Praise for Big Props
In Professional BoatBuilder No. 150 contributing editor Nigel Calder’s exploration of the inherent efficiency of larger propellers and their demands on propulsion engines introduced readers to the 78′ (23.8m), 150-ton Elmore, built in Astoria, Oregon, in 1890. Dee and Sara Meek have lovingly restored this former tugboat. Her current engine is a massive Atlas Imperial rated at 110 hp (82 kW) at 325 rpm and weighs 10 tons, for a power-to-weight ratio of around 1 hp per 180 lbs (82 kg). At 8 knots, fuel consumption is reported to be around 4 gallons (15 l) per hour. The Elmore is equipped with either a 58″- or 62″-diameter (147cm or 158cm) propeller (the engineer couldn’t remember which.
Big propellers have to turn slowly. The Elmore has a direct drive between the engine and propeller shaft, with a peak engine/shaft speed of 325 rpm and a “cruising” speed well below this. This video shows the Elmore maneuvering at engine speeds of about 160 rpm.
Below are two other videos of massive Atlas engines running. Note the significant weight of the rotating machinery—pistons, connecting rods, and, an enormous cast-iron flywheel. Once these parts are in motion, they provide the momentum necessary to drive through the inertia of the massive propeller on the Elmore. The net result is efficiencies at the propeller that we can only dream of in lightweight, modern engines.
Calder’s text also introduced the work of Norwegian naval architect Oyvind Gulbrandsen, who ran the numbers on Calder’s own 18-ton Malo 48 (14.6m) sailboat, Nada. Gulbrandsen theoretically optimized the boat at a cruising speed of 7.3 knots in 15 knots of wind speed with associated waves and compared two propellers—a matched 22″-diameter x 18″-pitch (56cm x 46cm) propeller and an oversized 26″-diameter x 20″-pitch (66cm x 51cm) propeller. The analysis is included in Calder’s story, but there was not room for Gulbrandsen’s equally interesting analysis of shifting the propeller design point with respect to small fishing vessels. The pdf document is available here.