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Need some advice on foam

Old 04-09-2019, 02:28 PM
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Default Need some advice on foam

Good afternoon, I have a 18ft 1954 Custom Duracraft Aluminum boat that I am restoring. I am at the phase of putting the flotation foam into bottom of the boat. I have watched so many videos on the different foams people are using. When I acquired the boat all it had was Styrofoam pieces all shoved up under the decking. I have seen a lot use the pour foam and I have seen a lot use the purple 2 inch insulation foam from the local hardware stores. A few points I have seen is the pour foam is great but a pain to remove down the road and it also burns pretty well if you had to weld the boat from a crack or something. The purple foam in my eyes is for the house not a boat but I am not expert at all. The purple foam I think could be ideal in my situation since the vertical stringers don't have a way to bring the water to the bottom of the boat and with the foam it could restrict the bottom of the water path return. I just think the purple wouldn't work if I was to poke a hole in the boat and it took on a lot of water.

So im at a wall of what product to use under the deck. I know there are plenty of reads and videos but I would like to really get some opinions on these foams.
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Old 04-09-2019, 02:43 PM
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Color means nothing with regard to performance of the foam. Its all isocyanate foam unless its actual Styrofoam, (the white stuff or usually white, they sell pink, blue and green).
Almost all the iso foam for insulation that you would find in a store is what is called closed cell. Should not absorb water.
The pour in place foam if you chose to use, should be closed cell for flotation. Some call it 2 pound.
All of them burn or melt very easy, so that does not matter.
Any of them will float. Depending on where you have access and where you want the foam to be will determine which one makes sense for you to use.
Any of them will work. However, if you want to add some rigidity, pour in place is will help with that.
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Old 04-09-2019, 03:05 PM
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So thinking about this I would need to fill almost 45 cubic feet, estimated and pour might be easier. Since some of the support joists to tack the decking down will have a screw protruding the opposite side. I think that would cause and issue with the closed cell (weak point). What would be your thoughts on on lining the voids with this ( FOAMULAR 150 2 in. x 4 ft. x 8 ft. R-10 Scored Squared Edge Rigid Foam Board Insulation Sheathing) type of foam then doing a pour over to fill in the void? I think it will still give me the rigidity but also help by removing some of those weak points from the main closed cell foam. Thoughts?
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Old 04-09-2019, 04:16 PM
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First off, do you need the foam?

Foamular requires epoxy not to Melt it, so that, along with 45 ft3 is expensive. Formular comes in about 8 densities and is available up to 5" thick.

2 lb/f3 of pour foam will require about 3 gallons, and that will also require some kind of sealer.
4lb/ft3 resists water absorbsion better, but its 2x the cost, and of course you are adding 180 lbs of unwanted weight to your hull.

Be careful not to confine the pour foam as its expansion force can push out walls, decks, etc.
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Old 04-09-2019, 05:28 PM
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Bullshipper I did not know about the sealer for the pour foam. Do you have a reference of a sealer for that application?
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Old 04-09-2019, 06:19 PM
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Originally Posted by jtullous View Post
Bullshipper I did not know about the sealer for the pour foam. Do you have a reference of a sealer for that application?
4 lb shouldn't need it and I believe that is the most practical material to use under the water line and deck.
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Old 04-09-2019, 07:13 PM
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any foam scored means the foam cells have been cut/compromised, very little, but will allow water absorption to that limited small area, not a big deal but something to consider. As bull mentioned above, do you need the foam? and if so, how much. If its only for flotation you do not need to completely fill the boat. Tons of old aluminum jon boats only had a little bit of Styrofoam filled in/under the seats. Just enough to keep it barely floated if flooded. Guess the big question is, what benefit are you looking for from the foam?

Also, Its not an all or nothing thing. You can just fill select voids for floatation and/or rigidity.
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Old 04-09-2019, 07:20 PM
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Pour foam in direct contact with the aluminum can lead to serious corrosion if the boat is ever used in saltwater.

Rarely does a manufacturer use anything but 2lb foam, most are even lighter.
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Old 04-09-2019, 08:16 PM
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I dont plan to put this boat in salt water at all. So the corrosion should not be a problem. I plan to put foam into for flotation and piece of mind if I accidentally run up on something that tears a hole or something in it. With a hole it still floats well. 45 cubic was an estimate it probably in reality more like 30 to 35. I know that the pour foam is expensive I estimated about 400 bucks. I'm not worried about the cost for that. If it last 10 years i'm happy with minimal water saturation. I currently also run an 85hp motor on this thing as well and only rated for 75. So i am taking that in account as well in this whole thing. Yes I know that can be bad but I have to take that in account as well. Now Bullshipper mentioned the sealing of the pour foam, which I haven't seen, and I haven't seen in any videos on people sealing the foam. Now why would i opt to use a 4lb over a 2lb in the bottom of the boat? Would that just create more weight? Or does it have a different property I am not aware of? I know if a closed cell is cut open there is a week point of water to get in so I wouldn't be shaving this stuff either or cutting down.
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Old 04-09-2019, 09:57 PM
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The higher the density, the more water resistant it is, and the stiffer it is after setting up. 6 lb can be used for stringers, 2 is usually for flotation, but 2 is also not resistant to gas and a lot of stronger chemicals.
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Old 04-09-2019, 10:07 PM
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Corrosion is an issue even in fresh water with the poured foams. I used blocks of polystyrene for mine which allows water to escape and the alloy to dry out when not in use.
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Old 04-10-2019, 03:40 AM
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Originally Posted by Bullshipper View Post
The higher the density, the more water resistant it is, and the stiffer it is after setting up. 6 lb can be used for stringers, 2 is usually for flotation, but 2 is also not resistant to gas and a lot of stronger chemicals.

2lb meets all the Coast Guard requirements, which means it’s required to be gas and oil resistant, that’s why 99.999% of the boat builders use it. Many use a 1.65lb version.
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Old 04-10-2019, 06:30 AM
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For your boat which is aluminum, there is no real structural benefit of higher density foam, so lower density foam is cheaper in the long run.

I would not pour expanding foam directly into a 1958 model year restored aluminum boat. The pour foam will stick to anything it touches, and servicing the hull under the foam will become very challenging, if not nearly impossible. Given it was built in 1958, to me it seems likely to need ongoing maintenance that other boats may not need.

The Foamular 150, which I believe you refer to as the purple foam available from home stores, is about 1.6 PCF and is an XPS material. Some of the other insulation materials are EPS and polyisocyanurate, they are all supposed to be closed cell, all will provide flotation, but they are not the same.

A while back I did a calculated cost comparison of the Foamular 150 to 2PCF pour foam from US Composites.
1) The Foamular 150 purchased as a 4'x8'x2" sheet for $37.83 (Home Depot?) is about 5.3 CFT and is $7.09/CFT and provides about 343 lbs of buoyancy, if you use the whole sheet and cut with zero kerf.
2) The US composites 2PCF poured foam 40 CFT kit at $264 ends up being $6.60/CFT and provides about 2560 lbs of buoyancy.
3) The US composites 2PCF poured foam 8 CFT kit at $67 ends up being $8.38/CFT and provides about 512 lbs of buoyancy.
4) The US composites 2PCF poured foam 2 CFT kit at $22.50 ends up being $11.25/CFT and provides about 128 lbs of buoyancy.

I personally would use the XPS / EPS hardware store foam, cut to fit and placed in. Distribute it around so you can try to maintain level flotation if the boat swamps.

Also figure out how much buoyancy you actually need. If you put 45 CFT of pour foam in your boat, you will have well over 2500 lbs of buoyancy. Does your boat even weigh that much?
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Old 04-10-2019, 06:32 AM
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Originally Posted by ondarvr1 View Post



2lb meets all the Coast Guard requirements, which means it’s required to be gas and oil resistant, that’s why 99.999% of the boat builders use it. Many use a 1.65lb version.
Then I guess the foam that absorbed water and was melted was something else in my Seaswirl years ago.
I also looked at some specs a while back, but perhaps I am recalling that incorrectly. I thought it just met their standard for flotation
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Old 04-10-2019, 03:20 PM
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The problem is that it breaks down over time, it eventually becomes like an open cell sponge. 4lb just lasts longer because the cells are smaller.
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Old 04-10-2019, 03:28 PM
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I would not be using a pour-in foam on an aluminum boat, due to corrosion concerns down the road. Use the styrofoam blocks or sheets, or the (any color) Extruded polystyrene sheets (XPS) sold as rigid home insulation. (pink, blue, whatever . . . )
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Old 04-10-2019, 04:13 PM
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Originally Posted by bluevein View Post
Color means nothing with regard to performance of the foam. Its all isocyanate foam unless its actual Styrofoam, (the white stuff or usually white, they sell pink, blue and green).
Almost all the iso foam for insulation that you would find in a store is what is called closed cell. Should not absorb water.
The pour in place foam if you chose to use, should be closed cell for flotation. Some call it 2 pound.
All of them burn or melt very easy, so that does not matter.
Any of them will float. Depending on where you have access and where you want the foam to be will determine which one makes sense for you to use.
Any of them will work. However, if you want to add some rigidity, pour in place is will help with that.
some do call it 2 pound. I only use 4lb because I like the extra rigidity and it is “ tighter” than 2lb.
my advice for foam is USE IT! I can save lives.
hope your project goes well!
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