How much inherent buoyancy required to keep a boat afloat... - Page 2

wavelength · 28 July 2012, 18:27

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

Alan Priddy · 28 July 2012, 18:56

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

chewy · 28 July 2012, 19:56

So the room full of bouncy castle balls sat at my parents house might not be such a bad idea after all?

chewy · 28 July 2012, 20:56

Is something better than nothing?

Boatnomad · 28 July 2012, 21:06

[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.

Searider · 28 July 2012, 21:16

Quote:

Originally Posted by Alan Priddy

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?

C-NUMB · 28 July 2012, 21:47

" 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.
"

Quote:

Originally Posted by tonto

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

m chappelow · 29 July 2012, 09:14

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 .

tonto · 29 July 2012, 12:06

Quote:

Originally Posted by C-NUMB

" 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....

wiLlfish · 29 July 2012, 12:13

Quote:

Originally Posted by Searider

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.

9D280 · 30 July 2012, 11:43

Quote:

Originally Posted by chewy

Looks like we're going down the closed cell foam route which can be poured in....

NOOOOOOOOOOOOOOO!!!!! Don't do it!!!!! One word for you - Dory!

Bottom line is 3 tons is 3 tons. What would youy rather lift - a ton of lead or a ton of feathers?

(Let's ignore inconveniences like the feathers absorbing water etc for the following discussion)

A ton of feathers will take up a load more volumethan a ton of lead. To support a tone of anything in water means slightly more than a ton of water needs to be displaced.

Let's use round numbers.... if a ton of widgets take up 10x the volume of a ton of thingumys, then you only need to submerge 1/10th of the block of widgets to get the same bouyancy. The problem you have with almost anything you can make a boat out of (bar possibly wood - although Th density of wood is so close to that of water the weight of the fittings is usually enough to overcome what little spare bouyancy there is) is a block of most construction material is more dense than an identical sized lump of water. Hence why boats are hollow!

Practical example - How does 300,000 tons of supertanker stay afloat? Replace the air in the tanks, with oil, and it submerges quite a bit. Pump the oil out, and load it with mercury (liquid metal) and it wouldn't still be afloat by the time you finish loading...

So, back to the orginal Q, presumably Chewy wants to keep his sunken 3 ton whatever boat afloat in a worst case scenario. Air is about as light as you can sensibly get (helium notwithstanding), so I would suggest either sealed compartments or airbags as used by sailing dinghies.

The "niceties" in the cals above are a bit like the anoraky pre-shot of oil in my fuel tank. Easy maths says "30L tank, throw in 600Ml of oil". But that oil takes up 0.6L of tank space, so you actually only need to to thow in 560 ml to get a "proper" 50:1 mix.

chewy · 30 July 2012, 12:30

Air tanks are the way forward maybe with removable bladders.....

Into The Blue · 30 July 2012, 13:22

I did read on a yachty website of a round the world type who solved this by stuffing the hull voids with as many empty 2ltr plastic drinks bottle as he could get in. With caps on of course. Worked out as a cheap and effective solution that was reversible. He did tie several together to ensure if the boat got holes they didn't all float out. Nets I seem to remember to bundle them up.

chewy · 31 July 2012, 09:25

Got aload of plastic bottles for that very purpose.... and some plastic balls.

9D280 · 31 July 2012, 09:36

That would explain the funny walk......

Razorbill RIBs · 31 July 2012, 17:41

Quote:

Originally Posted by chewy

Got aload of plastic bottles for that very purpose.... and some plastic balls.

Mate replaced the waterlogged foam in his Wilson with empty lemonade bottles (with lids)......seemed a good solution to me.....

Failing that, stick some sponsons around it

BogMonster · 01 August 2012, 10:20

How about a lot of the big-bubbled bubble wrap? Weighs nothing, traps lots of air, stuff in anywhere.

Did somebody say inner tubes

Rokraider · 01 August 2012, 10:58

Expanding foam is tricky stuff. Not only does it seem to absorb water, despite supposedly being closed cell, but it is very strong when it is expanding and is capable of distorting panels etc.. Personally, I wouldn't go that route.

Mythbusters once raised a sunken boat using ping pong balls. I would think that would be a good way to go? Be a bit of a mess if you ever ruptured the hull though.

I suppose another consideration would be if you increase the buoyancy and the boat rides higher in the water than designed, will stability be compromised?

tonto · 01 August 2012, 11:34

Quote:

Originally Posted by Rokraider

Expanding foam is tricky stuff. Not only does it seem to absorb water, despite supposedly being closed cell, but it is very strong when it is expanding and is capable of distorting panels etc.. Personally, I wouldn't go that route.

Mythbusters once raised a sunken boat using ping pong balls. I would think that would be a good way to go? Be a bit of a mess if you ever ruptured the hull though.

I suppose another consideration would be if you increase the buoyancy and the boat rides higher in the water than designed, will stability be compromised?

Anything you add to the boat will make it ride lower. You are adding mass to the boat, therefore it sinks.
The only way to make the boat (marginally) ride higher would be to fill the void spaces with helium.

However adding mass high up on the boat will raise the Centre of Gravity, Lowering stability, so adding foam high up will not be good, and will probably at best make it "wallow" more.

chewy · 01 August 2012, 12:41

Quote:

Originally Posted by tonto

However adding mass high up on the boat will raise the Centre of Gravity, Lowering stability, so adding foam high up will not be good, and will probably at best make it "wallow" more.

The RNLI have they buoyancy high up so in the event of a capsize it makes the boat unstable and self righting.
Looks like plastic bottles could be back on the cards...