Flotation foam

Redtruck12

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Jan 25, 2018
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i have been trying to find good information regarding flotation and am undecided on what to do. I’m thinking of a combination of rigid foam board and expansion foam for my 17 ft 74 Duo project.
My thought are that when it comes to flotation “more is better.
2 part flotation foam seems to go in in blobs but not fill the cavity ( at least what I have seen)
. I am also thinking of gaping it off of the hull to allow it to drain IF it were ever to get wet.
i will admit my experience with this is pretty much “0” but I am always trying to think of improving original designs.
Please share your thoughts and experiences🤥
 

Woodonglass

Supreme Mariner
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Dec 29, 2009
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25,924
If the 2 part foam is installed correctly, and under the right conditions, it WILL fill the void below the deck entirely. Most poor fill jobs are due to Installation errors. The issue of drainage has been discussed/debated, ad-infinitum here on the forum and up to this point NO GOOD SOLUTION has been found. That's not to say there isn't one, we just haven't found one YET!!! IMHO if you seal the deck properly and then maintain it properly there shouldn't be any water getting under the deck. Nothing significant anyway.
 

Redtruck12

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Jan 25, 2018
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I agree completely on the deck sealing and of corse nothing beats no water getting in.
From your experience does the foam add any advantage to the hull( sound deadening,stiffness etc.) Or is it really strictly just flotation?
 

kcassells

Fleet Admiral
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Oct 16, 2012
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8,576
All the above for 2 lb. 4lb is then going into structural needs.
US comp website gives a great breakdown on the the different foam per lb type.
 

Redtruck12

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Jan 25, 2018
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Secondly if it was completely sealed how do KNOW it’s sealed?
If it’s not sealed I guess the boat will eventually gain weight, which of course equals trouble!
so some seem to hope and attempt a perfect seal, while others make drainage holes. Foam without airflow will never dry!
drainage holes may allow water to drain out of the flotation cavities BUT also allow water in from the bilge channel.
just trying to get my head around different design ideas.
 

Woodonglass

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If you do a proper job of tabbing the deck to the hull sides, AND you always predrill and seal any holes you put in the deck then it will be sealed. A breach in the hull will be quite evident and should be repaired quickly. Wet foam almost always occurs due to neglect or POOR Workmanship. Factories DON'T do as good a job at glassing as you will.
Do the job right and you won't have to worry about it. All foam is primarily there for flotation. the 2 part Pour In foam Does add to structural support (even 2lb foam) and Quiets the hull significantly.
 

zool

Captain
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Aug 19, 2012
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When production boats are built, they MFG isnt concerned about longevity, they pour in X amount of foam to meet Y amount of regulations. Thats the reason for the blobs. If the boats lasted forever, you wouldnt need to buy another boat after 10 years.

Back yard builders dont have to meet those regs, but do a fairly good job with just well placed pour holes.

Some do very thorough jobs just weighing the deck loosely down after pouring, then remove the deck, shave level and filling in the gaps. Any remaining air gaps also add flotation inherently.

Larger boats like mine never had any foam, just air compartments sealed from above, with limber holes to channel water to redundant pumps, which IMO, keeps things dryer in any condition and also keeps the boat afloat for a similar time frame.

If your boat has stringers and bulkheads from hull bottom to deck, and reasonably spaced, then the foam isnt needed for structure and can be skipped altogether.
 

Redtruck12

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Jan 25, 2018
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🤔Maybe I am overthinking it (wouldn't be the first time)😏
I guess I keep seeing poor /assembly line work and forgetting that my rebuild (with Iboat forum member help) will be far superior to how it was dome at the factory.
My foam board scheme likely would have not stabilization and questionable sound dampening. Also I cannot find any rating for boyancy , only insulating value😕
I figure I will need to fill 2 triangular chambers 6 feet long,1 1/2 feet wide and about 5 inches deep .
so I figure that works out to about 4 1/2 cubic feet total. Or about 33 1/2 gallons.
Also work out to about 300 pounds of flotation vs 300 lbs of water
If foam expands "up to 30 X (20-30 times) then I'm needing a little over a gallon of foam to fully fill my cavities?
am I figuring correctly?
 

Woodonglass

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Given your dimensions of 72" x 18" x 5" that gives you 3.5 cubic feet per triangle. So a Total of 7 cubic feet. The expansion foam is rated at how much 1# will fill when fully expanded. 2# fills 1 cubic foot soooo you'll need a 16# kit to fill the voids. With shipping it'll cost about $100 bucks. I believe that flotation ratio is about 60-1. So the amount of foam you'll be adding should float approx. 750-900 lbs. As stated. You DON'T have to use it but it IS a good $100 insurance policy
 

Redtruck12

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Im just trying to follow you calculations.
I calculated as a rectangular tank, figuring it is the same as 2 triangular tanks
72x18x5 = 6480 cubic inches (3.75 cubic feet)
2lb foam is Rated at 63lb. / cubic foot
3.75 X 63 = 236.75 pounds of flotation (that would be total not per side) and assuming zero voids.
We seem to be a long way apart in our caculations😉
Even using your calculated 7 cubic feet (7 x 63 = 441)

As for expansion
1 cubic foot = 7.48 us gallons
3.75 cubic feet = 28.05 us gallons
so if 1 gallon of foam expands 30x that would be 30 gallons (expanded)

not understanding what you mean by "16# kit"???

just trying to understand how much I will likely require and what I will gain from it.
my 236 lb and your 750 -900 lbs of flotation are a huge difference.
 

Dennischaves

Chief Petty Officer
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Nov 9, 2016
Messages
439
I agree with woodonglass 100%
i completely filled underneath my deck with #2 foam
and I did my best to seal the deck so water never gets under there
the problem is not water from the ocean or the lake
it’s from standing rain water and the hull not sitting angled deck in the trailer so rain water will run out through the drain
 

Redtruck12

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I assume you are referring to 2lb foam?
how big are your cavities and how much foam did you require?
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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Im just trying to follow you calculations.
I calculated as a rectangular tank, figuring it is the same as 2 triangular tanks
72x18x5 = 6480 cubic inches (3.75 cubic feet)
2lb foam is Rated at 63lb. / cubic foot
3.75 X 63 = 236.75 pounds of flotation (that would be total not per side) and assuming zero voids.
We seem to be a long way apart in our caculations😉
Even using your calculated 7 cubic feet (7 x 63 = 441)

As for expansion
1 cubic foot = 7.48 us gallons
3.75 cubic feet = 28.05 us gallons
so if 1 gallon of foam expands 30x that would be 30 gallons (expanded)

not understanding what you mean by "16# kit"???

just trying to understand how much I will likely require and what I will gain from it.
my 236 lb and your 750 -900 lbs of flotation are a huge difference.

30 gallons / 2# = 15. They dont sell a 15# kit, they sell a 16# kit

Foam is sold by the pound, not by its expanded volume.
 

Redtruck12

Petty Officer 1st Class
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Jan 25, 2018
Messages
344
Where are you guys buying?
what do you mean by 15# or 16#???? 15 pound, ????

i see foam sold by liquid volume not weight. I must be looking in the wrong place🤔

looking on shop Iboats, I only see an evercoat kit that looks like 2 quarts?..possibly 2 gallons ???? Does not say.

i have been getting most of my supplies from Noah's marine, noahsmarine.com
they sell;
3 kits - 2 quart
- 2 gallon
- 10 gallon
 

kcassells

Fleet Admiral
Joined
Oct 16, 2012
Messages
8,576
Look at us comp website on foam. They have great description on qty/use. You can still order Iboats or wherever but they will answer all your questions.
IE:
[SIZE=-1]8. How much do I need to order?[/SIZE]
[SIZE=-1]You will need to calculate the volume of your cavity by multiplying the length x width x height
Example: 2 Ft x 2 Ft x 4 Ft = 16 CUBIC FEET

If calculating in inches convert all measurements to inches
Example 24 inches x 24 inches x 48 inches = 27,648 CUBIC INCHES. Now to convert to cubic feet you much divide that number by 1,728 which equals 16 CUBIC FEET.

See [/SIZE]
[SIZE=-1]FAQ question 10[/SIZE][SIZE=-1] to determine volume on a cylindrical type shape such a pontoon.[/SIZE]
 

Dennischaves

Chief Petty Officer
Joined
Nov 9, 2016
Messages
439
I had a 19’ Glastron tri hull
it took 5 kits of 2 lb foam
and pay attention to the air temp
it does make a difference on how much it expands
 
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