PURPOSE: To enable direct reading of a ship's speed (approach ship's speed) at the up and down sailing of a yacht, by enabling the display of a ship's speed obtained by multiplying the true speed by the cosine of a selected angle as desired.

CONSTITUTION: A ship's speedometer is made up of a ship sensor 1, a measuring section 2 thereof and a cosine computing section 3. A lead switch 6 is opened or closed by a magnet 5 rotating together with a rotating screw 4 towed underwater. Then, when with a switch 7 closed, the lead switch 6 is opened, a battery 8 is charged with a capacity 10 while with the switch 7 closed, the lead switch 6 is closed, the capacity 10 previously charged is discharged. When a rotary switch 17 is connected to a contact 18, a potential difference of a resistance 12 at the discharge is shifted to a high resistance R1, which is measured with an electronic voltmeter 16 to be displayed as ship's speed. In this manner, by changing the connection of the rotary switch 17, the ship's speed can be displayed as obtained by multiplying the true ship's speed by the cosine of an angle selected as desired.