While most sailors have propellers on their boats, few give it much thought until the prop meets an immovable object and requires a bit of mending. A basic understanding of the importance of this piece of equipment, however, and the important role its proper selection plays in the speed, range and fuel usage of your boat, can help enhance your voyaging experiences for years to come. For all propulsion systems, including propellers and sails, some basic principles apply. It takes a certain amount of effort to move any boat through the water, and that effort increases as the speed increases. The required effort is referred to as resistance or drag. It can be measured by towing the boat (or a model of it) and placing a scale in the towline to measure the pounds of pull being applied at any given speed. It’s very similar to weighing vegetables on the hanging scale at your local supermarket. The number of pounds and the speeds are then plotted to create a graph of speed vs. resistance, sometimes taking into account various heel and yaw angles that will be encountered under sail. This graph is used directly for sailboats, balancing pounds of resistance against pounds of force from the sails, but for powerboats the pounds of resistance are converted to horsepower. Since this is the horsepower that is actually effective in moving the boat, it is logically referred to as effective horsepower (EHP); it does not take into account mechanical and propeller inefficiencies. These must be figured in to get the required engine power, expressed as either brake horsepower (BHP) (measured at the engine crankshaft without reduction gear) or shaft horsepower (SHP) (measured at the reduction gear output flange), an amount which is much higher than the effective horsepower. Regardless of whether you’re under power or sail, extensive voyaging is always done at displacement speed or some lesser cruising speed. Attaining this speed with the least effort is your goal, and you can achieve it in a number of ways. The first requirement is to reduce the resistance of the boat as much as possible. Select a boat with a displacement or semidisplacement hull form; planing hull forms have much higher resistance at displacement speeds. While semidis-placement hull forms offer a slightly higher top-end speed to the voyaging yachtsman, they also have a bit more resistance at displacement speeds. This is a trade-off that can only be decided by the owner, taking into consideration his intended voyaging routine. Once the general hull form has been selected, the specific hull lines will also have an effect on resistance, but this is a concern that is left primarily to the naval architect; there is little the boat owner can do to change this once the boat is built.Displacement and drag Another area at least partially and sometimes substantially under the owner’s control is the displacement, or the total weight of the boat ready for sea with all supplies, spares, and crew aboard. All other things being equal, a lighter boat requires less power roughly in proportion to its weight (e.g., an 8,000-pound boat needs 20% less power than a 10,000-pound boat to reach a given speed, whether the power comes from sails or an engine, or a combination of both). When a boat comes into a yard for a major refit, it is not uncommon to see a truckload or two of the owner’s “stuff” going to a warehouse for safekeeping during the refit. The best thing the owner can do for himself and his boat is to sort through this collection that has accumulated over the years and leave the non-essentials ashore, either in storage or in a dumpster. If you’re going on a one-week trip, you don’t need to carry a case of 24 oil filters and seven five-gallon cans of lube oil with you. If your sailing is mostly coastal and near home under predictable conditions, you might consider leaving the extra sails, spare props, and major engine parts in the garage rather than in the lazarette or forepeak. In addition to reducing weight, keep your hull and appendages, including the propeller, shaft, struts, and rudder, as clean as possible. The added weight and roughness of marine growth has a substantial negative effect on resistance, and thus on speed and power. Also, keep in mind that a propeller’s blades are foils similar in section and scientific principle to an airplane wings. Marine growth on the propeller can destroy its thrust, just as ice on a wing can destroy its lift.Optimization Now that you’ve moved all the extraneous stuff ashore and have a clean, freshly sanded, newly painted hull, it’s time to turn your attention to the mechanical propulsion system, specifically the selection of propellers, engines, and reduction gears. Optimizing propulsive efficiency allows the boat to go faster and farther with less effort, saving time and energy. Powerboats use their engines 100% of the time, of course, but sailboats can also benefit substantially from attention to the basics. In recent conversations two leaders in the sailboat industry, designer Bill Langan at Sparkman & Stephens and builder Bob Johnson at Island Packet Yachts, estimated that voyaging sailboats may use full or partial engine power about 50% of the time and that for motor-sailers the figure may average 70% or more. A check of your logbook will let you know what your average is, and you may find it higher than you thought. For purposes of propulsive efficiency, as for navigation, a sailboat under power is a powerboat. We’ve already mentioned EHP, BHP, and SHP, but let’s revisit them for a moment. Engine suppliers normally quote BHP (brake horsepower, so named because it’s measured on a dyno brake hooked to the engine crankshaft at the shop) at full rpm, because it’s the highest number and looks good when you’re comparing Brand C to Brand D. The more up-front guys will also give you the figures for SHP (shaft horsepower, measured where the reduction gear connects to the propeller shaft) over a full range of rpms. Shaft horsepower takes into account gear friction losses and is typically about 3% less than BHP. Why then must engine power be so much higher than effective power, sometimes twice as high? The answer is propulsive efficiency, as determined by the propeller characteristics and gear selection. It is a basic but easily overlooked fact that the engine does not propel the boat, the propeller does. With a big engine and a bare shaft, you go nowhere. A higher-horsepower engine without the right propeller will not produce more speed, but it most certainly will burn more fuel and reduce your voyaging rangenot much of a benefit for all that extra money! Many times, the engine comes with the reduction gear ratio selected by the supplier, and that selection may be based more on what he has on the shelf than what the boat needs. Unfortunately, this is often as true for new boats as it is with repowers. The prop is then selected solely on the basis of getting the rpm up to engine’s limit, and the resulting efficiency can be poor. If you want to find out if your engine supplier or builder has done his homework, just ask for the overall propulsive coefficient (OPC) of the propeller/gear combination as installed in your boat. This is the ratio of the effective horsepower to the engine’s shaft power (OPC = EHP/SHP) and is almost entirely dependent on the efficiency of the selected propeller, as installed, operating at a rotational speed (rpm) and loading determined by your boat’s particular characteristics. The overall propulsive coefficient can range from as low as 0.40 for a very inefficient installation up to 0.70 or more for a finely tuned system using a modern propeller with the proper reduction ratio. If the figure is low, the supplier or builder needs to do better before you give him your money and your business; if it’s high, smile, because you’ll be saving fuel for the next 20 years; and if the figure is unavailable, you’re buying a grab bag. Put in absolute terms, if you need 100 effective horsepower to propel the boat, a system with an OPC of 0.40 will require a 250 shaft horsepower engine, while the system with an OPC of 0.70 will require only a 143 shaft horsepower engine. In addition to the higher initial cost of the engine, the less efficient system will also burn 75% more fuel to achieve the same speed and will have 43% less range. When you consider that the bigger engine will also weigh more, thus increasing the displacement and resistance, the situation gets even worse. This is a pretty harsh penalty on a boat owner due to the builder’s or supplier’s failure to do some basic engineering work.Wheel options The ultimate racing sailboat is shipped or towed to and from the racing venue and has no engine at all, but most weekend racers can’t afford this. For them, a propeller is a necessary evil, since reduction of drag under sail is the ultimate concern. A folding propeller, either two or three blades, is the solution. While sailing, it adds very little drag; under power, however, it’s not very efficient. Voyaging sailors also need to minimize propeller drag under full sail, but should give considerably more attention to propulsive efficiency since they will likely spend a significant amount of time under full or partial power. When the expected time under power is small, a feathering prop may be the best compromise. The cost premium is not dramatic, added drag is reduced enough to add a half knot or more under sail, and efficiency under power is acceptable. For the larger voyaging sailboat, a controllable-pitch propeller with full feathering capability should be considered. Under either full or partial power, the system can be adjusted to deliver the optimum engine rpm and propeller pitch combination for the current sea and weather conditions, boat displacement, and sail configuration. This capability does not come cheap, but for the yacht owner who canafford it,itmay be the ideal solution for serious voyaging under sail.Fixed pitch most economical For power voyagers, motorsailers, and voyaging sailboats expecting to spend considerable time under power, a fixed-pitch propeller is the most efficient and economical solution. Because modern boats no longer operate with the propeller in a small aperture behind a thick deadwood, the old standby two-bladed propeller is no longer the best choice in most cases. New stock propellers, three- and four-bladed, are now available with considerably improved performance. Some blade section shapes now include camber, progressive pitch, and other refinements, improving thrust over the older flat-face blades. Increased blade area is available to reduce the risk of cavitation, which can cause vibration and thrust breakdown. Highly skewed blades, similar in appearance to those on the “weedless” props used with electric trolling motors, reduce vibration.Another possibility created by this new technology is the use of a larger diameter, larger area prop without added draft, since it can be placed closer to the hull without causing excessive vibration. Because of the added area, the gear ratio and propeller pitch can be increased without overloading the blade surface, and increased propulsive efficiency is achieved because of the increased pitch relative to the diameter. The ratio of pitch to diameter, or P/D, used to lie in the range of 1.0 or less, often associated with a reduction gear ratio of 1.5:1 or so. Modern installations utilize a higher gear ratio, often closer to 2.5:1 or 3:1, with higher pitch props. The P/D ratio for some of these installations is up to 1.4 or 1.5. We have had considerable success with such installations in powerboats during the last few years, and recently were able to increase the top speed of one boat by more than 30% without repowering or increasing fuel usage, only changing the gear ratio and propeller.Forthevoyagingboatowner not interested in increasing top speed, this translates into a comparable reduction in fuel use and increase in range. As confirmation of this, we were pleased to receive reports recently of a number of repowering installations among the Caribbean sail charter fleet, at an increased gear ratio of 3:1, that showed a significant reduction in fuel use and a slight increase in top speed. For a given horsepower, torque at the propeller increases directly with gear ratio; i.e., double the ratio, double the torque. This results in a couple of additional advantages for the high P/D installation: improved acceleration and stopping power and the ability to slug through a tough head sea with less of theslowing and surging common with lowergear ratios. In getting from point A to point B, this has the same practical effect as a knot or two of extra speedat no increase in fuel usage. Much of what we’ve discussed can be understood and applied by the average owner, either alone or in conjunction with skilled yard personnel. Getting the excess weight out of your boat is simple and direct, and it’s a great first step. The successful application of the new propeller technology, however, depends on the use of sophisticated computer software and techniques which are best handled by a technician experienced in both propeller theory and real-world installations. Whether it’s your naval architect, boat builder, or propeller supplier, work with them to evaluate your personal voyaging routine and develop an installation that’s best for you. You may save money on the propulsion system up front, and you’ll certainly be happier with your boat’s performance, voyaging capabilities, and operating costs. n Dudley Dawson is president of Dawson Marine Group, Inc., which provides naval architecture and marine engineering services from offices in Greensboro, N.C., Dawson formerly served as chief naval architect at Hatteras Yachts and as vice-president of J.B. Hargrave Naval Architects.

By Ocean Navigator