57’ Catamaran with Twin Oxe Diesel 300 HP – 46% More Fuel Efficient At 18 Knots
BoatTEST evaluated a 57’ (17.37 m) catamaran, Voodoo, powered by a pair of 300-hp OXE diesel outboard engines. We ran her with custom Sharrow props and conventional 5-blade wheels made by one of America’s best outboard prop manufacturers. This was a particularly important test because there are now two brands of outboards being made for heavy-duty commercial, military, police and heavy recreational boat applications.
SHARROW PROPS 46% MORE FUEL EFFICIENT AT 18 KNOTS ON 300-HP DIESEL ENGINES
Because of the distances run each day by Voodoo, her ideal running speed is 18 knots. As can be seen in the chart above, she achieves that speed at 2800 ROM with the Sharrow props, over 700 RPM better than the conventional 5-blade wheels.
OTHER DIESEL/SHARROW PROP APPLICATIONS
A cursory look at other catamarans of about 40,000 lbs. (18,181 kgs.) powered by inboard diesels, both straight shaft and pod-driven, indicates that 300-hp diesel outboard engines equipped with Sharrow props are likely to be significantly more fuel efficient. Many charter fleets around the world are turning to diesel-powered cats, which means they will have the same downtime threats that the owner of the boat we tested faced — and that’s why he turned to outboard diesels.
PROP SHOWDOWN BOTTOM LINE
Virtually all gasoline outboards we have tested have ‘through-the-hub” exhaust. As a result, our test captains could never determine how much of the frothy bubbles emanating from the whirling prop was exhaust and how much was cavitation. But, because the OXE diesel does not exhaust through the prop hub, for the first time we could see the cavitation of a conventional 5-blade prop.
With the ideal running speed of 18 knots, Voodoo’s 2x diesel outboards burned 15.5 gal. with the Sharrow props, 4.3 gal. – or, 21.2%-- less than with the conventional props. Because Voodoo runs most days, the Sharrow props paid for themselves quickly.
The result of the cavitation was that the advance rate of the 17” (43.18 cm) diameter x 12” (30.48 cm) pitch 5-blade conventional prop advanced only 7.40” (18.8 cm) to 8.22” (20.88 cm) with every 360-degree rotation of the prop. On the other hand the Sharrow prop advanced from a low of 7.44” (18.9 cm) to a high of 10.05” (25.53 cm) with its 14.5” (36.83 cm) x 11” (27.94 cm) 3-blade prop. See the chart below.
Because the boat was advancing from 20% to 30% farther with each rotation of the prop with the Sharrow prop, it was naturally the most fuel efficient of the two designs at all rpm settings except one. Studies conducted by the world governing body of the boating industry (ICOMIA), showed that powerboats are used 40% of the time at idle. At that speed, the Sharrow prop was 27% more fuel efficient than the traditional design.
The greatest improvement in efficiency was precisely where the boat’s owner said he ran the boat most of the time — at 18 knots. For that reason, we were told, the Sharrow engineers dialed in the greatest prop efficiency at 18 knots — the 46% improvement in fuel efficiency at that speed was no fluke.
Our test of the two props was one of new technology vs. old technology. In fact, the conventional props used today are basically the same designs as the first ones that were invented in the 1830s. After nearly 200 years man has finally improved on the propeller concept.