Flooding calculator

[Ric]

As any fule no, flooding is proportional to the square of the diameter of the hole and the square root of its depth. Unfortunately most of the calculators found on the net use consistent units (either SI system or Imperial) which makes it difficult to work out whether your bilge pumps can cope with a through hull failure. Most bilge pumps are rated in USG/Hour, we still use inches for through hull diameters, but I at least am metric for depths. So boiling it all down I calculated that flooding (measured in USG/Hour) is equal to 2128 x root of through hull depth (m) x square of through hull diameter (inches).

With that formula you can work out whether your bilge pumps can cope with a through hull failure. Obviously the bilge pump probably won't pump at its rated value due to head, voltage drop, pipe skin friction, check valves etc, but it should give you something to work with.

As an example, my worst case through hull failure would be my sink outlet (1" through hull with outlet 0.15m below waterline, giving a flood rate of 824USG/hour. I have two bilge pumps, one nominally rated at 500USG/Hour, the other at 1100USG/hour. I am thus reasonably confident that my pumps can cope with the worst case failure.


Hope it helps!

 

[pvb]

As an example, my worst case through hull failure would be my sink outlet (1" through hull with outlet 0.15m below waterline, giving a flood rate of 824USG/hour. I have two bilge pumps, one nominally rated at 500USG/Hour, the other at 1100USG/hour. I am thus reasonably confident that my pumps can cope with the worst case failure.

You're impossibly optimistic. Bilge pump performance is quoted at zero head, zero pipe friction, and usually at 13.6v (engine running). Most pumps in typical installations will be lucky to achieve 20% of their quoted performance, so I think you could be unpleasantly surprised.

 

[pmagowan]

Unless you have a belt driven pump directly off your engine then the chances are your pumps will not keep up. The purpose of bilge pumps in a normal installation is to remove the water after you have resolved the leak not to keep up with the leak. Far better to look at prevention followed by detection followed by repair followed by floatation followed by bilge pumps. IMO

Test it. Fit a bucket of water in the bilge, pump it out until the bilge pump can't get any residual then put a known amount in and pump out again against the clock.

 

[Ric]

You're impossibly optimistic. Bilge pump performance is quoted at zero head, zero pipe friction, and usually at 13.6v (engine running). Most pumps in typical installations will be lucky to achieve 20% of their quoted performance, so I think you could be unpleasantly surprised.

Where did you get the figure 20%?

 

[pvb]

Where did you get the figure 20%?

I'm sure you can Google as well as most people, but if not see here for example - http://www.boatus.com/magazine/2014/december/bilge-pump-capacity.asp

 

[reeac]

As any fule no, flooding is proportional to the square of the diameter of the hole and the square root of its depth.


Hope it helps!

I assume that you mean the depth of the hole below the surface of the water but I'm not sure about that square root. For laminar flow the rate will be proportional to pressure difference between inside and outside and so will be proportional to depth. In the case of a hole through a thin sheet as for a boat hull (I.e. Non -laminar flow) flow will be proportional to the square root of the pressure difference. All in all, best to do some practical tests.

 

[prv]

we still use inches for through hull diameters

Just to lob another tomato at the calculations, the "inch" numbers for through-hulls are the BSP thread sizes, not the actual diameter. The nominal size refers to the inside diameter of a Victorian malleable-iron gas pipe which would have had the thread in question cut on its outside - your ' 1" ' through-hull probably sits in a hole getting on for 1.5" across.

Pete

 

[nigelmercier]

As any fule no ...

But this bit made me smile. For many years my online name was nigelmolesworth.

 

[alan_d]

But this bit made me smile. For many years my online name was nigelmolesworth.

... in which case I would have expected you to know that the quote ought to be "as any fule kno".