Hold on a second fellows. Before you talk a member out of this lets get our facts in line. I am going to assume a mercury 4.3 v6 is the standard replacement for this boat. With a alpha 1, it weighs 865lbs according to Merc's website and puts out 190hp. The gas 4cyl puts out 135 hp and weighs 635 also with an alpha 1. The 1.7 liter diesel puts out 120 hp and weighs 635lbs without the drive. I believe I have seen the diesel package with the alpha for $9995.00 advertised here in fla. You can go to this website and request a price quote.
http://www.brownmarine.com/mercdiesel.htm
I remember seeing a shamrock 20' with an inboard 120 yanmar a few years back in boating magazine. It got 30mpg! at idle and about 9mpg at cruise. The range was something rediculous like 1200 miles. Top end just broke 30mph so I would expect a 20' seacraft to do at least that with an io.
Do it! Be the first.