A Proper Bow / Stern Thruster Installation – Part 1
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. Why? Generally an electric problem, and generally an electrical problem that could have been avoided by proper installation and system design.
In todays boating world there may not be a more important piece of non primary use equipment on your boat besides a bow & stern thruster. 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.
Installation Guidelines and Tips
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. So the takeaway here is to mount the forward tube as far forward as possible and use the “T” stern tube option most manufactures offer.
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. But, as we saw above on a planning vessel sometimes it is not possible to have it as deep as desired, second on many other vessels power and sail there is tankage, boat structure, electrical, and or ducting in the path of where the “optimal” tube location could go. So lets move on to thruster selection.
Selecting the Proper Sized Thruster
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. i.e. is the vessel extremely heavy, does it have more windage than a normal X LOA vessel, or perhaps a full keel which prevents easy rotation.
Using the manufactures guidelines a model and associated hardware can be selected.
So? What size thruster do we need? Most manufactures publish multiple sizing charts just like the one above. This one is for the rage of Vetus thrusters, it shows the amount of force each model thruster produced and then the recommended vessel sizes it can service.
As mentioned above it does not take much smarts to figure out that the lower in the intended range you are, the better results you will most likely have and more effective the thrusters will be. Also, again as noted above, keep those special considerations in mind of any vessel peculiarities.
Based upon our example vessel being a heavy ocean going 40 foot trawler it seems that the 95 kg model would work well. A 40 foot boat falls low in the manufactures suggested range, suggesting that the additional weight of the trawler will be able to be handled. If one was concerned the 125 kg model could be considered, but keep in mind that going up in size means a a larger tunnel and higher amperage demands for operation which increases costs.
Thruster Critical Dimensions
Vetus publishes the following graphic for their thrusters showing the minimum proper dimensions and ratios they consider acceptable. The “D” dimension that the graphic hinges around is the diameter of the thruster tube it self which increases as thrust capability increases.
In our example trawler case we selected the 95 kg model, the 95 kg model specifies a tube diameter of right around 7 5/16 inches. With some simple math you can see what the other dimensions turn out to be in the graphic above.
As one can see here the minimum distance below the waterline Vetus specifies is half of the tube diameter. Another take away I see in this graphic is that they prefer the thruster propeller centered in the tube, which makes sense, but on some larger vessels this is just not possible due to installation limits as one needs to reach in to place the propeller gear box.
Generally the bow tube is supplied with the thruster, it is a pre made off the shelf part supplied by the manufacture of the thruster. This aids in overall quality and speed of installation.
Part 2 What Is Next?
From here installation manuals of most manufactures now go on to explain specifications for mounting the motors, and the specifications for connections of the tunnel to the hull it self and the electrical installation. Obviously care on the connection of the tube to hull is necessary for structural reasons, and a bit of fairing and thinking can reduce drag greatly but the design and installation of the electrical system is just as important for long term success.
This will conclude part 1, in part 2 we will have a look at what the considerations surrounding a bow and stern thruster battery, electrical, and control system are along with some details and video of installing the tube into the hull.
Captain Dustin Norlund
I have been a life long avid sailor with over 37,000 offshore miles around the world on sailing and power vessels. Check out my personal site with plenty of YouTube and TickTock videos with marine DIY tips, sailboat and powerboat handling advice and realworld navigation tips.
Recently I have made the change from sailing vessels to classic trawlers and have not looked back. Boats, sail and power, have given me the ability to see and do things all over the world. Some of my favorite highlights include transiting the Panama Canal twice, transiting the Trent Severn waterway twice, transiting the Welland Canal, racing in the Newport to Ensenada race multiple times, racing in the St. Maarten Heineken regatta multiple times, and locally to Lake Ontario racing in the Lake Ontario 300 and Susan Hood.