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Install
Tunnel thruster installation: the job, step by step
A A tube bored straight through the hull below the waterline, with a propeller in the middle of it. It is the conventional thruster. Fitting one is mostly a labor job rather than a hardware one. thruster install takes days, and the biggest line on the quote is labor: a number of hours that means nothing until you know what a yard does with them. This page walks through the job in the order a yard does it, from the Lifting the boat out of the water on a travel lift or a trailer so work can be done below the waterline. It is billed separately from the job itself. to the first run. Then, when a quote gives you a number of hours, you can see what those hours are for.
Where this comes from. Every step and figure here is read from five makers’ own installation manuals, on 19 September 2026. They are Sleipner’s SE series guide, Max Power’s CT100 and CT125 manual, Quick’s BTQ250 and BTQ300 manual, Lewmar’s Gen 2 manual and VETUS’s RIMDRIVE instructions. Each one is linked in the sources at the foot of the page.
The hours are our own band by boat length. The band runs between one itemized invoice and the job owners describe as typical, and every cost page prints the same figure. Treat this page as a map of the yard’s work. The makers put the hull work in a professional’s hands, and so does the DIY page.
The short answer
The tunnel goes through the hull below the waterline, so the boat comes out of the water. On a boat around 35 ft the hands-on work fits in two days. The glass has to cure between coats, so the boat stays ashore (on the hard, as the yard calls it) for most of a week.
On the cost page’s worked bill, labor is 45 to 53 percent of the total. The hours run from about 32 on a boat around 35 ft to about 70 on one around 47 ft.
Every maker’s manual says the tunnel work is for a professional. That covers the hole, the bonding and the Smoothing a repair or a fitting into the shape of the hull with filler, so the water flows past it without a step. On a thruster it is the work that rounds the tunnel mouth into the hull..
The battery bank, the cable run, the fuse and the control panel are within reach of an owner who can crimp heavy cable. Step seven covers them.
In our reading, a tunnel job quoted in a day is missing a step. Fairing the tunnel ends is the usual one.
The job in one table
There are eight steps. The right-hand column shows the line of a quote each step lands on, in the same words the cost page uses for its worked bill. Only one step is realistically yours to take on.
| Step | Who does it | Where it lands on the quote |
|---|---|---|
| 1. Haul out and block up | The yard | Haul-out and lay days |
| 2. Choose the spot | The yard | Labor |
| 3. Cut the hole through both sides | The yard | Labor |
| 4. Bond the tube in | The yard | Labor, and the tunnel tube |
| 5. Fair the ends | The yard | Labor |
| 6. Fit the gear leg, motor and propeller | The yard | Labor, and the thruster unit |
| 7. Run the cable, fit the fuse and the bank | You, or the yard | Batteries, cable and charger, and the control panel |
| 8. Antifoul, launch and check | The yard | Haul-out and lay days |
Labor is the big line. On the cost page’s worked bill it is 32 billed hours at $160 to $180 an hour. That is two people for two days on a boat around 35 ft.
The hours come from two points. One itemized invoice we hold billed 70 hours on a boat around 47 ft. Owners describe a boat around 35 ft as two people for two days, about 32 hours. The thruster finder, our free tool that matches a thruster to your boat, bands the hours between those two by your boat’s length. Steps three to six are where those hours go.
Step one: haul out and block up
The tube goes through the hull below the waterline, so the boat has to come out. The haul-out is billed on its own line. On the eight yards’ rate cards the cost page holds, it runs $620 to $1,690 for a 35 ft boat out 5 to 7 days. The days on the hard are billed as The days a boat sits out of the water in the yard, billed by the foot per day on top of the lift itself., by the foot, per day.
The glass work in steps four and five has to cure between coats. That is why the boat stays out for most of a week, even though the hands-on hours fit in two days.
On your quote, look for the haul-out, the lay days and the launch as separate lines. A quote that folds them into the labor hours is not wrong. You just cannot check it.
Step two: choose the spot
Every manual gives the same two instructions, in the same order.
As far forward as possible. The thruster turns the boat about her The spot the boat turns about when she swings. With the boat moving ahead it is usually about a third of the way back from the bow. In reverse, it moves further back.. A longer lever makes more turning from the same push.
As deep as possible. Depth stops the propeller sucking air down from the surface. Air in the tunnel is lost thrust and a lot of noise.
Each maker writes how deep differently.
- Lewmar asks for one tunnel diameter below the waterline, with 0.75 of a diameter the minimum.
- Quick asks for at least 0.75 of a diameter.
- Max Power wants the top of the tube at least 139 mm down, and ideally 185 mm.
- VETUS, for its rim-drive units, wants the centerline at least 250 mm down.
- Sleipner leaves the number to each model’s own measurements and says the deeper the better.
Many sailboats are narrow forward. A bow like that has no spot that is both deep enough and far enough forward, so the yard has to trade one against the other. The tunnel placement page works through that trade. It has each maker’s figures and the depth for every tunnel size in the database.
The tube’s length is set by the hull at the spot chosen.
- Sleipner says no more than 4 times the diameter, because friction in a long tube slows the water.
- Quick calls 3 to 4 diameters optimal and tolerates up to 6.
- Lewmar suggests at least 2.
Sleipner also warns against a tube that is too short. In a short tube, the water reaches the propeller before it has straightened out.
Ask this about the spot. Sleipner’s guide says the thruster must not go anywhere that means cutting a stiffener or a A long rib bonded inside the hull, running from bow to stern, that stiffens the bottom. Cutting through one weakens the boat, so a tunnel is kept clear of them.. If the spot the yard has chosen touches structure, ask what they intend to do about it.
Step three: cut the hole
This is the part of the job that makes an owner’s hand hover over the saw, and it is the part with the most tooling behind it. The hole is not round. The tube passes through a hull that curves away on each side, so each opening is an ellipse. The two ellipses have to line up through the boat.
Max Power’s method is the clearest.
- Mark the tunnel’s center on both sides.
- Drill a pilot hole of 8 to 10 mm through each side.
- Push a metal rod through both holes.
- Use the rod as a compass of 100 mm radius to trace the ellipse on each side before cutting.
The other makers have their own aids. Lewmar makes a jig to align the drill on both sides, and forms a wire guide to the tunnel’s diameter to mark the cut. Sleipner sells a special tool through its dealers.
Two owners who published their own tunnel job, on a Bayfield 32, describe how they kept the saw on line through a hull that curves two ways. Their account, in their own words, is on the sailboat retrofit page. The DIY page says which parts of the job an owner can take. For a yard, this step is a few hours.
Step four: bond the tube in
First the The smooth, colored outer skin of a fiberglass hull: a layer of resin over the glass. It keeps water out of the laminate under it, which is why ground areas are coated again. comes off around the hole, inside and out, down to bare glass. Sleipner says 12 cm around the opening, and Max Power 10 to 15 cm. The tube is offered up, marked to the hull’s shape, cut to length and sanded.
Then it is glassed to the hull with at least 8 layers. Sleipner’s are 300 g cloth, alternating mat and roving (two kinds of glass cloth). Max Power’s are the same count in isophthalic resin. Every gap between tube and hull is filled. Where a hand cannot reach to lay cloth, a resin and glass mix goes in instead.
Two things the manuals say not to do.
The first is glassing over the part of the tube where the motor bracket will sit. Both Sleipner and Max Power say so, because the bracket needs a clean, parallel wall. Sleipner wants that wall 4 to 6 mm thick.
The second is leaving ground glass unsealed. Sleipner says every area that was ground or molded must be gelcoated again, because bare laminate lets water into the hull.
On a steel or aluminum boat the tube is welded in. Sleipner draws angled tunnel ends for a metal hull in place of the rounded ones in step five. The aluminum and steel page has the weld, the wall and the isolation.
The tube itself is the cheapest part on the bill. VETUS’s glass tubes list from $90 to $456. In this step, the hours are what you are paying for.
Step five: fair the ends
A tube that stops flush with the hull leaves a sharp edge, and a sharp edge costs thrust. The manuals ask for the mouth to be rounded into the hull, with a radius of at least one tenth of the tunnel diameter. That is Sleipner’s minimum and Lewmar’s figure.
Sleipner’s guide gives the reason. A rounded mouth lets the propeller draw water along the hull side as well as straight in, and that pulls the bow sideways on its own. The guide puts that extra at 30 to 40 percent of the total thrust in a good installation.
A sharp mouth also makes turbulence at the propeller. Sleipner says that turbulence is the cause of any Bubbles forming on a propeller blade when it is working too hard or running too near the surface. The propeller spins and the push disappears. noise in one of its tunnels.
The back face of the tube faces the water flow when the boat is moving. On a fast boat or a sailboat, that is drag. Quick and Sleipner give the same two fixes.
- A recess in the hull behind the tunnel.
- A deflector ahead of and beneath the tunnel that pushes the flow out around the mouth.
Sleipner’s test is simple. Standing ahead of the boat looking aft, you should not be able to see the back face of the tunnel. On boats that run over 25 knots it asks for an extended deflector, because there the bow pounds and drives water into the tube.
Max Power leaves 1 to 4 cm of tube sticking out past the hull and Smoothing a repair or a fitting into the shape of the hull with filler, so the water flows past it without a step. On a thruster it is the work that rounds the tunnel mouth into the hull. with filler.
This is the step we would ask about on a low quote. A tunnel with square ends works, just not as well. Sleipner’s guide is specific about what it gives up in thrust and gains in noise.
Step six: fit the gear leg, motor and propeller
The hardware goes in through a hole cut in the top of the tube, on the tube’s centerline and the boat’s. Every maker supplies a template or uses the gasket as one.
Every manual says the same thing about that template. The clearance between the propeller and the tube wall is small, so the holes must be exactly on the centerline.
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. goes through with its gasket and sealant. The bracket and motor bolt to it. The propeller is fitted and turned by hand, to check it clears the wall evenly all round.
Sleipner has the installer check the drive shaft height against a template before the motor goes on. Max Power sets the play between the two The joint between the motor’s shaft and the gearbox below it. On many thrusters it is the part built to give way if the propeller jams, so the gears survive., and has you turn the propeller again with the motor fitted.
The motor can lean. Quick and Lewmar both allow any angle within ninety degrees of vertical. Both say a motor more than 30 degrees off vertical needs a support built for it. Max Power recommends supporting the motor whatever its angle.
The compartment has to be dry and ventilated, because the motor makes heat. Sleipner’s sign-off asks whether it is isolated from bilge water.
One small part matters later. The A block of zinc or aluminum bolted to the thruster that corrodes on purpose, so the seawater eats it instead of the gearbox. It is replaced when it is about half gone, usually once a year. goes on with the propeller, and Lewmar, Quick and Sleipner all say the same thing about it: do not antifoul the anode. Max Power’s composite leg is the exception. It is sealed for life, with no anode at all.
Step seven: the cable, the fuse and the bank
This is the step that can be yours, and the one the DIY page works through. The manuals ask for four things.
How a tunnel job splits, for example the Lewmar 110TT Thruster Kit
DIY 2 of 5: Wiring prep only. The yard fits the unit and does the hull work; you do the wiring prep.
You: 4 to 16 h Yard: 32 to 70 h Electrician: 2 to 4 h Plus a haul-out.
The yard’s part is the hull job, steps one to six and eight. The brass part is step seven’s prep, which can be yours. A marine electrician makes the final connections and tests them. Quick’s manuals say only qualified personnel may install its units, so a Quick tunnel is a yard job from start to finish.
DIY levels
- Level 5: Owner install. No yard, no electrician.
- Level 4: You fit it, electrician connects. You mount and wire it; a marine electrician makes the final connections.
- Level 3: Split with the yard. The yard does the below-waterline part; you do the wiring.
- Level 2: Wiring prep only. The yard fits the unit and does the hull work; you do the wiring prep.
- Level 1: Yard job. A qualified professional does all of it.
You Yard Electrician. The bar is labor hours; a haul-out is a note.
How we rate DIYA battery bank close to the thruster. Max Power asks for it as close as possible, so the cable run is short.
Cable sized to that run. The wiring calculator works it out from the maker’s own draw.
A fuse sized to the cable. Max Power is explicit that this is not the same thing as the motor’s draw.
A The heavy switch, usually near the batteries, that disconnects the thruster from its battery entirely. It is the only switch that stops a motor that has stuck on.. It should be visible, marked and easy to reach.
Max Power’s warning is the one to remember. Voltage drop from thin cable or tired batteries wears out the A small electrical switch that a low current closes so a bigger current can flow. A thruster panel works the motor through one, and a relay that sticks closed keeps the motor running. Relay, solenoid and contactor are three names for this one part. and the Small carbon blocks inside a motor that press on the spinning part and carry the current into it. They wear down, they shed dust, and they are the usual reason an old thruster motor stops..
On the worked bill, the batteries, cable and charger come to $1,000 to $1,400. The control panel and its cable add $248 to $547, and the main fuse, its holder and the battery switch $88 to $268, at one dealer’s prices. The An electrical switch worked by a small current that closes a large one. In a thruster it connects the battery to the motor when you press the joystick, and it is a common part to fail. Solenoid, relay and contactor are three names for this one part. comes with the motor.
The battery calculator gives the bank the maker asks for, in cold cranking amps. VETUS adds one more job: check every connection again after 14 days, because temperature changes loosen nuts and bolts.
Step eight: antifoul, launch and check
The gear leg and propeller get 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.. The anode does not. The boat goes back in, and the installation is checked for leaks.
Sleipner’s guide ends with a checklist the installer signs. It is a good list to ask any yard to work through, whatever the brand:
- The bolts holding the gear leg and main bracket together are tightened correctly.
- The bolts holding the motor to its bracket are tightened correctly.
- All electrical connections are clean, dry and tight, with the correct cable, fuse and main switch size.
- An ohm meter shows no electrical connection between the motor body and either terminal.
- Antifouling has been applied to the gear leg and propeller, but not to the anodes, seals or propeller shaft.
- The propeller is fastened correctly to the shaft.
- The propeller turns freely in the tunnel.
- The anode is in position and its screw is tightened with thread glue.
- The boat has been checked for water leakage around the installation.
- The drive direction matches the control panel.
- The user manual has been handed to the owner.
Hold on to the user manual from that last point, somewhere you will find it again. It is where the run limits and the fault codes live, and the troubleshooting page is built from one.
Reading a quote against the steps
A fair quote for a tunnel job has three parts.
- The haul-out, the lay days and the launch, each on its own line.
- The parts (the unit, the tube, the panel, the batteries and cable), at prices you can check against the dealers.
- The labor, as hours at a rate.
The hours to expect for your boat’s length are the band in the first section above.
Hours well above that band on a boat your size mean one of two things. The first is a harder hull than usual, such as structure in the way at step two or a deep fairing job at step five. The second is padding. To tell which, ask which step the extra hours are in.
Hours well below the band are the other warning. In our reading, a job quoted in a day is leaving something out. Step five is the usual candidate.
The quote checker puts your total against the by-length band for your boat, line by line. The cost page has the worked bill this page refers to. The DIY page works out what step seven is worth if you do it yourself.
Sources: Sleipner’s SE20 to SE40 installation guide, Max Power’s CT100 and CT125 manual, Quick’s BTQ250 and BTQ300 manual, Lewmar’s Gen 2 manual and VETUS’s RIMDRIVE instructions. All were read 19 September 2026. The hours are the site’s band by boat length, between one itemized invoice and the job owners describe as typical. Every cost page prints the same figure.