If you have a few cruises under your belt, you will know from experience that, in the Mediterranean, long “demotorations” during your summer vacation are the order of the day. As a result, a performance propeller will save you time (by increasing the speed of the motor boat) and fuel. Standard boats often come with fixed blade models: this means more friction, less maneuverability and, not least, faster axle bushing wear. So why not make an investment and throw in a propeller with retractable or steerable blades? Bruntons Propellers’ Autoprop is a true best-seller among cruise passengers: with automatic variable pitch, it adjusts it autonomously when motoring with sails hoisted, and when sailing, it can reduce drag by up to 95 percent. In addition, the shape of the blades is identical in both forward and reverse gear.
HOW THE TEST WAS CONDUCTED
We show you in the tables below the results of the test carried out, with a three-blade Autoprop H6 model with a diameter of 504 millimeters (propeller price is 2,898 euros + VAT), aboard a 2004 Jeanneau Sun Odyssey 43 Legend (Yanmar 75 hp engine with 35mm diameter shaftline transmission). This boat was chosen because it is a cruising model with good market penetration. The test took place in two different sessions, always with full water and diesel tanks, so that the propeller could be tested both with flat seas and no wind, and with wind and sea against and under sail.
IN PORT AND AT SEA
In port, the speed at idle engine speed, just engaging the gear, was 2.5 knots. This peak was reached with a slight but continuous progression, without creating problems for maneuvering in the harbor. Having then suddenly engaged reverse gear, the boat stopped within seconds. Autoprop blades automatically increase their pitch as the forward effort due to increasing draft decreases: this feature allows for good control of the mooring boat even in high wind conditions. Of the “classic” evolutionary effects of the propeller, no trace (the boat responded immediately and equally between starboard and port maneuvers). At sea, with little wind and flat seas, we went from a minimum speed of 6.4 knots at 1,000 rpm up to 9.5 knots at 3,000 rpm (pushing to a maximum of 3,200 rpm, we do not go any harder since the critical hull speed has already been reached), with a stiff breeze and rough seas, the speed, at the same rpm, dropped by about 0.3 knots on average. All test speeds and conditions were recorded in the summary tables above. At a speed of 6 knots, it took 13 seconds to stop the boat by engaging reverse gear.
THE GAIN WHILE SAILING
When sailing, turning off the engine with the forward gear engaged, Autoprop’s blades flag following the flow of water greatly reduces, compared to a fixed model, the brake due to the propeller surface. Each blade is independent of the others, and in this way each can be positioned according to the actual flow of water even as sailing gaits and maneuvers change. The effect of Autoprop on the Jeanneau 43 was noticeable, managing to increase the average speed, according to the owner, by almost a knot (sic!) over the previous configuration with a fixed propeller.
IN MOTORSAILING MODE.
With this propeller it is possible to carry out combined sail-engine navigation at minimal engine speeds. By releasing the propeller shaft and turning on the engine while sailing, the blades orient and position themselves on the correct pitch for the sail speed you have at that precise moment. It will then be sufficient to take the engine off idle speed to have a considerable help during sailing crossings with little wind, without sacrificing acoustic comfort.
Autoprop’s propeller has proven to be a reliable product for both motoring and sailing, significantly increasing (again according to the boat owner) cruising speed while simultaneously reducing engine rpm (and consequently fuel consumption). On the 75-horsepower Yanmar engine used for the test, the reduction in fuel consumption by more than 30 percent, for this owner’s specific boat use, generated a noticeable return in terms of fuel cost savings and increased cruising range. Autoprop is definitely not a racing propeller, but under sail it still manages to provide excellent performance when mounted on boats that are used for cruising, long sailings, or just day trips for fun.