M35B fuel consumption

[knitchie]

S/V Reverie, Hull#447, is in route from VT to our new home in VA. Much of the northern section is unsailable, so we have motored far more than we ever have. The benefit is that it gave us the opportunity to validate our fuel consumption.

At 2100 rpm, we are getting about 2/3 gallons per hour which surprised me, expected only about 1 gallon per hour. With the 39 gallon tank, that is almost a 60 hour (about 350 km) range without sailing. Since my fuel sender doesn't read accurately at all, this is a good data point for monitoring my fuel level.

What have others experience been with their aux?

Ken
S/V Revery
Mallets Bay VT, heading for Hayes VA
2007 Catalina 350 #447

[Air Mobility]

Ken

I have hull 158 with a Universal M35B engine. I normally motor at 2400 rpm and average .6 to .7 gal/hour.

George
s/v Air Mobility
Rock Hall

[Amnesia]

I use about a gallon per hour at 2700 rpms...with an auto prop

[TBOT422]

My experience has been much in line with what Air Mobility noted. Over 8 years, I have typically used something around 0.6 gal/hour as a rule of thumb. We typically cruise around 2300 RPM. But, of course, in a span of 4-6 months between fuel purchases, there is a lot of time below that and some time above that. Looking at the Universal Performance chart below, it seems to imply the fuel consumption at 2400 RPM would be over 1 gal/hour. So, I am assuming that the Universal chart is assuming 2400 RPM and maximum load. I am assuming that since most of us are not running at maximum load at 2400 RPM, our fuel consumption would be less than the Universal chart. Does that make sense? I assume that fuel consumption is dependent on both RPM and load. I.e. if the transmission is disengaged, the fuel consumption at 2,400 RPM would be significantly different than running at 2,400 RPM with the prop engaged. Different props create different loads and thus different fuel consumption at 2,400 RPM.

I am trying to work up an estimate on fuel consumption per mile rather than fuel consumption per hour in order to make a guess at an optimal operating RPM for long range cruising segments.

M-35B Performance.JPG (101.41 KiB) Viewed 3423 times

[knitchie]

I assume fuel consumption is also effected by the prop used. With the Flexofold, I find we get between 2/3 to 1/2 gal per hour, depending on conditions. A neighbor with the stock fixed 2 blade gets far less and can hardly go 5 knots at 2400.

[wolfe10]

While fuel consumption at a particular RPM is of interest, I am much more interested in fuel consumption at a given boat speed which equates to how far you go on that amount of fuel which equates to miles per gallon.

No question, fuel consumption with an undersized prop (i.e. engine under light load) WILL be less PER HOUR at a given engine RPM than a correctly speced or oversized prop (engine under load).

Kind of like running your car at 2500 RPM in 2nd gear (pretty slow, but almost no load) vs 2500 RPM in high gear. 2nd gear would give a far lower fuel consumption per hour for that same RPM, but also much, much lower speed.

So, yes, miles per gallon is more relevant to me, whether in a car or boat.

[TBOT422]

Yes Brett, that is what I am looking to do. I can easily get some data points of SOG at various RPMs, but how would I get gal/hour data by RPM that seems close to what I am actually observing. The Universal Performance chart seems to be about twice as high as what most of us experience in the real world.