Abstract: A planing type boat having a water ballast chamber provided at the bottom of the boat includes a partition wall provided in the water ballast chamber to divide the chamber into front and rear compartments and formed with an opening for communicating these compartments with each other. In the initial stage of planing, the water in the rear compartment is fastly discharged due to a combined action of inertia, gravity and aspiration induced by the outside flow whereas the water in the front compartment which is restricted in its rearward flow by the partition wall structure is discharged relatively slowly as compared with the water in the rear compartment. The weight of the water therefore substantially acts on the front compartment in the initial stage of planing thereby effectively preventing the bow of the boat from being excessively lifted with the result that the time required for entering a complete planing is minimized.