Bulkhead foam

I am replacing a bulkhead where the old tabs have started to lift and some rot.

I would like to put the trapezoid shaped foam in-between the plywood bulkhead and hull.

What foam do I use, is it available as a tape?

Thanks,

 

Tape ? You mean a flexible foam ? Then you are going to tab this bulk head in ?

 

Yes, The flexible foam. I will be tabbing in the bulkhead.

 

Welcome Ralf

Why are you adding the foam spacer?
Was it present from the factory build out?
Does your hull suffer from a "hard" spot?

 

Nothing wrong with the trapezoid filler to avoid a hard spot, but rigid foam will do the job.

 

Replacing the original plywood bulkhead. It was tabbed direct to hull, had some gaps, part of the bulkhead was only tabbed on one side.

No foam in original build, I did not see any hard spots on hull.

Replacing new plywood, use of foam spacer at plywood to hull fit, as this seems to be “proper” fit.

 

Proper fit may be too tight. You might want to post a photo.

too tight will telegraph through thin foam hull overtime

 

The only thing that matters is that the plywood doesn’t contact the hull. What you use to space it off the hull doesn’t matter much.

Most boat building companies don’t use anything, most don’t even space the plywood off the hull. Others just leave an open space under the plywood.

The foam is used as a convenient shape to space the plywood off the hull, and create a radius or shape that’s easier to glass over.

Old used diapers would serve the same purpose, only not as fun to work with.

So don’t overthink the process of creating the gap between the hull and plywood.

 

That trapezoid shaped foam is called a pad and cut to that shape. You can use marine foam or you could use a piece of xps as long as you tape it well with glass tapes.

It is typically made from marine foam offcuts, but it would be silly to buy a piece of marine foam for that.

 

Hi Ralph and welcome to the forum.

I'll post links to a couple of old threads (over a decade ago). These discussions had to do with stringer and floor fabrication, what you're calling a bulkhead. It might give you some food for thought. I fabricated my engine stringers and all of my floors out of 1708 biaxle mat draped and formed over 2" thick Owens Corning Formular 250 extruded polystyrene foam insulation board.

I didn't use a foam pad under plywood. I made the entire stringer/floor assembly out of the Formular draped with multiple layers of the 1708. I did fab up some triangular pieces that installed next to the stringers and floors. This helped the cloth drape nicely and form a smooth transition from the vertical stringer to the inside hull of the boat. There are photos in the links below.

I've been using the boat since 2014 and it's seen a lot of service over the past 6 years. Those stringers are as solid as they were in 2009. As for bonding to the inside of your hull, stringers or floors are done identically. Good Luck.

Yet another Stringer Post! https://www.boatdesign.net/threads/yet-another-stringer-post.28361/#post-288054

Stringer And Deck repair https://www.boatdesign.net/threads/stringer-and-deck-repair.29459/#post-302773

 

Just for the record, you can use mat with epoxy. Those threads mia linked go into all sorts of legalese [sic] about using mat tapes with only poly. total bs. The truth is mat does not break down in epoxy, but mat helps tapes keep shape for an 8 oz per sqyd penalty.

Foamular will be fine for the work. I like to abrade it with 40 grit on all sides if I am trying to keep the glass attached. It won't matter for the OP.

 

Good points fallguy......
For me the take away from those old posts was that while chopped strand mat can be used with epoxy it's not the best way to do it. Paul Ricelli's (PAR) point was that by using the 1708 biaxle stitchmat I built stringers/bulkheads that were certainly strong enough but more expensive and heavier than they needed to be. By using the mat/cloth combo I used much more resin than would have been necessary had I simply used cloth alone. I would have needed more layers to get the thickness I wanted but my resin usage would have been much less.
I also found that a lot of folks don't realize that the foam or plywood or whatever you use to drape the fiberglass over really adds nothing to the strength of the structure. It's all in the glass/resin.

 

well, not entirely right mia

if you use a ply stringer; it does add

if you use a cardboard stringer, you must add more glass

the glass over a hardwood or ply stringer is largely there to encapsulate it from ingress AND the reason for so much stringer rot is a few things; poor workmanship or polyster resins, post build holing, etc..

I had an old boat wth stringers poorly covered in poly; they were made from solid mahogany and had neary zero rot as they were so poory covered they could dry out. Of course, the real trouble was the end grain balsa core had eroded beyond economical repair so good stringers did not matter..boat was crushed

For the OP, they make sure water can't get to the ply BH.

 

I couldn't agree more about the poor layups & polyester causing so many issues with stringer rot. As for a plywood core adding strength we'll just have to disagree. If you look at the photos in the links I added above you'll see I added 4 good plywood inserts to my foam stringers. This was done to increase compressive strength since the engine beds bolt to the stringer in these places. My understanding is that the core material of the stringer itself adds absolutely no strength to the assembly. This was the reason that I made the stringers 2 inches wide (wider is stronger) and made a smooth radius where the vertical stringer meets the inside of the hull. That radius transmits the stringer loads to the hull much more easily and eliminates stress points in the laminate. I think it was Dave Gerr in one of his books that wrote about laying up hollow stringers. While not necessary, he said that you could layup glass over a foam core and then use solvent to dissolve the core. Provided the laminate was sufficient for the load your hollow core stringer would work fine. All loads are carried by the glass.

Regards,

MIA

 

The plywood may or may not contribute to the strength, it depends on the design.

It can be as used a low cost core/shape that isn’t intend to supply any support. Or it can be used as the primary support for the structure with a coating of glass to protect it and hold it in place.

And it can be anything in between.