There are three main points to consider in designing a boat.
POINT 1: BALANCE BETWEEN FLOATING FORCE AND WEIGHT
he floating force (buoyancy) of the hull needs to be bigger than the weight (gravitation) of the boat. Otherwise, the boat sinks. According to Archimedes's theorem shown in the following figure, the value of floating force becomes equal to the weight for capacity of the water which the hull pushed out. That is, in order to float the Stirling engine boat, which has total weight of 500 g on water, the hull must push out the water of 500 cm3 at least. You can imagine the size with visualizing of the size of can juice or a PET bottle.
POINT 2: STABLE BODY SHAPE WITHOUT OVERTURN
A flat wood board floats to lie on water. It cannot float to stand up on water. The floating board always sways by wind or waves. When the lying board leans somewhat, the board receives both a floating force (buoyancy) and the weight (gravitation) as shown in the figure to the right (a). So, the board receives a moment to keep the lying position. If the board is floated with standing up, the moment becomes to lying direction, and it overturns soon. Thus, in order to be stabilized and float the board material on water, the shape of a board and the center-of-gravity position is very important. When building a boat, you must consider the shape of the boat and the center-of-gravity position firmly.
Stability of a Boat
POINT 3: LOW DRAG BODY SHAPE
Even if a boat is stabilized and has floated, the shape like a flat board has a big drag force, then high-speed running cannot be desired. The drag force decreases with decreasing of the surface area between the hull and water. When boats have the same weight, the circular shape like the above figure (c) has smaller surface area than the squarish shape like (a) and (b). However, in considerations of the weight of a Stirling engine and its feature, it is difficult to keep the stability with the circular shape of the boat. Also, the drag force decreases with decreasing the waves which occured with the boat running. That is, the better boat has a pointed shape, smooth surface, no-bumpy in water, a slim and long body.