What temp is really "freezing" on the water?

[Dr. D]

On insurance:

My BoatUS/GEICO policy states it does not cover damage due to ice. Unless the boat is winterized by a commercial marine technician and a deicer is used (for a boat staying in the water), or under covered storage on the hard.

Don’t shoot me, I’m only the messenger.

[Shaneesprit]

I don’t do anything to winterise my boat - it’s in a very tidal area, on the UK true if the tide is in the relative warmer water than air will warm the Engine - however I did once go to see a Bowrider a few years back inland with an inboard engine - looking at buying it, but the engine block had cracked, frozen at some point for sure.

[DeValency]

In this screenshot you can see what my boat (docked in Boston harbor, first winter here after 15 years in the Caribbean) sends me (on demand) wherever I am. The temp and humidity sensor is located approx 15” (~37cm) above the floorboards in the main cabin more or less at the same level of the engine and water tanks. The ambient temp at nights over the last week was ~-1-3C each night but the cabin temp kept at least 3-5 C above at any time. During the days even higher as the boat is fully covered with a semi transparent cover, so gets nicely warm during the day while snow and ice should never accumulate on the decks. I have winterized everything as I don’t want to take any risk, but given the fact the ocean water never freeze here I’m less concerned but will keep monitoring through the winter and if necessary will turn on a ceramic low power electric fan heater to maintain over freezing levels.
Technically, there is no need to winterize but all the other owners here run a thorough process each year - so not taking any risk here. At least not this year and will monitor the actual temps by the spring.

[EngNate]

I live aboard so the interior is always warm, but there is the fresh water tanks, pump and plumbing in the unheated engine compartment (power boat). The surface water does freeze in the harbor here, up to 1" thick, with low temperatures down to 8F last winter, and I've had no issues. The boat does trap quite a bit of heat rising from the deeper water. A previous winter I had a small (500W) heater in there and it was particularly cold (for here), with lows to 0F and not reaching freezing for several days at a time, the boat was sitting in 2+" of ice for 2 or 3 weeks, and nothing froze then. However, a nearby boat with no heat had a thru-hull freeze and break. Note it's the fresh water on the surface that's freezing, it doesn't get cold enough here to freeze over salt water.

[capn ken]

Moyer marine is a wealth of information !

[Shaneesprit]

I just realised reading this that I haven’t winterised anything this year ( I haven’t winterised the Engine in 5 years anyway ) but this time I don’t think I even emptied the fresh water either - COVID has been killing my boating time - sails are in the cabin though I have left them up before.
No time to get to the boat until next year I am not in the UK and I fly back in Day days time only to leave again 48 hours later.
Based in South Wales UK 🇬🇧 boat moored in a tidal channel 80% of the time there is no water at all and no idea how salty the water is. Shall I just pray as there is little else I can do now.