Abstract: The invention relates to a small-sized shipbuilding. In particular, to a stern plug to be inserted into a stern portion of an inflatable bottom of a water jet boat, the plug is made such that it follows a shape of a front portion of the bottom of the boat, based on a hollow volume that is open at one base and comprises a closed second base and a lateral surface, the lateral surface is arranged along an outer contour of the closed base, and the closed base is made such that it follows a shape of the front portion of the bottom, and the lateral surface is made in a form of shelves, where an upper shelf is made straight, wherein the plug is made in a form of a monolithic cast piece of an elastic shape-retaining material, where the open base has a mesh cellular structure, where a lower shelf of the bottom is made straight, and plate-shaped guides for fixing to the boat are mounted on the shelves.