Hi Bob,
Apparently, we're in the same boat (figuratively). I'm not that concerned with top speed, but having the power on tap to drive the boat at 95% of hull speed gives you enough reserve at cruising speed to "punch through stuff, if the need arises. At 10,200 lbs, my boat should have a 5.1kW drive based on the 1kW/ton rule of thumb (1 ton = 2000lbs in the original rule, regardless of how others may want to rewrite it). So I installed a drive rated for 5.5kW continuous, and as installed, full throttle pulls 5100W, delivers 0.5 kts more top speed than the old diesel, and has much more thrust when I need it most. The only time that I've run at full throttle for any length of time is for performance testing.
As far as cruising speed, I prefer to go a little faster than some here. I typically motor at 4 to 5 knots. I can swim at 2kts (I once swam 4000 meters in 1 hour). I cruise at 3 - 3.5kts while paddling my kayak. 3kts in my sailboat feels like barely more than drifting. At 4kts, it takes me more than 20 minutes to get out of the marina and into open water. I prefer to spend my time on the water sailing, rather than dodging tourists in rental boats. So my system works well the way that I use my boat. Great range at cruise, and plenty of reserve power to motor into 20kt winds and significant swell. If I had installed a 10kW system in my boat, my top speed would go up by maybe 0.2kts. I've already got all the thrust that I've needed, but a bigger motor would give me more reserve power? I now know that my boat will cruise at 5kts at 2500W, so a 3kW drive would be a sufficient fair weather system, though I wouldn't feel safe in adverse conditions. Which brings me right back to the 1kW/ton recommendation. A number of boats, ranging from 2 tons to 8 tons, as reported in this group, are working well with drives that fall right around the guidelines. Their performance is right around what one would expect based on discussions that have been going on for years in this group. Based on our collective practical experience, your boat will work well with the recommended drive. Your boat will work with many drives outside our recommendations, but our sizing is a good balance between usability, safety and cost. If your use case is really unusual, then you may need a dramatically different drive, but most recreational boaters have similar needs.
If you have more questions, we're here to help.
Fair winds,
Eric
Marina del Rey, CA
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