Four inches would be enough.

Are the sides buoyant enough to keep the top that much out of water ?" "I do not think so, monsieur," Pierre said with a shake of the head.

"It would float nearly level with the water." "But see here, monsieur," Adolphe said eagerly; "I have an idea! The hatches are covered with tarpaulin.

If you could hide in the forecastle during the night you might cut away all the top underneath the tarpaulin and prop it up, so that if anyone trod on it in the morning they would not notice what had been done.

Then when they have pushed off you could knock away the props, the board would tumble down, and there would be only the tarpaulin cover on the sides.