W.F. McCann of Roswell, GA, asks:
I am closely following your adventures around the world! I'm also very interested in the Nordhavn 40. In the Nordhavn 40 brochure, the performance chart shows a range of 7,878 miles at 6 knots with a fuel flow of 0.78 GPH. At 7.01 knots the chart says range is 4,897 miles and 1.47 GPH.
The Nov 11th On Board Commentary entry says actual fuel flow at 5.8 knots and 1400 RPM is approximately 2.0 GPH. What accounts for the difference in fuel consumption on your passage to Hawaii?
The range graph in the original brochure 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 1700 and found 7 knots with consumption at 2.6 GPH. We have 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 (speed/length) ratio of 1 (5.95 knots) will require 30% more horsepower. At 6.54 knots and 7.14 knots (S/L 1.1 and 1.2) the increase is 32%.
are the numbers:
Boat Speed ----------------------------------------5.95 knots -- 6.54 knots -- 7.14 knots
Research Prediction at 40,000 lbs
Shaft Horse Power ------------------------------18.9 SHP --- 23.80 SHP --- 37.80 SHP
- using standard formula for 52,000 displacement
Shaft Horse Power ------------------------------ 24.9 SHP --- 31.45 SHP--- 49.95 SHP
performance prior to passage
Shaft Horse Power ----------------------------- 32.0 SHP --- 40.00 SHP--- 56.00 SHP
Comparing heavy weight calculation with the actual - we see a 28% increase in actual horse power required at 5.95 knots - a 27% increase at 6.54 knots and a 12% increase at 7.14 knots.
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 while running induce significant drag as the hydraulic pump which drives them consumes horsepower from the main engine. In addition, during moderate offshore sea conditions at 6.5 knots - with the fins turned on - the speed can drop as much as a 1/2 knot - to 6 knots - a percentage of almost 10%. 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.
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 as 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 an 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% 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 it on the 40 just prior to our departure. We noted an increase in performance of about 10% 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% 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. One of the reasons we are making this trip is to develop better methods of prediction that take into account the true performance of the modern equipped vessel, but also to know the effects on performance that the wind and sea will have.
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