Steering wheel abeam

Bow thrusters mounted transversely in the bow in the tunnel provide convenience for less experienced and more comfortable skippers: with a short press of a button, the helmsman can maneuver accurately and quickly in the desired direction, even in narrow passages and in rapidly changing winds. The anxiety at the helm and too rapid a change in thrust direction, which lead to loud protests from the gearbox, are a thing of the past - now people and materials are protected.

For the bow thruster to function properly and not fail at the most inopportune moment, the correct dimensions are critical. Some manufacturers try to analyze in tabular form the suitability of specific devices depending on the length of the boat. This information can only serve as a rough starting point, as there are too many aspects to consider and to consider the design: not only the length of the vessel, but also the shape of the hull (long or short keel), displacement, windage and last but not least, the shape of the front to choose the correct screw. If in doubt, in order to protect yourself, you need to adhere to the following recommendation: it is better to take the engine one number more powerful than to go to the limit values.

The most important thing is to place the tunnel as far as possible in the bow of the boat. The lever arm is then as long as possible in relation to the center point of the vessel, which ensures that the bow thruster operates efficiently with the least amount of malfunction. If the tunnel is set too close to the midships, the rotational movement causes the entire boat to move laterally. At a minimum, the rotation will slow down significantly and become sluggish, it will become much more difficult to maneuver, and it will take a long time.

Therefore, a tunnel with a relatively small diameter can potentially be better than a tunnel with a large diameter. This is also advantageous in situations where space needs to be saved, such as when a sufficiently flat body does not have sufficient depth in the area where the handlebars could be placed.

At the same time, several considerations must be taken into account when installing: the distance between the upper edge of the tunnel and the waterline should ideally be at least equal to the diameter of the tunnel. Otherwise, the propeller can draw in a lot of air, which drastically reduces efficiency and also produces a lot of noise.

The design of the tunnel edge in the hull has a great influence, on the one hand, on the hydrodynamic resistance of the hull in water, and on the other hand, on the efficiency of the engine or propeller. Behind the sharp edges of a narrow-radius tunnel, the circulating water is sharply blocked, which leads to a decrease in speed. There are two ways to eliminate this effect, or at least to reduce it: if the edges of the tunnel are laminated towards the bow and stern, while they are rounded, which is about 1/10 of the radius of the tunnel, the water flows more smoothly around the hole in the hull. , but the speed does not decrease. Another option is to install a spoiler in front of the body cut. For this purpose, the tunnel tube is not installed flush with the hull, but 2030 millimeters are left on the left and right sides. The space between the body and the pipe is about 60 millimeters in length filled with epoxy, so that a small ramp actually appears - just in the place where the water passes by the pipe. Here, too, the edge must be designed with a sufficient radius.

Round inlet edges at tunnel entrances have another positive effect: water is more evenly and linearly sucked through the edges into the tunnel as the bow thruster rotates. Less turbulence occurs in the tunnel and thus the propeller can work more efficiently. Whether the bow thruster needs its own battery, and perhaps even its own charger, or whether there is a need to run a cable from the existing batteries to the electric motor - these questions require thought. The electric thruster draws high current in a relatively short period of time because it can power the starter battery. Thus, if the distance from the engine battery to the bow is not so great (where large cable cross-sections would be required in order to prevent a sharp drop in voltage between the battery and the electric motor), this solution would be the simplest and cheapest. Finally, the engine always runs at high idle when maneuvering in port and always with the bow thruster, in which case a full charge will be ensured. A separate battery in the bow would have the advantage that thumb loops would not have to be pulled through the entire boat, and the charger cable would still be routed, provided the small charger did not fit in the bow. parts. Another advantage of an appropriately sized battery is that it can be used in conjunction with an anchor winch.

The motor must be positioned so that the screw is in the center of the tunnel in order to achieve an even stop on the right and left. Since these values can differ significantly due to the design of the propeller and the location of the gear in the water flow, some manufacturers recommend installing the propeller off-center, which may slightly compensate for the uneven ratio. Manufacturers approach technical equipment in completely different ways. Anodes, if necessary according to the type of design, to provide better protection against losses in some models are screwed between the screw and the gearbox, in other models they are directly mounted on the end of the wing hub. This makes replacement easier, since the entire turn does not need to be removed. With maximum energy, the anode is not even necessary, since the underwater part of the engine is made entirely of composite plastic with stainless steel or plastic screws.

The screw itself is tightened on the shaft in different ways: a strong nib is inserted across the shaft and transfers the force as confidently as a long stationary cracker or a shaft with coarse teeth. But there is a tendency for the pin to deform slightly and this makes it difficult to remove the screw during maintenance. The only weak point in the system when the propeller is jammed is the safety pin in the engine, which is located where the transmission shaft engages with the electric motor. This safety pin breaks to avoid damage to the engine. During this transient moment, torsion-elastic couplings can be found from certain manufacturers to reduce vibration and reduce operating noise.

In addition, this acoustic decoupling protects the motor and gearbox, so this part makes sense. Almost all manufacturers talk about filling the gearbox with oil, which should last throughout the entire period of operation. However, there are a few exceptions to the way the grease is used. Only the use of the Sidepower system convinces in its quality, and it is better to refuse the easy possibility of adding lubricants.

When buying the engine itself, you need to find out about the presence of fire-resistant materials on it, since during long-term operation it heats up very much, which can lead to imperceptible short circuits and smoldering. During the test, our engine started to smoke, since in the immediate vicinity of the positive pole there were two negative conductors, which were loaded. Of course, it is better when plus and minus are separated from each other, as, for example, in Lewmar. The recommendations regarding the cross-sections and the corresponding cable length should also be followed very precisely. If the load conductor is too thin, it can quickly lead to smoke or even more damage if the fuses do not fit or fail. For safety reasons, it is advisable to use some technical tricks, such as an automatic shutdown or at least a visual warning at low voltage (below 10.5 V), since in this case the switching relay can remain permanently in the “on” position. Then the engine can no longer be stopped from the cockpit. However, manufacturers recommend installing an emergency switch in the load conductor close to the electric motor, through which you can at least turn off the current in the bow of the boat. A temperature sensor or timer registers the running time and overheating of the engine and, if necessary, turns it off.

1. The further in the bow the thruster tunnel is, the better the rotation and the lever will work. Setting the tunnel close to the bow results in significantly worse performance

2. The location of the tunnel must be chosen carefully so that the distance to the water surface is at least equal to the diameter of the tunnel. If the propeller is set too high, it can easily suck in air, and the efficiency drops dramatically.

3. Rounded edges (left) and laminated spoiler (center) improve hydrodynamic properties. Small radius rigid leading edges (right) in the bow-side position cause turbulence and brake the boat

4. Composite case without anodes (top), others are installed in front of / behind the screw. Rounded edges at the inlet of the flow towards the bow and aft lead to the fact that the water drawn in by the propeller at the entrance to the tunnel is less prone to turbulence, so the propeller works more efficiently

5. Electric motors in the bow can be installed, depending on the space, not only around its vertical axis, but also around the axis of the hub. The tilt of the chassis post must be at least 45 degrees

Torsionally flexible clutch reduces vibration and ensures smooth starting

THZ / batteries

A lot of energy is required to operate the bow thruster for a short time. Therefore, it is important to have a suitable battery type for this purpose. The decisive factor here is not so much the capacity of the battery, which is usually expressed in ampere-hours and is indicated on the battery itself, but the strength of the so-called "cold start current", which is indicated as TCA (CCA) according to the European standard (EN). The DIN data is determined under different conditions and is approximately 50 percent lower. The use of mains batteries is excluded, in this case the classic wet starter batteries with liquid electrolyte, since they are also used as an additional starter for the engine or winch, are the most affordable alternative. Magnetically powered batteries are also suitable, but they are significantly more expensive.

Exturn

If you do not want to cut huge holes in the area of the bow thruster, a Marinno "Exturn" screw under the engine can be installed as a bow or stern thruster. The propeller, gearbox components and motor are torpedo-shaped to assist in maneuvering. The required relays are installed directly in the ship. The advantages, according to the manufacturer, are quick and easy installation (within one day) and higher efficiency, since the bulb can be installed in front and / or at the same time deeper than a tunnel rudder. Thus, the power loss of the engine is comparatively low. The resistance of the bow thruster should be barely noticeable.

This is how we tested

Under the guidance and equipment of the manufacturer Vetus we have tried to determine the loads of the bow thruster. The equipment of the experiment was represented by a rotating cage with an external frame, in which various iron boxes with an integrated tunnel were suspended. We installed a load cell (Tedea Huntleigh 614) between the moving and fixed parts of the system, which was calibrated to measure thrust and sent its data through the data logger to the Dataq software. Due to the asymmetric design (distance between points: support-load cell / load cell-steering wheel) of the device and the different path lengths of the lever passage in the boxes, the readings had to be multiplied by certain factors to obtain real values. These coefficients are difficult to determine, so we were unable to compute absolute values. Some of the values obtained in this way, for example, significantly exceeded the data specified in the manufacturer's instructions - this was the case with brands Craftsman and Maxpower, other indicators were very different when moving forward / backward, for example, in Exturn - the manufacturer explains this with new carbon brushes. As a rule, shear forces are also indicated among the measured values.

Nevertheless, in the table we have indicated them as dimensionless quantities. The bow thruster was powered from a 225 Ah battery with a 1060 THZ cold start in accordance with the European standard (EN). As a result, when conducting separate experiments, the output voltage did not differ much, although after each operation we connected the charger, and also charged the batteries for at least 20 minutes. The number of revolutions was measured using optical detection. At a certain point, we also measured the noise level on each engine. These measurements, however, can only be partially compared, since the metal boxes in which the bow thrusters were mounted had different shapes and wall thicknesses and thus produced different sounds. For this reason, we decided not to present these results. Since the engines came partially in their original packaging, we started them up and ran them according to the starting program for two minutes: we operated each thruster in a continuous mode in forward and reverse directions at three intervals of ten seconds, as well as for thirty seconds in long-term operation. To take the actual measurements, each motor was run twice for 20 seconds in a continuous mode, running forward and backward. The obtained values (A, V - each indicator is measured on a battery, and an emphasis) we have indicated.

When turned on, power consumption quickly increases (right) and battery voltage drops (center). With decreasing stress, the pulling force decreases (left)

Charging with 225 Ah battery with 1060 A cold start

Motor installed inside the box and connected to the cables just before the warm-up phase

Pool with a rolling frame bolted to the drawers

After each measurement, the boxes were lifted and filled again.

We screwed the Exturn "180" thruster directly under one of the boxes, which was then attached to the rotating posts for measurements

Common defects on bow thrusters

Bow thrusters, like all electrical appliances on board, differ in specific types of malfunctions. Especially in conditions of being in a potentially wet tank, where the engine must share space under certain circumstances with a winch, chains and cables, this topic is especially relevant. In addition, insufficient electrical installation or weak batteries have a negative impact. Johann Buschmann from the company "Marine Sales and Service" highlights the following most common violations:

• if the engine is running at low voltage, the collector and carbon brushes wear out prematurely
• in addition, the relays can start popping, which means that they close and open (when turned off) quickly and haphazardly. However, they can remain in the "on" position, and the steering wheel can no longer be disconnected. In the event of an emergency, a suitable switch must always be installed in the load conductor.
• electrochemical corrosion: when the metal body and jacket are connected, currents pass through them that destroy the material
• Cannot be operated without load: relatively small AC motors have a very high power density. The number of revolutions can increase to more than 8000 rpm, leading to rapid overheating of the engine
• If foreign objects such as tree branches, etc., enter the tunnel, the shear pins or clamps may be damaged
• over time, panels can be destroyed by UV light - this can only be fixed by replacement

Shaft gear with fork end and shear pin weak point in engine

Screw Hook Pin Solutions

Gear with refueling capabilities (in red circle) and without

Approximate pole configuration: they are far apart, no short circuit caused by the tool

Additional installation

On almost every boat, you can additionally install a bow thruster, if in the bow under the berth boards, i.e. far in the nose, there is enough space. It will take about three days to complete the installation, as apart from the working steps it will take a long time to thoroughly dry the laminated areas. Only after complete drying can the device be sewn up inside with a surface coating, and on the outside covered with gelcoat and antifouling paint. In addition to special tools, experience and a high level of craftsmanship will be required, so this installation must be carried out by specialists. Depending on the complexity and design of the installation, including the thruster, batteries, cable and switches, the cost of the whole idea will be at least 4500 Euros.

Choosing a place for installation inside: there must be enough space for the tunnel, which is fully laminated, as well as for the engine. Then you need to check from the inside with a thin drill, which should go out, whether the chosen place is suitable. To symmetrically drill holes for the centering post on the right and left, take the spirit level as a guideline as a reference. Inline straight lines of inline elements are rarely actually parallel and cannot be relied upon. The centering post is now inserted (1) and the corresponding radius marked (2).

In addition, with the help of a larger template, a second circle should be marked, which will indicate the areas for the outside of the lamination work (3). Then the diamond head can be used to cut a suitable piece of laminate out of the housing.

Now you need to install the tunnel (5) and laminate it from the inside completely around the entire pipe (6)

Outside, about 30 millimeters of the tunnel remains, which protrudes beyond the side wall (7). This small edge serves as a support for the spoiler, which is currently molded from epoxy resin with fiberglass. At the final stage, a layer of silicon oxide is applied to a well-ground surface (8)

The tunnel and laminated surfaces are now carefully sewn inside the surface coating. Outside protection is provided by Epoxiprimer primer, gelcoat and anti-fouling paint, which is also applied in the tunnel, on the screw and gear, an anti-osmosis material and reduces build-up

Text: Gerald Sinshek

17.06.2014