Why don't most boats blow up eventually?

[jonjo]

Butane is heavier than air and it is impossible to light a cooker without some escape of gas before combustion, hence any yacht must accumulate a growing pool of combustible gas in the bilges.

So why don't most boats explode within 10 years of use?

I have a theory and hopefully someone with more scientific knowledge will clarify. At school many of us will have seen the O level chemistry demonstration of Brownian motion. I suspect that air molecule agitation will in a 24 hour period tear away a small percentage of any stagnant pool of butane and bounce a few heavy butane molecules up into cabin air currents and eventually waft them outside. Over time the bilges will be cleared.

http://www.youtube.com/watch?v=D2SpYlkTDsc

 

[DaveS]

I think that's basically right. Diffusion. I have a vague memory that partial pressures may be involved, but someone who actually knows will no doubt be along soon.

 

[sailor211]

Is it not because you pump it out with 10 strokes of the bilge pump when emptying the bilge. That's what I was told as a youngster

 

[Blueboatman]

Surely propane is displaced by the crews selfless, noxious emissions over the years ?

 

[Neil_Y]

Is it not because you pump it out with 10 strokes of the bilge pump when emptying the bilge. That's what I was told as a youngster

Me too, I always manually pump the bilges first when the boats been sat (before making a brew) and every morning when on board.

 

[Searush]

Butane is heavier than air and it is impossible to light a cooker without some escape of gas before combustion, hence any yacht must accumulate a growing pool of combustible gas in the bilges.

So why don't most boats explode within 10 years of use?

(snip)

Ahh but, ah but, ahh butt.

Lighting the gas invariably produces a small plop of ignited excess gas & is probably one reason why the area under the burners is usually dished. I doubt that very much ever escapes that way.

But, although butane & propane may be heavier then air, but they are not as heavy as water & even that will evaporate over time so why wouldn't LPG dissipate over time - uless there was an ongoing leak.

 

[lw395]

Gas expands to fill the space available.
If you mix two gases in a container, they will intermix.
That's why gases explode so well, the oxygen and the fuel are perfectly mixed, unlike a liquid where they are only in contact at the surface.
Gas will be a bit more concentrated low down, but will diffuse away over a short time if there is much ventillation.
When gas evaporates and expands as it leaks, it becomes cold, which also means it initially concentrates more in the bilge.

There is a school experiment, collect some hydrogen, lighter than air, in a testube with the opening at the bottom. After a minute it will light with a pop, after a few more minutes it won't light, it has diffused away. I forget the times, but it shows the idea.

Another illustration is the air around us. It's a mixture of nitrogen and oxygen mostly.
There is oxygen at the ceiling as well as the floor, it does not settle out, despite being 14% heavier. The concentration will be a little different, but you need a very serious centrifuge to change the concentrations much.

It does not take ever such a big gas leak for it to build up faster than it can diffuse away.
Likewise it doesn't take a big source of carbon mon, particularly with the washboards in.
Take care!

 

[lw395]

Me too, I always manually pump the bilges first when the boats been sat (before making a brew) and every morning when on board.

It is usually completely ineffective!
Dry bilge pumps mostly don't work well, you have to go like crazy, until the valves get wet they don't seal. We of the leaky dayboats know about bilgepumps

 

[VicS]

Me too, I always manually pump the bilges first when the boats been sat (before making a brew) and every morning when on board.

Wasting your time and effort

Quite apart from the fact that a manual bilge probably wont pump air or gas effectively it will take far more pumping that you ever do to reduce a dangerous gas concentration in the bilges to a safe level .

Even a small bilge blower will shift something like 4 m³ /minute ( thats 4000 litres / minute. A typical small manual diaphragm bilge pump will only shift 50 litres/minute.
So even if it worked you'd be pumping for best part of 1½ hours to equal a minutes operation of a bilge blower ............ God you'll need that cuppa!

 

[Seajet]

It is usually completely ineffective!
Dry bilge pumps mostly don't work well, you have to go like crazy, until the valves get wet they don't seal. We of the leaky dayboats know about bilgepumps

Hi lw395;3603176]

As a qualified, experienced engineer and sailor I know a bit about pumps & valves; when encountering the 'stirred up smelly bilges' phenomon say 20 seconds on the pump did seem to reduce the smell.

Whale 8 -10 gph manual pumps usually.

Call me old fashioned, I know power boats do it all the time and I am aware of the precautions one can take, but I prefer not to mix petrol, vapour & electrics in a small locker space on my own boat !

 

[peterb]

I think that's basically right. Diffusion. I have a vague memory that partial pressures may be involved, but someone who actually knows will no doubt be along soon.

You're quite right; diffusion will gradually clear the boat. The rate of diffusion doesn't depend directly on partial pressures, but it does depend on the concentration gradient. The effect on a pool of butane gas, with a high concentration gradient at the butane/air interface, would be for a greater number of air molecules to move down than up, while a greater number of butane molecules would move up than move down. Assuming that there is a vent to atmosphere at some point then the butane concentration will gradually move towards the atmospheric concentration, effectively zero.

I once studied the burning of propane gas under zero gravity conditions, as in a spacecraft. We had a porous burner head through which the propane percolated into the surrounding air. With no gravity there was no convection, but the fire continued to burn with oxygen diffusing in to a spherical combustion zone while the combustion products diffused out. Interesting job; the only time I've been allowed to light a fire in an aircraft cabin in flight!

 

[VicS]

You're quite right; diffusion will gradually clear the boat. The rate of diffusion doesn't depend directly on partial pressures, but it does depend on the concentration gradient. The effect on a pool of butane gas, with a high concentration gradient at the butane/air interface, would be for a greater number of air molecules to move down than up, while a greater number of butane molecules would move up than move down. Assuming that there is a vent to atmosphere at some point then the butane concentration will gradually move towards the atmospheric concentration, effectively zero.

I once studied the burning of propane gas under zero gravity conditions, as in a spacecraft. We had a porous burner head through which the propane percolated into the surrounding air. With no gravity there was no convection, but the fire continued to burn with oxygen diffusing in to a spherical combustion zone while the combustion products diffused out. Interesting job; the only time I've been allowed to light a fire in an aircraft cabin in flight!

FWIW
Rates of diffusion of gases approximate to Grahams law of effusion which tells us that the rate of effusion of a gas is inversely proportional to the square root of the relative molecular mass.

( effusion I think would be the correct term for the process which you described as "percolated")

 

[Neil_Y]

It is usually completely ineffective!
Dry bilge pumps mostly don't work well, you have to go like crazy, until the valves get wet they don't seal. We of the leaky dayboats know about bilgepumps

I'm not so sure, a large whale pump moves air and as its pumped regularly there is no build up, we used to do 100 dry strokes before switching batteries on (when my dad was on board back in the 70's) Now I'll do 40-50.

It's all limiting risk and easy to do, trouble with blowers is you need the batteries on and they are usually low down, a poor connection at the terminal could be where your problems start.

I also insist on the practice of (with match lit stoves) lighting the match before turning gas on and rather that turning gas off at the cooker knob turn it off at the pipe.

I think the danger is returning to a boat that has had a leak whilst you've been away, hopefully you'd smell something before you switch the batteries on.

 

[Boathook]

Whilst butane is heavier than air it will take a lot to settle in the bilge from lighting a cooker. The slightest bit of air movement will normally dispearse the gas which is why you can smell it from a distance when someone is a few seconds slow in lighting the burner. A leak in a pipe in the bilge is another matter though.

 

[lw395]

Hi lw395;3603176]

As a qualified, experienced engineer and sailor I know a bit about pumps & valves; when encountering the 'stirred up smelly bilges' phenomon say 20 seconds on the pump did seem to reduce the smell.

Whale 8 -10 gph manual pumps usually.

Call me old fashioned, I know power boats do it all the time and I am aware of the precautions one can take, but I prefer not to mix petrol, vapour & electrics in a small locker space on my own boat !

I completely agree about mixing vapour and electrics in a small space.
We used to use Whale thru deck bilge pumps on the keelboat. These boats take water over the deck, so we used the pumps often and bought new ones from time to time. It was very common for a pump to take a lot of vigorous pumping to prime, particulary the first time you use them in a day. I guess the valve flaps are covered in salt when the pump dries out?
I suppose a bilge pump that never pumped sea water might be different, but frankly, I don't like to rely on that.
Just moving the air back and forth will do some good though, getting the air mixing.
As VicS says, the volume these pumps move is very small in air terms.

For a boat on a swinging mooring, I think its good to aim for through ventilation. But the key thing is keeping the gas system in good nick, not storing petrol or outboards in the cabin space or any locker that vents to it.
Spotted one of the OYT ketches in Lymington Wednesday, I noticed they store their outboard fuel in cans lashed to a grating behind the transom. Looks OK on a 70ft boat, not sure about 26ft.

 

[DaveS]

I once studied the burning of propane gas under zero gravity conditions, as in a spacecraft. We had a porous burner head through which the propane percolated into the surrounding air. With no gravity there was no convection, but the fire continued to burn with oxygen diffusing in to a spherical combustion zone while the combustion products diffused out. Interesting job; the only time I've been allowed to light a fire in an aircraft cabin in flight!

Now that sounds like fun!

 

[wklein]

The amount of gas needed for an explosion is absolutely minimal, so i am surprised it doesn't happen more too. When i bought my previous boat i lived on it for about a month when i was working on batteries in a cockpit locker with the kettle on. i heard a quite loud hissing noise. Turns out every (6) joins in the gas pipe had been made with wrong size compression rings.

Still from my experience gas is far safer than a taylors paraffin cooker.

 

[KenMcCulloch]

I once worked on a boat where the configuration of bilges and location of the gas alarm sensor meant that when a full boat of, say 18 people all went to sleep after a dinner of, say, beans and cabbage, the gas alarm would go off at about 4 in the morning even though the gas was turned off at the cylinder overnight. Explain that if you can.

 

[lw395]

The amount of gas needed for an explosion is absolutely minimal, ......

The lower explosive limit for both butane and propane is roughly 2%

The volume of yacht cabin might be say 8m^3.
Density of air is about 1.2kg/m^3

So the minimum for an explosion is going to be roughly 200g of gas.
If it is reasonably mixed with the air.
That is actually quite a lot of gas, say about what a single burner will use in a hour?

 

[lw395]

I once worked on a boat where the configuration of bilges and location of the gas alarm sensor meant that when a full boat of, say 18 people all went to sleep after a dinner of, say, beans and cabbage, the gas alarm would go off at about 4 in the morning even though the gas was turned off at the cylinder overnight. Explain that if you can.

Farting and burping methane!