fuel consumption

[Glenn D]

I have a 30 sedanbridge with twin 270hp and it feels best at about 3200 rpms and according to gps if I put my tabs down a bit I pick up speed ??? does that sound correct
Glenn D

 

[NorCal Boater]

I have a 30 sedanbridge with twin 270hp and it feels best at about 3200 rpms and according to gps if I put my tabs down a bit I pick up speed ??? does that sound correct
Glenn D

I also pick up a bit of speed with a bit of down tab. I was told it's because it pushes the stern up and the props are at a more efficient running angle.

 

[sprink56]

I would imagine any RPM range, cruising, that keeps the secondary's on the carbs out of the equation is a good start. I would think that a 20K-22K cruise around or just under 3000 RPM would give you the best economy..

Well by 'economy' I mean around 30-32 gph... @cruise...

When I was shoping for my boat I was seriously looking for re-powers with Multi Port Fuel Injection. I knew that a Quadra Jet carbed Big Block Chevy would be brutal on fuel consumption. Those secondaries aren't opening as a function of RPM. The secondaries on a Rochester Quadra Jet (or any other vacuum secondary carb) open when the manifold vacuum reaches a certain level that overcomes a spring that holds the air valves on the air horn shut. Multi Port FI engines are truly delivering fuel based on many more factors including load, throttle position, prop selection, temperature, etc. Better system...period.

I had previous experience with this vacuum secondary business in the '80s with a 242 SS Formula w/a single 260 Mercruiser Alpha 1. I made great gains with that set up by replacing the intake manifold with an Edelbrock "Torquer" and using a square bore Holley "Double Pumper" (USCG version) manual secondary carburetor. I had complete control over when the secondaries opened...it was strictly a function of throttle position. I don't remember what the numbers were GPH wise but I do recall that there was a vast improvement. The boat also performed much better. I think I pcked up 3~5 MPH with no other changes.

I find that my 390 likes 3300~3400 RPM with full tab and with medium load gives me 21.5 kts (about 25 MPH). I top out at about 4400 RPM at 28.5 kts ( 32.75 MPH).
I am running 19 x 18 prop with a cup on B/W 1.88 gears.

My boat is 16000 lbs dry so add easy 2000 to that with 1/2 fuel (150 Gals) and 50 gals H2O w/passengers.

This brings up two interesting points:

Larger gas engines like the 8.1 Merc MPI Horizons kick standard power up to 385 and torque up from 450 ft. lbs. to 525. These motors don't use any more GPH than the 7.4.

Diesel!! The diesel version of my boat w/ twin 3208 Cats push my boat to 36 MPH at 2900 RPM thru 1.5 gears turning a 20 x 25 prop at probably 20~25 GPH.

From a fuel consumption point of view, diesel wins every time because of the torque produced.

What will be interesting is how the new generation of supercharged 6.2 liter gas motors perform in this arena. They weigh half of what the big blocks weigh and produce 550 HP & 550 ft. lbs of torque...3208 Cat territory at 1/3 the weight.:thumbsup:

Stay Tuned...

I have no real idea what my fuel consumption numbers are but would bet that it's quite a bit better than the quadrajet version of a 340 HP 454.

 

[swift1]

The most efficient is just up on plane because you are not pushing all that water out of the way. With my single big block and B3 I burn 6-8 gals averaging 8-10 MPH
or about 1MPG but at 25 MPH it burns around 12 GPH or 2 MPG.



1995 290DA