But So What? She'll Cross Oceans Handily

The announcement that Pacific Asian Enterprises was introducing a new, small trawler was met with excitement and no small dose of skepticism.

This was the offending paragraph, which we and other media published:

With 900 of fuel and a cruising speed of 9.5 knots, the N41 should have a range of 972 miles. If you slow that down to an ocean-crossing speed of 6 knots, that range extends to more than 8,200 miles. 

If true, Nordhavn and its design partner Vripack would have created the fiberglass equivalent of the Impossible Burger--tastes great and good for the planet. Alas, that statement of performance lacked context.

I caught up with PAE Vice President Jim Leishman the other day to talk about the 41. Leishman and I got to know one another during the Nordhavn 40 around-the-world voyage back in 2003. We spent two weeks together on the Oman-Djibouti-Suez leg of the record-setting circumnavigation.

Since the 41 is replacing the now-discontinued 40, there was some basis for comparison. The single-screw 40 arrived at Hawaii from California with just 40 gallons of fuel remaining in its 920-gallon tank. The distance: 2,100 nautical miles. Yet the 41 with twin 75-horsepower diesels was being touted as being able to nearly quadruple that range with 900 gallons. What gives?

Leishman explained that these were Vripack's numbers, an advanced fluid dynamics analysis that did not try to factor in the real world. For example, he said, wind and waves are not factored, nor are underwater appendanges such as stabilizers and thruster tunnels that cause drag. Another form of drag is on the engines in terms of alternator loads and hydraulic take-off loads for the stabilizers.

"There’s no question, it's going to be radically efficient but it’s not going to be that good," Leishman said. "We’re going to have wait until we do some in-the-water testing. The predictions I would make would be significantly less but much better than that 40."

Leishman said he is hoping to see 3,500-mile range, which is still outstanding and way more than needed to make the passage to Hawaii, which is among the longest in any circumnavigation.

Below is an analysis Leishman wrote almost 20 years ago comparing the real-world performance of the original Nordhavn 40 with the tank test predictions done by BC Research. Those original BC predictions also showed astonishing performance but comprehensive testing before the start of the Nordhavn 40 circumnavigation showed numbers that were considerably different:

The range graph...is based upon the predictions of BC Research in Vancouver and is a result of their tank testing of a scale model of the 40 which they built. In the test configuration the predicted weight was a 1/2 load displacement of 40,000 lbs. In actuality the 40 which is going around the world weighs in at about 52,000 at the beginning of a passage.

Additionally the test model was not fitted with stabilizing fins, bow thruster tunnel or wing engine shaft, strut and folding propeller. I spent a considerable amount of time testing our boat prior to the passage to Hawaii and found that running at 1,400 rpm and consuming 1.9 gph, we averaged 6.3 knots in moderately calm water. 1500 rpm gave us 6.7 knots and consumption was 2.2 gph. I ran tests at 1600 and found 7 knots with consumption at 2.6 GPH.

We had been running the boat all summer to and from Alaska - primarily at 1800 RPM - burning about 3.6 GPH and making about 7.8 knots (the boat was always lighter for the Alaska cruise). There is a significant difference in performance from the extreme load condition where I conducted the tests and the predictions developed by BC research. As far as I can figure the difference in weight can explain a lot.

Using a standard formula of performance calculation, I found that by increasing the weight of the vessel from 40,000 lbs to 52,000 lbs, the prediction of horsepower to drive the boat at a S/L ratio of 1 (5.95 knots) will require 30 percent more horsepower. At 6.54 knots and 7.14 knots (S/L 1.1 and 1.2) the increase is 32 percent.  

There is no calculation used to predict the drag of the active fin stabilizers, the bow thruster tunnel or the wing engine shaft and prop. The stabilizers are the biggest drag component with two 6 square foot fins (total 12 square feet) deflecting up and down - through an arch of about 60 degrees. These fins not only induce significant drag while running as the hydraulic pump, which drives them, consumes four full horsepower from the main engine. The power consumption accounts for an additional fuel burn of almost .25 gallons per hour at any rpm selected.

In moderate offshore sea conditions at 6.5 knots--with the fins turned on--the speed can drop as much as 1/2 knot - to 6 knots - a percentage of almost 10 percent. At lower speeds frictional or drag resistance is the primary force to overcome however as speed increase up to 1.2 to 1.3 times the square root of the waterline (speed length ratios) the primary resistance force to overcome transitions to wave making - thus we see only a 12% reduction in performance at 7.14 knots,  illustrating that drag from these appendages is not hurting us as much at these higher speeds.

Another big consumer of power is the alternator, which are known to be very inefficient. I have heard it said that a DC alternator is only about 38 percent efficient – which means that for every horsepower consumed by the alternator – only 38 percent of that power is converted into wattage. In other words to produce one horsepower of wattage (750 watts) 2.6 horsepower has to be delivered to the alternator. On our around the world boat – with the large freezer aboard – I estimate that we were consuming an average of 70 amps of DC power – 24 hours a day. 70 amps times 14 volts equals 980 watts is 1.3 horsepower multiplied by 2.6 equals 3.4 horsepower – plus there is inefficiency due to the belt loss. I suspect that the alternator is drawing approximately 4 horsepower – again adding about a quarter of a gallon per hour.

As with the hydraulic draw (maintaining 1200 PSI of pressure–regardless of whether the fins are moving or not) the alternator draw is consistent and at lower power settings –the draw becomes a larger percentage of the fuel consumption.

The balance of the performance reduction is probably the propeller - we spin a 4 bladed 28 by 24 inch prop. A huge amount of effort has been put forth to make the boat a quiet and smooth running as possible. Over the years we have done testing on propellers and found that a three bladed prop will give better performance at lower (ocean crossing) speeds S/Ls between 1 and 1.2. Above that the three bladed propeller becomes more highly loaded and begins to cavitate. Normally the NORDHAVNs are run at a S/L ratio of 1.3 (for the 40 this is just under 8 knots) and the three bladed prop offers no advantages and is quite noisy. We installed a three bladed prop on Salvation ll for the final leg of her circumnavigation from Hawaii to California. The three bladed prop gave a 20 percent increase in Salvation ll's range at 6.5 knots however it was subsequently removed and the four bladed prop was reinstalled for coastal cruising because of the vibration and cavitation. I did order a new three bladed 30 by 24 inch prop and tested in on the 40 just prior to our departure.

We noted an increase in performance of about 10 percent but also noticed the characteristic vibration, which was anticipated. Despite the performance improvement - I reinstalled the four bladed propeller in the interest of a quiet and vibration free boat. The point of this is that the BC research predictions were based upon achieving propeller efficiency of 50 percent and I don't believe we are achieving that with our present propeller selection and don't believe that we can unless we're willing to accept noise and vibration.

To recap: We've got a much heavier boat than what was originally tested and it has a lot of drag because of the accessories we install to make voyaging safer, more comfortable and easier. We have been aware of the effect of weight and drag on all of our vessels and find that oceans are crossed at much lower speeds than what the same vessel makes during coastal passages. 

Nordhavn 41 rendering.

Nordhavn 41 rendering.

"So the predictions for the new 41 are likely overstated by a considerable margin in our opinion," Leishman said. "These are not our predictions but done using established methods." He said the main differences to consider when comparing the new boat (N41) with the proven results of the Nordhavn 40 are the following:

  • N41 has reduced displacement over the 40’s real 52,000 full load displacement which we believe will be controlled during the build process bringing the 41 in at her designed displacement
  • The N41 hull has been modified based upon a complete Computational Fluid Dynamics analysis performed by Vripack Yacht Design. This will be a more easily driven hull than the N40
  • The N41 has better propeller efficiency due to clean water flow into the propellers
  • The N41 has a lower profile and reduced wind resistance than the N40
  • The N41 has a much longer waterline than the N40

The Nordhavn 41 purchase contract explains the predictions provided by Vripack and NavCad are not guaranteed and the actual performance will only be determined during seatrials and even then the ocean crossing performance will only be confirmed after oceans are crossed. 

Drawing

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