RPM & Fuel usage

[ChromeDome]

Overpropped...

You sohuld (read: must) prop your engine to achieve rpm witihn the specified range at Wide Open Throttle - (WOT), at your boat's normal gross weight.

 

[Montemar]

Overpropped...

You sohuld (read: must) prop your engine to achieve rpm witihn the specified range at Wide Open Throttle - (WOT), at your boat's normal gross weight.

Thanks and agreed

 

[WFA]

Engine performance curves may help you choose an optimum cruise rpm ie correlate OEM's torque and fuel \ RPM curves but, as has been stated by others, once an engine leaves the OEM and is installed in a boat, endless performance limiting factors come into play.
You will never mirror an OEM's published figures. However noting fuel added and engine hours run will provide your particular boat's historic fuel consumption curve ie Litres \Hour. For performance, a mental note of RPM and SOG achieved will help build a picture of how well your installation is performing in various conditions - fouling, wind, tide etc. - for comparison to previous trips.
I own a motorsailer and a sport fishing boat, both with diesel inboards. Engine use in either can vary considerably from trip to trip but in general fuel consumption per engine hour remains pretty static within the bounds of my normal use patterns. What is noticeable however, on both boats, is if there is a need for extra revs to achieve an expected boat speed ie passage through the moorings, pushing against the tide or most noticeable, a lower SOG than normal during a WOT thrash.
When planning a trip I just need to know there's plenty of fuel in the tank to meet all eventualities. If I notice once underway more revs are required to attain the required SOG it confirms I picked the wrong trip for the day and\or the bottom needs a clean.
We all like challenges such as the OP's when not boating (hence my post) but a note of fuel used and engine hours run will IMHO provide the most useful guide on which to base your judgement of the fuel efficiency of your particular boating habits.

 

[James L]

If you check that your max revs match design WOT revs then the fuel/rpm curve will be close enough.
Probably worth noting down the speed at each rev point and use that as an indicator as to whether there is more load or not.
i.e. if your speed at a certain rev count is lower then you know the engine is working harder and thus burning more fuel.

 

[Never Grumble]

I'm in the process of making a boat speed/fuel usage chart. Am I correct in assuming that fuel usage is directly proportional to engine RPM? Does anything else influence the fuel usage at a given RPM?

Not really as long as you don't engage the gears.