for Salalah, Oman, 1,377 miles to the northwest. The Indian Ocean continued to provide ideal conditions, light winds out of the northeast and mild seas. With clear skies and temperatures in the low 90s, we were glad to have our air conditioning, which kept our staterooms a cool and dry 70 degrees. The gentle breeze flowing through the open doors and window of the saloon and wheelhouse kept temperatures within the upper decks comfortable. These ideal conditions allowed for
plenty of rest, and the galley and aft deck barbecue were in constant use. Brian Saunders emerged as the official ship's chef, however, he acknowledged my barbecuing expertise, and the cocktail hour usually began about the time the Magma was lit. With comfortable reclining chairs, a balmy breeze, setting sun, dinner on the grill and a cool beverage in hand, voyaging-- and life--didn't get any better. Each morning Brian and Paul Grover would monitor prearranged frequencies on the SSB to receive
information from sailboats around Salalah. We were hearing more and more discussion of boats leaving Salalah in convoys, that there were some 30 yachts anchored and waiting to depart, and talk of German navy ships gathering at the southern end of the Red Sea. To enter the Red Sea we would have to pass through the Gulf of Aden and clear Socotra Island, just west of Somalia and the Horn of Africa, which has been a dangerous area for years with many reported pirate attacks. A course to Salalah
PRIMER Do you have adequate range to reach your destination?
C
rossing an ocean in small motorboat is much like flying across a continent in a light airplane. You want to be absolutely certain of how long you have before you must refuel. One of the chief challenges of crossing an ocean in a trawler yacht is the careful management of speed and fuel consumption. It’s a vital function that leads to great satisfaction at the successful conclusion of a long passage. Our circumnavigation underscored the importance of knowing the range-speed capabilities of your particular vessel. It demonstrated that tank tests are just tank tests and computer predictions of range are merely predictions—and only for calm conditions. BC Research in Vancouver tank-tested a scale model of the 40. In the test configuration, the predicted weight was a 1/2 load displacement of 40,000 lb. BC Research produced a range graph that indicated an economical speed of 7 knots would give a range of up to 4,000 nautical miles. In real life, the circumnavigating 40 weighed in at about 52,000 lb 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—nor was air conditioning running. Prior to the passage to Hawaii, I spent a considerable amount of time testing our boat. The N40 fuel system has a supply reservoir that is designed to allow precise fuel-consumption checks over 15-minute intervals. Running at 1,400 rpm, we averaged 6.3 knots in moderately calm water and consumed 1.9 gph. At 1,500 rpm, we averaged 6.7 knots at 2.2 gph, and at 1,600 rpm, 7 knots with consumption at 2.6 GPH. We had run the boat all summer to and from Alaska primarily at 1,800 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 74 · CIRCUMNAVIGATOR 2003
formula of performance calculation, I found that by increasing the weight of the vessel from 40,000 lb to 52,000 lb, the prediction of horsepower to drive the boat at a S/L ratio of 1 (5.95 knots) will increase by 30 percent. At 6.54 knots and 7.14 knots (S/L 1.1 and 1.2) the increase is 32 percent. Here are the numbers: Boat speed
5.95 knots 6.54 knots 7.14 knots
Speed/length ratio
1.0
1.1
1.2
BC Research prediction at 40,000 lb in shaft horsepower
18.9 shp
23.80 shp
37.80 shp
Adjusted using standard formula for 52,000 displacement in shaft horsepower
24.9 shp
31.45 shp
49.95 shp
Tested performance prior to passage in shaft horsepower
32.0 shp
40.00 shp
56.00 shp
Comparing heavy weight calculation with the actual, we see a 28 percent increase in actual horsepower required at 5.95 knots, a 27 percent increase at 6.54 knots and a 12 percent increase at 7.14 knots. There is no calculation 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 6.5 full horsepower from the main engine. The power consumption accounts for an additional fuel burn of about .325 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 which is almost 10