How much heat is needed to provide frost protection?

Tiny, tiny. Use heating tape directly on the pipes and under their insulation.

Use rubbery closed-cell insulation, armaflex IIRC (not the open-cell foam stuff which can absorb moisture and thus freeze).

Most important thing then is to eliminate draughts, try to partition the area being frost-heated.

ET05 is a good digital frost thermostat, but quite expensive this time of year (=A322-24)... wait, TLC Direct have them on special offer right now (=A313 or something like that).

Before my Broom European became a victim of divorce I put 2 of the tubular greenhouse heaters in the engine space run off a frost stat and that worked fine.

Difficult to say. You would need more when the outside temperature is seriously cold than when it's merely just below freezing.

I use a 200W tubular heater on permanently, plus a cheap 240W convector with a thermostat in it, in a 10m uninsulated GRP boat.

That's with the boat ashore, of course. Were it afloat, it might get the odd bit of light deck frost, but the interior would not get cold enough to freeze anything.

How big were they? From CPC 2ft long is 80 watts and 4ft ling is 180 watts.

Well, possibly, but the boat is new (to us) and so I haven't a really good idea yet where there may be water that can freeze. In addition we're aiming to use/visit the boat occasionally all through the year so total drain down won't be very convenient.

From memory 4' - definitely longer than 2'.

OK, thanks (and to all the other contributions here).

probavly 50-100W or less.

If 5KW is uncomfortably warm - say 25C and its -5 outside, then 1KW will get you to 1C.

But whole house can be heated on 5Kw so I doubt you would neeed that for just a boat.

Best to have a lot of heat and a frost stat.