Someone awhile back questioned my concern that a popped seacock could sink a boat despite the presense of bilge
pump(s), so I decided to dust off my 50 years ago fluid dynamics course notes and also scrounge around the internet
on the issue. I actually found one formula on the internet
that has gallons per minute and per second confused.
The starting point model I'll use is a 1" popped seacock located 3 feet below the waterline. That computes out to 34 gallons per minute. That calculation is easy and pretty robust; it's the lift
by the pumps and particularly the friction loss in the hose and fittings that gets hazy.
Pumps are rated with no intake obstructions and no hose attached, so 1600 GPH = 27 GPM, 2000 GPH = 33 GPM and 4000 GPH = 67 GPM (Johnson 4000, the largest I could find on the ready market). Even at rated capacity, only the 2000 GPH and up are going to keep up with our 1" hole.
When BoatUS looked at them, they could not find one that did better than 75% of its rating at 12.0 volts. Let's assume 13.4 volts and give them full rated output. But, these little centrifugal pumps only put out about 6 PSI, so you don't have much to work with either for pure lift
(goes to zero PSI at about 14 feet of lift) or friction loss (varies with hose diameter, smoothness, bends, and length).
At an assumed 4 foot lift, through 10 feet of smooth hose and no sharp bends, the answer is about 60% of the rated capacity for the smaller pumps. Part of that is head
, which costs you about 2 PSI, and part is friction loss in 3/4" hose. Standard friction loss tables for lined fire hose say that at 30 GPM the friction loss is 10.5 PSI per 10 feet, so you see where we're going - the larger the pump, the greater the friction loss percentage. Even a very big pump won't put 30 GPM through a 3/4" hose 10 feet long. 1" hose helps immensely, dropping friction loss at 30 GPM to 2.6 PSI for 10 feet.
Then put the stream through a necessarily constricting thru-hull (think of it as a nozzle), and you get less than 40% of rated output. These are not fire pumps. A different assumed model used by BoatUS and actually tested discharged 26% of its rated capacity.
Let's call it an optimistic 50% and move on. That gets you 13, 16, and 33 GPM. Only the Johnson might keep up with our 1" hole, and it better have 1" hose, but the others may slow things down enough to give us time to get to the seacock. The 1600 optimistically slows it to 20 GPM into the hull
That leads to a second but I think the central question for cruisers. How many gallons can enter your hull
before the tops of your battery
bank are reached? Once that happens we don't have electric
pumps. Given a popped 1" seacock, how many seconds do you have to realize the problem and stop it? I know that for mine, the answer is "not very many" if the hole in in the forward cabin
with the battery
bank, but I only have two seacocks, and they're in the engine
room. One stays closed (sewage) and the other can be closed by pulling a wire in the main salon
above it. I do need to remember to keep the engine
room door dogged shut, and to exercise my gasoline OMG pump frequently.
I'll have to agree with BoatUS on this issue - They say the priority must be stopping the leak, because the pumps we have are not likely to keep up, and pump only a fraction of their rated capacities.