Types of Foam Under Deck

[Quik Fix 16]

When I was making skiffs, we put bottles everywhere we could. Chop over to hold in place and styrofoam, cut to fit, in heavy plastic garbage bags.

 

[leaky]

I'm wondering if the best solution is to fill a void with plastic bottles, temporarily hold them in place with a piece of hardware cloth, then pour foam over them. The foam would fill the voids between the bottles, the hardware cloth would allow you to pour foam through it as well as stop the bottles from floating in the foam before it cured.

With bottles held in place with foam, if you were to have a catastrophe where you run up on some rocks and the hull is breached with a big chunk taken out, all your bottles won't just float out and you loose all your buoyancy when you need it most. Plus the foam will keep them from rattling around and if the foam was to somehow get waterlogged, there won't be as much of it in there to become heavy. Also, the foams not cheap so it would be good to use less of it and this way you'd end up with even higher buoyancy.

Maybe but the pour foam is actually cheap.

 

[Quik Fix 16]

Nothing has more buoyancy than an air pocket.

 

[leaky]

Nothing has more buoyancy than an air pocket.

That is true, but kinda like that pic of the soaked keel foam, an air pocket needs a way to be cleared should water get in there.

 

[MAArcher]

Nothing has more buoyancy than an air pocket.

I don't think anyone disputes that. The problem is if there's a hole in it. The same hole that allows water to get in and saturate foam is the one that lets the pocket fill up with water. At least if there's foam there, you won't loose all buoyance unless the foam is 100% waterlogged, which from what it sounds like just won't be the case with the newer closed cell foams. I doubt the Coast Guard would require it for small craft if the benefits didn't outweigh the faults.

 

[Quik Fix 16]

If an air pocket is truly an air pocket, no water can get in. Air would need to be displaced.

 

[MAArcher]

If an air pocket is truly an air pocket, no water can get in. Air would need to be displaced.

If no water can get in, then why not have foam in there? It only adds a little more weight, but if a rock or metal channel marker punches through your hull, you'll still float. The only argument I've heard against foam is that it can get waterlogged over time and make repairs harder than the would be without the foam. But if the argument is water's never going to seep into the air pocket, then foam will never become waterlogged and always be there for that extra safety factor, right?

My 22 foot boat has ten bulkheads under the deck running port to starboard, there's no longitudinal stringers. If I hit a rock and the hull was breached at one of the bulkheads, that would be two of the eleven compartments compromised and I'd loose almost 20% of my buoyancy just like that. If they are foam filled I'd only lose some smaller fraction.

I can understand thinking that the risk of having to deal with soggy foam at some point outweighs the likelihood of having to depend on the foam for floatation at some point, that's sort of an individual risk assessment like whether to wear a life jacket at all times or not, but I don't think you can say a boat is safer without foam than it is with it. And on a small boat like a 21' pointer, I think the safety benefits far outweigh the small risk of causing repair headaches years from now.

At least that's how I'm looking at things, I'll have to make the decision "to foam or not to foam" in the next next month.

 

[MAArcher]

Maybe but the pour foam is actually cheap.

If I have it figured out right, to 100% foam under my deck, I'd need about $1,375 worth of foam. A 2 gallon kit of 2lb foam produces 8 square feet of foam (actually less because I can't wait until its 80 degrees out to do it). I'm estimating 88 square feet of void below deck. Divided by 8 = 11-2 gallon kits at $125 = $1,375. So if using free bottles cuts that by 2/3rds, that's a $900 savings and you could argue you end up with a better product.

 

[Genius]

Nothing has more buoyancy than an air pocket.

a hydrogen pocket may

 

[Genius]

If I have it figured out right, to 100% foam under my deck, I'd need about $1,375 worth of foam. A 2 gallon kit of 2lb foam produces 8 square feet of foam (actually less because I can't wait until its 80 degrees out to do it). I'm estimating 88 square feet of void below deck. Divided by 8 = 11-2 gallon kits at $125 = $1,375. So if using free bottles cuts that by 2/3rds, that's a $900 savings and you could argue you end up with a better product.

If you are worried about sinking, maybe you shouldn't go to sea?

 

[leaky]

If I have it figured out right, to 100% foam under my deck, I'd need about $1,375 worth of foam. A 2 gallon kit of 2lb foam produces 8 square feet of foam (actually less because I can't wait until its 80 degrees out to do it). I'm estimating 88 square feet of void below deck. Divided by 8 = 11-2 gallon kits at $125 = $1,375. So if using free bottles cuts that by 2/3rds, that's a $900 savings and you could argue you end up with a better product.

So as far as cost, here is a ballpark from the local guy:

Merton's Fiberglass & Marine Supply: Floatation Foam & Cores

10 gallons, $450, 45 cubic feet if I've got that all right, somewhere ~$900 to do your entire hull.

But let's do some math first, I don't think you need or want to do your whole hull. There is probably a formula for this stuff, I'm just going to work through it in layman's terms.

Think about what makes an object float. It floats if it's less dense than water. That density is 62 lbs a cubic foot (salt water I think is less so, but that ballpark anyway). Water is referred to as a density of 1.0 - it's the universal density everything else is compared to basically. So why does wood float? Well it's a ballpark of 30 to 50 lbs a cubic foot, < 1.0. Water technically I think is 1 gram per ml or CM^3 - but we need not get into all that.

You only need enough flotation to keep your boat above water. Things like fuel, they float. The wood in your hull floats. Without even counting what is already positive buoyancy in your hull, that weight, it can be disregarded (although it actually reduces the overall need for foam)..

The fiberglass doesn't float, windows/glass doesn't float, metal doesn't float. Out of those, how much foam is needed is not a direct weight to foam calculation - it's dependent on the density of those things.

Fiberglass parts for instance, hand laid, has a density in the ballpark of 1.2, to an absolute max of 2.0. A round worst-scenario number for fiberglass part density is 120 lbs cubic foot, although some might use 100 lbs as the estimate. How many cubic feet of foam does it take to float a cubic foot (or 120 lbs) of fiberglass then? Just enough so the average density, weight per cubic foot, or however you want to calculate that, is less than water. 1 cubic foot of foam per cubic foot (or 120 lbs of) fiberglass more than does it then - ie a cubic foot of foam, plus a cubic foot of fiberglass, weighs like 122 lbs, less than the 124 lbs of 2 cubic feet of water. Would you use a little extra foam to be sure it actually floats above the water somewhat, sure, but that's the formula.

Sooooo if your hull has about 1500 lbs of fiberglass in it - that's 12.5 cubic feet of glass, needs at least 12.5 cubic feet of foam to buoy that.

Aluminum is a density of about 2.7, or 168 lbs a cubic foot. Takes about 2 cubic feet per 168 lbs then to buoy your outboard.

In short - sure you might want to use 30 cubic feet of foam, make sure the boat floats above the water, based on what is convenient, where it fits well, etc.. But you do not need 88 cubic feet of it, as far as I know anyway, to assure your boat is unsinkable.

 

[Quik Fix 16]

Plastic bottles are air pockets.

 

[MAArcher]

Plastic bottles are air pockets.

So are the bubbles in foam

 

[John Allin]

Leaky is an intelligent man...

 

[TwisterRI]

I know you know what you are doing more than I, but I get skeptical when I read pour foam referred to in a structural context..

Are there more structural ones out there at a higher price point maybe than the 2 lb I've just been playing with?

My impression was, the stuff is damaged kinda easily, doesn't really adhere well even to a prepped surface - basically if you had something cored with it, once those skins move from a stress the foam is permanently damaged, versus say a foam core material like corecell that bounces right back (to a point as well of course, just can take an infinitely greater change in shape before its destroyed too).

None of this changes how suited it is for the positive floatation aspect. And at a low cost too.

yes there are different pourable foams, the more dense it is the less expansion and the stronger they are. 2# is just a cavity filler not structural at all.

 

[leaky]

yes there are different pourable foams, the more dense it is the less expansion and the stronger they are. 2# is just a cavity filler not structural at all.

Wow I just looked into that and you can get it all the way up to 16 lb density. Although that'd certainly end up way too heavy for use as flotation, I can see how it'd be potentially really really structural - ideal in some situation. And I see 4, 6, and 8 lb as well.

 

[Quik Fix 16]

So are the bubbles in foam

I give up, y'all should fill the entire hull with foam.

 

[AtoZ]

Just another version that I am working on. French fries!

 

[John Allin]

Just another version that I am working on. French fries! View attachment 155402

I don’t think you could put enough foam in there for positive floatation.
Just keep a raft handy in the event your feet start getting wet.

 

[AtoZ]

My bad, I’m taking that out. Gonna leave it open. Let it breath. That was heavy with water. There is always a cooler to grab on to if needed!