Fuel economy and range: Open Ocean Crossings

I found this post by James Hamilton interesting. He wrote software to monitor fuel consumption with realtime feedback at the helm. The guy certainly is a techno geek!

Fuel economy and range | MV Dirona


Jim
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I think what he did is great. If and when I cross oceans I'm going to want something similar.

Richard

A nice read.

The display is amazing. Almost too much information.

The average speed was 6.6 knots for a Nordhavn 52..

Next he needs to integrate the program with his throttle to automatically adjust the speed

The average speed was 6.6 knots for a Nordhavn 52..

About right perhaps 49 ft LWL, so 7K is sq rt.

Multiply by .9 for LRC add ,,,,6.6 is about what would be expected.

FF said:

The average speed was 6.6 knots for a Nordhavn 52..

About right perhaps 49 ft LWL, so 7K is sq rt.

Multiply by .9 for LRC add ,,,,6.6 is about what would be expected.

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Is that a standard formula for optimal fuel burn?

menzies said:

Is that a standard formula for optimal fuel burn?

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There is no formula for "optimal" fuel burn.

If you want greater fuel efficiency (higher MPG), go slower. It always gets worse the faster you go, and better the slower you go.

If "optimal" means the best trade off between fuel consumed and getting to your destination within your life time, then it's purely a personal decision. The more fuel you are willing to burn, the faster you can get to your destination. That was part of the point of James's post. They discovered that they can actually run faster on long passages that originally thought, partly because at low power their engine reports higher fuel burn that actual.

I think the optimal fuel burn for m/v Dirona (from reading the blog) was to arrive at their destination as quickly as possible while still having 200 gal in the tanks.

Richard

From what I know so far, for me to take Stillwater to Hawaii I'd probably be running in the 6-7 kt range. I like what they did on Dirona.

Richard

"Is that a standard formula for optimal fuel burn?"

Close the sq rt of the lwl times .9 to 1.15 is the most common speed sweet spot.

Fat heavy boats will go slower than the lighter skinny ones, but as a simple rule of thumb, its fine.

Note it is almost IMPOSSIBLE to set up a boat cruise with good efficiency with out a fuel map, or BMEP graph.

It would take a flow-scan EGT gauge , GPS and a dozen prop swops to figure out the info the mfg usually refuses his customers.

The usual power and prop graph assumptions will not do the job.

Britannia said:

I think the optimal fuel burn for m/v Dirona (from reading the blog) was to arrive at their destination as quickly as possible while still having 200 gal in the tanks.

Richard

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Yes, for a passage that's exactly what's needed. The distance is fixed and the available fuel is fixed, and the result gives you an MPG number that you need to meet or exceed in order to make it. Then you go as fast as you can while remaining above the required MPG number. They ended up varying speed quite a bit due to ocean currents.

"I think the optimal fuel burn for m/v Dirona (from reading the blog) was to arrive at their destination as quickly as possible while still having 200 gal in the tanks."

WHY , do they not enjoy the blue water?

We have usually only varied speed to arrive in daylight when customs is open , to avoid overtime charges.

James Hamilton did something else that is interesting. He went one size up on the JD engine from what is normally installed in the N47/52. In doing this his range was potentially shortened so fuel curves became doubly important.

Usually engines are over sized as car or farm conversions are far less expensive for the boat assembler than an industrial style engine.

They seem oversized as they are rated at peak, short term power ,not 24/7 power.

The rule of thumb 3 cubic inches for each HP at the normal cruise setting works for simple 4 stroke non electric injection ,no turbo, no inter cooler engines .

FF said:

"I think the optimal fuel burn for m/v Dirona (from reading the blog) was to arrive at their destination as quickly as possible while still having 200 gal in the tanks."

WHY , do they not enjoy the blue water?

We have usually only varied speed to arrive in daylight when customs is open , to avoid overtime charges.

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I can't speak to that. But considering that we're talking about two weeks at sea doing 6.6kt on a boat capable of 9kt, it's understandable not to want to go any slower than necessary.

Richard

In addition to the fuel economy, comfort is the other consideration. We've had to slow down or go off course just from a comfort standpoint. Going a knot under hull speed is noisy and can be very tiring on long trips.

A 58 ft Roughwater, Sawn Song, cross to Hawaii, averaging about 6 kn. She had anti rolling tank for stability. She was for sale. Fuel tank size is 1200 gallons, but carried an extra 200 on deck in a blader. It you want to read about the anti rolling tank and the crossing, do a search of Sawn Song or Roughwater 58. The anti rolling tank was mounted on the pilot house roof so it did not effect the speed or range.

sunchaser said:

James Hamilton did something else that is interesting. He went one size up on the JD engine from what is normally installed in the N47/52. In doing this his range was potentially shortened so fuel curves became doubly important.

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I believe due to EPA emission standards that engine is now standard on the 52. Not to mention he had 500G in bladders.