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Install
Sailboat bow thruster installation: the right route in
A sailboat’s bow is narrow, and the keel often starts right where a tunnel would find its depth. There are four ways in, and the easiest bolts on above the waterline.
- External unit: no hole below the water
- Tunnel: the forefoot decides
- Retractable: the biggest hole
- Ways in 4 › External unit, tunnel, retractable or half-tunnel.
- Tunnel, 32 ft sailboat $9,700 to $11,700 › Installed by a yard. The cost by boat type row.
- One owner’s DIY bill $2,367.90 › A tunnel in a 1985 Bayfield 32C, unit included, Mar 2023.
- Lever at the very bow 36 percent longer › An external unit turns her with less thrust than a tunnel.
- Retractable opening 3.8 times › Lewmar’s smallest cut, against one tunnel mouth.
- Yard labor $160 to $180 per hour The rate our estimates bill.
The short answer
There are four ways a thruster goes into a sailboat’s bow. Start with the easiest one your hull will take.
- An external unit at the bow. It bolts on above the waterline, with no haul-out and no hole below the waterline. It works on a lever about 36 percent longer than a tunnel’s, so it does the same turn with less thrust.
- A tunnel, where the forefoot allows it. The tube needs depth below the waterline and hull on both sides of it. On a narrow bow those rules push it back until the keel stops it. Cutting the hole is the hard part.
- A retractable, where a tube will not go. It leaves the hull clean under sail. It also needs the biggest hole: Lewmar’s smallest is about 3.8 times one tunnel mouth.
- Sleipner’s half-tunnel. A tunnel with a deflector for a bow too shallow for a full one. It is a builder’s method more than a retrofit.
Power is the other job. The motor is at the bow and the batteries usually are not.
The four routes
| Route | What it asks of the hull | What it gives | Where the figures are |
|---|---|---|---|
| A tunnel where the forefoot allows it | Depth for the tube by the maker’s rule, hull either side of it, and enough beam that far forward | The cheapest hardware, and the permanent answer with no moving parts. It leaves a mouth in the flow both ways. | Here |
| Sleipner’s half-tunnel with a deflector | A hull too flat or shallow forward for a full tunnel, and a builder or yard that has done it | The tunnel in the right place on a hull that would not otherwise take one, at the cost of a fairing job | Here |
| A retractable | A watertight compartment on the centerline with room for the housing, a rectangular hatch cut in the bottom, a yard and often a naval architect | A clean hull under sail, and the propeller deep when deployed. It is the most expensive hardware and the biggest hole. | Here |
| An external unit at the bow | A place at the very bow to bolt a track or a mount, and the willingness to carry a unit outside the hull | The longest lever on the boat, no tunnel, and on one product no wiring. | Here |
Sleipner’s guide says a tunnel goes in most sailboats: “However, it is possible to install a tunnel thruster in most sail boats, even when the hull does not directly support the installation of a tunnel.”
It also says whose job the half-tunnel is: “This installation is being used by some of the world’s largest sail boat builders and has proven to give little to no speed loss during normal cruising.” The placement page has the method in full.
The thruster finder, the tool here that matches a unit to your boat, puts the retractable higher for a sailboat. A tunnel mouth is a hole in the flow under sail.
The external route
A unit at the very bow works on a longer lever than a tunnel can. The lever is about 36 percent longer, so the unit does the same turn with less thrust. The sailboat guide has the arithmetic and the table by length.
It asks two things of you. The hull needs a place at the bow to bolt a track or a mount. And you need to be happy with a unit that lives outside the hull.
A removable unit stays on its rail and is lifted clear of the water by hand, so nothing is in the water while you sail. DockStar’s carries its own battery, so no cable runs forward.
Sleipner sells its external range for “a practical thruster solution for displacement and semi-planing boats up to 60 feet, independently of hull form, hull-material, propulsion and depth”, and a sailboat is a A hull that pushes through the water rather than climbing on top of it. Most sailboats and the heaviest trawlers have one, and they carry a lot of weight for their length.. The Sleipner external units we list are stern units, though. Its EX series is not on our list, so the finder offers no Sleipner unit for this route.
Finding the spot for a tunnel
On a sailboat the makers’ rules pull against each other.
Forward, for the lever. The tube wants to be as far forward as possible.
Deep, so it does not suck air down from the surface.
Hull on either side, for the tube to bond into.
Here is the depth each maker asks for.
Lewmar: one tunnel diameter below the waterline to keep air out of the tunnel, with 0.75 of a diameter the minimum.
Quick: in the bow and as low as possible, at least 0.75 times the tunnel diameter below the waterline, to avoid cavitation at the propeller.
Sleipner: as deep as possible under the waterline, and as far forward as possible. The depth for a given model is in that model’s own measurements. Deeper prevents air being sucked in from the surface and raises the water pressure at the propeller.
On the keel side, keep at least a quarter of the tunnel diameter clear of the keel. The length has a rule of its own, and it is Sleipner’s. Avoid a tunnel longer than four times its diameter. Friction inside a long tube slows the water and cuts thrust. Too short, and the flow reaches the propeller before it has straightened, which the manual says causes cavitation and noise. The propeller must sit entirely inside the tube.
That rule stops the tube going through the widest part of the boat. A narrow bow is often deep enough only where it has already grown wide, and a tube there is too long.
Angled motors need support. Quick and Lewmar both say a motor more than 30 degrees off vertical needs a support built for it.
Two owners marked their spot with a template, a model of the bow and magnets. The detail at the foot of this page shows how they did it.
The retractable route
Where the forefoot will not take a tube, a retractable is the other way in. It puts the propeller below the hull only when it is working.
The price of that is the opening. Lewmar’s smallest, the RT140, needs a cut of 220 by 265 mm through the bottom, at 45 degrees on all four sides.
Quick’s BTR wants its support welded or laminated in, and a hull no thicker than 35 mm at the 185 mm size. Open and close it under 4 knots.
Lewmar recommends a naval architect for the compartment. The housing takes the unit’s weight, its thrust and the water under way.
On a sailboat that compartment is the The space right in the bow, inside the hull, usually a locker for sails, fenders or the anchor chain. or the space under the V-berth. Sleipner’s retract manual puts its housing in one sentence: “The thruster must be installed in the center line of the keel and as far forward as possible while following the minimum tunnel depth requirement.”
The retractables page has every maker’s hatch, speed and price.
Power forward
The motor is in the bow and the batteries usually are not. VETUS’s manuals put it plainly: “We also recommend that each bow thruster is powered by its own separate battery or batteries. This allows the battery bank to be placed as close as possible to the bow thruster; the main power cables can then be short thus preventing voltage losses caused by long cables.”
A bank in the forepeak, and you carry the weight in the bow.
A long cable run, the length of the boat.
The wiring calculator sizes the cable and the battery calculator sizes the bank. The battery page has the makers’ positions on where the bank goes.
The bill, and the checks after launch
The two owners did the whole tunnel job themselves: “The entire project — bow thruster unit, resin, hatch, remote, parts, and pieces — cost us $2,367.90.”
A yard on the same 32 ft boat runs $9,700 to $11,700, on up to 32 billed hours. Their figure is about 24 percent of the bottom of that band. The difference is the labor they did not buy. The DIY page says which parts of the job you can take on.
After launch the checks are the makers’.
VETUS asks for every electrical connection to be looked at again after 14 days.
Sleipner’s sign-off list has one check about water: “The boat has been checked for water leakage around the installation.”
It has another about isolation: “An ohm meter shows no electrical connection between the motor body and either terminal.”
The detail
One boat’s tunnel retrofit, in its owners’ words, and where this page comes from.
One boat’s retrofit, in its owners’ words
William Koehne and David Jolly put a 6 in tube through their A keel that runs most of the length of the hull, with the rudder hung off its back edge. The boat holds a straight course well and turns slowly. 1985 Bayfield 32C. They told the story in Good Old Boat, Issue 149, March/April 2023. It is one boat’s job, printed as that.
They chose a tunnel over a unit outside the hull for one reason: “We wanted a through-hull, not an add-on under the hull, to reduce the chances of the thruster bumping something.”
Finding the spot
They used a template off the waterline, a 3D model of the bow section inside the hull, and prototypes of the tube and the unit. First they made room: “In order to see the amount of space we had to work with, we removed the blackwater tank”.
The model showed how little room there was: “We found out that we had only about an inch tolerance up, down, forward, and aft.”
It also set the motor’s angle: “We were also going to have to angle the thruster motor precisely so we could have access to service the motor while not having it protrude into the top of the front V-berth deck.”
Then came the mark on the hull: “we confirmed the position of the bow thruster using rare earth magnets”. One owner worked outside and one inside, dragging the magnets together to the projected center.
Cutting the hole
They say why the hole is the hard part: “The hull angles wider toward the top and narrower at the bottom, creating a compound angle that forms a goose-egg-shaped hole.”
So they made the saw follow a line through the hull. Two quarter-inch pilot holes went on the projected centers, with a board outside holding a third. Then the tool: “We bought a 6-inch carbide hole drill, removed the ¼-inch drill bit, and fastened a ¼-inch diameter, 36-inch-long rod instead.”
They cut halfway from each side. The hull was thicker than most: “We were amazed by how thick the hull was — about an inch.”
And the result: “It fit with less than ⅛-inch of tolerance around the tube.”
Bonding the tube
They used epoxy and cloth, not the polyester The layers of glass cloth and resin built up one over another to make or repair a fiberglass hull. the manuals draw. They changed the manual’s order on one point, and said so: “The manufacturer instructions suggest doing this after the tube is fully externally and internally bonded. With our very critical fit and limited working space, we needed the saddle in place.”
Then the layup: “We bonded the tube to the hull with West System 105 Epoxy resin and several layers of biaxial fiberglass cloth.”
The makers ask for at least eight layers on a fiberglass hull, and a rounded mouth. The install page has the layup step by step.
They 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 forward edge into a flare: “The eyebrow allows water to flow freely over the tube opening, reducing drag.”
Sleipner’s guide backs that up: “A possible problem in sail boats or fast powerboats is that a non-rounded surface can generate drag from the back face of the tunnel” The placement page has its deflector drawings.
At the bow
They added one thing the manuals do not: “We purchased a third-party wireless control to operate the bow thruster at the helm, plus a portable control that we attached to a lanyard that you wear.” Now whoever is at the bow with the lines can use the thruster.
Where this comes from
The makers’ installation manuals, as the install page and the retractables page read them, plus Sleipner’s SE20 to SE40 guide and SR retract manual. The owner account is the one published sailboat tunnel install we have found, read 20 September 2026.
Sources: William Koehne and David Jolly, “Going Sideways”, Good Old Boat, Issue 149, March/April 2023. Sleipner: SE20 to SE40 guide and SR retract manual. Lewmar: RT manual and Gen 2 manual. Quick: BTR manual and BTQ manual. VETUS: installation manuals, as the battery page cites them.