Blockage effects due to the restriction of the tank size of the rectangular section are studied expermentally and analytically. Double model flows (DMF) around Wigley and Wigley-type models are numerically analyzed with or without tank boundaries. Two analysis methods, mirror image method and "Box" type boundary source method are introduced. Through these analyses the next 3 remarks are given; 1. The experimental resistance increase due to the blockage seems to be much more than the estimate by the restricted DMF. 2. In a medium or high Fn range, increase of wave resistance due to the blockage is significant. 3. Mean sinkage increases remarkably with increasing blockage effect.