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There were four people in each sailboat, together. Not ALL together... This "word riddle" would have been easier if there had been proper punctuation.
When it is being overtaken
A sailboat will have positive buoyancy if the weight of the boat is less than the weight of the water it takes the place of. The weight of the water that is being occupied by the hull is displacement.
Wind powers a sailboat.
Personally, I think "seaworthy" is a relative term. A sailboat that is seaworthy on a lake might not necessarily be seaworthy crossing the North Atlantic. I know that doesn't help much, but I think it probably boils down to whether or not a sailboat is capable of handling the sea conditions most likely to exist where (and when) you intend to sail.
The sailboat was invented in Ancient Mesopotamia.
The force may be gravity if the cargo or ballast are incorrectly stowed. On a sailboat it is more likely to be the wind.
The Egyptian people were said to have made the first sailboat.
Barcă cu pînze is a Romanian equivalent of 'sailboat'.
The correct spelling is "sailboat" (boat propelled by sails).
Being carried.