I Love Classes That Make You Think! Part I

Today is Thursday and I have a bareboat class going as I write this. Class started Sunday and will continue next Saturday and Sunday. Last night I received an email from one of the students with a list of questions she had come to mind over the past few days. Each and every question was really good. Some were easy and are already in the lesson plan for the next two class days.  Others made me think.  The two questions that made me think the most are “What is the fuel burn rate to operation for the boat we will be taking?” and “How do you figure the estimated amps used vs. battery capacity and charging.”  My answers in the email; as to the first question, 3/4 to 1 gallon per hour; with the second question I tried a delaying action by saying “This answer is too long for an email.  We can talk about it in class.”  For 9 out of 10 people, these answers would have been good enough.  Not this time.  I was quite impressed when I got the next email asking for more in-depth information.  That’s how “a captain” approaches things.  So, for you captains out there, here is a more complete explanation.

Fuel Burn Rate:  Unless you happen to be in Death Valley, running out a gas in a car isn’t much more than inconvenient.  Running out of fuel in a boat puts the boat and everyone on board at risk.  Unfortunately, without an accurate fuel flow meter (not something most boats have) how much fuel you are using is a question that is always going to be a best guess.

In looking at the engine manual for a 50 horsepower motor found in many Catalina 42’s, fuel consumption rates are listed as varying from .55 to 1.3 gallons per hour.  A number of variables enter into the calculation.  Propeller pitch, revolutions per minute, wind, current, and waves are all factors.  Even the condition of the bottom is a major factor, meaning the same boat in the same relative conditions may consume more fuel per hour if the bottom hasn’t been cleaned in three months.  Over time, experience with a specific boat is going to give you a good idea of the normal fuel consumption for that boat.  For example, I crewed on a trip from Cabo San Lucas to San Francisco on a Catalina 42.  We found that we averaged .75 gallons per hour (GPH).  In that case, we were motor sailing close hauled at approximately 2700 RPM.  This was moving us nicely along at about 7 knots.  2700 PRM is a good cruising speed for that particular motor, so I would keep that part of the equation.  If you are not motor sailing, then add some consumption … how much I am not sure, however I would think .25 GPH might be a reasonable overestimation.  On that trip, we had some great conditions.  If things were a little “bumpier” maybe add another .25 GPH.  At this point, we are up to about 1.25 GPH, and I would be comfortable using that figure for most conditions on that specific boat.  Now, lets use that figure in something practical.  That same Catalina had a 48 gallon tank.  Always leave a reserve … in this case lets say 1/4 tank, or 12 gallons, giving us 36 gallons of usable fuel.  At 1.25 GPH, that allows motoring for 28.8 hours, at 7 knots a range of about 200 miles.

That works great if you are motoring straight through.  How about when you motor for a day, then sit at an anchorage for 3 or 4 days.  During the time in the anchorage you run the motor to charge batteries.  Maybe that’s where the .55 GPH comes in.  If so, don’t forget to count that time when figuring range.  So let’s say you anchor 8 days over a two week bareboat charter, running the motor 3 hours a day to charge your batteries (whether or not that’s enough is the topic of “Part II.”)  You just lost about 13 gallons of your 36 available gallons, meaning you only have about 23 gallons available to use.  A range of approximately 130 miles.

How does all this work in real life?  I love Mexico.  Chartering out of La Paz is outstanding.  A great plan is to go as far north as Agua Verde, about 100 miles away.  I hope you have good wind, because if not, given the above numbers you are going to be pretty much out of fuel about 70 miles short coming home.  I know, I know, that extra 12 gallons of reserve will get you 67 of those miles.  Close but no cigar.  You are still out of fuel.  And you are taking a chance on sucking all sorts of nasty stuff (like algae and water) off the bottom of the tank, clogging the fuel filters and possibly the injectors, meaning a sizable repair bill.

Even on a day sail all of this is good stuff to know.  Always check your fuel level.  I would recommend not trusting the gauge.  Always check the tank itself.  Know how much fuel you are starting with and an estimate of your hourly consumption.  I like to use 3/4 GPH for the Bronze diesel powered boats, 1 GPH for the Silver Fleet boats, and 1.25 GPH for Gold Fleet boats.

Be safe out there.  As a friend of mine likes to say, there are three types of sailors.  Beginning sailors … paranoid sailors … and retired sailors.  A little paranoia regarding fuel consumption is not a bad thing.

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5 Responses to I Love Classes That Make You Think! Part I

1. Bruce says:

Really useful information for potential cruisers. Thanks.

2. Wayne Matzen says:

Don; Great article! I crewed on a boat out of Astoria, Or. several years ago and the skipper relied on a survey to determine fuel tank capacity. It was incorrect leaving us out of fuel off the Oregon coast with no wind after 48 hours of motoring. After a tow by the Coast Guard it made me pay even closer attention to fuel capacities and burn rates on boats on which I am crewing and not just trusting the skipper to know these things. Another factor that seems to increase burn rate is the loaded weight of the boat with lots of extra gear loaded for an extended offshore cruise.

3. Dougal Maclise says:

I have some issues with your explanation. Mainly, fuel consumption RATE should be based on RPM only, unless your octane or air quality changes (such as exhaust leaking into the engine compartment or motoring at high altitude such as Lake Tahoe). DISTANCE travelled per gallon is another thing and is affected by all the points you bring up. That’s the engineer in me talking. The somewhat novice sailor in me wonders if boaters consider fuel consumption rate to be more like what I’d call mileage (gallons per mile). This of course is affected by throttle, drag, octane, air quality, currents, weight, etc. Where did I go wrong?

4. Don Gilzean says:

Dougal … Even though I’m not an engineer, common sense told me the same thing. Apparently it’s not quite the case. RPM is definitely the major contributing factor in calculating gallons per hour, however, the load on the propeller, which is influenced by a bunch of things can cause GPH to vary.

5. Greg Gorbach says:

I don’t know if I understand the previous comments. I am not an engineer but I know that the load that an engine is under affects the rate of fuel consumption. There are many applications that require an engine to maintain a constant RPM. The greater the load the greater the fuel consumption so the weight of the boat, condition of the bottom etc do affect the fuel consumption. Likewise going uphill in a car requires more fuel than driving on the flat in the same gear hence the same RPM.
Having said that sometimes a small increase in engine speed can result in a great decrease in fuel economy. Sometimes there is a good reason to push the speed in a boat such as tides and daylight. Many times easing off a few hundred RPM can save fuel and wear and tear on the engine and drive train. Charging the battery at anchor is basically no load and coupled with reduced RPM results in greatly reduced fuel consumption.
I have been victim of believing a previous owner as to the size of the (day) tank. Also not only can the fuel Guage be inaccurate, the tachometer can be too