I need this explained simply: how does foam attached INSIDE the boat help flotation?

[DumDum]

I'm not usually an idiot, but I'm trying to wrap my head around this: I have an aluminum boat. There are blocks of foam in the spaces between the hull and the floor decking. I get the Archimedes' principle of water displacement, etc, but if the flotation material does not come in contact with the water, at what point does it matter what that below-deck material is made of? The blocks/noodles/poured-in foam/whatever has some weight. It doesn't exert upward force, like a helium-filled balloon. So where does the actual material come into play for flotation? What happens if I pack the same space with an equal weight of books, or golf balls or Raisin Bran? The boat in the water weighs the same. Why does it float differently if that space is packed with 20 pounds of Styrofoam or 20 pounds of bicycle tires? The same amount of aluminum hull is touching the water. Who cares what is ABOVE that part?

Does the enclosed space have something to do with it? Like, if I had a simple aluminum jon boat, would it be more buoyant if I glued foam blocks to the floor inside? How do the rules of physics know that I attached 10 pounds of foam to the floor, instead of ten pounds of shoes?

Oh, my head. Can someone make this clear for me?

 

[jbcurt00]

Volume and weight

20lbs of shoes occupies little space and offer an unknown flotation value or weight it will 'support' so they dont displace much water.

20lbs of 2lb flotation occupies a great deal of volume (10CuFt), and offers a know flotation value [60lbs of flotation per CuFt (so 600lbs)]. So 20lbs of 2lb flotation foam is significantly better suited to keeping a boat level and floating above water if it becomes swamped then shoes or any of the other choices you proffered.

 

[ahicks]

The part you're struggling with is boat weight/displacement floating vs. submerged.

Sure the flotation inserted into interior cavities has weight, but it also provides flotation by assuring those interior cavities can't/don't fill with water when the boat is submerged.

The sealed air pockets/cells in the types of foam normally used for this purpose will displace more water that water soaked Wheaties.

 

[Brandon5778]

Hi, welcome to the forum. I can understand why it confuses you, but to my understanding the theory is that the foam is for an emergency situation. Capsize your boat rip your transom off, and your hull should still float.

 

[ondarvr]

As eluded to, its only function is to keep the boat afloat when it fills with water, during normal boating the foam just gets in the way.

 

[DumDum]

For some reason I had it in my mind that the foam beneath the decking played a role in the boat's normal, regular-use flotation or stability. I'm dumb.

 

[shaw520]

'Closed cell' foam is preferred,..less saturation

 

[ondarvr]

The only stuff used is closed cell, no such thing as open cell flotation foam.

 

[shaw520]

'Flotation' foam, yes,...styrofoam, or HD insulation board as indicated above would be a bad choice.

 

[jimmbo]

When the foam is mainly low in the boat, the boat if it does sink it will tend to place the foam at the top, in other words, the boat is going to capsize

 

[ondarvr]

'Flotation' foam, yes,...styrofoam, or HD insulation board as indicated above would be a bad choice.

Those are all closed cell, and can be used in some locations.

 

[444]

For some reason I had it in my mind that the foam beneath the decking played a role in the boat's normal, regular-use flotation or stability. I'm dumb.

Previous owner removed all the old flotation foam when the floor and transom were done on it. Pretty sure it was never replaced. If she goes down one day it will be to the bottom of the lake!

 

[bob johnson]

'Flotation' foam, yes,...styrofoam, or HD insulation board as indicated above would be a bad choice.

why do you say that??????

 

[JASinIL2006]

Insulation board (e.g., the pink foam board available at home centers) works great as floatation foam; if you read many restorations threads elsewhere on this site, you'll see that lots of folks use it, particularly if they don't want to deal with pour-in foam. I used the pink foam boards to build a couple floating dock sections. The dock spends five months or so in the water each season and there is absolutely no water saturation of the foam after 5-6 years of use.

 

[shaw520]

why do you say that??????

The op was asking why foam is better than "old shoes"...or "bicycle tires"... Ive seen foam become water saturated and do exactly the opposite of what it was intended for. Basically 'floatation' can only be achieved when air pockets seal out water, if the material used becomes water saturated then youve achieved the exact opposite. Empty 2 liter soda bottles with their caps glued/sealed would be a great/cheap form of floatation.

 

[shaw520]

Ive used the pink board in a couple build also and has proven to be quite water resistant,.. But even closed cell foam. once cut with a saw or knife will allow some extent of water penatration no?

 

[bob johnson]

The op was asking why foam is better than "old shoes"...or "bicycle tires"... Ive seen foam become water saturated and do exactly the opposite of what it was intended for. Basically 'floatation' can only be achieved when air pockets seal out water, if the material used becomes water saturated then youve achieved the exact opposite. Empty 2 liter soda bottles with their caps glued/sealed would be a great/cheap form of floatation.

still don't know why you think the two items you mentioned would be a BAD CHOICE..........ie Styrofoam and HD insulation board..........

 

[shaw520]

Some foam can act more like a sponge then floatation...whats so hard to get.

 

[bob johnson]

Some foam can act more like a sponge then floatation...whats so hard to get.

well the two you listed don't act like a sponge at all....so wondering why you think they are a bad choice?................that's what hard to get! ive gone through , the tearing apart of the boat and pulled out fully soaked foam and rebuilt my boats with new foam.... and I cant think of a better choice that STYROFOAM and the HD foam boards...... Styrofoam billets are used as dock flotation, where they spend 100% of the time in contact with water and they absorb no water at all......

 

[shaw520]

You may want to read this link below "styrofoam" is a very general term, the white expanded cell styrofoam sold at hd would be better at soaking up water then keeping it out..
https://www.boatdesign.net/threads/polyurethane-foam-water-absorption.35963/page-2