David,
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Julian and Andy have summed it up well. I use a 2.5hp Suzuki for my coastal cruising and really rate the engine - very light, economical, pushes a BRe at 4 knots and starts first time. But if I was doing longer more exposed cruises or sailed in Poole Harbour like Julian and wanted the security of being able to punch against a string tide in the harbour enrance then I suspect would get a bigger motor. Most manufacturers' 4hp, 5hp and 6hp engines seem to be the same model with the 4hp and 5hp versions de-tuned. Given that in the EU at least we can only buy 4 stroke engines in these sizes and hence they are bulky units you may as well go for a larger model.
Incidentally, I liked the simplicity and lightweight nature of 2 stroke engines and felt indignant when these were banned. However, having seen that 5 mins of flushing my 4 stroke OB in a bin leaves the water clear whereas my 2 stroke (one of the last produced, so 1:100 mix, etc) turned the water milky I am now pleased to be using a 4 stroke.