Re: Twin Volvo 305's in 1994 Silverton?
Twin small blocks in that Silverton are an appropriate engine package for the size and weight. Obviously, your speed & fuel consumption are going to vary accoriding to the total weight (and how it's loaded), hull condition and cleanliness, water conditions, current, wind, engine tuning, maintenance, etc. You didn't specify what HP ratings they had and it also will depend somewhat on the transmission packages (gearing ratio) and the size/pitch of the props. It will also make a difference if they're carburetored or fuel injected.
When doing your survey sea trials (you ARE getting a survey...AREN'T YOU???), the surveyor will have a spec sheet for your vessel to determine if it performs according to standards. You (guessing here) will probably get a top speed of around 25-28 MPH and a 3/4 throttle cruising speed of 18-20. I have found that the best part of owning a bigger boat is the slowing down and lightly cruising the waterway. I used to love to fly around in a boat, now I'm in in for the ride.
BTW, not sure what your economic standpoint is, but at 3/4 throttle +, you're looking at .75-1.0 MPG (equates to 25-40 GPH fuel burn)...juss' sayin'. When you slow down and enjoy the ride, you can easily get 1.5+ MPG.
