 What naval architect in his/her right mind would sacrifice the joy of an easily controlled vessel under full sail for slow speed handling in a congested harbor? Now, thrusters have become popular enough to warrant some pretty good competition among the manufacturers, which has brought prices down and quality up, allowing sailors to enjoy the best of both worlds. T H R U S T E R S by Bruce McDugle |
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Over the last 30 years I've watched, and been involved in, various devices designed to create better maneuverability. These have ranged from rudder designs, keels, skegs, twin screws, drogues and thrusters. One of the more memorable ones was a gentleman who had a single screw 30' powerboat. He had two old trailer tires on eight-foot lines cinched to amidships cleats, one on the port side and one on starboard. From the aft helm station he could launch the proper tire overboard to enhance last minute control. His success was often regulated by the amount of vodka needed to build up his courage. I was the one who had to patch the odd scratches and gouges from mis-timed tire launches. Since then, my experience in augmented control has notably increased. A giant step forward was the fireboat we built which used high-powered water jets. A diesel engine powered a large pump that fed four 3-inch diameter hoses. They terminated in a nozzle at each corner of the boat. After much trial and error the system worked well. This system did the required job but was heavy, expensive, noisy, and took up too much space. Several years later, we installed another water jet on a yacht. It used an electric motor to drive the main water pump. I ran two 3-inch hoses forward to nozzles on the port and starboard sides. The pump, motor and hoses, when charged, were between 500-600 foot-pounds. It was awkward finding room to fit very stiff 3-inch hoses behind cabinetry and through storage areas to the forward nozzles. The electric motor was a 24-volt but still drew a tremendous amount of amperage and again it was too heavy, expensive and bulky. Both of these water jet thrusters were installed on boats under 36 feet and left me convinced that in their present state of technology, they didn't belong on yachts. In the time between installing the above water jets, I installed two hydraulic bow thrusters on commercial fishing boats. By today's standards, they were a bit heavy and awkward to install. The mounting surfaces didn't match up well with the supplied fiberglass tunnels. It took two people to actually bolt the units in place and then there was the cost of hydraulic pumps, oil reservoirs, controls, hoses and fittings. Even with their shortcomings, all four of the above units are still in use today. I do believe that hydraulic thrusters are the best you can get, but for vessels less than 45-feet, are just (usually) too expensive. If you already have a hydraulic pump setup, like some sailboats with hydraulic drives, then you are the exception and the installation could be relatively inexpensive. The best alternative is DC electric. Two varieties are available. Direct gear or belt drive. I remember an early drive that required the boat to be hauled out to replace the belt when it broke. The lower unit had to be disassembled and the prop shaft pulled out to fit a new belt. I am sure the owner could have found other things to spend his money on. There are belt drives that can be repaired in the water but eventually belts wear out or have to be readjusted. A good gear driven unit only needs the oil checked and zincs changed periodically. The simple facts are that direct drive electric thrusters outsell all of the others by a wide margin. As an example, of the last 30 bow thrusters we installed, 4 were hydraulic and the other 26 were electric gear drives. There is another choice for powerboats but it won't work on sailboats. Dickson Thrusters makes a nice bolt-on hydraulic stern thruster. The unit is mounted on a stainless steel transom bracket and it doesn't use a tunnel. It is less expensive than a good bow thruster and would almost have to be engineered retractable. Currently, it is not a viable option for sail boaters. Historically, the majority of our thruster installations have been on powerboats ranging in size from 26 to 75 feet. What has really caught me by surprise is the number of sailboats installing bow thrusters and they seem to be the most pleased with the results. In retrospect, it is very understandable. What architect in his/her right mind would sacrifice the joy of an easily controlled vessel under full sail for slow speed handling in a congested harbor, especially when a bow thruster will allow him the best of both worlds. It has been challenging to say the least, fitting bow thrusters into places they were never designed to be. Let me share with you a few of the hurdles that have to be overcome and some of the solutions. We will concentrate on the DC electric thrusters. 1. The power source: Ideally, this will be the engine starting battery. The requirements for a thruster are very similar to an engine starter and have the same problems. The effectiveness of direct current (DC) is limited by wire size and length. Simply put, if the thruster is too far from the battery then we install one at the closest point. The necessary cold crank amps regulate the battery size for the thruster. The new spiral cell batteries work very well and can normally be integrated into most ship's charging systems. (A word of caution here. DO NOT use existing windlass wiring. The wire size will be wrong, the breaker/fuse will be wrong and the power to run the thruster will probably not be adequate, resulting in the motor being underpowered or motor failure.) 2. Tunnel Location: The most favorable position is as far forward and as low as possible. The problems are chain lockers, water tanks and shallow draft. Let's use an example of an average 32' sailboat. If the tunnel can be placed forward position". The tunnel would go a couple feet aft and we would probably use a 6 horsepower unit. That equates to an additional $1000.00 but is still less expensive that rebuilding a water tank. (Lack of leverage arm can be overcome with horsepower.) Another consideration is the depth. A good rule of thumb is the top of the tunnel should be at least one diameter below the waterline. The deeper the better. Higher water pressure equals higher efficiency. 3. Cavitation: If a thruster cavitates it loses thrust and makes noise. We've worked hard to learn how to prevent this. Here are a few things to consider. Water always follows the easiest path and its flow can be directed into the tunnel via proper radiusing. The path is created by plan not chance. A normal installation will cost over $50.00 per each pound of thrust. Why throw it away, when by proper design of the tunnel installation you wont lose any thrust. 4. Vessel Trim: The new electric thrusters are surprisingly light for the amount of power they develop. A good example would be an 8 horsepower unit in a 40-foot sailboat. The brand we use weighs 68 pounds. With the addition of a spiral cell battery, overall weight is 118 pounds. Miscellaneous fiberglass and accessories finishes this installation at a weight gain of approximately 135 pounds. A 4 horsepower unit in a smaller vessel that did not require an additional battery could be as low as 60 pounds overall. There is a new 3 horsepower unit that develops 77 foot- pounds of thrust, yet weighs in at a mere 22 pounds and uses a 5 inch tunnel. This thruster works well in a 30 foot and under vessel. Trim considerations are always important, but as you can see, these weights are pretty easy to deal with. As I said, sailboats have been challenging, but the results have been worth it. Thrusters have become popular enough to warrant some pretty good competition among the manufacturers, which has brought prices down and quality up. Let me finish with a couple of observations. Boats with thrusters generally sustain less cosmetic hull damage, charter companies prefer them and boat brokers report they are easier to sell. Bruce McDugle works at Cap Sante Marine, Ltd. He may be contacted at: (800) 422-5794 Fax: (360) 293-2804 TRUSTERS -- installation photos page |