It is specific to the dingy.
These engines are typically prop sized for putting on small fishing
prams etc. Typically they have much longer waterlines etc.
With all engine/dingy combos it is the pair that much work together. The purpose of the account was to reinforce that as a fairly minor change in pitch had a significant change in performance. A smaller pitch allows less initial load on the engine
so it 'spins up' faster allowing for more HP to be delivered. An increased pitch has more bite with each revolution produces more thrust but the engine
works harder. If the boat planes very easily then the increased pitch will give a higher top end, conversely if the boat does not get up on plane easily the engine is overproped for the boat and decreasing pitch has the effect shown.
Net is that the boat planes easier but I sacrificed a little bit of top end speed.