Fuel burn rate

[Robob]

I have a 175 Osprey with a 35 gallon gas tank and an Evinrude 90 horsepower carbureted 2 stroke engine.

I have read on various websites that an outboard like mine will burn 10% of the horsepower rating (or 9 gallons) per hour at full throttle or about 5,500+ RPM.

If I did my math correctly and I run at 80% of throttle or 4,500 RPM (25 miles per hour) can I can expect to burn 9 gallons X 80% = 7.20 gallons per hour?

Thanks,

Bob on Tampa Bay

[RickK]

You could try out your formula on the way down to and back from the gathering this weekend  
If your calcs are correct you could top off your tank (28 gal?) and it should take you no more than an hour to cruise down at 80% throttle and ditto on the way back leaving about a half tank.
Now you have a legitimate reason to attend  

[Robob]

I would like nothing better than to go boating this weekend but can't get away:-(

Bob on Tampa Bay

[fitz73222]

Hey Robob,
You are correct with the 10% of the rated HP @ full throttle in gallons per hour equation. It seems to work on 2 strokes and I`ve varified it with my flowscan and GPS to calculate fuel consumption for a given trip. The $64 question seems to lie in finding the sweetspot in miles per gallon and gallons per hour in the off full throttle calculations. The flowscan is an incredible tool for doing this. I`ve had my 22-2 with twin 115`s as low as 10 gallons per hour on plane at about 27 mph @3400 rpm`s. and 50 gallons of fuel on board with a light load (3 people). The boat is propped for this with four blade propellers that deliver lots of bite in the midrange. I strongly recommend the flowscan for tweaking the max miles per gallon.

[Robob]

Thank you very much for confirming the info.

Even I can't justify spending almost 500 bucks on a Flo scan instrument.

The old Evinrude seems to have sweet spot around 4,000 -4,500 RPM & mid-twenties MPH on the GPS so I'll just use that and keep track of what I replace in the fuel tank. That's close enough.

Thanks again,

Bob on Tampa Bay

[RickK]

You can get a Lowrance fuel metering package for around $125 - I think that is what I paid for mine.

[John Jones]

I can tell anyone to definitely stay away from the Northstar F210 fuel meter.  It's a piece of crap.  I have one with a failed sensor I'll give someone for the postage.  



I bought one.  Just out of warranty the sensor failed.  They wanted more for a sensor than the entire package cost online.  I bought another package.  That sensor failed after one trip out.  They sent a new one under warranty.  That sensor failed about a month out of warranty.  I let one meter and failed sensor go with the CCP.  I have a NIB meter and bad sensor collecting dust in the garage.

It was downstream of a 10 micron Racor filter/water separator so I don't know what else I could have done.

[John Jones]

Oh yeah, the 10% of HP held true for my 225 OceanRunner.  When the meter worked it showed 23 gph @ 5600 rpm.

[Robob]

Thank you.

I'm done.

Bob on Tampa Bay getting good mileage:-)

[GoneFission]

The flow and mileage numbers get a little screwy as you approach Wide Open Throttle (WOT).  It is typical due to hull drag and unburned fuel going through the cylinders - so the fuel burn rate goes up as a square of RPM and speed toward WOT.  Fuel injected (especially direct injected) engines do better than carbs, and four strokes do better than two strokes, but gas piston engines are just not going to be very efficient at WOT.  :evil:

Notice the numbers below for my CCP - burn rate doubles from 4000RPM to 5300RPM (from 9.1 to 18.7), while the RPMs go up 1300 and speed only goes up 13 miles per hour.  The sweet spot for my boat is 4000 RPM and 27-30 miles per hour (depending on trim), and it seems most Aquasports do best between 25 and 30 miles per hour.     Here are the data for my 22-2CCP:

 

Hope this helps - see ya on the water!

[John Jones]

good info Cap'n John