The joys of thrust
Bow thrusters
“So just going to cut a big hole right through the bow of your boat, don't worry, we do it all the time”
Retrofitting a bow thruster
Having been involved with 3 retrofits of bow thrusters in the past few months I thought I would take this opportunity to write about it, what´s involved and the benefits of having a bow thruster fitted. I cannot go into too many specifics on every type and installation as that would require a book, this is concentrating on the common or garden tunnel type electric thruster, the options for sailboats apart from this type include retractable thrusters and even ones that can have the motor and propeller fixed to the outside of the hull, negating the need to make a big hole through the hull.
What is a bow thruster?
A bow thruster is a propeller fitted below the waterline at the bow (front) of the boat, the propeller is located in a fixed tunnel that runs from side to side of the boat, it´s designed to push the front of the boat from side to side, the idea being to assist with low speed maneuvers, especially it windy conditions, they aren't designed to be used for prolonged periods, only in short bursts. The big names in thrusters are Max Power, Vetus, Sleipner and Quick, if you are having a thruster installed it´s worth comparing prices and availability, the last thing you want is to buy one and not have service and parts backup.
Bow thruster components
Tunnel, this is usually glass fiber, it´s cut down to size to fit the boat, a “eyebrow” of sorts is left on the outside or one is glassed into place on either side of the tunnel to act as a water deflector, without this at high speeds water rushes into the tunnel and makes a terrible amount of noise and potentially could add stress the the components inside the tunnel.
If your hull is cored (as in the photo above), that is has a sandwich construction, often of balsa “sandwiched” in between the inner and outer layers of glass fiber, then a section of the coring around the tunnel might be removed and glass fiber put in it´s place.
Gearbox, this is the “leg” than goes on the outside, fitted in the tunnel it holds the propeller(s) in place, some are made of bronze and will have a sacrificial anode to help with corrosion, others are made of composite (plastic) materials and will probably not have an anode. Some propellers have a sheer pin or similar behind the propeller designed to break if something should get caught in the propeller.
Thruster motor, this is mounted inside the hull, either electric or hydraulic and fitted directly coupled the gearbox, often they can be fitted at angles to allow for different installation considerations. The second component of the thruster motor is the solenoid system, this is electromechanical device that causes the motor to spin one way or the other depending on what direction you want the bow to go, solenoids can be fitted directly on the motor or sometimes there is the option to mount independently.
Control panel or joystick, this is located at each of the helm positions, usually in the form of a joystick, sometimes a flat panel with arrows to port and starboard, some high end units are available with wireless remote controls too.
Installation considerations
Before any works begin, it´s important to consider quite a bit;
What's in the bow?
Depending on the model of boat, some boats were available from the factory with a bow thruster installed, if this is the case then half the battle is won because there will likely be a void or empty space under the floor or forward berth to install the tunnel and thruster motor.
If the boat wasn't originally optioned for a thruster then you´ll have to start taking up floor panels and removing the mattress and possibly any storage under the forward berths will have to be given up in favor of having the bow thruster (as was the case of one of the installations recently).
Batteries and cables
In order to make your thruster work, you are going to need quite a bit of battery power, how that power is provided will vary from boat to boat;
Using existing batteries, this means tapping into the batteries already installed on the boat, usually the house or service bank of batteries, this might work on smaller boats, some bow riders as small as 27 feet have thrusters installed at the factory and this is the method they would usually use. This setup works well in small boats, where the distance from the batteries to the thruster motor is short, negating the need to install large battery cables, it´s also more likely that a smaller boat will have fewer electrical demands, barring some amplifiers and stereos, so the electrics are not so likely to get pulled down by the electrical demands of the thruster when its used.
On larger boats, careful consideration must be made to using the existing battery bank, a larger boat will need a more powerful thruster, the thruster motor will be more electrically demanding, you might find the current house bank is too small or perhaps requiring upgrading from lead acid batteries to a different chemistry, also don´t forget that all those amps of current needed by the thruster motor translates into larger cross section battery cables and terminals for the cable ends, those large battery cables are expensive, heavy and often difficult to thread through.
I have seen cable installations where two smaller identical cross section cables were used to equal or pass the requirements of the thruster motor current demands simply because those smaller cables were more readily available and easier to work with, they were obviously fused (circuit protected) as though they were just one cable.
I have been on a few larger boats that when the thruster was used the electronic navigation aids turned off due to the electrical demand and in one instance one of the engines stopped. I never did get to the bottom of why this happened as the boat was shipped away, I can only imagine the batteries were undersized or possibly an engine battery was being used for engine start and bow thruster use.
Independent battery group only for bow thruster use, this is how the ones recently installed were configured. Take a look at the specs of the thruster to find the battery requirements, even on 12 volt equipped boats, it´s not uncommon to install a 24 volt thrusters, with two batteries connected in series, completely independent of the other electrical systems on the boat, the thruster batteries have their own independent charger. Please remember if installing batteries inside the boat, do not use lead acid batteries due to them venting explosive gases during charging, AGM batteries are much safer.
Further to the batteries and cables, you will also need a battery disconnect switch and a way of fusing the thruster motor. You install a battery disconnect switch mainly in case of needing to isolate the thruster motor and also just in case the motor should get stuck or the thruster control become unresponsive (exceptionally rare), you would need to cut the power to the motor.
When choosing a battery disconnect switch, please choose one that will be suitable, the one that was sent for fitting on one of the recent jobs was rated to only 100 amps (a cheap switch with a red plastic key & NOT the quality Guest branded one on the photo above), the fuse that was protecting the thruster was rated at 250 amps, so in theory the battery switch could have melted or failed way before the fuse and possibly caused a fire.
The most common fuse is the ANL type, they allow the initial inrush or sudden demand of the thruster for a short enough period of time to not break but if that current (amps) should persist the fuse will break protecting the circuit.
Fire. Not to be alarmist, fortunately fires are fairly rare on boats, given the number of boats is storage, on hard standing and in the water you´d be forgiven for thinking fires would be more common. I would always recommend taking precautions to extinguish a fire in the compartment where the thruster motor and/or batteries are located, usually in the form of an independent automatic fire extinguisher, it´s good practice and cheap insurance, talking of which, if you have a thruster fitted post construction, you should inform your insurance company of this change.
Doing the work
Actually doing the thruster installation is not for the faint of heart, it involves taking apart some of the interior of the boat, cutting holes on both sides of the hull below the waterline, fiber glassing in a tunnel and then installing a gearbox and thruster motor, cutting a mounting hole in the newly installed tunnel. Pick a company or a yard that has a good reputation and has done this kind of work before, if you are in doubt, get in touch with the thruster manufacturer or distributor for your zone and ask them if they have any recommendations or partner networks who can do the installation.
Spares
Although there is in the most part little you can do if there is a serious problem, it´s worth carrying a few simple to replace spares on board, there's nothing more annoying when it´s a simple fix but you just don´t have the simple items.
Spare fuses, the main fuse that protects the thruster motor, usually a high amperage rating, make sure you have a spare, spare fuses taped in a bag or cable tied to the fuse cover to find them easily.
Blade fuse, these are common to lots of things on a boat, they are the same as those often used in cars and vans, some thrusters have a blade fuse that protects the control panel or joystick in the circuit that controls the thruster motor.
Overtemperature protection, these small devices go by one than one name, clixon is one, basically, in the event that the thruster motor gets too hot, this type of device breaks a negative cable from the thruster control panel, I have yet to see one that resets on its own when the motor cools down, all of the ones I have seen required replacing, on some installations its more fiddly than it should be.
Sheer pins, these go inside the hub of the propeller of the bow thruster, in the unlikely event of something getting stuck in the thruster tunnel and jamming the propeller, a small pin designed to break at a certain force will stop any damage to the thruster gears, to replace this pin the boat would have to be hauled out. Carrying spares might save a bunch of time and money, even if you don´t replace them yourself, the boat can just be suspended, the pin replaced, the thruster tested and you´re back in the water quickly.
I hope you have enjoyed reading my article, please feel free to follow me on social media, Facebook, Twitter, Instagram, abuxboater.
