In a message dated 2/26/05 12:01:55 AM, John writes: << testing 80'-240' double-ended vehicle ferries, some single and some twin screw, have shown that your quoted tests are correct except that a locked prop presents more drag than a slowly rotating one. One caveat: it may well depend on the prop size and vessel speed, and what speed the "slowly rotating" prop is turning at. Our tests were done at approximately 9 knots. I'd be pleased to have more info on your sources for these tests. >> The free spinning vs. locked prop drag tests were performed by Douglas Phillips-Birt, a British NA. They were reported in Yachting World over 20 years ago. Similar tests were reported over the years by other boating journals, mostly devoted to sailing. Douglas Phillips-Birt anchored a boat in a rapidly moving tidal stream and used a test rig consisting of a propeller and a prony brake arrangement that could let the prop spin with various amounts of resistance. A spring balance measured the amount of prop drag. The drag was least when the prop was allowed to spin freely, then increased rapidly as turning resistance was added. Drag was at a maximum when the prop was spinning at 1/2 to 1/3 of its normal speed at that flow rate. Drag then decreased with a locked prop to a value slightly greater than with a free spinning prop. With a locked prop, the drag is roughly equivalent to that of a flat plate with an area equal to the prop blade area. Ideally, one would let the prop spin freely for minimal resistance but in the real world there is enough friction from the shaft bearings and the reduction gear to slow the prop down to the high drag range. In addition the slowly turning prop makes noise and may harm some gearboxes. For many boats, the best compromise is to simply lock the prop. How much drag does a locked prop cause? Unlike most of the scholastic arguments on the TWL, I have some practical data on this one. I have an older Willard Horizon motorsailer, PUFFIN, essentially the Willard 30 trawler hull with a mast and sail. The Willard has a full keel displacement hull with gently rounded chines, a high bow and a rounded stern. The LWL is 27.5' with a displacement of 16,000 lbs of which 4000 lbs is internal ballast in the keel. Beam is 10.5' and the draft is 3.5'. The design is similar to that of small sail working boats of a century ago with all that that implies. It is easily driven below hull speeds and has good seakeeping qualities but tends to roll in beam seas. Power is supplied by a Perkins 4-107 driving an 18" x 14" prop. The boat carries 260 sq. ft. of sail on a low aspect rig, a large foresail and a smaller main. This is only about half the sail that a cruising sailboat of similar specifications would carry and PUFFIN can be considered to be sailing under perpetually reefed conditions. It can best be considered a 30/70 motorsailer. On a calm day and with a clean hull it requires 23 hp. to drive PUFFIN at its 7 kt. hull speed. This estimate is confirmed by careful fuel consumption measurements kept over several years. The best speed I have ever gotten under sail alone in a beam wind is 5 kts. It takes approximately 8.2 hp. to move the boat at this speed under power. Sailboat designers estimate that sails can produce about 1 hp. for each 27 sq. ft. of area under good conditions. The 280 sq. ft. of sail on PUFFIN should generate about 10.4 hp. of propulsive effect. The 1.4 hp. difference between the 10.4 hp. generated and the 8.2 hp. required to move the boat at a 5 kt. speed is undoubtedly due to the drag of the large non-feathering prop. In essence, the prop drag costs 21% of the generated sail power. Most sailboats minimize prop drag by using the smallest blade area prop that will propel the boat, efficiency be damned, or by using folding or feathering props. Motorsailer owners compensate for prop drag by keeping the engine ticking over at a speed just above idle. This spins the prop at about its freely rotating speed but does not contribute significantly to propulsion. Larry Z