Searching for a calculator

[dwiggl]

Anyone know of a good online calculator to chart Fuel Burn/Hull Speed/RPM?
I want to know a bit more about my hull efficiency.
I can do a spreadsheet but don't' feel the need to reinvent the wheel when someone has already geeked out on this topic.
Thanks, D

 

[Shrew]

Too many variables to create a calculator. Each hull is going to run differently. Each engine is going to burn fuel differently in the same hull. Prop size and pitch, gear set ratio. Even tide, current and load will all vary fuel burn.

 

[OldDan1943]

The results reached today may not be true tomorrow.
Some biggies are tide state, currents, wind, temp, cleanliness of hull and fuel loading, water loading, speed, ‘crap on board, etc.
You can create your own, more accurate fuel burn rate chart.
I have a commissioning, sea trial burn fuel burn rate chart. I dont consider it ‘carved in stone’, merely a suggestion.

 

[dannc]

Skene's Elements of Yacht Design and Bebee's Voyaging Under Power have equations that determine HP needed to go at a given speed/speed length ratio(SLR). From the HP, one can look up the needed fuel burn for that HP on a given engine.

Beebe's equation is a easier to use since part of it is looking up values in a curve in the appendix.

Which equation is correct? I have no idea.

There is a variable to tweek in Skene's equation but what is the correct value? I have no idea.

I think one could tweek that variable and get pretty close to Beebe's numbers but is that correct? I have no idea.

Having said that, I do think the equations are useful in comparing different boats, but they numbers have to be taken with a cube of salt, and the numbers are only for flat water, no current, and no wind conditions.

Later,
Dan

 

[rgano]

Look for somebody with the same boat who has a FloScan or similar system installed and get their data. I have a FS and ran the boat in still water to get a good set of data and plotted it in Excel to give lines for GPH, GPM, and SOG vs RPM.

 

[Jeff F]

Often manufacturers will provide prop demand curves that show consumption by RPM. If your boat is propped properly and a semi-displacement design these are very close in my experience.

Just tell us what kind of boat and motor(s) you have

Sent from my moto g play (2021) using Trawler Forum mobile app

 

[dwiggl]

Thanks for the info. I'm not going to overthink it.
I will look at Beebe's book. Haven't read it in years so a good reason to dust it off.
The plan is to pick a calm summer day, w/full tanks and record data. All the uncontrollable variables are to be left out.
I will record: SOG, FloScan burn rate, RPM with the engine sync activated. That's it as all other conditions vary daily.
I just want to see a trend as I've never done any of this research before. Hoping to see a sweet spot for cruising: economy, comfort, vs cost of being in a hurry. I have been told that on some hulls a few hundred RPM could make a big difference in the burn rate. This is based on the relationship with "pushing water" and the engine torque curve.
It's a Carver 45 w/ twin 2 stroke DD turbos. She will go up on plane, but in reality, she is just punching a big hole in the water. At planning speeds (yuk) economy is a non-issue. But somewhere between 700 RPM 1500 RPM there may be a sweet spot. Just want to find it if I can. And it would be a fun, simple analysis. Then I can know Gals/mile to estimate fuel needs for longer trips, maximum safe miles between fill-ups, etc.
And I don't trust FloScan readings as accurate. Just sort of a gauge.
Guess I will run a spreadsheet/chart. Thanks for your comments!