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Install
Sailboat bow thruster installation: the right route in
A sailboat is the hull most likely to have nowhere obvious to put a thruster. The bow itself is narrow. Worse, the The part of the hull just behind the bow, where the front of the boat curves down toward the keel. It is the shallowest part of the underwater hull, so a tunnel there has little water above it. is often cut away, and the The lowest part of the hull, running along the middle from bow to stern. starts about where the depth arrives.
This page walks you through the four ways in, starting with the easiest, and shows what each one asks of your hull. Then it follows one boat’s tunnel retrofit from the first template to the bill, in its owners’ words. The sailboat guide helps you decide whether to fit one at all.
Where this comes from. The makers’ installation manuals already on the install page and the retractables page, plus Sleipner’s SE20 to SE40 guide and SR retract manual. The owner account is the one published sailboat tunnel install this site has found: William Koehne and David Jolly on their 1985 Bayfield 32C, in Good Old Boat, Issue 149, March/April 2023, read 20 September 2026. Their account is one boat’s and is printed as that.
The short answer
An external unit 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. One kind stays on its rail and is lifted clear of the water when you sail.
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. The hole itself is the hard part.
It leaves the hull clean under sail. It also needs the biggest hole: Lewmar’s smallest is about 3.8 times one tunnel mouth.
Two owners fitted a tunnel to a 32 ft sailboat themselves for $2,367.90, unit included. Our yard band for the same boat starts at $9,800.
The motor is at the bow and the batteries usually are not. You either put a bank forward or run cable the length of the boat.
Plan the job in four steps
- Pick your route. Start with the easiest one your hull will take. The table in “The four routes” below sets out what each asks of the hull, and the external route is the one with no hole to cut.
- For a tunnel, find the spot. The makers’ depth, keel and length rules pull against each other on a sailboat. The section “Finding the spot for a tunnel” has each rule and how one pair of owners marked theirs.
- Cut the hole and bond the tube. On a sailboat the hull curves two ways at the bow, so the hole is not round. The section “The hole and the tube” shows how one boat did it.
- Bring power forward and check it after launch. Decide between a bank at the bow and a long cable run, then work through the makers’ checks. The sections “Power forward” and “The bill” cover both.
Not sure which unit fits your boat? The thruster finder, our free tool that matches a thruster to your boat, lists every unit that fits a sailboat your length. The size calculator gives the thrust and shows its working.
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 |
Before you plan a tunnel, look at the bow. An external unit bolts on above the waterline. It needs no haul-out, no glasswork and no hole below the waterline. A removable unit stays on its rail and is lifted clear of the water by hand on a rope and pulley, so nothing is in the water while you sail. You lower it again to dock. The external route has more.
Sleipner’s guide names the sailboat problem: “Some sail boats have a flat bottom and shallow draft in the bow section. This can make installing the thruster as far forward from the boats main pivot point difficult.”
Then it gives its own answer: “However, it is possible to install a tunnel thruster in most sail boats, even when the hull does not directly support the fitting of a tunnel.”
The method is the half-tunnel: “Instead fit the tunnel halfway into the underneath section of the existing hull. Strengthen it with a deflector/ spoiler directing the water flow around the tunnel.”
It also says whose job that 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.”
So the half-tunnel is a builder’s method more than a retrofit. The placement page has it in full. The other three routes are the ones a yard will quote on a boat that already exists.
The thruster finder moves the retractable up its order for a sailboat. The reason is that 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. On this site’s geometry 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.
Sleipner’s external range is sold as “a practical thruster solution for displacement and semi-planing boats up to 60 feet, independently of hull form, hull-material, propulsion and depth”. 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.. This site holds no Sleipner external unit for the bow, though. Its EX series is not in our product list, and the Sleipner external units we do list are stern thrusters. So the finder will not offer a Sleipner unit for this route.
DockStar’s unit carries its own battery, so no cable runs forward.
Whether one unit covers your sailboat is this site’s own arithmetic, worked on the sailboat guide. No maker claims it.
The Bayfield owners chose the tube over an external unit for one reason. They did not want anything under the hull to bump. The section “The hole and the tube” quotes them on it.
Finding the spot for a tunnel
The rules are the makers’, and on a sailboat they 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.
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 tube’s 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-bowed hull is often deep enough only where it has already grown wide, and a tube there is too long.
How one boat found its spot
The Bayfield owners 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 by taking out the holding tank. In their words: “In order to see the amount of space we had to work with, we removed the blackwater tank”.
The model told them how far back the unit could sit and still leave the tank its space. It also showed how little room there was: “We found out that we had only about an inch tolerance up, down, forward, and aft.”
And it 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.”
Angled motors need support. Quick and Lewmar both say a motor more than 30 degrees off vertical needs a support built for it.
Then came the check that turned the model into a 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.
The hole and the tube, on one boat
Why a tunnel at all, on a 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. cutter? In their words: “We wanted a through-hull, not an add-on under the hull, to reduce the chances of the thruster bumping something.”
The hole is the hard part on a sailboat’s bow. They say why: “The hull angles wider toward the top and narrower at the bottom, creating a compound angle that forms a goose-egg-shaped hole.”
Their answer was to make the saw follow a line through the hull. They drilled two quarter-inch pilot holes on the projected centers and set a board outside with a third. Then they describe 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, keeping the center reference. 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.”
They bonded the tube with epoxy and cloth rather than 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 also departed from the manual 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 itself: “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.
These owners 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. They explain it: “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”
Its deflector and recess drawings are on the placement page. The noise page has what the mouth does to cavitation and to the propeller’s spin under way.
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, and leaves the bottom clean under sail.
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. That is about 3.8 times one tunnel mouth of the same tube size.
Quick’s BTR wants its support welded or laminated in, and a hull no thicker than 35 mm at the 185 mm size. Its manual also says to open and close the unit 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. Lewmar’s manual puts the deployed hub 1.5 propeller diameters below the hull, deeper than any tunnel can go on the same bow. 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.”
On a sailboat that leaves two choices:
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 trade between them is yours. The battery page has the makers’ positions on where the bank goes.
The Bayfield owners 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, as well as whoever is at the helm.
The bill, and what came after
Here is how the owners sum it up: “The entire project — bow thruster unit, resin, hatch, remote, parts, and pieces — cost us $2,367.90.”
That is the whole project, unit included, with two owners doing the work themselves. Their account appeared in Mar 2023. They published no yard quote to set beside it.
This site’s tunnel band for a 32 ft sailboat fitted by a yard runs from $9,800 to $11,700, on up to 32 billed hours. The cost by boat type page shows where that comes from. 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 an owner can take and which a yard should.
Checks after launch
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 survey page turns the same list into what whoever buys the boat will look at later.
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.