Calling all Fluid Mechanics Engineers

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OK, we all know about the sunk P65 (or was it P56) at Salterns and the prognosis seems to be that an engine intake hose failure caused the accident. However, it has been suggested that the bilge pumps should have coped with the influx of water. So we need to test this theory
Who out there is brainy enough to tell us what the flow of water would be through the seacock (assume, say, a 75mm diameter?) at a depth of say 1.5m?

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the calc you need is for discharge under pressure.

discharge in litres/min = 2/3 x Diameter of hole squared x square route of the pressure (which at 1.5 metres should be about 0.15 bar or at 1 metre 0.1)

L= 75x75x0.39/3x2

or 1162 litres/min @ 1 metre

or 2193 litre/min @ 1.5 metres

(should have called the fire brigage, they could have pumped it out at a rate of 2250 L/min)

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Re: Bernouille

ok

The average speed of water through a pipe can be roughed as half the maximum flow.The max speed is worked from Bernouille

as P1 plus (0.5 rho (V1squared)) = P2 plus (0.5 rho (V2 squared))

P1 is what drives the static water through the pipe. Once through, the pressure is atmosperic pressure but it's that speed V2 that we want to find. Water density rho is 1000kg/cubic metre

so disregard terms with V1 and P2

so V2 = sqrt (2x height x density x g / density) = sqrt (2 x height x g ) = sqrt 30
= 5.47 metres/sec

But this is max speed, edge effects and friction reduce this, approx one half,lets sayspeed is 3 m/sec through pipe dia 75 mm so volume ofwater flwoing in cu metres per second is

3 x pi x dia x dia /4
= 3 x 3.1412 x 0.075 x 0.075 /4

and in litres/min is this x 60 (to get 1 min) and multiplied by 1000 (for litres)

= 795 litres per minute which means towards four-fifths of a cubic metre per minute. yeah, sounds bout right.











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At an empirical level, how many bilge pumps do you know that can throw a solid jet of water out of a 75mm outlet. Most bilgepumps I know only have 38 to 50 mm outlets. The engine inlet is going to admit at least 2 1/4 times more water than a bilge pump can shift through a 50mm hose, assuming both operate at similar pressures. Add to that the frictional losses in a pretty long bilge pump hose, and the figures get worse.

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Lets go with some wild assumptions plus some maths.

The 75mm seacock, if fully open, will have a surface area exposed to water of 0.00442 sq Meters. Assuming the flow rate is one meter per second through that pipe, then 0.265 cubic meters per minute will flow inward. Thats just over a quarter of a tonne of water per minute, which sounds rather a lot to me because if that flow continued for an hour, that would ingress just under 16 tonnes of water. Hmmmm....

A bilge pump that operates at 2000L an hour is removing 2.0 tonnes of water - hence sunken boat!!

My guess would be that the battery isolator switch was in the off position, that water was leaking in and not being pumped out and given bad weather (the picture of the weather looked awful) all of these conspired to sink the boat. Using my cranky sums above, you can see that even with much smaller quantities, an over-night sinking is entirely possible.

I think I'll just go and check my boat......

<hr width=100% size=1>Madoc Yacht Club
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Mmm, big assumption of 1 m/s velocity but good try. Bilge pumps should be wired independent of battery isolator so should have worked

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Yeah, but we assume there were 5 bilge pumps on a boat this size

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Typical, little wonder you can never get a bloody plumber, they are all on here answering boating questions. Here is another one.

The wife wants a new kitchen sink, which is best, stainless steel or ceramic?

Just as an aside, when I take out the little paddle wheel thingy I am always suprised how LITTLE water comes in?

<hr width=100% size=1>CJ
cjs@stone.uk.com


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Very good. Whats the name of this theorem? Anyway, seems plausible to me
So, if we assume that there are 5 bilge pumps on board which typically can handle 1000gph each gives a total bilge pump capacity of 375 l/min which is nowhere near enough to handle the ingress of water
Therefore, boat sinks. QED

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According to Aztec's Theorem of Fluid Mechanics No1 the flow of water is dependent on the square of the hole diameter so, given that the paddlewheel sits in maybe a 1" hole, flow may not seem so big but it always scares the s**t out of me, I have to say
Stainless, definitely, the ceramic one will chip

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I assumed there were 5 because I have four (so do you?) and I thought another 10 foot would warrent another.

As you say the bilge pumps are wired directly to battery (or should be) so the isolation answer is a non starter.

Nice people, nice boat, very sad.

Going over at lunchtime so may hear what really happened.

<hr width=100% size=1>CJ
cjs@stone.uk.com


<A target="_blank" HREF=http://www.stone.uk.com>http://www.stone.uk.com</A>
 
to be honest a three inch hole five foot below the waterline is a bit of a bad day out in my book. unless there's someone there to stem the flow, or a water depth alarm.

one other thing to consider..

about five years ago, i was on a crew sent to firefight on a large cruiser, cause of the fire was thought to be a defective bilge pump (left swithed on) as it was the only thing that was wired past the isolator.

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Seems I am poorly endowed as I have only 3 bilge pumps on my T48 but on something around 60' I think you're right about having 5 pumps

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Bilge pumps

What also must be considered is the" Lift" the bilge pump has to pump, I.E the higher the outlet above the pump the less volume of water will be pumped, bilge pumps are rated flow at zero lift so a pump rated 1000gph with an 18inch /2ft lift will in practice only pump 6/700 gallons

julian

<hr width=100% size=1>If it can, It will.
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Re: Bilge pumps

Yes, a good point and also any friction losses in bends, valves etc
What this goes to prove is that the average bilge pump system has'nt got a prayer of dealing with an engine intake hose failure

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.. but I'd guess only about 3 will be called into service - most power boats are trimmed by the stern, so water will flow aft, trimming even further. I doubt whether any pumps fwd of the fwd engineroom bulkhead will see any meaningful water before it is far too late. By this stage the batteries are likely to be submerged rather denting the efficiency of the pumps.

We only have two pumps on a 50 footer, but the electric pump shifts 4500 litres/hour. A really good pumping system will massively outperform the little centrifuge types normally fitted.

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While on the subject of sinks and basins while all the plumbers are here - can anyone tell me a manufacturer somewhere in the world makes small ceramic bathroom hand basins that will fit into a 300mm deep bench. Looking for 6 of them (boaty use).

Thanks in anticipation.

John

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Re: Bernouille

Ahah. So here's a bloke who spent most of his time at lectures and not in the student union bar. Swot!
Since result is same order of magnitude as Aztec's, I am prepared to accept it despite sweeping assumptions
Next question is - how long did the boat take to sink?

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Re: Bernouille

Approaching the problem from the opposite end - the big Rule pump (8000 GPH) delivers 560 litres/minute through a 76 mm pipe, which ties up nicely, allowing for the pressure element.

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