher than the tension number.  Being a good conservative engineer, I would advocate for filets on both sides.  

From a practical perspective, I have no doubt that a foam bulkhead glued only on the cockpit side with a half dozen backing blocks and some of Howard’s “compression fit” force would certainly hold.  On the other extreme, I would be concerned about a loose fit bulkhead secured only by a minimal filet of 5200.  If overloaded by water (or that accidental kick during a wet exit), I could see the bulkhead breaking free because the foam itself tears at/near the filet.              

 Enough thinking for today, time for Captain Morgan.  Cheers!