Why foam?

You don't necessarily want the foam in the bilge anyway, it might turn the boat upside down when full of water. On the sides and up toward the gunnel is good. You're trying to make the boat float level and upright when full of water, so you're not clinging to a slippery bottom or where the motor sinks and the boat is straight up and down and you're left clinging to the bow.

Some people pack spaces with plastic soda and water bottles (with the caps on) for flotation.

Polystyrene dissolves in gasoline and a few other things.

 

I will never forget inspecting a glass boat that hit a large log, it was busted from bow to stern, though the speed couldn't have been more than 25 knots, the installed power wouldn't have pushed it any faster. Watertight compartments would not have saved this boat from sinking, the damage was everywhere you looked, and it wasn't a flimsily built boat by any measure. What did save the occupants was block PS foam, otherwise they'd have been like the boat, a write-off. In watertight compartments i do not trust !

 

Some will say 90% of the time watertight compartments will suffice, it is not the 90% you worry about, but the 10%, that is what will do you in. I know people who have had a boat sink they "thought" had sufficient watertight compartments, whether there was insufficient volume or their integrity had been breached we will never know, the boat is on the bottom, luckily there was another boat nearby to witness their sinking.

 

Thanks fellas, all concepts with merit and a bit to think about now. I have roughly 1 cubic metre of polystyrene foam. Prolly not enough to float the boat. But with the sponsons I could fit something workable

 

1 cu metre = 1 tonne of fresh water, a bit more of salt water, work it out from there, sounds like it is all you would need for a 4.3 m boat, then some.

 

ive heard of people using the plastic balls in a play area. like 3 inch all colors. but like bottles how you going to contain them in a major accident. they will float out a big hole. im torn my self on this. I may
use sheet foam laminated in large blocks and sealed with epoxy. you can put forms in the hull for drainage and cover with plastic and pour the foam in. then seal it with epoxy. so ive been told

 

The bottles can easily be contained in a mesh net, I don't think they will escape that way, if the whole lot floats off your boat has probably been hit by a smart bomb.

 

I have been looking at the gunwales and other cavities like decks and things. Shaping polystyrene to fit would be pretty easy and the purbond will swell and hold it nicely if it doesnt melt it. Otherwise epoxy

Combination of Sponsons and the styrofoam should be plenty I hope. Damn if I had that sheet of PE foam here in Fiji I would have fitted that to all the deck surface and gunwale sides. Cushion and flotation!

 

Why foam. Most boat builders use foam, in particular two part pour foam (or spray foam), because it is easier to inject into compartments when building boats on an assembly line. The compartments can be any shape. If using block foam the foam has to be cut to fit in the compartment which is labor intensive, and the top of the compartment has to be open until the foam is in. This works well on small aluminum boats that use boxes under the seats, but does not work well for fiberglass boats. So you see block foam on some aluminum boats but rarely on fiberglass boats.

Having tested hundreds of boats for flotation, it is true that some foam absorbs water. But the problem is almost 100% confined to two part pour foam. It rarely occurs with pre-manufactured block foam.

Again, why foam? The US Coast Guard requires certain types of boats to have flotation. They do not tell them what to use, except if they use air chambers, the boat must pass the flotation test with the two largest air chambers punctured. The exception is boats under two HP or manually propelled. So again it is easier for the manufacturer to just pump foam into the chamber. But they could use anything that traps air and provides flotation. I have seen milk bottles, soda bottles, ping pong balls, beer cans, inflatable bladders and other unique ways of providing flotation.

Does foam float better than an air chamber? No. Air supports 64 lbs / cu. ft or 1025 kilos per cubic meter. If you use foam then you have to subtract the weight of the foam. So it supports slightly less, but it is not significant.

On the boats I have built and on my 18 ft 1972 Sea Ray I used block polystyrene, 2 lb density foam sealed in plastic sheet. I bagged it. Water can't get in so it will not degrade. It will probably last longer than the boat. But I should mention that some of the original foam was still good. I removed it because about 75% had absorbed water. But two factors were involved. One was gasoline had leaked into the foam (it really smelled of gas) and the boat had been sitting with water in it for years. The sole and underlying structure had rotted out as well. So while replacing the sole and supports it was easy to also replace the foam.

 

Thats a classic straw man argument... I could show you dozens of boats that survived huge collisions with no bouyancy foam, only sealed compartments. I could also show you dozens of boats where the waterlogged foam has caused rotting of the timber beneath it, not to mention all the discusting smells and mould that breeds in that environment. Foam for emergency buoyancy sux IMHO...

I can also show you many modern boats, particularly the more upmarket and better designed models, both private and commercial that dont use any bouyancy foam, only sealed compartments...

At the end of the day, the problem of reserve buoyancy in a catastrophe can be achieved many ways - you cant simply say foam is the only way to go - there are many alternatives...

 

Straw man my aarse, the owner of the wrecked boat wouldn't be enjoying his retirement now, but for those foam blocks, he'd have been a dead man. It is your increasingly dogmatic assertions that "suck" !

 

Wow, are you really going to make it personal over this Mr. E?

This is a forum, im entitled to my opinion and i offered it accordingly.

You sensationalize your anecdotes with stuff like this;

And yet you follow me around this place with something to say about everything i post, then have the audacity to say

I suggest you take it up with the man in the mirror...

 

Sensationalize ? Not at all, just factualize. There are instances where watertight compartments just don't do the job, it is up to the individual to work out whether there is an extra level of safety to avail themselves of, and I include in that retro-fitting of old boats, and foam is a lot easier to install than watertight compartments, and praying they remain watertight is not needed. Anyone who goes to sea deciding they don't want foam because it might get waterlogged, really needs a good fright to help them realize its value, especially since this supposed vice is easily circumvented. The commonly used life jackets employ a type of foam, there are inflatable vests that are better than nothing, but they must remain watertight to work, it is an analogous to the situation with boats, of chambers of air, versus buoyancy materials that are not subject to being rendered useless by penetration.

 

I suggest you use plastic soda bottles (I would add glue to the threads on the lids before tightening them), then foam the soda bottles in place with two-part pourable closed-cell foam. This significantly reduces the volume of the expensive foam used, recycles the soda bottles, reduces waterlogging possibilities, keeps the bottles in place, reduces issues with solvents, little to no penalty in weight, etc.

 

Use the plastic acid bags available from lead acid battery supply centres.
Blow them up with a vacuum cleaner in place and cap firmly. Tough and resist damage if floatation space ruptured. I have used them in all my boats over the last 50 yrs with no problems. .