Fuel (In)Efficiency

[BTreanor]

Greetings. New Catalina 36 owner. New to cruising on my own boat (as opposed to friends' boats). New to the forum.

As I say, I have a Catalina 36 and I'm starting cruising this month. Looking for rough, general guidelines on how efficient the engine will be. I've got a slightly larger fuel tank (32 gal.), and I'm trying to estimate just how much fuel I'll be burning per hour at half throttle. One gallon? Three-quarters of a gallon?

Thanks for any advice/help.

Best regards,
Brian

 

[SFS]

Many diesel engines burn between 0.38 and 0.45 pounds of fuel per hour for each horsepower developed. Diesel weighs 7.2 pounds per gallon. You can calculate your approximate burn rate at various throttle settings (rpm settings) if your engine manual has a chart or graph of horsepower vs. rpm settings. If you don't have such a graph, and you post your engine make, model, and rated horsepower, it is likely that someone here has that information.

Remember as you progress on your journey, Google searches are your friend. Welcome to the forum.

ETA: Said another way: Diesels burn about 1 liter per hour for every 10 horsepower being developed. This was borne out by our experience. Our Yanmar was rated at 23 hp at 3600 rpm. Therefore, at 2800 rpm (our cruise setting) I suspect I was developing about 17 hp, which would be 1.7 liters per hour. That converts to 0.449 gallons per hour.

THE best way to know how much fuel you have burned out of your tank is to keep a spreadsheet over time of your fuel fillups. Over a long enough usage, the different throttle settings involved for the activities of cruising, docking, anchoring, speed reductions for bridges and traffic, etc. will coalesce into an "overall fuel burn" rate, which is extremely reproducible. See Stu Jackson's post here (and maybe check out the whole thread):

Fuel consumption and navionics

Hi, I am setting up the boat info in Navionics and I have come to the fuel consumption part. I have a 2007 Hunter 33. I know that asking what fuel consumption would be is like asking how long is a piece of string but I have to put a figure in. So any suggestions? I am new to this boat and at...

forums.sailboatowners.com

 

[Davidasailor26]

Welcome to the forum and to boat ownership!

What engine do you have? @SFS’s numbers sound pretty good to me. Our 30 HP Yanmar 3YM30 on a 37 foot boat uses about 0.7 gallons per hour on average with every fill-up, cruising at 2800 RPM’s. We go about 7 knots at that engine speed.

 

[dmax]

If it's a Universal M-25 or m-25XP (3 cylinder), it is generally about 1/2 gallon per hour, which jibes with SFS's calculation. You should consider joining the C-36 association, lots of boat specific knowledge: Welcome Aboard! | Catalina 36/375 International Association

 

[Stu Jackson]

If it's a Universal M-25 or m-25XP (3 cylinder), it is generally about 1/2 gallon per hour,

I've been keeping diligent track of my M25 since July 1998. It's 0.5 gph. I've "collected" input from many, many others that confirms this.
SFS is right. It's what I did way back then, and still do.

 

[BTreanor]

Thanks for all the advice. Much appreciated. The engine is M25-XP.

I'll definitely keep a spreadsheet as we go to get a better sense of how my particular boat operates. The question came up because our fuel indicator gauge seem to be a bit wonky (as they seem to be on basically every engine--terrestrial, aerial, or aquatic--that I've ever used). Want to be conservative in my estimations on fuel consumption, and everything else, during my first few weeks, and develop some experience before I start "pushing the boat further out," no pun intended.

Thanks again for the prompt answer and friendly advice.

 

[Hayden Watson]

This is a spreadsheet from a C42 I found on a forum somewhere and thought it revealing. This guy did very careful measurements of fuel, miles traveled and speed over many years. His process was after each fill-up, he would pick a motoring rpm for that tank and always motor at that speed and take recordings of distance, average speed etc. He is much more methodical than I am, so I have just used the principals he found. I typically motor at 1900-rpm at 5.4-kts with my M25XP and get a safe 200 miles with a good reserve from the 17-gal tank on my C30.

 

[Stu Jackson]

I've read maybe hundreds of these fuel gph posts over the years. Many of them "promote" checking gph at different rpms. I think we all can understand that an ICE always uses less fuel at slower rpms. I believe, however, that it is a meaningless fact when that engine is used in a boat. Why? Because unless you ONLY motor at close to idle speed all the time (for whatever reason) then whenever you are using your engine you ARE going to be at a throttle range of beyond 50% to 80% of WOT which is where the average fuel consumption occurs. I have been observing boats for 35 years. Rarely do I see them merely "idling around" when out of their harbors. I motored out of the Oakland Estuary for 30 years and rarely saw dawdlers. I don't see any here in the Gulf or San Juan Islands either. Point being: doesn't matter about the reality of less fuel used at lower revs 'cuz that's not how we operate our boats.

Here's my longer story:

Fuel Log 101

Fuel Log

 

[SFS]

It's also difficult for many new sailors to think in terms of fuel usage in terms that are so different from cars. How "far" I could go on a tank of fuel never entered my mind while on the boat. There are so many factors (tides, winds, currents, wave effects, bridge closures, etc.), each of which can be helping or opposing you minute-to-minute, that the concept of "miles per gallon" simply doesn't work.

Train yourself now to think in terms of how much running time is left in the tank, rather than miles. The only way to do that is to become thoroughly familiar with your overall fuel burn rate, and to know how many hours you've run your engine since you last filled up. Also note that many sailors will not burn the last 1 or 2 gallons out of the tank, in order to avoid introducing chemical and/or physical contaminants into the remainder of the fuel system and engine. Those last bits of fuel are not reserves, they are repair avoidance buffers.

 

[jssailem]

Train yourself now to think in terms of how much running time is left in the tank, rather than miles. The only way to do that is to become thoroughly familiar with your overall fuel burn rate, and to know how many hours you've run your engine since you last filled up.

Welcome @BTreanor to SBO. Congrats on your new to you Catalina 36. They as great boats.

What SFS shares is very practical. Some owners need to know all of the engineering specifics about their boat's performance. Maintaining detailed quantities of fuel to the 3rd decimal. That is more work than I am comfortable maintaining. I am of the running time clan. If I put 20 gallons in the boat I know I have approximately 26 hours of run time till Bingo, with a 2 hour reserve. This is at cruise which is about 80% WOT.

I did not know these numbers the first time I put fuel in the tank nor the second or third time I untied the lines and took the boat out on the water. Those first several adventures were like sea trials. I look at the fuel gage like an estimator. When I want to know detail I dip a stick into the tanks that I have marked at full and then divided the dip stick into quarters then eighths. After several fills I was able to learn my engine consumption and the impact this had on the gallons consumed. With each fill I got numerical usage to back up my dip stick visual reference.

While the .7 gallons per hour worked for 5 years, I am back to the start this year. I just had the engine rebuilt. Together we will be testing the new engine's fuel usage. She (the engine burning the fuel) and me monitoring her performance.

The information and detail above is spot on data. You get to decide how you feel comfortable calculating and using it.

One thing I have learned is write your notes down in some fashion. Do it every time. Memory is something not to be trusted when the seas are big and the stress of reaching port is rising.