An experiment was carried out to measure sloshing pressures and forces in a prismatic chamfered tank. The tank was excited laterally for regular and irregular excitation and the results were compared with numerical computations based on a Finite difference method. The tank has a Length to breadth ratio near 1, and especially in intermediate filling levels, at around the tank's 1st mode sloshing frequency, the occurrence of a rotational flow, i.e. swirling, was observed. Based on the results from both model experiment and numerical simulations, there are cases where swirling loads can be significant and therefore cannot be neglected. After checking the tank dimensions of real LNG carriers, it was noted that many actual designs have tank with the length to breadth ratio near 1 and therefore the problem can be relevant. The rotational flow characteristics and its occurrence is investigated and its pressure distribution is compared with the 1st mode sloshing, finally, considerations about the tank safety are inferred concerning the swirling problem.