Fuel Flow Calculation for Univ. M25

[Pete]

Where can I find fuel flow rates for my Universal M25 engine (at different RPMs)? I want to create a range / fuel usage chart.


Thank you,

PC

[Tom in Cambria]

There is a nice graph showing fuel consumption at various rpm in the parts manual on the bottom of the page before the pictures of the parts start. Why it's in the parts manual and not in the operating manual I've never understood, but there it is. The top of the power curve is right around 2000 rpm btw.

[Pete]

Wow, parts manual... who would have thought?

Thanks Tom!

[Pete]

I pulled my service and parts manual for my M25 (at least the copy the PO gave me) and I can not find any chart that speaks to the fuel consumption rates at different RPMs.

If someone has this chart for the Univ. M25 -- and can make a copy of same--- please contact me.

Thanks PC

[Tom in Cambria]

Pete,

I have it and can photocopy it and send you a hard copy via snail mail or can scan it and email it if you send me your address to:
TomCambria at fastpop dot net (supply the symbols for the typed words.)
Sorry, should have offered before.

[Tom in Cambria]

I haven't heard from Pete with an address. Anybody out there know it or can log on and get it for me? I'd be happy to send it to anyone else who's interested in the M25 as well. Just send your address to the above and I'll forward you a copy.

[Matt Cawthorne]

The reason that you can't is that the amount of fuel used is proportional to the torque at a given RPM. Just because an engine is capable of delivering, say, 20 hp at a given RPM does not mean that the boat needs that much power to be driven at the speed that particular RPM will drive it. Therefore, as you drive at lower speeds, your engine is delivering much less torque than it is capable of, and the fuel consumption is much less. For my CD-36 I did speed trials and at the point where my engine started to belch black smoke I consulted the engine curves to determine the power output. I then back calculated the power at lower RPMs and used the cubic power law for a propeller (maybee it was a squared law, I can't remember right now). I then related the power to the fuel consumption and the change in speed at the lower RPM to determine a fuel consumption curve. At hull speed I figure that my boat get's little more than 3 mpg. At 4 knots it is close to 12 mpg. Needless to say, when on long trips where range is an issue I drive at 4 knots.

Matt

[Pete]

Thank you Matt and Tom for your updates. I have been out of town and am just now back.

[Tom in Cambria]

Matt,

You're right, of course and the chart has a footnote that says prop load will affect results. Still, it's some rough indication of how rpm affects things, isn't it? For a given prop load the consumption goes from three quarters of a gallon up to 1 gallon perhour beween 2400 and 2800. It's not something you could count on for navigation purposes, but it seems like it would give you some idea of how rpm is going to affect fuel consumption. And it's handy to know that you get maximaum torque at 2000 rpm and it starts falling off above that, and at 2800 rpm it's down to about 36.5 foot pounds from a high of 40 at 2000. BTW I couldn't find anywhere that the manual is copyrighted.

[Matt Cawthorne]

Tom,
Using the engine curves will be conservative at all but maximum RPMs. You will severely underestimate your range if you cruise at speeds below your maximum powering speed in calm water. On the other hand, if you will be powering into waves, the engine curves will give a reasonable estimate.

Matt

[Tom in Cambria]

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