How does one cut cushion foam?

[grampianman]

I am re-cushioning my boat; v-berth and a double berth. I have purchased mildew and water-resistant foam, 4 inches thick, and now have to cut it to the original foam shapes.

I was thinking of using the sharpest knife I own to cut the straight lines, but thought I would put it to the forum's experience first. Will a knife tear the sponge? Would it be better to compress the foam, then cut?

I did look on the forum and found the hints of how to put the foam into the covers using wax paper or garbage bags, although I thought of dry-cleaning bags as they are some of the slipperiest bags around.

I look forward to your thoughts,

Cheers,
Ian
Wing & a Prayer

[dmairspotter]

I have to say that I have never had good results cutting any kind of thick foam with any sort of knife. It always comes out jagged. I think the production way to do it is with a hot wire.

Might not be too hard to rig up with a 12 volt battery and a length of nichrome resistance wire.

[Summersdawn]

An electric turkey knife. Or a bandsaw. The foam can be "sanded" on a bench sander.

I haven't done this myself, but have seen it done. The trick would be to practise on a scrap first.

[Jason K]

Go with the electric knife. If you use a regular knife, you'll get a very ragged edge as you saw through the foam.

Most fabric stores that sell foam use an electric knife as well.

[Tim]

The electric carving knife works fairly well, but you can still get a ragged cut if you're not careful. The blades on these tend to be pretty smooth, though, so with a little practice you can do quite well.

Serrated bread knives also work; typically they leave a ragged cut as well, but you can get something acceptable for cushion foam. The key is to make fine strokes.

We cut all the foam for our boat with these two tools. It's not perfect, but it doesn't need to be either, since the foam is covered. The main thing is to get the shape just right. Minor roughness of cut doesn't make a huge difference.

You could also compress the foam with a piece of plywood or a 2x4 and cut it with a utility knife or some such, at least for short, straight cuts, like cutting fiberglass insulation.

[Quetzalsailor]

Commercial foam sellers cut their huge (5' x 4' x 8') blocks of foam with appropriately huge bandsaws.

Your bandsaw will work just fine, but speaking from experience, take the time (that I did not) to build/cobble together a big enough slippery table to slide the foam through on. (no project here, probably polyethylene stapled to plywood would do; the professional's tables are plastic laminate) I tried various knives with a great lack of success. A belt sander or even an angle grinder works fine to clean up the mess or for finer shaping.

By the way! Foam comes in several degrees of stiffness. Pick one that will float your hips off the settee surface. And that means it'll probably be stiffer than the original. Another trick you might consider, which worked for us, was to have the new cushion tapered, thicker under the knees, thinner at the butt. Makes a more comfortable seat, and for us, worked to transition to the bunk filler I made for between the settees on the Morgan 27.

[Hirilondë]

I used an electric carving knife. It is probably the best tool that's cheap. You can still get a corduroy effect but it can be controlled to where it is minimal. Batting can do wonders to hide it as well. The technique seems to be close to what works on a band saw. Don't over steer and let it flow.

[CharlieJ]

We moved my small (12 inch) bandsaw up onto our front deck and cut all the foam for inside the boat in one evening. If you have access to a band saw, that would be the smoothest and quickest way- you can even cut angles to match the sides up in the Vee berth using the band saw.

[grampianman]

Many thanks for all of your prompt suggestions. I'll be taking photos and will let you know how it all turns out.

Thanks
Cheers,
Ian

[Robert The Gray]

I used the electric carving knife with some success. Creating a stable work surface and good blade angle control are the keys to good cuts. For the beveled sides of the v berth hull edge, where the angle can change as you go bow to stern, I suggest creating two templates from the old cushion, one for the bottom and one for the top. You will end up with two curved lines. To see both lines at the same time have the cushion either on edge of off the edge of a table. Go slow. I kinda straddled the thing and used my hand on the back of the blade to guide obth ends ofthe knife.

The foam can be wrapped with a lighter foam batting. If you wrap it twice you can get a softer look to the cushion. On the triton I cannot sit up in the fore peak berth to read. My head hits the overhead if I am leaning against the mast beam bulkhead. I also do about 7 - 10 nights of over night sleeping a year, the rest are day sails. I never sit on the v-berth cushions I just lie. I went with the low density 4" foam for the fore peak and the best high density foam for the high traffic seats in the salon.
I also feel that the boat surfaces are so small that I do not mind using high end long wearing furniture fabrics on the areas that contact the eye or skin but cheaper sunbrella like acrylics under the cushions. No cottons, but rayons silks. both can be rinsed of salt without to much shrinkage. I have been thinking of storing my v berth cushions off the boat this summer until I go on my august cruise.

r