A glitchy shower pump and learning how to stay afloat

Pearl has had a problem with the shower pan in the head filling up for a while. This has driven me nuts. Sometimes it fills faster than others, other times, it’s fine and bone dry. The fundamental problem was that water was siphoning back through the shower pump, but given several other factors, none of this made sense. The problem could be easily fixed by installing a loop with a vacuum break to prevent the siphoning but didn’t answer why the problem occurred intermittently.

A couple of days ago, I decided to dismantle the self priming diaphragm Jabsco Shower Pump that seemed to be the source if the problem. Once taken apart, the problem was obvious and simply resolved. Hair had got through the pump guard (filter) and was breaking the seal on the pump that prevented back flow.

While aggravating, this project taught me a lot. The first is that diaphragm pumps are a pretty good solution for situations where there might be back pressure, as well where the pump needed to be self priming, and/or possibly run dry without burning out. The second, which is more important, is that the same pump (more or less) empties out Pearl’s bilges. The third, is that backing plates on the through hull fittings will be a very good addition when the boat gets pulled to have the bottom painted.

For the most part, the Jabsco diaphragm pump is a works well to keep the bilge clear of drips. In the event of a real problem, however, like failure of a through hull fitting, I didn’t think there there is no way it would keep keep up with the influx of water. So, i decided to run the numbers.

On average, Pearl’s through hull fittings are between 1″ and 2″ in diameter, and they are somewhere between 6″ and 12″ below the waterline. My question was how many gallons per minute would that generate, and what sized bilge pumps would I need to accommodate that influx of water. As it turns out, this is the inverse of the “emptying a tank” problem that many of us had to solve in college. I have a vague recollection of how to calculate the answer, BUT as it turns out there is an easier solution, an online flow rate calculator (see http://www.efunda.com/formulae/fluids/draining_tank.cfm#calc).

The net of this story is that I need to size my bilge pump(s) better. Fortunately, this isn’t too hard a problem to solve. West Marine had 2000 gph bilge pumps on sale for $100 ea this week, and the locating them and running the required hose isn’t too much of a problem. This is, however, and important consideration even if you leave your boat parked at the dock and aren’t planning to go sailing.

Depth of spout (ft):
0.50 1.00 1.50 2.00 0.50 1.00 1.50 2.00
Spout exit diameter (in): 1.00 1.00 1.00 1.00 2.00 2.00 2.00 2.00
Exit Velocity, V (ft/s): 5.67 8.04 9.84 11.35 5.67 8.02 9.83 11.30
Volume Flowrate (gal/min): 13.60 19.20 23.60 2.72 54.40 77.00 94.30 109.00
Volume Flow Rate (gal/hr) 816.00 1152.00 1416.00 163.20 3264.00 4620.00 5658.00 6540.00
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