by Earl R. Hinz
Even a proficient boathandler will be pleasantly surprised at how a side thruster adds to vessel’s maneuverability. A small thruster will provide enough side power to make most difficult situations more manageable.
Recently, my wife and I decided that we should give up wandering on the high seasandmdash;we had been at it in one way or another since 1970. With reluctance, we sold our Out Island 41 ketch, Horizonandmdash;which had carried us faithfully through 40,000 miles of the best and the worst Pacific weather. It was not our intention, however, to swallow the anchor. So we started looking for a comfortable liveaboard boat while continuing to sail on other people’s boats. The latter proved to have many advantages.
Our liveaboard selection turned out to be a fine Monk 36 single screw trawler yacht. It met all of our requirements as ex-sailors: modest speed to enjoy the travel, seakindly qualities and low operating cost. But I couldn’t back it down in a straight line or glide it gracefully alongside a dock no matter how hard I tried. Cross currents, gusty trade winds, and a high superstructure guaranteed onlookers a striking display of poor seamanship. Black skid marks began to decorate the topsides where I bounced Kumulani off rubber-tired fendering on pilings and piers.
Is a Monk design any less controllable than other popular trawler yachts offered with single screw propulsion? After watching a number of them ricochet off pilings, piers, and wharves while attempting to dock under adverse conditions of wind and current, and after seeing numerous skid marks and rubrail abrasions on their hulls, I felt we were all in the same boat, so to speak. The usual handling techniquesandmdash;such as prop walking and coasting with part or full rudderandmdash;work reasonably well when current and wind are not adverse and where there is plenty of space for maneuvering. These methods just don’t work, however, in the face of vigorous crosswinds, currents, and narrow fairways.
The whole thing baffled me at first. My cruising ketch, Horizon, had a single screw and was docile as a kitten. But then I got to thinking about the differences in the design of the two boats. The ketch had a large rudder, a small propeller, a long keel and was of low profile. The trawler has a small rudder, a large propeller, less keel, and lots of windage. Furthermore, the trawler’s propulsive thrust and torque at idle are probably as much as the ketch at cruising power. This appeared to be a whole new ball game, and I began to feel less guilty of my seamanship.
Nevertheless, I was truly envious of the maneuverability of a twin-screw boat, but not of the cost nor the crowded engine installation that I often saw during my search for a 36-foot LOA trawler. There had to be a better way to steer a single-screw boat than luck and bouncing off hard objects. That challenge led me to an investigation of side thrusters for low-speed maneuvering. To make a long story short, Kumulani now sports a side thruster giving it the handling qualities at low speed equal to twin screws while maintaining all the benefits of the single screw installation.
Seamanship is a vital part of maneuvering a boat, but proper equipment is also essential. Thrusters are not a new bit of technology in the maritime world, they have been around on ships for many years. As you pass a ship in a berth, look for the telltale sign painted on the hull near the bowandmdash;-a large circle with a propeller inside of it. That means there is a bow thruster below the waterline. The 156-foot topsail schooner Tole Mour is fitted with a 24-inch diameter bow thruster, enabling her to maneuver in the passes of Pacific atolls (see andquot;Easing a schooner through a coral reef,andquot; Issue No. 36). Only recently, however, has the technology and price of side thrusters made them practical for small craft.
The matrix of features available in side thrusters attests to the ingenuity of the boating industry. We have bow and stern thrusters; electric and hydraulic drives; single and dual propellers; in-line and right-angle drives; propeller or jet drives; fixed and variable pitch propellers; and fixed and retracting designs. Just about anything is available. The one thing they all have in common is that they turn a cantankerous single-screw vessel into a docile pussy cat. Bow or stern thruster?
In choosing a side thruster, the first decision to make is whether it should be an original equipment installation or a retrofit. Kumulani’s was a retrofit and that favored a stern thruster which could be installed while the boat was in the water. While the effective moment arm would be less than for a bow thruster, there would still be more than enough side thrust to turn the boat handily.
A bow thruster requires professional boatyard installation because of the two large holes required for the forward hull. Typically, a 40-foot boat using a single bow thruster would require a nine- or ten-inch diameter tunnel. While installing such a tunnel is not a difficult task for a fiberglass or metal hull, it is not an easy job in a wooden hull. My experience indicates that the fewer underwater holes one puts in a hull, the fewer problems one will have in the future. It should also be noted that the bow thruster holes are always present and add an element of drag that is paid for in either speed reduction, increased fuel consumption, or both. A retracting thruster may solve the open hole drag problem, but it does so at the expense of complexityandmdash;something worse than disappearing headlights on a car.
The size of the boat also enters into the problem. The fine bow lines of most powerboats require the thruster to be set quite far aft to assure proper tunnel submergence and water supply to the propeller. This lowers the moment arm advantage of the bow thruster as compared to the stern thruster. The installation problem is both structural and hydrodynamic. For the small boat (under 40 feet or so) it can mean valuable forecastle space is taken up with the unit and weight is added forward where buoyancy is the least. Typically, in a 40-foot boat, the tunnel and drive assembly would weigh more than 75 lbs and, if electric, a battery would weigh another 65 lb. OEM installations can be designed to minimize the loss of space, but with most retrofits, there is little way to avoid it. Electric or hydraulic drive
There is no consensus by builders or users on whether electric or hydraulic drive is the better choice. To a large extent it depends on the power sources already available on the boat. If the boat is already well-electrified and has an electric windlass installation, then a forward bow thruster powered by an electric motor may be the better choice. However, it should be remembered that the power demands of the thruster are more severe than an anchor windlass, requiring that the electrical system be beefed up to handle the increased load.
The current draw of a 5-hp motor operating on 12 volts would be in excess of 300 amps, or about twice the draw of the anchor windlass for a typical 40-foot boat. This much current would require 0000 wire gauge size for a 20-foot run at 12 volts.
Another consideration is the length of time that high power demand that can be sustained. Electric motors, especially at low voltages, heat up very quickly, limiting operating time. Total battery capacity (which should come from deep cycle batteries) may also be a limiting factor, that is, without excessive battery weight. In adverse maneuvering situations the available cycle time may be exceeded and the thruster motor could shut itself down just when it is needed most.
Solutions to the electric power demand problems include: 1) placing one or more deep cycle batteries near the thruster, 2) running very heavy copper cables from the batteries to the thruster, 3) placing an auxiliary generator near the thruster, 4) increasing the operating voltage of the electric motor. None of these are serious problems for the OEM installation, but they do pose difficulties in a retrofit.
A hydraulic drive, on the other hand, is easier to retrofit and also makes a good OEM installation. Its one drawback is leakage at the many joints that constitute a hydraulic system. This can be avoided by having it installed by a qualified hydraulic mechanic. While leakage inside the boat poses a possibly messy cleanup problem, leakage outside the boat poses a $5,000 fine problem. This latter concern is circumvented if the thruster uses a right-angle drive that places the hydraulic motor inside of the hull.
Hydraulic drive thrusters have none of the duty cycle limitations of electric drive and, in fact, variations on the side thruster include their use as get-home or trolling motors.
My own installation is hydraulic, installed by a qualified person. I have found it to be completely trouble-freeandmdash;which is more than I can say for my OEM electric windlass. Efficiency aspects of designs
There are a variety of design features in side thrusters that effect system efficiency, some are within the purview of the designer and some are just inherent in a more basic choice. Take the issue of a ducted propeller (often called a nozzle), which is more efficient than an unducted propeller. One automatically receives that benefit in a bow thruster, provided it is installed according to recognized design criteria.
But that doesn’t mean stern thrusters cannot take advantage of ducted propellers. The Michigan Wheel Corporation has done extensive development on ducted units for ships as a way to improve propeller efficiencies. Port Kent Marine, Inc. has applied such technology to outboard motors with their andquot;Handlerandquot; nozzle. Such ducted propellers show that performance can be increased by as much as 20%, which is enough to make them useful for stern thruster propellers.
Single, dual, and dual counter-rotating props are also seen on bow thruster designs. Each has something in its favor: The single propeller’s advantage is simplicity. Dual propellers have the advantage of being able to absorb more horsepower in the same diameter. Although dual propeller thrusters are mechanically more complicated, a smaller tunnel has its advantages in less drag and better internal packaging. Counter-rotating propellers, meanwhile, are able to recover some of the thrust that is otherwise lost by a corkscrewing water discharge. The latter can also be corrected through the use of stator blades to straighten the flow, but this creates other losses due to water drag through the stator. Sizing considerations
Stern thrusters come only in one size (approximately six horsepower), but they can be operated in parallel to double the side thrust available and make them usable on larger vessels. Bow thrusters, on the other hand, come in a variety of sizes, from four horsepower to more than 100 horsepower. The manufacturers of all thrusters, bow or stern, want the potential user to contact them for recommendations since thrusters come in discrete sizes. Among the factors that must to be considered in a selection are: vessel length, displacement, and windage; power available from the prime mover; skill of operator and degree of maneuverability desired; and type of weather conditions anticipated.
Performance must be the principle criterion in selecting a side thruster. Compared to the cost of a second-engine installation, side thruster costs are minor. Kumulani’s 10-inch stern thruster kit excluding hoses, fluid, and installation cost $4,000. A 24-inch diameter hydraulic bow thruster would cost about $30,000 uninstalled. Electrically-operated thrusters may cost less provided the required electrical power source is already available and adequate. Except in the smaller sizes, side thrusters are not off-the-shelf items and pricing is highly dependent on the individual installation. Anyone succumbing to the advantages of side thrusters should contact the manufacturers for customized information on installation and cost. Stern thruster installation
The beauty of the stern thruster is that it can be installed with the boat in the water and there are no holes put in the hull below the waterline. It is an ideal retrofit device. The Dickson Stern Thruster, which I installed on Kumulani, comes from the factory as a complete kit minus only hydraulic hoses and belts. They can be procured from the factory after measurements are made, or, as in my case, I procured them locally. With so much industrial equipment being hydraulically operated, there is no problem finding a supplier to make up the hoses to one’s specifications.
Dickson’s Stern Thruster comes in four packages: motor and external mounting hardware; engine-driven hydraulic pump and mounting bracket; control panel and reservoir assembly; and the helm control panel(s). This division also coincides with the four assembly steps.
Step 1: Installation of external equipment. The hydraulic motor and propeller assembly comes with an L-shaped bracket long enough to place the upper propeller arc at least one diameter below the water’s surface to minimize ventilation. The bracket is thru-bolted to the transom at the boot topping, and the hydraulic lines penetrate the transom at approximately the same height. The stainless steel hydraulic lines are connected between motor and the thru-transom nipples before the motor bracket is fixed in place.
Step 2: Installation of power takeoff equipment. The manufacturer provides a universal mounting bracket for installing the hydraulic pump on the engine. It has sliding and rotating provisions for aligning and tightening belts. The bracket attaches to the front end of the engine using existing bolts or studs. Belts are procured after the pump installation is completed since their length is dependent on the particular installation made.
Step 3: Installation of control panel/reservoir assembly. The control panel and reservoir come preassembled and ready for mounting. Shuttle valves (port/starboard control), reservoir, and filter are prewired and preplumbed. The control panel baseboard is bolted to a bulkhead in a location convenient for connecting with the engine-driven pump. With only a single engine in the engine compartment, there is plenty of room for the small control panel. The hydraulic hose connections between the transom, control panel and engine pump are then completed. Hose routing should be as direct as possible, but, unlike electrical wires, they do not need to avoid potential bilge water splash areas.
Step 4: Installation of helm station control panels. Control panels are placed at each helm station for convenience, although if the owner uses only one station for docking maneuvers only one thruster control panel is needed. These panels are prewired with sufficient tail lengths to reach the electric power supply and the shuttle valve terminal board. The panel provides a toggle switch (joy stick) for port/starboard maneuvering control, a power switch, and a fuse.
Installation time for the Dickson Stern Thruster on Kumulani was approximately 18 man-hours. Two people were required for installing the external gear and making hydraulic connections through the transom, otherwise it was a one-person job. Once installed, there is nothing to adjust or maintain. Hose joints should be monitored for possible leakage, and the drive belts should be kept properly tensioned. Otherwise, the toggle switch is one’s only contact with this system in use.
Side thrusters, either bow or stern, have made precision maneuvering of the single screw boat in close quarters a reality. While I did not anticipate the problems of maneuvering a single screw boat when I bought Kumulani, I found out quickly that to keep a decent appearance and avoid insurance claims, I had to improve her maneuverability. In my case, the stern thruster with its in-the-water installation design, was the answer. I no longer have to worry about contrary winds and currents carrying Kumulani into pilings, docks, and other boats. She can now be fitted into my narrow slip with ease. Side thrusters have reached a state of development where they are a practical and economical addition to single-screw boats as well as many larger twin screw boats needing more positive maneuvering control in close quarters.
Earl Hinz has voyaged extensively in the Pacific and written six books on marine topics.