Foam in hull?

[kd6nem]

I've see a lot of posts with folks asking advice about putting foam under the floor, etc. Is this desired mostly from a structural/soundproofing point of view or for flotation? If flotation, is there actually enough bouyancy to keep the boat from going down if totally swamped? I know Boston Whaler advertised their boats as unsinkable and proved it by taking a chain saw to one. But doesn't their foam run all the way up the sides of the hull? I'm wondering whether the floor alone would give enough displacement to offset the weight of the hull, motor, batteries, etc. I suppose it could be calculated (better than pulling the plug in the middle of the lake to see what happens, right?). I'm trying to decide what the formula for calculating this should be. The volume of foam would displace an equal volume of water. Would the equivalent weight of displaced water be the figure to compare to the weight of the entire boat as it sits on the water (less trailer, in other words)? Or perhaps I am missing the point as to why the foam is there in the first place. What is the best way to view this concept?

 

[rickdb1boat]

Re: Foam in hull?

Floatation is the first and foremost purpose of the foam. Any benefits after that are a plus. More info:<br /><br /> web page

 

[flydangler]

Re: Foam in hull?

Most of the USCG approved two part foam I've used gives you about four cubic feet of foam per gallon of mix. Each cubic foot of this foam provides about 62 lbs of positive bouyancy. If used properly it can also provide some structural reinforcement, but that isn't real easy to get just right.<br /><br />What you've got to watch out for is the expansion of this foam. When the two parts are mixed it expands to about four times the volume of the mixtures. I've seen folks pour large amounts of this stuff into confined spaces and being unpleasantly surprised with the force exerted by the expansion process.

 

[18rabbit]

Re: Foam in hull?

Any boat required to be unsinkable must be so with passengers aboard. However, I am fairly certain the requirement does not apply when cutting your boat in half. In a nutshell, a totally swamped boat must stay near the surface with payload. Figuring most of the boat will be below water, you can think of it as everyone sitting in a floating saltwater Jacuzzi … just not nekked or having very much fun … not that you have to be nekked to have fun but one does seem to follow the other… <br /><br />2-lb foam seems to be commonly used for floatation. As stated above, each cu-ft of water displaced by foam yields 62 lbs of lift, LESS the weight of the foam. So 1 sq-ft of 2-lb foam nets 60-lbs of buoyancy, 4-lb foam nets 58-lbs of buoyancy, etc. The idea is to float the boat AND the max payload (the motor, people, ice chest full of beer, batteries, etc). The max payload should be on the boat’s certification label.

 

[JasonJ]

Re: Foam in hull?

Usually the under floor volume is enough to keep a boat at the surface. You have to remember that an object that weighs a certain amount on land is lighter in water. If you have an object that is one cubic foot and weighs 65 pounds, you can say it has neutral bouyancy as one cubic foot of water weighs 65 (ish) pounds. That object will stay at the surface. It will not sink or float. Obviously if the 1 cubic foot object weighs less than 65 pounds, it will float, and if it weighs more, it will sink. Objects displace water when submerged, so if you have a boat that weighs 1400 pounds on dry land, that boat, even submerged will weigh less due to the various parts of it that is displacing water. Say the submerged boat displaces 600 pounds of water, or about 9 cubic feet, then the manufacturer only needs to add enough foam to displace the remaining water needed to keep the boat at the surface. They also account for the boats payload minus the average displaced water volume and weight by the payload (people, gear, whatever). When you think of it this way, you start to see that it does not take much foam to keep the boat at the surface. Also, boat manufacturers will spray foam up under decks, gunnels, all sorts of hidden areas you never know about, to further displace water. Boston Whalers can't sink because they are nothing but foam, they have extreme positive boyancy. Now, if the foam starts to absorb water due to age and leaks, then the weigh goes up, which lessens the foams effectiveness. Hope this helps a bit....

 

[kd6nem]

Re: Foam in hull?

I should mention that this is a theoretical issue to me at this moment, but the info will be filed away for possible future use. I like to know a little about the ins and outs of various aspects of what I choose to get involved with. Comes in mighty handy at times.<br /><br />OK, the numbers mentioned are convenient. Thank you. Since the floor of my boat remains intact and in place, I'm not going to try to figure out just how much net bouyancy might be gained- that will wait till whenever I do need to repair the deck so I can measure the space down there. I really wonder whether the average boat has enough room under the deck to get enough of the stuff in there to keep it from going to the bottom should enough bad things happen? It seems that it would be a good question to ask myself before starting that project. I, of course, have NEVER done anything so silly as fail to figure out all the details before plunging in with all four..... (at least not with a boat floor yet)

 

[kd6nem]

Re: Foam in hull?

JasonJ, you answered while I was typing. All you guys had good answers. Thank you.

 

[edandq]

Re: Foam in hull?

Well, as I read through these old posts, all your posts just make me want to ask more questions! When I removed my floor (replacing floor on 16' 1970 Starcraft), it had a lot of styrofaom, in long lengths. It was old and dirty, but still in tack and not water-logged. Should I replace this under the new floor, or employ another method (cost being an issue). I know there is some respect to boyancy, but also noise, and it is an aluminum boat.. Thoughts?

 

[JasonJ]

Re: Foam in hull?

I guess it falls under the "if it ain't broke, don't fix it" category. If the foam has stayed dry, I can't see why you can't clean it up and put it back in. Free up some loot for other things.