In the November entry, Capt. Tom Caywood referred to issues they were having with fuel management. An experienced captain who conducts many seminars on trawlers, Tom frequently receives questions regarding fuel issues. He decided that the topic was worthy of an entire separate entry and enlisted Boundary Waters owner Ron Benson to help explain the fuel challenges they faced.
Chapter 1 - Enroute, San Diego to Cabo San Lucas:
Lacking a fuel maker (it was backordered), we're being even more careful about that necessary commodity than we are about the water issues. There are calibrations on the sight gauges on the fuel tanks but, because this is a new boat to all of us, we're being very conservative so that there are no unpleasant surprises.
To begin quoting Ron, "Our objectives on the fuel issue are to determine 1) total tankage and 2) fuel burn rate at various rpm's."
This Nordhavn 50, Boundary Waters has 4 fuel tanks:
|Main, Port and Stbd
||535 gals each
|Aft, Port, Stbd
||125 gals each
|Total reported capacity
"Each main tank has a sight gauge. Aft tanks do not have gauges. The typical procedure would be to transfer fuel from aft to main tanks (via a fuel transfer / cleaning system already installed, although each tank can be set up as the supply tank). As you know, diesel engines return unburned fuel to a tank for later usage. Nordhavn recommends returning unburned fuel to the same tank it was drawn from to avoid the chance of overfilling a tank.
"In preparation for departure from San Diego, we visited the fuel dock. Prior to fueling the sight gauges indicated a total of 320 gal aboard in the main tanks. Aft tanks were assumed empty. The fueling process is straight forward. As suggested by Nordhavn, I estimated the amount needed by subtracting the amount in each main tank from its stated capacity. Accordingly, I added a total of 720 gal. to the main tanks [720 added to the existing 320 gal, (plus an estimated 30 gal space to allow for expansion). Fueling the main tanks proceeded as expected. (It should be noted that this main tank sight gauges do not extend to the top of the tank which requires the estimating process.)"
"Fueling the aft tanks presented a surprise. The port tank took 89 gals and the starboard tank took only 70 gal. (instead of the expected 125 gal each). The optimist in me says there was extra fuel in that tank, placed there before I took possession of the boat. The pessimist in me says that the tanks are not as large as advertised. The only certain way to determine is to pump them dry with the transfer pump and then fill them again, which we plan to do on this trip. Incidentally, it should be added that the N-50's vent tubes are immediately adjacent to the fill tubes so a warning of a full tank can be determined by sound. Not a drop of fuel reached the water at my fill tubes . Nice touch, Nordhavn!"
"With respect to fuel burn rate, more data is needed. The N-50 is not offered with a 'day tank' and I chose not to include a fuel burn meter as accuracy is (reportedly) not the best. I hope to develop some speed / fuel tables during this trip for future trip planning. We are currently running at 1575 rpm with the speed over ground (sog) generally in the 8.2 to 8.4 knot range. Some ground speeds have reached 9.2 to 9.4 knots no doubt reflecting the impact of the south flowing current.
"Initial guesstimates of fuel burn were in the 6.5 to 7.5 gal per hour range, which is well over the 5 to 6 gph expected. However, accurately reading sight gauges in a boat underway is not something I have completely mastered. Also, I transferred fuel from the aft tanks for 1 hour each, which I estimated (from sight gauge changes) transferred 30 to 40 gal per tank. The transfer pump is rated at 65 gph, but normal losses, including passing through a transfer Racor filter no doubt limited the amount transferred. By Tuesday I should be able to empty the aft tanks and, when they are filled, we will answer the aft tank capacity issue. Also, once we reach port, (target is now Cabo San Lucas) we will be able to read sight gauges in calmer water and get a better feel for the burn rate at 1575 rpm.
"Incidentally, the target speed of 8.3 knots will result in 200 miles per 24 hours, a nice easy number to remember!"
Chapter 2 - After Fueling In Cabo San Lucas
"The fuel capacity issue is much closer to being resolved. During our two-day stay, I transferred remaining fuel from the aft tanks to the main tanks with the fuel transfer system. When we refueled, each aft tank then took 117 gal. Most noteworthy, the main fuel tanks appear to hold more than the expected 535 gal. By adding together fuel purchased to what I read on the sight gauges, the starboard main holds 590 gal and the port main holds 548 gal. Therefore, if correct, the total capacity of this N-50 is 1,372 gal, some 52 over the advertised total capacity. Needless to say, main tank calculations are subject to sight gauge variations, depending on the observer.
Chapter 3 - After Fueling In Barra de Navidad
"With the relatively short passage of some 361 nm, from Cabo, we just topped off the main tanks. Again depending on my sight gauge reading ability, I estimated total capacity of 550 gals. for starboard main and 535 for port main. Not as much as estimated in Cabo, but well within advertised total capacity. We will further refine these tank capacity estimates by refueling in El Salvador, beginning with one main tank pumped completely empty.
"With respect to fuel burn, we averaged 6.4 gph for this leg. This included 31 hours on the generator (a 12 kw Northern Lights). Assuming (that word, again!) 25 gals. for the generator (3/4 gph), our main engine burn rate was 5.8 gph.
Chapter 4 - After Fueling at Barillas Marina
Interesting update: we've discovered that the generator is supposed to be using 1.2 gph which changes all our previous computations!
We fueled here at Barillas yesterday, Sun 12/15, putting a total of 842 gals. (@ $1.85 per gal) into the boat. Most interested in the capacity of the port main tank, we'd completely emptied it so that we could find out exactly how much our 535 gallon tank actually held. After putting in 611 gallons, we were amazed to calculate that it held 76 gallons more than advertised!
There was also excellent news on the main engine fuel consumption. Subtracting 166 gals. for the generator (1.2 gph for 138.5 hours) gave us a fuel burn of 676 gallons or 4.95 gph for the main engine over the 1,025 miles since our last fill up.
We intend to make our next leg to Balboa (the port area immediately adjacent to Panama City) in Panama at a steady 1,500 rpm so that, after fueling there, we'll have a still more information for our fuel burn log.