PURPOSE: To make improvements in rigidity of the bottom as making it lighter in weight by forming the bottom into a double structure, and charging a buoyant body consisting of foaming material to an interspace between both first and second bottom plates, while attaching a mounting bracket for setting inboard equipment tightly on the second bottom plate.

CONSTITUTION: When it is executed to a hull construction of a small jet propulsion boar, the hull 11 is constituted of its bottom 12 and an upper deck 13 mounted on this bottom 12, and an engine room 14 is partitioned off by those of bottom 12 and upper deck 13. The bottom 12 is formed in to a double structure with both first and second bottom plates 25, 26, and a buoyant body 27 consisting of foaming material is charged to a interspace between both these bottom plates 25, 26. In this case, these bottom plates 25, 26 are made of glassfiber reinforced plastics, and the second bottom plate 26 is extended almost in flatness in the boat length direction from the bow of the hull 11. Then, a base plate 30 as a mounting bracket for inboard equipment including an engine 20 or the like is clamped to a specified position on this second bottom plate 26.