I’m surprised at Dave Gerr. In his otherwise excellent article about freshwater systems (Freshwater system quality, Issue 137, Ocean Voyager 2004) Gerr says that incoming dock water should be pressure-regulated to prevent the tank bursting. How does this happen? Did someone close the tank vent? He is correct that tanks are usually tested at 3 to 5 psi and then they often make an alarming “boing!” sound as the plates flex, so, of course, they can’t hold the average 40-psi pressure of municipal water — they are not pressure cylinders. In addition, dock water can often reach 80 psi if a water-tower tank is the source, because the marina is usually the lowest point in town. However, I believe what you are trying to avoid by limiting the pressure is the rupturing of hoses and blowing apart of fittings, which of course would fill the boat if unnoticed.
Larry Battaglia is a dive-boat captain in the Florida Keys who built a Bruce Roberts 434D design in steel. He and his wife plan to voyage in the Caribbean in the next couple of years.
Dave Gerr responds:
Interestingly enough, it is possible to burst a water tank — even if vented — when connected to shore pressure water. At a potential of 60 or more psi, the vents may not be able to relieve enough pressure before the tank fails. I imagine watching the spray jet out of the vent openings would be a good early warning, though! Nevertheless, Mr. Battaglia is correct. Bursting the tank is less likely than bursting the piping. Either way, you can sink the boat.