Why 10 knots
Chris, You sold yourself short on the DD argument. They shouldn't be the same price as the teeny weeny yanmars. Rebuilt DD's could be a 50% savings over new tenny weeny anythings. THAT's where your arguement gains some real weight (get it - DD's = weight - lol).
OK, we have to do a little math here; this could get interesting. Why pick 10 knots? If you're buying mini-power, why not hull-speed? Could it be when a person decides to buy 'low power', it is not primarily for cost reasons? I'm from that in-between world of trawlers - not willing to depend on the wind like a snailboat (hey, I like that term), but willing to lope along at hull-speed.
How about some fun with numbers. A little 'rule of thumb' math says hull speed is between 1.2 and 1.34 times the square root of a vessel's waterline length. In the case of the 46 in question that turns out to be 7.7 to 8.2 knots. As you pointed out before, and I agree, these wide body Roamers are not the 'best displacement design', so lets take the less efficent figure 7.7 kts. The shaft power needed to push this boat to hull speed is about 5 HP per ton, and we'll figure the 46's loaded weight is around 20 tons (guestimate). This is using the low end of efficiency, but still comes out to only 100 SHP. Figure for 3-5% loss in drivetrain, 108 crankshaft hp is needed to reach hull speed (18% of 600HP - tw300's). Now to get to your selected 10 knots. Another 'rule of thumb' I've read is double the HP for every 2 knots of additional speed (until the boat 'planes). So 216 HP (total) gets 10 knots. Still a bit short of the 300.
In a few years I'll have my boat in Florida and you can blast past me. I'll accept that. I will smile and wave, because you are a boater and you use your boat (of course I wave at the 'dockers' too, but my smile says something else).
Hey, I'm coming to Stuart, FL in January for Trawlerfest. You wanna see how the other half lives? If we can get together, it could be a very interesting 'cocktail hour'.