posted 08-11-2009 09:23 AM ET (US)            

http://www.whaler.com/site_media/Performance_Data/305CQ_EnginePerf.pdf

The report shows the fuel economy at optimum cruise speed of 26.4-MPH to be 1.35-MPG. This is at an engine speed of 4,000-RPM. The data was collected with the fuel tank level at 150-gallons and with only 355-lbs of crew weight.

In your configuration the boat has 150-gallons of additional fuel, or about 940-lbs more fuel. Your crew of five probably weights 1,000-lbs, or about 645-lbs more than Boston Whaler's test crew. At a minimum we can figure the boat is probably 1,600-lbs heavier than when tested by Boston Whaler. In addition, if you are heading out to fish, I suspect you have additional equipment aboard. Let's say your boat weight is 2,000-lbs greater. How can we assess the influence of this weight on the performance?

To assess influence of weight on performance, we turn to Crouch's Calculator. First we look at the performance in the test report. We see:

LBS = 12312
MPH = 26.4
HULL FACTOR = 180

Using the calculator we predict the horsepower to be 265-HP. We compare this with the fuel flow data, 19.6-GPH. This gives us a Brake Specific Fuel Consumption figure for the engine at optimum cruise of

19.6-gallon/1-hour x 6.25-lbs/1-gallon x 1-hour/265-HP =0.46-lbs/HP-hour

That is a reasonable figure for a four-cycle fuel-injected engine operating at optimum.

Now we increase the weight on the boat to 14,312-lbs and see how much horsepower we need to get back to 26.4-MPH: the calculator says 308-HP. If we assume the BSFC is the same, we will be burning

308-HP x 0.46-lbs/HP-hour x 1-gallon/6.25-lbs = 22.7-GPH

This fuel flow will reduce our fuel economy to

26.4-miles/1-hour x 1-hour/22.7-gallons = 1.16-MPG

We find that the additional weight being carried has reduced the fuel economy to 1.16-MPG from the test data's 1.35, a decline of about 15-percent in fuel economy. This demonstrates that fuel economy is inversely proportional to weight, and there is quite a sensitivity. You can further expect that the fuel economy reported in the test data was likely produced in optimum sea conditions. Also note that the report has an explicit disclaimer about other boats being able to reproduce this same performance.

Now we turn to your request. You report an average fuel economy of only 0.9-MPG, but you have not given us any data about the boat speeds being used when this fuel economy is obtained. You wish to improve the fuel economy to 1.3-MPG, an increase of 0.4-MPG. This is an improvement of 44-percent. Can we obtain a 44-percent improvement in fuel economy simply by altering the propeller selection?

There is a reasonably consistent trend with four-blade propellers that mid-range fuel economy is typically better than with a three-blade propeller. That is a very general statement, and subject to many arguments. Based on this general trend, you might try four-blade propellers such as the Mercury REVOLUTION4. This propeller is now being made in low pitches like a 15-inch pitch. However, I would be surprised if the fuel economy improvement was as dramatic as the 44-percent boost you are seeking.

The only way you can really get back to a fuel economy figure close to the test data's 1.3-MPG is by reducing weight in the boat and operating it at the optimum speed for fuel economy.