There are plenty of "purists" on here who think all motors are bad or cheating. Personally rowing flat water in a 30 mph headwind, is not the experience I seek. I regularly bring a motor for (slightly) off season trips, where wind is likely.
As stated already a 5 is sufficient. It will push one or two boats well, or 5-6 boats poorly.
A bigger motor won't go much faster, unless you're a cat, but it will have much better abilities when steering a big pod.
I went the cheaper route and got a 5hp Briggs and Stratton. It works fine, but it is louder than a nicer water cooled motor.
I've used a 2hp from my parents small sailboat, and it worked about the same as one strong dude rowing his ass off. The strong dude rowing his ass off did not have a free hand for his beer.
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