Guide to Bow and Stern Thrusters
Bow and stern thrusters are propulsion devices built into or mounted onto the boat's hull in order to make the boat more maneuverable at low speeds or preferably, at full stop.
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Bow and stern thrusters aids in docking and fine maneuvering of the boat by providing side thrust when needed, without providing forward or backward thrust. If you want to steer the boat using just main drive and helm, then you will create some forward/backward thrust. Also, when fighting game fish, especially when trolling, bow thrusters can be of great help when positioning the boat toward the fish direction and enabling people on the stern to - fight with the fish!
On the other hand, using small outboard engines that can rotate shaft 360° in all directions (usually petrol or electric outboard engines/motors), stern thrusters are often not needed.
Medium size and medium class boats often have only bow thrusters. Combined with main engine and main helm, sometimes even with electric trolling motors, bow thrusters offer great maneuverability at acceptable cost. Bow mounted electric trolling motors can be used as bow thrusters, if needed, but they are not installed permanently and for such purpose have many drawbacks.
Bow and stern thrusters are most effective when boats don't move - when boat moves forward or backward, water jet changes angle and thus lateral force is decreased. When boat is moving faster than 5 knots, bow and stern thrusters almost have no effects on the boat. Feel free to try :)
Source of Power
Fishing boats usually have electric thrusters - they are powered by main battery and can be used even when main engines are off-line. Usually, they are 12V, 24V, 36V, 48V or even 72V systems, requiring hundreds of amperes to provide required lateral force.
Medium sized boats usually have 12V systems with output power of 2-4 kW and they require 200-400A strong currents. Lead acid, deep cycle, modern marine batteries can withstand such currents with ease. However, during normal docking maneuver, it is common that main engine works and supply at least a fraction of current to the electric system.
Operating time of boat thrusters is usually limited, few seconds or, during strong winds and currents, few minutes.
Example: 12V 3kW electric thruster that consumes around 300A, theoretically will drain around 5 Ah (in practice, little bit more) of your battery, per minute of constant operation. But, if you have main engine with alternator capable of providing 200A continuously and your thruster is consuming 300A, then your battery is drained at around 2Ah per minute. If you have 12V battery with capacity of at least 80Ah, you can maneuver your boat for at least 30 minutes, without draining battery entirely. Note that most of the electric thrusters will overheat if operated for 30 minutes continuously. After all, if you are unable to dock your boat in under 30 minutes, then find a skipper that can ... :)
Larger boats can have hydraulic bow and stern thrusters. Hydraulic thrusters are usually stronger, require less space and are more complicated than electric thrusters. Hydraulic pump is usually connected to the main engine, but there are systems that use auxiliary petrol, diesel or even electric motor.
Tunnel Thrusters vs. Externally Mounted Thrusters
Tunnel thrusters are usually bow thrusters - reversible electric motor provide power to the propeller which is usually positioned in the middle of the tunnel. Electric motor can be serviced without requiring boat to be dry docked, while propeller is protected by wire mash from various debris that can float around.
On the other hand, tunnel can increase hull resistance and introduction of the tunnel itself can cause weaknesses in the hull and other problems in boat construction.
Externally mounted thrusters can be easily added or removed, but such operations usually require that the boat is in dry dock. They are often positioned on the bow's or stern's ends and thus create larger momentum during operation. Also, externally mounted bow and stern thrusters on fast boats are entirely out of water when boat is planing - there is no increase in hull resistance when it is most important.
When boat slows down, thrusters are submerged again.
Tunnel Thruster Diameter vs. Boat Length
Tunnel diameter and speed of water in tunnel determine the amount of water that passes through the tunnel every second and that is - thrust.
Little bit of math and physics:
Thruster Force (N) = Amount of water per second (kg/s) x speed of water in the tunnel (m/s)
Required Thruster Power (W) = Amount of water per second (kg/s) x (speed of water in the tunnel (m/s))2 / 2
If we want to double thrust force, we can double water flow (larger tunnel diameter) and keep the water speed at the same level - we would require to double the power of electric motor.
On the other hand, if we want to keep the force at the same level, but use the smaller tunnel diameter (half the size of cross section area), we would have to double the water speed which would require double increase in power for the same thrust.
Likewise, if we keep the same diameter of tunnel and we increase the water flow by factor 2, we would increase the thrust two times, but we would require four times more power.
In practice, tunnel diameters on boats of various sizes are approximately (tunnel diameter in mm vs. boat length in meters):
- 125mm - 6-9m
- 180mm - 9-12m
- 210mm - 12-18m
- 250mm - 15-24m
- 300mm - 18-30m
In order to keep tunnel diameter to acceptable size, larger boats/ships can have multiple tunnel thrusters.
This list is provided just as guidance, since docking conditions do vary from port to port significantly (currents, winds, maneuvering space etc.).
For example, most of the time, 9m boat will maneuver easily with 180mm/2-4kW bow thruster, but during strong winds and in marina with strong currents, not even two of such thruster would be enough :) Of course, in such conditions, good skipper will dock 'easily' even a boat without a single thruster. What to do? Practice how to maneuver your boat with and without thrusters - even use bow and stern thrusters to simulate wind by pushing boat away from dock and use only main drive to counter your thrusters!!!
Bow Thrusters for Small Boats
Although externally mounted thrusters have their advantages, I prefer tunnel bow thrusters for several reasons:
- tunnel with wire mash protects propeller from collisions and various debris floating around,
- tunnel is always under water, leading to increased hull resistance, but that increase in resistance is mostly negligible due to small tunnel diameter,
- although there is always a lack of space on small boats, access to the motor and its maintenance while the boat is still in the water is very important,
- bow thruster for small boat should be electric, forget hydraulics even if you have engineering background in this field - deep cycle battery is more than sufficient source of energy for thrusters on small boats, even if your main engine is not working and you are using your electric trolling motor and electric bow thruster to dock the boat,
- finding suitable spot for tunnel on the bow can be sometimes problematic, but using modern materials, especially modern epoxy resins, hull strength and integrity is not weaken, on contrary, it can be even increased.
- due to small power output (compared with larger boats) and small boat dimensions, electric cables are not so thick nor long - however, we are still talking about hundreds of amperes and those are still very serious currents.
- using dedicated battery for bow thruster on small boat is not absolutely needed. Rather invest some money in a good and reliable lead acid marine battery that can supply required current without significant drop in voltage, since such drop can negatively affect other electric and electronic equipment on the boat. If you do have sensitive equipment and have issues when using thrusters, then you can get thicker cables, have dedicated battery for sensitive equipment (charged from the main engine, but having that part of electric grid/wiring protected from voltage drops by some form of one-way protection circuit - and no, generally, fast switching silicon diode is not enough :o) - paired, matching high-power diodes can be used (on heat sink) and they are often used, but their use and sizing depends on many things, so don't do anything on your own if you are not absolutely sure what are you doing), have dedicated battery for thrusters (again, similar story with having dedicated battery for electronics), get better battery, position battery closer to the thruster etc. Anyway, if you run into problems, be sure to ask professionals for help - in the long run, it will save you both time and money.
- be sure to check your trim - these systems are not very heavy, but they are mounted on the boats end and can lead to 'wet bow'. So, before mounting bow thruster, be sure to mark real actual waterline (not the boot stripe) - after bow thruster is mounted, waterline should be the same. If it is not, rearrange some of the equipment on your boat or if possible, move your ballast toward the stern.
- be sure to keep your battery (batteries) and heavy, thick cables away from your fuel tank(s), metal fuel lines and fuel pumps. Strong currents can cause induction in nearby metal objects, even sparks, and that is not desirable near petrol or diesel fuel, to say the least.
Of course, there are many other reasons why one should use tunnel bow thrusters on small boats and what can go wrong. But generally, they are proven and reliable piece of boat hardware and can serve well for a long time.
Controlling the Thrusters
Thrusters are controlled easily with joysticks or touch pads.
Some cheaper models have off/on only functions, while output power of most models can be varied, depending on the situation.
Joysticks and touch pads are often positioned near the main boat controls and can be used at the same time with the main engine/helm.
Also, there are wireless models that enable a skipper to move around the boat and control the position of the boat while not being at the helm - this is especially useful during, for example, trolling when big game fish is hooked and every hand is needed on the stern, without losing possibility to finely position the boat.
Such wireless models with extended range can be used for docking the boat remotely or even for steering/navigating/driving few boats from the one boat at the same time etc.
Possibilities are endless, but wired controls are wired, and wireless controls can have issues and interferences from other equipment - high quality, digital wireless remote controls are very reliable, but can cost some decent money. If you do need such option, go for digital wireless, but I still prefer ordinary wire :o)
Long story short - boat thrusters are great aid in maneuvering your boat, regardless if you are in cramped port or out there in the open, trying to position your boat so that angler on the stern have best odds of landing the fish. They can be costly, require regular maintenance, add to the weight of the boat, but they can be very useful!