February cold snap, burst tank

The morning of Tuesday 18 February 2026 came in at 19°F at the Boat Haven, the third consecutive night below 20°F on the Port's weather log. By 0710 the harbormaster's office had logged three separate calls from boats in the inner basin reporting frozen lines, frozen heads, or unexplained alarm faults. Two were resolved by phone (a frozen head pump on the F/V Petrale, a frozen freshwater dock connection at slip B-7, both walked through with the owners). The third was the 1971 Cal 39 Halibut Bay, whose owner called at 0742 to report that his bilge alarm had been screaming since 0630 and that he could not find the source of the water.

By 0815 the editor was at the boat's slip with a 2-amp Rule emergency pump and a 25-foot extension cord. By 0840 we had identified the source as the freshwater system. By 0855 the owner had isolated the freshwater pump at the breaker and we were pulling sole panels in the saloon to find the tank.

The diagnosis

The boat carries a 30-gallon polyethylene freshwater tank under the saloon sole, original to her 1971 build but replaced by the second owner in 2003. The tank had been topped off at the dock the previous Saturday (10 February) to about 28 gallons. Over the next eight days the boat had sat unoccupied with the cabin heat off; the owner is a weekend visitor whose primary residence is in Olympia. The Wednesday-through-Friday cold snap (low 21°F, low 19°F, low 19°F on three successive nights) had cooled the tank's contents to within a degree or two of freezing. The Friday night, the coldest, had been enough.

The tank had not burst the way a copper line bursts — with a clean rupture and a clear failure point. The tank had split along the internal baffle seam, on the long axis, about 2/3 of the way down. The split was perhaps 6 inches long. Water had been seeping from the split into the cabin sole at perhaps a half-gallon per hour for three days, accumulating in the bilge until the float switch tripped.

The Cal 39's bilge is shallow and runs forward; the freshwater that had accumulated had pooled in the cabin sole rather than the bilge proper, which is why the alarm came late. By the time we arrived, the cabin sole was wet to about six inches up the saloon settees on both sides. The galley was the same. The owner had been on the boat the previous Sunday and had noticed nothing. He had not been back since.

The decision

We had two options. The first, a quick patch: drain the tank, dry the split, apply a marine-grade epoxy patch (3M 5200 over a fiberglass tape), refill to half capacity, monitor. The second, a tank replacement: order a new tank from Ronco or from BGS Industries on a custom-fab basis, pull the existing tank, install the new one. The first option would cost about $200 and take two hours. The second would cost about $1,400 and take two weeks (the lead time on a custom tank in February).

The owner chose the first. He has six weeks before his April charter season starts (the boat is in the Salish Sea Yacht Charters supplemental fleet). He will run the patched tank through the season and have the replacement done in the October haul-out. We documented the patch in the install report and gave him a copy with a one-page advisory: this is a temporary fix; the tank needs replacement before another cold snap.

The patch went on Tuesday afternoon. The tank was refilled to 15 gallons (half capacity) on Wednesday. On the Friday we checked it; it was holding. The owner has, sportingly, agreed to winterize the boat properly next year — an empty tank, an antifreezed plumbing run, the cabin heat set to 45°F on a thermostat — for which we billed him $180.

The lesson

The lesson is not subtle. A polyethylene tank with water in it at 19°F is going to freeze, expand by approximately 9% of its volume, and split somewhere. The split happens at the structurally-weakest point, which on a tank with internal baffles is the baffle seam. The mitigation is to not leave water in the tank during a known cold snap.

The two-paragraph winterization checklist we hand to all our subscribers each November: (1) empty the freshwater tank to below 1 inch. (2) Disconnect the freshwater pump. (3) Run RV-grade propylene-glycol antifreeze through the supply manifold until pink fluid runs from every faucet. (4) Set the cabin heat to 45°F on a thermostat, with the breaker on. (5) If the boat is on shore power and you are willing to risk it, leave a 60-watt incandescent bulb under the cabin sole in a wire cage; the bulb will hold the bilge above freezing through any reasonable cold snap, at a cost of about $5/month in shore-power amperage.

The checklist is in the Tankage primer at the back of the issue, and in every annual maintenance call sheet we hand to a customer. The owner of Halibut Bay had not read it. He will from now on.