View Full Version : Foam under deck?

G Jacobson
11-17-2003, 12:49 AM
Hello all, I am new to the forum and from what I have read so far I should have checked it out long ago! Anyway, here is my situation. I am building a 17’ vee hull plywood skiff of my own design (actually it’s based on a superb design of my dad’s.) The hull is nearly ready to pull off the jig and I must now consider how I am going to finish the interior. My concept is a very simple, open plan, for a utilitarian fishing boat. However, there are a few important decisions to make before I get started. I would like to have a sealed floor and raised fore deck with foam underneath. Unfortunately, I am having serious concerns about long term water and rot issues in that I can’t believe anything can be truly “sealed off” regardless of how much epoxy is used (I’m using West System.) So tell me, am I being paranoid, or are there serious problems with this method? Any input would be appreciated. Also, my apologies if this is a redundant subject.

11-17-2003, 01:57 AM
What kind of construction method did you use? Is it all epoxy/ply or are you just using goo here and there?
If it's all nails, screws and neat fits then putting goo anywhere is generally a bad thing. It has to stay ventilated to last.
However, if it's wood reinforced plastic (my favourite) then you can pile in the foam anywhere you like. Do a search on the forum using keywords like foam and bouyancy - a few regulars have wrestled with the problem and even posted photos!

11-17-2003, 10:30 AM
If the foam is for flotation, than the only time you really need it is if the boat sinks. If that happens, heavens forbid, you'll be ripping the boat apart to remove the soaked foam. If the boat sinks with no foam, you'll take her up and let things dry out and that's that. If the boat never sinks, you have the added advantage of air circulation in there which is a good thing.

11-17-2003, 02:52 PM
Gaffman, if the boat sinks without foam it may not be recoverable at all. Or did I miss something. I also think that in the unlikely case of a sinking or capsize it may save lives or at least make the boaters more comfortable as they will of course be wearing their PFDs.

I personally like the idea of positive flotation But to allow some airflow would be inclined to use the blue styrofoam stickered like a wood stack.


Bruce Hooke
11-17-2003, 03:02 PM
One thing for sure -- don't use beadboard (the white stuff with little "beads" that stick to everything)...I seem to recall pulling some of that out of a boat and finding that after 25 years or so of knocking around in the bilge it was heavy enough to SINK on it's own!!! This was in a boat that had never been swamped but the foam was located such that it was sitting on the bottom of the boat and so could soak up any bilgewater that was present.

11-17-2003, 08:07 PM
The two part poured foam will also absorb water over a long period of time. This is a common problem with older fiberglass and aluminum boats. However, it is often used in plywood/epoxy boats. the key is to keep the foam, and your bilges, dry.

G Jacobson
11-18-2003, 08:05 AM
Thanks all for the input. To answer Aramas, all plywood and structural elements are bonded with epoxy. I also intend to seal the interior wood surfaces with epoxy and exterior surfaces with glass/epoxy. I did a search on “foam”, and to my surprise there does not seem to be a “standard” method or approach although the 2 part liquid foam is popular with some. There are also a lot of opinions regarding rot issues in watertight compartments whether foam is used or not. One thing I did not read (or may have missed) is discussion of the strength benefits provided by foam filling in between plywood layers. I would think the strength to weight characteristics of “foam sandwich” construction would be almost as important as the floatation benefits. A few years back I made a pair of “boogie boards” for body surfing using 1 ” blue board insulation material sandwiched with epoxy between 1/8” lauan. I have been amazed at how strong these boards are given the flimsiness of the materials used and their light weight. Similar weight to strength characteristics should apply to boat construction using foam bonded between hull and deck. I’m thinking that if the wood is properly sealed with epoxy (perhaps thinned for better penetration) then this may be an acceptable approach for a boat that is in the water a few times a week at most and never overnight. Of course, I don’t like the idea of having to dig out foam to access parts of the hull for repairs, but that is of minor consequence if you can make it home after punching gaping hole in your hull on a submerged piling. Some useful information I did find was that standard styrofoam (white beaded) does indeed absorb water (hydroscopic) where the blue board foam actually repels water (hydrophobic.) This is according to Dow Chemical so I think I will go with blue board.

Bruce Hooke
11-18-2003, 09:52 AM
There's no question that such a "laminate" or composite structure such as you described could be very strong. I can think of a couple of reasons why it isn't done more often.

1. There's not a lot of data on the specs for such a composite. For regular plywood construction it's relatively easy to figure out what thickness of plywood to use because so many similar boats have been built in the past. For a composite such as you describe you will need to do a lot more work to figure out what layup of materials gives you the right properties.

2. It seems to me that it really only makes sense as a replacement for thicker pieces of wood. If the piece of wood being "replaced" by such a composite would only need to be 1/4" thick, for example, then as a composite you would likely be talking about something like 1/16" plywood on each side of the foam, which would be very prone to puncture damage.

3. You need to think carefully about where you can use such a composite. Many of the parts on a small boat are most vulnerable to puncture damage, which a composite like the one you are talking about would not be very good at resisting.

4. As you noted, repairs could be considerably more "intersting."

5. The look would mostly likely be distinctly non-traditional, unless it was very well hidden.

The obvious place where such a composite might work well is for thwarts: the risk of puncture damage to thwarts is relatively low, they normally need to be rather thick because they must resist a bending load, and if they fail it will not cause the boat to swamp or sink so seat of the pants engineering would be a reasonable way to go.