PURPOSE: To improve appearance and strength of a dinghy and easily mount and remove its mast, by stopping the bottom end of a plastic cylindrical mast sleeve to a ship bottom plate and connecting the mast sleeve to a hole for fitting deck mast in the bow.

CONSTITUTION: In a small open boat of dinghy type made by fiber reinforced plastic (FRP), a mast fitting hole 4 is extensibly formed cylindrically facing downward of a deck 2 in the bow of said boat. A mast sleeve 6 consisting of a cylindrical body made by plastics of nylon or the like is connected by an adhesive agent 10 to the bottom end of an extension 5 of said hole 4. A bottom end part of the mast sleeve 6 is connected to a stopping member 7 provided to a ship bottom plate 3. A bed part 71 and a protrusive part 70 are formed to the member 7 by coating a seal material of FRP on an external surface of a wooden core material 8, and the mast sleeve 6 is fixed by a screw 9 or the like. A supporting part easily operating mounting and removal of a mast can be obtained, further appearnace and strength of a ship body can be increased.