As a marine surveyor I see lots of bow and stern thrusters. I would guess that over fifty percent of the installations I see are not working during a survey or perform poorly during a sea trail. Generally an electric problem, and generally an electrical problem that could have been avoided by proper installation and system design. The number of boaters that can not control their boat without a bow thruster or even a bow and stern thruster is numerous thus many docking damage events occur when an operator has become dependent upon a thruster system that then fails and does not work when they need it most.
Second and even more difficult to deal with is a bow thruster that is stuck on! This is a very common failure mode and very difficult to deal with. Lets have a look at what it takes to install a bow or stern thruster and how to properly design the supporting electrical system and controls. For the purpose of this series lets assume we are going to install a bow and stern thruster system on a heavy oceangoing 40 ft LOA fiberglass trawler and use that as our example vessel.
The trawler has a strong 12V house electrical system with dedicated battery bank, as well the trawler has two starting battery banks consisting of separated 4D starting batteries, all batteries are located mid ship in the engine room of the vessel. First lets have a look at what the manufacture a good tip for any installation says for physically installing the tunnel.
One of the first items to note is the further forward or aft the thruster is the more effective it will be in rotating the boat, this makes perfect sense because the lever arm of the thrust gets larger and larger as the thruster moves away from mid ship. The next item that needs to be considered is moot in the case of our ocean going trawler example as it is a non planning vessel. On a vessel which planes manufactures recommend that the thruster location is forward and above the planning waterline.
This may take extra effort but a safe location can easily be determined by taking pictures while following the subject installation vessel on plane from a chase boat. As a general rule of thumb a bow thruster should go as far forward and as deep below the waterline as possible. So lets move on to thruster selection. Based upon the vessel size manufactures recommend different sizes of thrusters, the various sizes of thrusters are sized to provide adequate performance for most boats of the size range indicated, although consideration should be given to where in the manufacturers recommended rage the vessel falls and IF the vessel is abnormal for its LOA.
