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Old 28 July 2012, 13:27   #21
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the long term problem with closed cell foam that doesn't absorb water... is that it does!!
We have had loadsa boats in the club here with a weight problem in later life, hmmm sounds familiar!
We have cut the floor out of boats that could no longer struggle up onto the plane to find that the foam was sodden and heavy. Once stripped out and the floor relaid the boat was fine. Two or three folk bouncing on the deck at sea, compressing the deck will break down any foam down eventually then its just wet deadweight with no useful floatation properties left. Be better off sticking a loada empty closed plastic pop bottles etc etc in the voids.
If ya do put foam in despite things be careful how much, it can get a tad warm when curing
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Old 28 July 2012, 13:56   #22
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This is all very well but if you are talking about RIB design, everything is relevant to how the tubes are attached. The buoyancy v displacement becomes void if the size and strength of the attachment is not good enough. Alan P
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Old 28 July 2012, 14:56   #23
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So the room full of bouncy castle balls sat at my parents house might not be such a bad idea after all?
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Old 28 July 2012, 15:56   #24
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Is something better than nothing?
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Old 28 July 2012, 16:06   #25
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[QUOTE=tonto;478269]Nope!

Mass does not vary, so the mass of a 3 tonne boat is the same wherever it is, but the weight is different. Weight is affected by the environment it is in, Mass is not.

Way i see it also.
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Old 28 July 2012, 16:16   #26
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Quote:
Originally Posted by Alan Priddy View Post
This is all very well but if you are talking about RIB design, everything is relevant to how the tubes are attached. The buoyancy v displacement becomes void if the size and strength of the attachment is not good enough. Alan P
I wonder how many know what you are referring to?
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Old 28 July 2012, 16:47   #27
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" the weight of a 3 ton steel boat weights in air exactly as much as a 3 ton sandwich GRP boat. But for flotation You need much less volume for the sandwich GRP one....if any.
"
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Nope!
Mass does not vary, so the mass of a 3 tonne boat is the same wherever it is, but the weight is different. Weight is affected by the environment it is in, Mass is not.
I don't get what is the disarrangement here...If we scale a steel boat in air and the figure is 3 ton and then scale the sandwich GRP boat in air with a equal weight of 3 ton, think its fair to say that in real life applications the weight (and mass) is same for both boats(in air)?

And as the density of the hull in the sandwich GRP boat is much less, for sure we need less additional flotation for the GRP to keep it afloat compared to the steel one. Thus this fully confirms that weight is affected by the environment it is in, Mass is not.

One foam I believe will keep dry in the long run is good quality XPS foam (not to be mixed with EPS). The downside with XPS compare to PU foam is that it does not handle solvents very well and it is not (as far as i know) available in liquid form for household use.
Blue Styrofoam is XPS as an example
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Old 29 July 2012, 04:14   #28
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Just been talking to a couple moored up with a steel Dutch barge this morning waiting to go through some locks ..
As the boat looked fantastically well being only just built the last few months or so

we then got onto the subject of fitting out and
he was saying that the insulation foam is polyurethane closed cell foam and was sprayed on to the steel initially on his boat for thermal and sound insulation,

The company that did his also use it on steel trawlers , and thinks the company also do flotation foam for marina floating pontoons and the like
He had his boat done by the boat builder but thinks the company also does DIY Spray kits for home built steel boats barges / trawlers / yachts .

Forgot what he said the company was called but think it was just called ,,
spray foam boat company or Narrowboat spray foam !
No chance of his boat floating if it ever got holed even with the foam think he said it had 14 tons of ballast in it .
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Old 29 July 2012, 07:06   #29
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Quote:
Originally Posted by C-NUMB View Post
" the weight of a 3 ton steel boat weights in air exactly as much as a 3 ton sandwich GRP boat. But for flotation You need much less volume for the sandwich GRP one....if any."

I don't get what is the disarrangement here...If we scale a steel boat in air and the figure is 3 ton and then scale the sandwich GRP boat in air with a equal weight of 3 ton, think its fair to say that in real life applications the weight (and mass) is same for both boats(in air)?
Not really, We are not weighing the boat in air, but in water, or more specifically, fully sumbmerged in water.

If you take the 3 tonne boat made from steel, the density of steel is approx 8tonnes per M/3, therefore the volume of the steel will be 0.375M/3, which when multiplied by seawater density of 1.025, will give a displacement of 384Kgs of seawater. This displaced water will give 384Kg's of bouyancy to the 3tonne boat, so it will "weigh" 2.616 tonnes underwater, requiring over 2.616 tonnes of bouyancy to make it float or 2.55M/3

Now do the same thing for a 3 tonne fibreglass boat of density 2.5 tonnes per M/3, and you get a volume displaced of 1.2 M/3, and a displacement of 1.23 tonnes, meaning the boat will "weigh" 1.77mt underwater, and require over 1.72 M/3 of bouyancy.

The weight of the boat will be 846 Kgs lighter in water.

It is why all ships are measured by their displacement, not by weight.

However it is all useless theory, as a boat is made of many materials, and trying to find the correct average density is almost impossible, so as a basic rule if the reserve bouyancy in M/3 is the same as the boats displacement it will float, as the materials themselves will add a small amount of bouyancy, even if it was made from lead.

So to answer the OP, yes 3 cubic meters of reserve bouyancy would float your boat, but as other posters have pointed out, it might well make it handle crap, be tender, unstable, and be rubbish! it might make if float upside down, on its side, bow in the air or stern in the air....
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Old 29 July 2012, 07:13   #30
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I wonder how many know what you are referring to?

Itt wazunt mee wot dun itt. Itt wazunt mee.

Itt woz dem rokketeers from dennmark.

Onist.
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