Foam or Gap between bulkheads and hull?

[Rich P]

Do any of you subscribe to the strip of foam between the bulkhead and the Hull, or leaving a gap so as not to form a hardspot. To me it seems sensible in theory, but in practice its hard enough getting the piece of wood cut accurately anyway, let alone adding a foam strip thats going to push it all out again!
What are your thoughts on this?

[Hirilondë]

The goal is to avoid the hard spots. The foam itself does nothing once the tabbing is complete. What it does is assure you have no contact between the bulkhead and hull during the lay up. If you can fit the bulkhead well (uniformly to the hull), or assure that there is no contact (hard spots) during the lay up, then it isn't needed. How necessary this is, is a matter of opinion and/or your skill.

.....let alone adding a foam strip thats going to push it all out again!

Huh? What is going to push what out?

[Rachel]

I think Rich is saying that he gets the bulkhead cut to fit well without the foam, and then the addition of the foam pushes the bulkhead "out" away from the hull (since the bulkhead was cut to fit tightly without the foam).

Do I have that right, Rich?

If so, then perhaps you could cut the bulkheads to fit less perfectly/closely, so that you will have room for the foam/air gap. If I read it correctly, Tim uses a few dots of hot glue (from a glue gun) to hold the works in place prior to the fiberglass tabbing going on (since there are only a few, you can just leave them there when you tab).

Rachel

[Rich P]

Hmm well generally speaking the part thats touching the hull is a very good fit for me, so i'll probably leave it like that.

Your correct in your assessment Rachel, problem is with just me, i can't hold a bit of foam in place and then the bulkhead accurately on top of it, whilst maintaining it square and marking of where to cut the other side!

[Idon84]

There are defiantly different ways to make sure there is not a hard spot, but for me, I have found that using 1" blue foam is the easiest for me.

I use a tick stick to get the general dimensions of the bulkhead, I then cut what I measured out of cardboard and test fit. When done right you just about a perfect bulkhead from the tick strip. Trim as necessary to get a close fit (not more than 1" off against the hull for the foam). Once that is done I use a piece of cheap scrap ply and cut out the bulkhead from the cardboard pattern. I mount the part with sacrificial cleat stock (cheap hemlock for home trim) to other bulkheads in the boat to keep it steady. I use scrap wood and foam to put against the hull to get the bulkhead exactly where I want it (level, plumb etc). Then I take a piece of the 1" foam and bend it to the shape of the hull right next to the bulkhead (sometimes with big parts I tack it with hot glue to keep it against the hull). Then I trace the curvature of the foam against the plywood. Take the bulkhead out, trim to that line, tack foam with hot glue back onto the bulkhead and test fit again. When I'm happy with the fit I then make the bulkhead out of Marine Grade Ply, add 3 coats of neat epoxy to the edges. After the cure I firmly attach the foam to the edge bulkhead which is sealed by the 3 coats of epoxy, attach the cleat stock again to the other interior bulkheads and tab to the hull...

Hirilonde said,

The goal is to avoid the hard spots. The foam itself does nothing once the tabbing is complete.

After it is bonded the foam could dissolve away for all I care as it was there to create fillet and to keep the bulkhead off the hull while tabbing.

If you do have a tight fit as Rachel mentions, great! Now just temporarily mount it where you want it. Secure it so it doesn't move, make sure it is level. Lay your foam right next to it and trace the line onto the plywood and you should have a perfect fit!

Just my 2 cents. But I've been quite busy this year doing the same job.

Good luck!

[Idon84]

Here are some pictures...

[Rachel]

Bryon, I'm curious: Once you've got the bulkhead fitting properly, and if you know the foam is, say, 1" -- why couldn't you just cut 1" off the appropriate edges of the bulkhead to account for it?

I know I'm probably missing something obvious here, so I'd like to know what it is :)

Rachel

[Idon84]

When I get a bulkhead fitted just right the edge that is against the hull is not always the same distance from the hull so just cutting 1" from that measurement would create high and low spots. Sometimes the gap is 3/4" sometimes only 1/4". When I fit the bulkhead the 1" of foam doesn't care about the little gaps and I can trace the exact line onto the bulkhead. My tolerance for imperfection is then 1". As long as the bulkhead fits with less that 1" between the hull I have no issues.

The forward bulkhead in my picture above (the first one I built) had 1 3/4" wide gap along the bottom 10", so I had to add some thickened epoxy to compensate for the 3/4" that the 1" foam didn't cover. After that issue I made all subsequent bulkheads to fit tight against the hull and then I used the 1" of foam as my guide line. Worked wonders since.

I hope that answers you question...

[Rachel]

Thanks. Now I understand why you do it.

I didn't think those sorts of gaps were an issue once the tabbing was on, but then I say that never having fit large bulkheads. I've only tabbed other/smaller stuff.

Appreciate the explanation and photos.

Rachel

(Edited to add: Of course there's a certain satisfaction in having things fit well.)

[Oscar]

i can't hold a bit of foam in place and then the bulkhead accurately on top of it, whilst maintaining it square and marking of where to cut the other side!

You could use self stick door seal......

[Rich P]

Cheers for the info guys!

[Triton106]

Byron,

Seems like you use the same blue foam for bulkhead as Tim. Do they come with trapezoid already cut or do you have to cut to size and shape? In any case where do you get them? I cannot seem to find them anywhere on web. Maybe I am not googling with the right name.

Thanks and regards,

[Idon84]

1" foam (the brand I found is from DOW) from the hardware store sold in 4x8 sheets. I run them through the table saw... Takes a bit getting use to but once you figure out the technique it goes quite fast.