Captain's Post #10 - “Whatcha' Burn?”
By Larry McCullough
Having grown up in California the question you hear is a bit different, “Whatcha’ burning Dude?”. In the boating world, as opposed to California, it is referring to how much fuel do you burn when cruising.
A couple of days ago while here in Fort Lauderdale a 55 foot Nordhavn pulled into the marina next to us. We invited the owner, who just took possession of the previously owned boat, and his captain over for a drink. The owner was doing a shakedown cruise with a paid captain to go over systems and practice the handling of the boat, just as we had done.
We got to talking about fuel burn and I must say the conversation was enlightening. What surprised me was that at Displacement Speed our boats burn a similar amount of fuel (I thought my larger engines would result in much higher consumption).
What is displacement speed you ask? Displacement speed is determined by obtaining the square root of the boat at the water line, multiplied by 1.34. Independence is 51 feet at the water line, with a square root of 7.14, multiply by 1.34 equals a displacement speed of 9.5 knots (about 10.9 Statute MPH). A term that people like to use is where is the sweat spot,, which would be that speed where you are achieving an efficient speed without pushing up the fuel burn disproportionately. This number is less than the displacement speed, about 10% less. For Independence I have found that spot at just under 9 knots, around 8.7 knots.
I have had many hours at the helm figuring these numbers out, keeps the mind active, in many different situations. Probably the most accurate calculation was when we were on the Great Lakes on a windless day with very calm water. No tides or currents to deal with but the boat was in fresh water, not salt, which I would assume could be another factor. At 8.7 knots I was burning 8.7 gallons per hour, or 1 gallon per 10 statute miles. At 9 knots we were burning 10 gallons. So to go just 3.4% faster we were burning 11% more fuel!
Displacement vs Semi-Displacement: Independence is a semi-displacement boat, which means that if you give it more fuel, and have the Horse Power, the boat will partially rise out of the water and achieve a greater speed beyond 9.5 knots. A boat with a displacement hull can not go faster than it’s displacement speed, no matter the amount of horse power or fuel fed to the engines. If you were to look at the hulls of a semi displacement boat compared to a displacement boat you would notice that near the bow they are very similar, but as you go aft the semi starts to flatten out, giving it the lift.
The lift comes at a price (remember from a prior post I am parasomnias), a pretty steep price. We have had Independence up to 22 knots, the fuel burn, 75 gallons an hour! Below is a chart that I received from Nordhavn when I was doing research on possibly buying Independence. As you can see from the chart, the faster you go the MPG drops dramatically.
(Cost per 10,000 miles based upon $3.00 per gallon)
Why have a boat that burns that much fuel, you ask? It is nice to have that extra speed when you want it or need it; out running a storm, or trying to get into an unfamiliar area before dark. The most glaring example was when we were on the East River, just off Manhattan approaching Hells Gate. The current was well over 5 knots and we were only making 5.6 knots and burning over 15 gph.
Back to the discussion with the new Nordhavn owner. We started talking about fuel burn and I was surprised at how similar our burn rates were. I did some more research and came upon a performance review article of a Nordhavn 60 from Boattest.com
“The Nordhavn N60 has a LOA of 62’6” (19.05 m), a beam of 18’ (5.49 m), and a draft of 6’8" (2.02 m). With an empty weight of 138,000 lbs. (62,596 kg), full fuel, water and three people onboard, we had an estimated test weight of 156,905 lbs.
“With the single 325-hp John Deere run up to a max 2200 rpm, the speed topped out at 9.8 knots. Now no one in their right mind would run her at that speed for an expedition passage. The 16 gph fuel burn is almost double that at 1800 rpm while gaining only 1.1 knots. At 1800 rpm and 8.7 knots she burns 8.7 gph, and can keep running for just under 2250 nm”
Performance of Nordhavn 60 from Boattest.com
I must admit I was surprised that our boats both burned 8.7 gallons at 8.7 knots. The chart Nordhavn provided me for fuel burn at a slow speed, I found to be way off, but I am still happy with the burn and the additional distance we can achieve. I found that at 7.5 knots Independence burns about 5.5 gallons per hour (a range of about 1,400 miles with a 10% reserve. At 7 knots I have found I burn about 4.5 gallons an hour for a range of over 1,550 miles).
This is an apple to orange comparison and, to each his own. Nordhavn is known for its’ displacement yachts, aka trawlers, that have circumnavigated the globe and have made many ocean crossings. Independence is not made for that type of cruising, that is why they call it a “Coastal Pilot”, but it is perfect for our cruising lifestyle.