My sailboat boat is quite similar to your example, overall length 30 feet, waterline length 25' 6", beam is 10'6", draft 5'10", displaces 9,000 lbs, engine is a 16 hp diesel, fuel
tank 15 gallons, cruising speed 6 knots. Although I haven't tested it, the manual says I can go 175 miles per tank of fuel
. Of course this would depend a bit on conditions--current, waves, wind, etc.
On most sailboats, the engine is called "auxilliary", with the sails being the main mode of transport, so I figure if I can get 175 miles, that will get me out of most "spots", and to my next destination if I can't sail for some reason.
Hope that helps.