A Proper Bow / Stern Thruster Installation – Part 1

A Proper Bow / Stern Thruster Installation – Part 1

Bow & Stern Thruster Components

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.

Thrusters At Work

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.

Planning Vessel Example Location

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

Vetus Sizing Chart – KGs of Force vs Recommended Boat Lengths

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 Installation Specifications

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.

Written By:

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.

The Sea Trial – What is a sea trial? Do I need one?

Taking a boat for a sea trial—or basically a “test run”— I feel should be an essential part of the boat-buying process for higher value vessels. What to look for during this test, however, isn’t always clear to new owners even if they are experienced boaters due to the new boats complexity and new systems. This said there are several areas worthy of your attention during a sea trial, but the final decision to do one is up to the buyer.

Here is Ontario sea trials do not seem to be as popular as I have seen in the Caribbean or the United States boat markets. I am not exactly sure why this is but I suspect it has to do with the fact the boating season is so short and many buy & sell transactions happen during the off season when the boat is out of the water, or the weather is not suited for a sea trial. Weather and boating season aside I still stand by the recommendation for a sea trial on more expensive vessels. Does it make sense to sea trial an 18k boat with minimal systems and a single engine, maybe, maybe not, does it make sense to sea trial a 150k boat with twin engines, two generators, water maker, inverter charger, integrated control system, and a whole pile of other components, yes 100%

Basics of a Recreational Boat Sea Trial

Whenever possible, we believe in testing a boat like you plan to use the vessel. We see too many sea trials that are done on a boat loaded with low fuel, no passengers, no passenger belongings, empty holding tanks and an absence of gear. This just does not paint a true picture of the vessel being tested and should not be considered a valid trial. As well consider the weather including heat and sun, do you plan to operate in the summer heat? Find out if your air conditioning can keep up. Do you plan to operate late into the fall? Sort out if the heat works. Basic number one – setup a realistic situation.

Second Skiing and wakeboarding? Think back to the basics and put the heaviest person you intend behind the boat and see how the performance is. If you dealer or seller will not let you do it… move on.

In actual use, odds are the boat will be loaded with all of the above items and probably a few things I forgot, thus adding hundreds or even 1,000s pounds or more to your boat’s load. Keep in mind that the performance you feel during a lightly loaded test ride will never equal the real world performance at a much heavier operational displacement.

what are sea trials

The next basic we like to focus upon is the operational parameters of engines, gensets, and other equipment. In the cases of engines and gensets a real world test should be accomplished per the manufactures ratings. The ratings are easy to find for nearly every engine and generator on the market today. Simply hit the manufactures site, search the model, and download the PDF. Within the PDF you will find the operating parameters for the equipment.

Engines and drivetrains can potentially be some of the most expensive equipment to repair on the vessel. To check the engine health the engine should be operated in a method to see if it will deliver the rated RPM without overheating or having other abnormalities for the specified time within the manual. Many manuals will specify a temporary power rating, and a continuous power rating. The engine and transmissions should be able to run without issue at the continuous power rating, this will not damage or harm the engine if it is in good health. Does the engine overheat? Transmission temperature and pressure ok? Will the engine not make RPM? Excessive smoke or steam? Any yes answers and it is time to start diving deeper into the engine and drive system and head back to the docks.

Generators should be tested in much the same way, gensets have specifications for power out put and RPM. During the sea trial load up the generator and observe how it behaves. Again the generator should be able to run continuously at the rated power without any issue, RPM should not flux or cycle, voltage output should be steady, it should not overheat, when loads or added or removed the genset should quickly fix any variations in RPM or voltage. Personally I find that using the vessels air conditioners, water maker, and refrigeration components provide a great opportunity to load the generator on a sea trial. Second, while testing the long term loading on the generator it give you a chance to review the performance of the items you have the generator loaded with.

After a check of the other operational equipment not used to load the genset this should conclude the basics of sea trialing the engine room. Next lets talk about boat performance and ground tackle performance.

Next up for the basics of the sea trail are focusing on actual boat performance, because at this point we have decided that the engines are in good health and we are ready to move on to some more complex maneuvers and tests. At a minimum we would be seeing if the boat will make the advertised performance speeds from the manufacture, does the boat track in a straight line at idle?, at power?, are there abnormal noises shifting from forward to reverse, what do the engine mounts do shifting from forward to reverse, what do the mounts do at higher speeds shifting from forward to reverse (with time for the transmission to spin down)? Any findings here should be documented and or recorded with video for a qualified mechanic to review.

At this point of the sea trail we like to start getting into the actual boat performance, checking steering, ground tackle, and deck equipment. We will save these items for a future post.


Based in the GTA, Toronto Marine Surveyors serves central Ontario as far north as the Muskoka’s, East to Ottawa, and West to Sarnia.  We will travel to the USA as required.

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