Fuel Range

Sans Souci is now about a third of the way up the Pacific Coast of Baja, running in fairly calm seas, and having a great time. The weather is cooler, at 77 degrees, and the fishing has begun. When I phoned the boat this morning, they were slow answering the phone. I couldn’t figure out why, until they explained that they were just reeling in their third Dorado of the morning. Jeff said the fish were jumping all around them. Morale is at an all-time high.

I asked if they dug any deeper into yesterday’s problem with the steering, and he said they had done a lot of experimenting and could not get it to repeat. All is working flawlessly. This implies the problem could be nothing more than “operator error” .. but, it’s too soon to say. 

On a different topic…

David Sidbury, owner of the second Nordhavn 68, just completed a two-day east coast passage. He had onboard another N68 buyer, who is still waiting on his boat, but wanted to see how the N68 handled rough seas. From David’s description, the buyer met his goal of finding rough seas, and the N68 performed like a champ. Jeff said the same thing this morning when I asked about how the boat handled bucking the 25 knot headwinds all day yesterday. Jeff said they were a non-issue, and he was continually amazed how well the boat performs in rough conditions.

One of David’s goals for this passage was to get some good data on fuel consumption and range for the N68. I’ve done a limited amount of testing, but I haven’t had the discipline to run at slow speed for an extended period. Most of my data is based on 10-20 minutes of run-time. David’s is the first test of extended cruising in real-world conditions. His goal was to determine what speed he would need to run at to do a 2,100 nautical mile passage (the distance across the Pacific to Hawaii).

Here’s his numbers:

Fuel 3,000
Reserve 300 10%
Available 2,700
RPM 1,200
Speed 8
Fuel Burn (gallons/hour) 7.02
Fuel Burn (gallons/NM) 1.14
Fuel Burn (nm / gallon) 0.88
Range 2,369
RPM 1,025
Speed 7
Fuel Burn (gallons/hour) 6.25
Fuel Burn (gallons/NM) 0.89
Fuel Burn (nm / gallon) 1.12
Range 3,024

Both David and I are VERY pleased with these numbers. They indicate that the passage should be easy at around 7.5 knots.

That said, assuming Nordhavn saw no issues with the added weight, I would probably still load on an additional 500, or 1,000, gallons of fuel, in a fuel bladder, so that higher speed would be possible. We crossed the Atlantic at roughly 8.2 knots, and it felt like a good speed.

-Ken W

7 Responses

  1. The chart for 8 knot speed should have 9.1 gallons per hour fuel burn rate if per gallon distance traveled is .88 nautical mile.
    Just my thoughts.

  2. Rob: I looked at this a year ago, and found a formula which sizes sails based on horsepower needed. Unfortunately, a sail that would move Sans Souci would be enormous, and totally impractical.

    As you mention, the kites are starting to be viable on large freighters. I looked into these at the time, and decided that the deployment mechanism for a giant kite is too large for my boat (or, that’s my memory).

    My primary interest was in kites as a “get home” capability, not to save fuel. Sooner or later, if I cruise long enough, my boat will be struck by lightning. I don’t like the idea of being “out in the middle” with no ability to move. A kite that would move me at any speed would be better than nothing. However (and, once again, this is from ancient memory), if my boat has its electrical and hydraulic systems knocked out, I won’t be launching massive kites.

    Fuel prices are high, but overall, fuel is far from my largest expense. When looked at over the life of the boat, as a percentage of the total cost of boat ownership, I doubt fuel ranks very high on the list. The big costs are: depreciation (what you buy it for, less what you sell it for), moorage, maintenance, spare parts, electronics, tenders, crew, delivery fees, property taxes, sales taxes, etc). In the life of my boat, let’s say I travel 40,000 miles, that would be around 60,000 gallons. At $6 a gallon that equates to $360,000. That is a huge pile of money, but it is roughly the same as the sales tax was in Washington State, and that’s only one of the costs.

    -Ken W

  3. I’m a N62 fan and love the traditional lines. With all this fuel consumption algorithms being formulated for long distance voyages I’ve been giving some thought to Wind Power… I know this is a reach…but has anyone ever talked about using a Dynarig or Kitesail?
    I wonder with all the foredeck space on a N62, would it be possible to install a midsized Dynarig to aide in propulsion for long hauls or even as an emergency get-home option. I’m not talking about Tom Perkin’s Maltese Falcon, but would it be interesting to have a basic rig that could harness the wind. I’ve seen many N62 with forward masts, which are mostly used as tenders hoists, but I’m sure this type of rig could be more securely fastened to support some canvas or modern day square rig.
    The other method is kites. Many large commercial vessels are experimenting with kits to reduce fuel consumption and prices.

  4. Mark: I looked at the fuel calculations again, and still don’t see the error. Are you sure there is one? The miles per gallon increases as the rpm declines, as it should. And, the numbers feel consistent with what I’ve observed.

    As you mentioned, the numbers given are in US gallons. If anyone British is reading this, then the fuel efficiency is actually about 20% better than these numbers. I should also mention that these numbers are based on a twin engine N68. A single engine boat would probably have 15 to 25% greater range. Lastly, David’s and my boat are both heavier than a standard N68 might be (if there is such a thing). Both of us equipped our boats for worldwide cruising, and have virtually every option one could imagine.

    One other thought on this topic which is probably the most important one, and the one I really focus on – Range at low speed isn’t really as important as range at high speed. Here’s what I mean… If you spread out all the “trips” you are going to make during the boats life, and look at them, you are going to see that most trips are short. I have zero stats to refer to, but my hunch is that 99% of boats, even Nordhavns, spend most of their time making trips of under 200 miles. Then, of the other 1% of trips, those that we call “passages”, I would bet that 99% of those are under 1,500 miles. The number of passages that are made, that are over 2,000 miles in power boats, is staggeringly low. Roberta and I fit into the “crazy” category of worldwide cruisers, and during the nine years we had our N62, which included crossing the Atlantic, we never made one. Our N68 is likely to do one or two, but its not something that happens every day. The key stat to me, that is where I really believe the range focus should be is on: Does the boat have range adequate to do the runs you are going to do 99.9% of the time, at full speed? David’s fuel stats focus on runs at 7 and 8 knots. To be honest, I’m using David’s data because I don’t have data of my own. I like to go fast, and am more focused on “What’s the fuel burn at 10.5 knots?” than I am concerned about 7 knots. I do back off a little, for fuel efficiency, but generally I cruise from 9 to 10 knots.

    So, particularly because we will be crossing the Pacific next summer, I am paying attention to range calculations, but I do hope to never really take them too seriously. As currently planned, our longest passage on the crossing from Seattle to Japan is only 1,100 miles. I can do that at any speed with almost half my fuel left over.

    The bottom line: Range is important, but it’s not at the top of the list. For me, safety is the #1 issue, followed by comfort. Nordhavns are amazing in both these categories. And, on the “range” issue, I can go 1,900+ miles at 9.5 knots. This gets me anywhere I want to go 99.9% of the time, and for the other .1% of trips, it takes me an extra day to get there. No big deal.

    -Ken W

  5. Ken,

    I think that there is an error with the fuel consumption data. Gal/nm and nm/gal are the wrong way around on scenario 1. End result is better range than the calculation. Strangely the higher speed produces more efficiency in your figures.Vaiable generator use/load or probably a result of the difficulty of obtaining accurate readings.
    Worth reminding European readers of your blog that you are talking about US gallons (smaller than European ones).This fuel consumption will look worse to Europeans than it actually is.

  6. Ken-When you go to David’s website it comes up ok, but there is no data. Any ideas as to why. Maybe you could post the website address again, just in case I have it wrong.

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