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The simplest way of 'calculating' output is to get a leaflet on radiators from a DIY shop or a plumbers merchant, and try to match the sizes and number of layers in the radiators on the leaflet to the ones in your boat. Then (see the other reply today) correct the output for the temp drop across the radiator (or more correctly, for the difference between the mean radiator temperature and the cabin air temperature). Use a 'linear' approximation, that is, if the temp diofference is half the specified value, your output is half the rated value.