I fitted a pump to the heating on our boat at the beginning of the winter with this in mind. It works very well, what helps is to run the eberspacher during the first hour as the engine warms up, the radiators will then stay at a reasonable temperature for the rest of the cruising day, warm rather than hot. Not as hot as when the eberspacher is running but still enough to keep the cabin at an acceptable temperature. The other solution is to fit a heat exchanger, which was discussed on a thread a couple of months or so ago.