40 hp vs 60 hp on a small RIB

Short answer. Twenty horsepower sounds like a small jump on paper and is a large one on the water. It is usually the difference between planing slowly with a full boat and planing confidently with one, and between a boat that struggles pulling a skier and one that does it without drama.

01 · Power is a margin, not a top speed

The most common mistake in comparing horsepower is thinking about top speed, when the number that changes daily life on the water is how much reserve power is left once the boat is loaded. An engine that gets an empty boat on plane easily can struggle to do the same with four adults, a full fuel tank and a cooler. The extra twenty horsepower is mostly spent on that margin, not on a faster cruise.

02 · What a full boat does to the number

A boat's dry weight is not what the engine actually moves. Four adults at roughly 180 lb each, a full fuel tank, and gear can add close to 900 lb to a small RIB's displacement. On a 40 hp engine that additional weight is a meaningfully larger fraction of what the motor has to push than on a 60 hp engine, which is why the same boat can feel adequate empty and marginal loaded on the smaller engine.

A hull's maximum horsepower rating is a structural and stability ceiling, not a recommendation to run it there. The ADB 420 is CE rated to a 70 hp maximum, and the builder's recommendation is 60 hp, because that figure is where the hull, the deck load and everyday use balance out. Running lower than the maximum by that margin is not underpowering the boat, it is the number the boat was actually designed around.