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 Why, What, When, and How of Marine Surveys?

Marine surveys are conducted on boats or yachts for a variety of reasons, and for various entities or people. Marine surveyors are, by nature of their industry standards, supposed to be impartial and unbiased. Their job is to tell the truth about a boat or yacht, as pleasing or painful as that may be.

American Boat and Yacht Council


To be very blunt, if you are a recreational boater, you most likely put the most precious cargo you have in your boat… your family! Then you trust the boat to safely take you to a destination or provide enjoyment through touring or watersports while on the water. A marine survey can help you assure your boat is safe and reliable for these fun times.

Second, if you plan to insure your boat most insurance companies will require a recent survey. You may be thinking, what a drag, but keep in mind a properly documented survey will protect the owner in the event of a major claim. By having proper documentation of the vessel overall, equipment on board, condition, and any upgrades the owner can prove insured value to the insurance company in the event of a claim.


Marine surveyors use Transport Canada, US Coast Guard, federal laws, and published industry standards as guidance for evaluating boats and yachts. These are a mix of mandatory standards defined by government entities and voluntary standards recognized as best practices in the yachting and boating industry.

  • The Code of Federal Regulations (CFR), what US Coast Guard regulations is based upon, is very specific as to certain construction requirements, especially for smaller and/or gasoline powered vessels, as well as for equipment such as navigation lights, flares, fire extinguishers and life jackets. Transport Canada and Canadian regulations follow much the same standards.
  • The American Boat & Yacht Council (ABYC) Standards and Technical Information Reports for small craft. This set of standards provides guidance on certain aspects of the vessel’s design and construction, such as visibility from the helm position, and installation of many of the systems and equipment found on a vessel, but does not delve into hull design, structural standards or rules.
  • The National Fire Protection Association (NFPA) is concerned with the things that can cause, prevent or put out a fire, as well as with carbon monoxide poisoning.
  • The International Organization for Standardization (ISO) has developed boatbuilding standards that are used in some areas outside the US, especially in Europe.


The following are the general cases and times when a marine survey would be required or desired by a boat or yacht owner.

  • A Prepurchase Survey is conducted when someone is contemplating buying a vessel. It is a very comprehensive inspection that includes operational testing of all systems and equipment, in many cases a pre purchase survey will also include a sea trial. The vessel is typically inspected both in and out of the water. The overall goal of the pre purchase survey is to provide the potential buyers with the information that they need to make an informed purchase decision.
  • An Insurance Survey is done specifically for insurance purposes at the request of the insurance company. An insurance survey may be done with the vessel either afloat or ashore, depending on the insurance carrier’s requirements, an insurance survey generally does not require a sea trial or operational testing of all equipment and systems. Every insurance company has their own guidelines as to what age/size vessel needs a survey, how often a survey must be accomplished, and what the survey report should include.
  • Valuation & Appraisal surveys are completed for application to financing companies, estate and divorce requirements, and possibly donation of vessels to charities.
  • Damage Surveys are performed for insurance companies, vessel owners and other parties after the unfortunate incident causing harm to the vessel or surrounding objects. The marine surveyor will work to document the nature, cause and extent of damage to a vessel.


At Toronto Marine Surveyors we work with vessels both in the water and on land. Depending upon your own personal needs or your insurance companies requirements a decision will be made on the location for the survey. If you boat is in the water and it requires haul out we can help you with arranging a quick haul to complete the out of water requirements.

Our surveys generally cover the following topics and base inspections on the following format:

  1. Hull material, construction, and condition
  2. Deck material, construction, and condition
  3. Propulsion – engine
  4. Running gear – shafts, struts, outdrives, propellers
  5. Fuel system – tankage, fill fittings, valves
  6. Ground tackle – anchors, anchor rode, windlass
  7. Standing rigging – if a sailing vessel, mast, boom, and related hardware
  8. 110/120 volt system – shore power feed, main electrical panel, switching, generators, air conditioners, battery chargers, wiring type
  9. DC electrical system – battery security, wiring type, alternators, chargers, switch panels, battery switches
  10. Navigation systems – navigation lighting, entertainment systems, RADAR, chart plotters, GPS, compasses, instruments, autopilots
  11. Bilge pumping systems – electric pumps, float switches, related plumbing, manual pumps
  12. Accommodations – general condition of accommodations, galley installations including LP stoves, ovens, sinks, water heaters, refrigerators, ice makers, freezers, etc
  13. Safety equipment – inventory of safety equipment on board compared to Transport Canada guidelines for the applicable vessel

After the surveyor is completed with the physical inspection a report will be delivered to the owner or prospective buyer. For pleasure craft these reports range from 25 to 50 pages and include a both a written and visual description of the vessel.

Findings or recommendations from the surveyor will be categorizes as HIGH, MEDIUM, AND LOW.

Recommendations are provided based upon the logic of high priority items not meeting the above stated standards, medium level items impacting safety of the vessel per the surveyors opinion, and low priority items impacting long term integrity and value of the vessel.



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.

Start the process by contacting us through our web form, or phoning us at: 647-812-4117.

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