Polyurethane foam water absorption

[Mr Efficiency]

I have heard a few horror stories over the years about poured bouyancy foam absorbing water over time. I can't say I have come across much supporting evidence, but the rumours persist. What is the truth here ? Does using a denser grade of foam protect against such possibilities ?

[Steve W]

Its true,when you pour the expanding foam it cures with a skin which seems to resist water but if that is compromised it sucks up water like a sponge, i have seen many examples of this.
Steve.

[Herman]

The foam is water tight, do not worry.

But it is not water-vapour tight. So moisture gets in, and condensates, never to come out again.

I have fixed a couple of boats with water logged foam. Basicly it is splitting the hull from the deck, or whatever is holding the foam, removing the foam (a smelly, messy job, but luckily the foam will be degraded as such that it is not that hard to remove, you can dig it away with your hands). Kilos and kilos of foam and water. I once did a 4 meter boat, which had gained 145 kgs. (over the total weight of 85 kg which the boat was when new)

PU foam? On board only for refridgerators, please.

If you do feel the urge to fill a cavity with foam: make sure no water can enter from whichever side. Foam is no replacement for not being able to find a leak.

[bulk-head]

Water and freezing conditions,Ice, are particualary hard on foam. Keep it watertight.

[Mr Efficiency]

Seems like Polyethylene foam might be a safer alternative, or will someone give it the thumbs down ?

[CDK]

There is a PU foam formulated for underground use like sewage systems and wells. It has 95% closed cells so doesn't absorb a lot of water, just a bit. But it cannot cope with freezing conditions: the ice in open cells damages the adjacent walls.

Styrofoam has no problems with water intrusion, maybe a mix of Styrofoam balls and PU foam is a suitable compromise.

[Mr Efficiency]

Ice shouldn't be a problem where I am, if it is Global Warming is a false alarm. The styro beads mix is new to me, how do you achieve that ?

[Steve W]

What application? as CDK said,the good old cheap styrofoam does not absorb water so if you can cut and fit chunks it will do the job. In the US the Macgregor line of sailboats all have full floatation in the form af every available space filled with styrofoam blocks. I had a Mac 36 catamaran which had enough so that "sunk" was a mere annoyance. I had one compartment which the rudder tube passed thru which had a leak for years before i bought it and when i removed the foam to repair it the foam was fine. On the other hand i have had exactly the same experience as Herman with the PU foam.
Steve.

[Herman]

On another forum I sometimes visit now we have a guy that found out his Sailhorse (5.5 meter sporty daysailer) is 100 kg (220 lbs) overweight. Water logged foam...

Also a nice one is rudders: Many rudders are just a polyester shell, filled with polyurethane. Boats in the winter are hauled ashore. Winter strikes, and CRACK, there goes the rudder.

Due to the heavy frost very early this winter, I expect good business fixing rudders this spring.

Originally Posted by bulk-head

Water and freezing conditions,Ice, are particualary hard on foam. Keep it watertight.

You cannot keep it watertight when the foam is open to vapour, thats nonsense! And PU is open, as polyester is. Hence the massive problems, some encounter with "cheap" products.

The poured PU has another problem. Due to its brittleness it does not hold the bond to the structure and falls off, or even becomes just crumbles.

[Herman]

I do not agree.

A compartiment, closed completely in sufficiently thick polyester (yes, even polyester, Richard) can survive without taking unacceptable amounts of moisture during the life span of the product. (30 years?)

Adhesion of the PU foam is not a problem. It adheres to about anything. If you break it (and yes, it will break if loaded dynamically) it will show PU on both surfaces.

[poleguy]

Originally Posted by apex1

You cannot keep it watertight when the foam is open to vapour, thats nonsense! And PU is open, as polyester is. Hence the massive problems, some encounter with "cheap" products.

The poured PU has another problem. Due to its brittleness it does not hold the bond to the structure and falls off, or even becomes just crumbles.

???? What are you talking about ? Have you experience with boat construction ? Do your boats naturally leak ?

I regularly observe 15 year old boats fitted with pour foam PU buoyancy tanks that show no sign of moisture or dis bonding . Atmospheric moisture is insignificant.

Positive flotation...buoyancy, is the law and in many cases pour foam is the only way to achieve it. You do boat owners great disservice by spreading poorly conceived opinion concerning this viable option. .

If PU is being considered for buoyancy, use best craftsmanship to insure that the void being filled is indeed watertight and will stay watertight. Areas which have hull perforations, particularly the center line of a hull and areas aft in the area of engine installations are best left free of foam. Using foam under sheer clamps , foredecks, seating , and off center line in the chine area under the sole has proven to be very reliable and greatly improves stiffness and sound deadening . .

[Herman]

Tell that to the people that use PU foam to "solve" leaks. I certainly believe PU foam has a use, when used properly, (for me, not in boats...) but there are so many builders that make a total mess. I think even more builders make a mess of PU foam, than of installing cores.

Water + PU foam = trouble. Do it right, or don't do it at all...

[Steve W]

Poleguy,you cant be sure that a compartment where you have installed pour in foam will not leak at some point in the boats life,as an example thousands of runabouts and fishing boats in the 16 - 20 ft range have a plywood sole with pour foam under it,i have repaired plenty of these where they have been damaged by the trailers keel rollers and the foam was waterlogged,the soles are not usually tabbed to the hull anyway so these areas get wet anyway. Small dinghies/tenders often have air tanks fore and aft for bouyancy which works fine until the forefoot wears away from people running them into the beach before disembarking, and/or the skeg wears through from people dragging them up the beach, now those air tanks which were once air/water tight are no longer providing bouyancy,if they were filled with pour foam they would be waterlogged,if they were filled with styrofoam blocks/peanuts in a mesh bag etc all would be fine until you fix it. Im not a big fan of air chambers for bouyancy for the same reason as the pour foam, you can not be sure it will remain protected from water or vapor.
Steve.

[TeddyDiver]

Originally Posted by poleguy

I regularly observe 15 year old boats fitted with pour foam PU buoyancy tanks that show no sign of moisture or dis bonding . Atmospheric moisture is insignificant.

Reckon your experiences are from sunny florida.. so little you know..