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Which Bow Thruster Type is Best? Tunnel vs. External vs. Retractable vs. Water Jets

Imtra thruster expert Peter Nolet reviews four common types and describes the pros and cons of each style for recreational boats.

Bow and stern thrusters come in many shapes and sizes and are used to help maneuver boats in tight quarters by providing thrust in a sideways direction. Different styles of thruster will work best for different types of boats, depending on the cost, complexity and maintenance you are willing to bear. To help sort out what’s best, let’s review this list of the pros and cons of each.

In full disclosure, at Imtra we sell Side-Power thrusters for the North American market and Side-Power has a broad product line matching three of the four styles described—Tunnel, Retractable, and External. To learn more about thrusters generally, read our earlier article “What are Bow Thrusters and How Do They Work?”

Tunnel Thrusters

Prop Tip Clearance

PROS: Thrusters in “tunnels” are the most common type of thrusters, mostly because they provide the most efficient way of moving water. For every horsepower delivered by your thruster motor, you receive 132 pounds of thrust, pushing your bow or stern in the direction you want. The tunnel concept has been accepted and integrated into boat designs by boat builders worldwide, and manufacturers offer tunnel thrusters in the largest range of sizes with multiple types of propulsion power: DC electric, AC electric and hydraulic.

Tunnel thrusters are also the most cost-effective type of thruster, and are known for their reliability, in part due to a minimum of moving parts. One other benefit is that the tunnel serves as a tubular stringer in the bow, adding strength to the hull.

CONS: The most obvious negative to a tunnel is that you will have a good-sized hole cut through your hull, although well-installed tunnels do not cause issues. Tunnel holes create a small amount of drag but if faired properly, this is of minimal or no concern to most boaters. A more significant con to a tunnel thruster is that it is somewhat more expensive to install. It also adds some weight and takes up some space in the bow of your boat.

Tunnel Thruster Rating

External Thrusters

External Thruster

PROS: External thrusters fit when all other styles don’t. For example, boats like sailboats or trawlers often won’t have the space to install a tunnel thruster in the bow or stern. This is when the external thruster becomes the obvious choice.

At the same time, externals still have their own tunnel, so they remain efficient, and the fact that they are water-cooled extends their run time. Installation is also least expensive as the units are bolted on and require few holes in the boat.

CONS: A clear knock against external thrusters is that they create more drag, but that’s only the case in the bow, not the stern, where they live behind the transom. The product itself is more expensive than internal thrusters, delivers lower initial thrust due to its motor design, and generally requires more maintenance, since all functioning parts are outside the boat. Finally, the range of products is narrower than with tunnels, and if you have a larger boat, you may need to install a pair of thrusters to have the power to get the job done.

External Thruster Rating

Retractable Thrusters

Retractable Thruster Animation

PROS: For those with performance boats, sail or power, who are worried about the smallest amounts of drag, a retractable thruster may be the best solution. Retractables have the second greatest range (next to Tunnels) and work with all types of propulsion power—DC, AC and hydraulic. Further, because the gear leg is inside the boat, they need less maintenance and there is less need to replace the anodes.

CONS: Retractable thrusters are the most complex of thrusters and have a lot of moving parts. As a result, they are the most expensive thruster product and the most expensive to install. They are also tricky to install, making a DIY project next to impossible, and there aren’t many boatyards with experience installing a retractable thruster; whereas tunnel & external thrusters installations are far more common.

Retractable Thruster Rating

Water Jets

Jet Thruster

PROS: Water-jet thrusters are an older technology and are the least expensive to buy and the least expensive to install on the hull. They also allow for long run times.

CONS: Water jets only deliver 10 pounds of thrust per horsepower, far less than the other types. Installation requires running large water hoses through the boat to the through-hulls, which may not be practical in the case of a refit. Boats with water jets also will need to replace their hoses every 7 to 10 years.

Screen Shot 2020-07-22 at 3.19.29 PM

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