Which foam for stringers

[JohnC]

I am rebuilding a 77MA 20 and I want to re-foam the stringers. I assume 4# foam would be the best bet to fill the box stringer but I'm not sure. US Composites has 8# available but I believe that's a but overkill. I believe the stringers will be around 12 cubic feet or less so weight of foam is no big deal; the cost of the foam goes up though.
The rest of the space under the deck will be 2#. I will be cutting as much of the top of the stringers out as is needed to remove the old stuff and then re-glassing.

I would rather ask then assume! Is there a reason to use one foam over the other? Thanks.

[Bushwacker]

The foam adds very little strength; it's only there for flotation. The glass carries all the load so I'd go with the lowest density foam you can find, provided it's closed cell foam.

[gofastsandman]

Originally Posted by JohnC

I am rebuilding a 77MA 20 and I want to re-foam the stringers. I assume 4# foam would be the best bet to fill the box stringer but I'm not sure. US Composites has 8# available but I believe that's a but overkill. I believe the stringers will be around 12 cubic feet or less so weight of foam is no big deal; the cost of the foam goes up though.
The rest of the space under the deck will be 2#. I will be cutting as much of the top of the stringers out as is needed to remove the old stuff and then re-glassing.

I would rather ask then assume! Is there a reason to use one foam over the other? Thanks.

You don't need to cut the tops off. just drill 3 inch holes every few feet and blast it out
with a pressure washer. Messy but fast and you`ll save lots of time glassin`.
I wouldn't foam under the deck unless you use epoxy foam. The stringers will keep a swamped hull afloat.

Be real careful using that foam in this heat. You have about 45 seconds before it kicks now and it takes
about 20 seconds to mix... Go with multiple small batches. Buy some dollar store plastic cups to mix.
You must mix exactly 50/50 so use graduated cups for the measure and pour into dollar store cups.
Walk from your mixing station to the hull whilst stirring quickly like a rabbit.

You can only clean dribbles for about 2 minnows so tape/plastic off areas of danger Will Robinson.
It has tenacious grip after about 2 minnows.

You also have to pour in the shade as hot surfaces will cook the foam and stop the expansion.
Bet they didn't tell you that at US composites...

[JohnC]

Thanks for the tips! I have used foam before and I know it's a rapid process. I didn't know about the heat cooking it though. That's very good to know as my boat is in the sun at this time.
I will be putting foam forward of the fuel tank and outside the stringers. I see the advantage of no foam (nothing to get waterlogged) but I have decided I want the extra flotation in areas I can seal off from the bilge. Project stalled due to work and funds (bought a fixer upper house a while back) so there is still time to change my mind but that's my preference. I should be done with my house and back to the boat by end of June.

[gofastsandman]

Originally Posted by JohnC

Thanks for the tips! I have used foam before and I know it's a rapid process. I didn't know about the heat cooking it though. That's very good to know as my boat is in the sun at this time.
I will be putting foam forward of the fuel tank and outside the stringers. I see the advantage of no foam (nothing to get waterlogged) but I have decided I want the extra flotation in areas I can seal off from the bilge. Project stalled due to work and funds (bought a fixer upper house a while back) so there is still time to change my mind but that's my preference. I should be done with my house and back to the boat by end of June.

Closed cell foam is only closed until it is not.
You get water outside of the stringers as well in the invisible bilge that used to drain into the bilge until you stopped that egress.

Your swamped turtled hull will just float a little higher...

[JohnC]

Originally Posted by gofastsandman

Closed cell foam is only closed until it is not.
You get water outside of the stringers as well in the invisible bilge that used to drain into the bilge until you stopped that egress.

Your swamped turtled hull will just float a little higher...

I was on a friends Ryborunner a few years back when two bilge pumps failed. It seems that someone had done some electrical work in the console and didn't clean up after themselves. The ends of the zip-ties found their way to the aft bilge and got stuck in the pumps impellers (Both of them!). The livewell drain had come loose under the deck, effectively pumping water into the bilge at 500 GPH. We noticed the boat started feeling heavy sitting on the anchor and started checking things out. When we discovered the problem, the water was a foot or more high bow to stern in the bilge and the 30' boat felt like is would tip from the weight of two guys standing on one side of the boat - very unstable! Dry boats are stable but partially swamped boats are relatively easy for a wave or even a mistake at the helm to finish off. Disaster was averted by a manual bilge pump mid-ship. I decided then that I would rather have foam under the deck than air, especially on a boat that may spend a day or two in the water. Foam can get heavy, but not as heavy as open space under the waterline. Most boats turtle because of unstable load after filling with water. I think there are pros and cons both ways but I prefer to displace as much water from under the deck as is practical.

[Mshugg]

These threads always seem to devolve into two camps:

You need the foam for flotation or the foam will break down; skip it. The foam for flotation croud goes a bit far sometimes suggesting billets of EPS, empty soda bottles even ping pong balls as a sub for foam.

The thing is, the foam was there not just for flotation. It also provided structure in the same way that the center web of an Ibeam provides structure. Original spec for your stringers was 4lb. 2 pound everywhere else. Some folks leave the flotation foam out, but I would never leave it out of a box beam stringer.

Yes, the foam will eventually absorb water if subjected to prolonged water exposure. But it doesn't have to. Just protect it from water. The foam in the stringers of my 1978 Aquasport was dry.

Replace the foam or not, but it does provide more than flotation.

[gofastsandman]

Originally Posted by JohnC

I was on a friends Ryborunner a few years back when two bilge pumps failed. It seems that someone had done some electrical work in the console and didn't clean up after themselves. The ends of the zip-ties found their way to the aft bilge and got stuck in the pumps impellers (Both of them!). The livewell drain had come loose under the deck, effectively pumping water into the bilge at 500 GPH. We noticed the boat started feeling heavy sitting on the anchor and started checking things out. When we discovered the problem, the water was a foot or more high bow to stern in the bilge and the 30' boat felt like is would tip from the weight of two guys standing on one side of the boat - very unstable! Dry boats are stable but partially swamped boats are relatively easy for a wave or even a mistake at the helm to finish off. Disaster was averted by a manual bilge pump mid-ship. I decided then that I would rather have foam under the deck than air, especially on a boat that may spend a day or two in the water. Foam can get heavy, but not as heavy as open space under the waterline. Most boats turtle because of unstable load after filling with water. I think there are pros and cons both ways but I prefer to displace as much water from under the deck as is practical.

Limber holes are nice to have and I like access with vision.
High water alarms are your friends.

[JohnC]

I think in our situation the limber holes were what allowed water to leave the flooded bilge area and fill other spaces under the deck. It's rare to lose bilge pumps so getting water into the bilge area is almost always a good thing. Water alarm is a must! It doesn't do much for an unattended boat at dock but it is definitely a must and would have tipped us off of a problem when it was more manageable.
Foam provides flotation in all situations but there us a down side to it, as with most things.