Burn-out was observed in two simplified atmospheric models of marine water-cooled reactor core-loops on which cyclic heaving motions and constant listing were applied by a heaving apparatus. Cyclic heaving motions up to ±1.1 gravity amplitude and 30° list were employed. The following thermal and hydraulic performance was observed: 1. (1) The critical heat flux decreases linearly with increasing heaving acceleration, and is accompanied by violent cyclic fluctuations of void and flow velocity when the inlet flow velocity is low. 2. (2) When the loop lists, the critical heat flux decreases locally near both sides of the loop channel. A sharp concentration of void also occurs on one side. Listing helps generate fluctuations in the sub-cooled condition. 3. (3) By theoretical consideration of these phenomena, minimum values of resonant heaving frequency and of sub-cooling of the core-exit water are found above which no void fluctuations appear. 4. (4) By experimental and theoretical considerations, a semi-empirical prediction of the rate of decrease of critical heat flux is obtained. 5. (5) An approximate equation is obtained for predicting the amplitude of void-fluctuations as a function of the initial flow speed, resistance, dimensions and other items of the channel. It is shown that the void fluctuation due to cyclic heaving is inversely proportional to the fourth power of the inlet velocity. To obtain marine reactors which can operate safely on very rough seas it will be necessary to incorporate the following suggestions into the thermal and hydraulic design of the core loops. 1. (1) The decrease of critical heat flux by heaving and listing should be included among the hot channel factors. 2. (2) Out-flow water at core exits should preferably be sub-cooled below the critical sub-cooling temperature. This means that the PWR type is to be preferred for severe marine use rather than the BWR type. 3. (3) The circulating water velocity should be high. 4. (4) The total height of the core and riser should be low.