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Install
Aluminum and steel boat bow thrusters: what changes
An aluminum or steel boat takes a bow thruster well. The tube is welded in with no laminating, and the size you need does not change. This guide walks you through what does change, from the tube to the corrosion rules every maker prints.
Where this comes from. Sleipner’s SE20 to SE40 guide, SEP80 to SEP150 manual and 2015 SE80 to SE130 manual, Lewmar’s Gen 2 manual, Max Power’s CT HYD 125 manual, Quick’s BTR manual, Sideshift’s monohull manual and Yacht Thruster’s FAQ, read 20 September 2026, or earlier for the brand reviews. No maker publishes a metal-hull price. The one figure is an owner’s.
The short answer
The wind pushes on the topsides, and no maker’s sizing method asks what they are made of.
Metal gets angled tunnel ends where fiberglass gets rounded ones. Sleipner says they perform about the same.
The hull conducts, so the motor and its bracket must stay isolated from it. Lewmar gives two A hand-held meter that reads volts and resistance. It is the tool behind the makers’ isolation tests. tests to prove it.
No copper-based The paint on the underwater hull that stops weed and barnacles growing. It goes on the gear leg and the propeller too, and never on the anode. near the unit, and an aluminum primer under the paint.
On the one welded bill this site holds, the tube and the welding came to $1,000. That bill is from December 2016.
Fit one to a metal hull in four steps
- Size it the way you would any boat. The hull material does not move the number. The section what does not change explains why, and where a heavy steel boat does show up.
- Give your welder the tube details. The ends are angled, and the wall has to suit the The underwater part of a thruster: the housing in the tunnel that holds the gears and the propeller. The motor sits above it, inside the boat.. Get both from the maker’s manual before the tube is made. The section the tube has the slope and the wall table.
- Isolate the motor, then test it. The motor body and its bracket must stay isolated from the hull. The section isolation has Lewmar’s two tests and Sleipner’s sign-off check. For an external unit, see a unit bolted to a metal hull.
- Get the paint and the anode right. Aluminum takes its own primer and no copper paint. The section paint and anodes has the makers’ lines.
Want the answer for your own boat? The thruster finder, our free tool that matches a thruster to your boat, lists the same units for a metal hull as for a fiberglass one. The size calculator gives the thrust and shows its working.
What does not change
The size does not change. The wind pushes on the The sides of the hull above the water.. No maker’s sizing method asks what they are made of. Sleipner puts it in one line: “Thruster power is mainly determined by a boat’s design and not by its weight.”
Where a steel boat does show up is in weight. The size calculator takes displacement as an optional entry. It adds a note and leaves the figure alone.
The thruster finder lists the same units for a metal hull as for a fiberglass one, because it never asks what the hull is made of. It asks the boat type instead. A small aluminum utility boat comes up there as a fast, light hull, and gets the rules that follow from that.
The tube: welded, angled, thicker
Welded in
Sleipner’s guide draws the joint two ways on the same page. On a fiberglass hull it is labeled “Layers of fiberglass and resin”, at least eight of them. On a steel or aluminum hull it is labeled “Welding”.
You can order the tube from Sleipner too. Its 2015 manual says: “Steel & aluminum tunnels available at request.”
Angled ends
Fiberglass gets a rounded mouth, with a radius of at least 10 percent of the diameter. That rounding is worth real thrust, and the placement page has the figure.
Metal gets a An edge cut at a slant instead of left square or rounded. instead. Sleipner says it does the same job: “For steel/ aluminium hulls angled tunnel ends also offer similar performance as a rounded connection.”
The slope has a size, and Sleipner gives it: “For steel/ aluminium hulls make a slope with a length of 10-15% of the tunnel diameter.”
A thicker wall
The gear leg is built for a range of wall thickness. Every Sleipner manual prints it, 4 to 10 mm across the models below. The maximum is the figure for “other GRP, steel or aluminium tunnels”. A welder working from plate needs that number before the tube is made.
| Models | Tunnel | Thinnest wall | Thickest wall |
|---|---|---|---|
| SE20 to SE40 | 110 and 125 mm | 4 mm | 6 mm |
| SEP80 to SEP130 | 185 mm | 6 mm | 8 mm |
| SEP120 to SEP150 | 215 and 250 mm | 7 mm | 9 mm |
| SE80 and SE100 (2015 manual) | 185 mm | 6 mm | 8 mm |
| SE130 (2015 manual) | 250 mm | 7 mm | 10 mm |
Max Power comes at the same thing from the other side. Its manual is the one that most often meets a metal tube, and it says: “When using tunnels of different thickness (example: metallic tunnel) it is imperative that the area between the drive leg/gasket and the motor support, matches the thickness as indicated in the table on the drawing "Positioning & Measurements" at back of manual and that the motor support is stable.”
For a tube under 6 mm, Max Power also wants a hard rubber gasket between the motor support and the tunnel.
A retractable’s housing
A retractable’s housing is welded in the same way. Quick’s BTR manual sells the support with the unit: “Quick® can provide stainless steel, aluminium alloy or GRP supports, which are essential for a fast, sturdy and precise installation.”
On a metal hull that support is welded too. Quick says: “For aluminium alloy hulls or stainless steel hulls, the support needs to be welded to the hull.” It adds what a boatbuilder would: “If well done, the installation of a box structure such as that of the support can give greater strength to the hull.”
The retractables page has the hatch sizes.
Isolation: the rule every maker prints
A metal hull conducts. A thruster is a bronze or aluminum leg sitting in the water, with hundreds of amps running to a motor bolted a few inches from the plating. Every maker’s manual has a line about that.
On a fiberglass boat the motor body sits surrounded by an insulator. On a metal boat it is bolted to the hull through a bracket. Lewmar writes the rule with a capital must:
“To prevent electrolytic corrosion or faults, the thruster motor body and assembly MUST remain isolated from any power supply or grounds.”
Its manual gives the installer two multimeter tests.
The first is done with the battery disconnected. Lewmar says: “If a connection is measured between the –VE stud and the motor body, remove any bonding straps attached to the assembly and check as before.”
The second is done with the battery connected, reading the volts between the negative stud and the motor body. Lewmar says: “If the supply voltage (12V / 24V) is measured, disconnect power immediately and inspect the assembly for faulty installation or damage.”
Two smaller lines belong with those tests. One is about the parts in the box: “Ensure the insulating boots, supplied with the unit, are correctly fitted.”
The other is in the service schedule: “Brush out carbon dust from top of electric motor especially on aluminium boats.” The manual gives no reason for naming aluminum boats there. Carbon dust conducts.
Sleipner builds the isolation into the bracket and then checks it. Its 2015 manual specifies the motor bracket as “Motor bracket: Seawaterresistant aluminium, galvanicly insulated from motor” (its spelling).
Its eleven-point sign-off list is on the install page. One of the eleven is the test: “An ohm meter shows no electrical connection between the motor body and either terminal.”
On a hydraulic unit Lewmar isolates the oil side too: “Place insulator in between saddle and motor flange.” It isolates the pipe as well: “It is advisable to fit insulated pipe sections to prevent thruster corrosion.”
Lewmar also sells two of its larger units with an aluminum gearbox beside the bronze one (the TT 250 at 8 kW and 24 V, 140 kgf, and the TT 300 at 10.8 kW and 24 V, 250 kgf, the AL part numbers in the Gen 2 ladder). The manual does not say which hull each one is for. The Lewmar review holds the ladder.
Whoever wires the boat should know what these tests are for. Lewmar’s word for what goes wrong is electrolytic corrosion. Its second test is written to catch a motor body that has gone live to the battery. Do not skip the isolation test on a metal boat. The wiring page has the cable and the fuse.
A unit bolted to a metal hull
An external unit avoids the tube. It meets the same problem on the outside of the plating. Yacht Thruster’s housing is “seawater-resistant special treated aluminum”, and its FAQ is blunt about metal hulls: “It is necessary to assure that there is no direct contact between Yacht Thruster™ and boat hulls out of steel or aluminum.”
Sideshift’s pontoon and houseboat units bolt to aluminum by design, so its manual goes further.
It sets the plate. The manual says: “Ensure the mounting surface of the thruster is at least ¼” aluminum or steel plating, or attach rails (see image below) to create an appropriate mounting platform. The mounting surface must be completely rigid with no flex.”
It deals with the metals touching. The manual says: “If for any reason the motor is removed from the mount, it is highly recommended to replace the motor strap kit to prevent the chance of dissimilar metals touching.”
It says how often to look. The manual says: “It is recommended to inspect your thruster every few weeks (when the boat is in use) to ensure the motor is not being attacked by stray currents, causing paint failure and premature corrosion.”
The failure it guards against is the same one a tunnel unit faces. Picture an aluminum motor and an aluminum hull, with stainless fasteners between them and the sea around both.
DockStar’s unit sits on a track and carries its own battery, so no boat wiring reaches it. The track’s bolts are its one contact with the hull.
The no-hole page compares the external units. The houseboat guide is the one place on this site with a welded aluminum hull priced.
Paint and anodes
Two rules that matter on any hull matter more on aluminum.
The first is paint. VETUS prints “do not use copper based anti-fouling” on every manual, and the barnacles page says why. On an aluminum hull the plating around the tube goes by the same rule. Yacht Thruster’s FAQ says what to use instead: “Therefore use an appropriate aluminum primer and then the according antifouling.”
The second is the anode, which follows the hull’s own scheme. Quick fits an aluminum anode on a gearbox of aluminum-bronze alloy. Sideshift says how fast it goes: “The rate at which the anode deteriorates depends on the water temperature, type (salt or fresh), and stray currents around the boat”.
On a metal boat, Electricity leaking into the water from a fault or a poor connection on a boat, yours or a neighbor’s at the dock. It eats metal under the water. is the thing to watch. The anode page has every maker’s interval and the one rule they all print.
The bill
No maker prices a metal-hull job. The yard hours on the cost page are fiberglass hours.
The one itemized welded bill this site holds is a Summerset houseboat, December 2016. The aluminum tube was $200 and the welding $800, so the hull work came to $1,000. The battery bank, the charger and the cable were the expensive lines. The houseboat guide has the whole bill and its date.
That is one boat, and its prices are a decade old. It still tells you two things for any metal boat. The hull work was the cheap part of that bill, and the trade doing it is a welder. A fiberglass yard’s quote is the wrong one to compare it with.
Sources, by maker. Sleipner: SE20 to SE40 guide, SEP80 to SEP150 manual and 2015 SE80 to SE130 manual. Lewmar: Gen 2 manual. Max Power: CT HYD 125 manual. Quick: BTR manual. Sideshift: monohull manual. Yacht Thruster: FAQ. The welded bill is from a Wayne’s Words thread (December 2016).