"Hull Speed" is a broad based method of giving an approx. speed of a boat.
Generally it is the square root of the waterline length X 1.4.
It is very accurate with wide beam, heavy displacement type boats.
A general rule
is 5 hp per ton is needed to achieve displacement speed.
For a 30ft , 5 ton boat displacement speed is 7 knots using 25 hp.
This is with no current
or wave action.
Extra power will produce a bow wave and stern wave which forces the bow of the boat upwards and the stern down (going uphill) Speed can be increased but large amounts of power will be needed.
For a boat to plane you need a minimum
of 50 hp per ton (more like 70 hp) and a hull shape which helps to hydrodynamically lift
Hull width and hull shape can improve performance.
A narrow boat with a fine bow entry, will delay the formation of the bow and stern wave.
This is called a semi displacement boat.
I once had a 36ft semi displacement power cruiser which weighed 7 ton and a 280 hp diesel
power and a maximum speed of 18 knots. (40 hp/ton)
A 36ft planning hull of 8 ton (heavier motors) needed 550 hp but had a higher top speed (25knots)
These are real life examples.
However it is possible defy the "hull speed" rule
A narrow hull design of 16-1, 40ft X 2ft 6 inches wide does NOT form a bow wave or stern wave at high speeds. (even at 30 knots)
The boat is NOT planning but is considered "Fast Displacement".
Power requirements are 25 Hp per ton, approx 1/3 of a Planning boat.
This is the concept
used in Catamaran
ferries and the new generation of power catamarans.