A good comparative exercise but it won't tell us much about thrust or BHP.
Take a heavy displacement vessel which will not pop up on semi displacement or plane and calculate its hull speed
Tow the vessel with a heavy line having minimal stretch (Small nylon rope is of no use here) Fit a load cell into the line and tow at the calculated hull speed and again at 75% of hull speed
Tow the vessel at constant speed for each test in two directions over a set distance and record the load cell and speed.
The result will provide an accurate figure of the actual force required to tow the vessel at hull speed and 75% of hull speed.
With these results, connect your test engine and again carry out the same test over the same distance and course. Carefully monitor the speed to match the tow test.
Record engine revs and boat speed (accurate engine reves are critical). Conditions must be the same as when towing therefore smooth water is essential
The force required to propel the vessel at the corresponding speeds will therefore be equal to force required to tow the vessel.
These tests are also carried out in places like Marin, Wageningen, Holland see http://www.marin.nl
in tow tanks. in this case the tow vessel is connected to an overhead trolley that pulls the vessel down the tank, different wave and wind conditions can be impute to achieve a wide spectrum of conditions.
When the force is obtained it can be used to calculate the engine power. By having the two speeds, coupled with the engine revs we can then compute engine performance curves.
Points to note: the test vessel must have a clean hull and not be dragging an outboard engine. The tow line must be short and kept taut, it must not be so long that the catenary drags in the water. The towed vessel must not be so large that the test engine cannot reach its calculated hull speed. The towed vessel must be loaded and trimmed identically for all tests