Antifreeze testing

I popped out to the boat today to check it was OK pending the return of the freezing conditions.  No frost damage so far, but how can you be sure you have enough antifreeze in the engine?  Apparently too much (ie over 50%) is not a good idea.  I had to top up with a few litres of fresh water when we had the boil up on the Stort so I'm not sure how we stand.

I have one of those little antifreeze testers from Halfords - three little plastic balls in a dropper tube, and that shows two balls floating, which according to the label says probably enough, but check.  Then I remembered a great tip I read on canal world forums.  Take a sample of your coolant and stick it in the freezer at home.  So I did, remembering to run the engine first to give it a good mix.  After four hours in the freezer it is still liquid although there are ice crystals at the bottom of the sample which is is a pottery jug.  I Googled freezer temperatures and found that most UK freezers operate at about -18 degrees C, so I guess I'm alright unless it gets extremely cold out there.  The engine is reasonably well insulated from the outside world anyway.  The min max thermometer I left by the front water tank stopcock showed a low of -1 deg C over the last 3 weeks or so, so that's not too bad.

Whilst I was there today I also stuffed a hot water tank insulating jacket round as much of the calorifier as I could reach.  These are really cheap at the moment in DiY stores as they get a government subsidy like roof insulation.