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Boat guide · Sailboat
Sailboat thrusters: bow, stern or both, and what fits your hull
Under power at a walking pace, the bow goes with the wind, and the rig makes sure of it. This guide checks whether your hull takes a tunnel, sizes a thruster for your boat and rig, and shows the one cost figure a sailboat owner has published.
- Usually one engine
- The rig catches the wind
- External units go on afloat
- How she is driven Single screw Almost no steering at docking speed, and prop walk one way only astern.
- Wind on her Medium The wind has a fair amount to push on.
- Thrust to steer, 48 ft 150 lb From a tunnel, in a 15 knot wind. An external unit at the very bow needs 113 lb.
- Lowest installed $4,540 › DockStar Smart Thruster, bow, put on by a yard.
- Haul-out $960 to $2,320 You pay it only for a unit below the waterline. Worked from yards' own rates at 48 ft, with her out five to seven days.
- Models that fit 43 › At 48 ft, in the thruster finder.
The short answer
Your 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. decides that, well before your length does. A fine entry and a deep forefoot often leave nowhere to put the tube.
A sailboat of 36 ft needs about 85 lb from a tunnel to steer in a 15 knot breeze. From an external unit right at the bow, the same boat needs about 63 lb.
An external unit at the bow needs about 26% less thrust than a tunnel for the same turn, because it works on a longer lever.
By our arithmetic from their published thrust, one unit covers a sailboat to about 44 ft. Two, one at each end, cover her to about 50 ft.
The only sailboat figure an owner has published is $2,367.90 for a job he installed himself. Sideshift’s fixed external kits start around $4,495 before installation, and DockStar’s bow kit is about $4,380. A fixed external unit also needs batteries, cable and more hours to fit than a self-powered unit, so compare the installed estimates.
What fits a sailboat
For a sailboat of 48 ft, in a 15 knot wind. This is what the thruster finder lists when you answer only the boat and the length. Your own answers can change it.
External
Mounts on the outside of the hull, at the very bow or the stern, so you cut no tunnel through the boat.
4 models fit
Tunnel
A tube through the hull below the waterline, with the motor inside. A yard usually does the job.
39 models fit
Bow, stern, or both on a sailboat
The bow is usually the end that needs help. It is the end the wind takes as you slow down. You do not need a tunnel to fix it. An external unit bolts on above the waterline with no haul-out. One kind lifts clear of the water on its rail, so nothing is in the water while you sail.
At the bow, you have three kinds of thruster to choose from.
- A fixed external unit bolts on above the waterline. It needs no haul-out, no glasswork and no hole below the waterline, and it sits right at the bow on the longest lever. Sideshift makes the fixed kind.
- A removable external unit stays on its rail and lifts 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. DockStar makes the one on the market, at about $4,380 for the unit and its bow track. It carries its own battery, so there is no boat wiring.
- A tunnel needs a spot the forefoot may not give you. The section will a tunnel fit forward?, below, works that out. A retractable drops a leg through the hull and lifts it clear under sail, and it needs a hole cut too.
Our page on external mount thrusters (bracket, rail-mounted and pod) compares the bolt-on types side by side.
At the stern, the transom will take an external unit. It moves the back of the boat when you are backing into a slip, which on a sailboat is the move with the biggest audience.
At both ends, an external unit at each end turns the boat harder than one tunnel and needs no hole below the waterline. The section a unit at each end, below, has the numbers for a sailboat.
Buying only one? Our page bow or stern first? helps you choose, and our page on what a stern thruster costs has the dated prices for the back of the boat.
Will a tunnel fit forward?
A A tube bored straight through the hull below the waterline, with a propeller in the middle of it. It is the conventional thruster. Installing one is mostly a labor job rather than a hardware one. needs somewhere to go. Next time she is on the hard, stand under the bow and look for it: a flat, deep patch of hull with room either side. That spot has to meet three conditions at once:
- Deep enough. It sits far enough below the waterline to stay fed with water.
- Wide enough. There is enough hull either side of it to bond into.
- Far enough forward. It is placed where it is worth having.
A sailboat is the hull most likely to fail all three. The bow is narrow (a fine entry, in a yard's words), so there is little width forward. 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 deep and often cut away, so the depth is there, only not where you want the tube.
A fin keel and a long keel also give different answers on the same length of boat. That is why no chart organized by length can give you an easy answer.
Find out whether your hull has a tunnel location at all before you price anything. If a yard says it does, ask where and how deep. One powerboat owner, with a 41 ft Formula 41PC, was quoted tunnels for both ends, then found they would not fit the hull and canceled. A sailboat’s fine bow and deep forefoot raise the same question.
Asking first is the cheaper way to learn the answer. What the yard is looking for, in the makers’ own figures, is on the tunnel placement page. The sailboat retrofit page has the job itself, route by route, with one owner’s account.
If the answer is no, or the quote is too high, you still have options. An external unit bolts on above the waterline and needs no haul-out. One kind lifts clear of the water on its rail, so nothing is in the water under sail. The next section, when a tunnel will not go, shows why it can steer as well as a tunnel.
A thruster at each end
On a sailboat the case is unusually strong. The forefoot that refuses a tunnel forward will usually take an external unit right at the bow, and the transom will take one at the other end. A 45 footer that was told “no tunnel location” still has this option, and it out-turns a tunnel.
A tunnel goes where the hull is deep enough to take a tube, around 6 feet back from the bow on a 40 footer. An A thruster that bolts to the outside of the hull, up near the bow or on the back of the boat. Its propeller hangs in the water below it. Nothing is cut through the hull, so there is no tunnel. bolts to the outside of the hull instead, with its propeller hanging in the water, so it can go right at the very end.
Put a 90 lb DockStar at each end, 39 feet apart. That pair turns the boat about 67% harder than one 150 lb tunnel, in both directions: 3,510 lb·ft against 2,100. It needs no hole below the waterline and no haul-out. You can fit one end now and the other later.
Two units buy you more turning. They do not buy more push, and a pair will not pin her against a dock in a blow.
DockStar is the figure used here because it is the one external maker that publishes a thrust rating. Sideshift, the other, publishes none for any model in its range, so there is no figure to do the arithmetic on.
The drawing of all three puts one tunnel, one external unit and one at each end on the same boat, with every distance marked. The page it sits on has the whole argument and every route to a pair priced side by side.
A thruster at each end steers both ends of her, and can hold her against the dock. Size the pair for steering, our default, or for holding.
Every knot figure here is wind speed, not boat speed or current.
Why we size on steering, not holding
To hold her against a 15 knot beam wind, the bow alone has to push about 234 lb from the very bow, or 268 lb from a tunnel set further back. To steer her in the same breeze it needs 113 lb. That means a bigger thruster, battery and cable, and usually a tunnel: on this boat the cheapest pair from one maker goes up from $8,970 sized for steering to about $15,600 sized for holding.
The engines and a dock line do much of the holding, so most owners never need it. It is a good extra. To see the pairs sized for it, pick Holding at the dock on the switch above.
Steers both ends while the engines move her
Each thruster puts its own end where you point it, on the way in and on the way out. A pair is only as good as its weaker end, so each pair's figure is the strongest wind both ends still steer her in.
Beam wind, with way on at docking speed.
The pairs that steer her, and the wind they steer her in
Solid mark: one maker at both endsOutlined mark: two makers
Matching pairs: one maker at both ends
One remote, simpler wiring and controls, and one maker to call. We list these first, even where a two-maker pair costs less. DockStar is listed only as its own pair: you lower each unit into the water to use it, so both ends work the same way.
- 1 BowDockStar Smart Thruster 90 lb bolt-on removableSternDockStar Smart Thruster 90 lb bolt-on removable from $8,970 Steers her at 13 kt
- 2 BowMax Power CT80 (12 V) 176 lb tunnelSternMax Power CT80 stern pod (24 V) 165 lb pod on transom from $15,304 Steers her at 16 kt
- 3 BowSleipner SE80/185T 212 lb tunnelSternSleipner SX80 212 lb pod on transom from $21,649 Steers her at 18 kt
Two-maker pairs
Two remotes or panels and two sets of controls, so docking means a hand on each. Worth a look where a matching pair is missing, or costs much more. The five cheapest, one per maker pairing.
- 4 BowLewmar 185TT GEN 2 Package 128 lb tunnelSternMax Power CT80 stern pod (24 V) 165 lb pod on transom from $15,386 Steers her at 14 kt
- 5 BowSleipner SE80/185T 212 lb tunnelSternMax Power CT80 stern pod (24 V) 165 lb pod on transom from $16,826 Steers her at 18 kt
- 6 BowVETUS Bow Pro Boosted 57 126 lb tunnelSternMax Power CT80 stern pod (24 V) 165 lb pod on transom from $17,675 Steers her at 14 kt
- 7 BowMax Power CT80 (12 V) 176 lb tunnelSternSleipner SX80 212 lb pod on transom from $20,127 Steers her at 16 kt
- 8 BowLewmar 185TT GEN 2 Package 128 lb tunnelSternSleipner SX80 212 lb pod on transom from $20,209 Steers her at 14 kt
Every pair here steers her at 13 kt or more at both ends, priced as one job. Sideshift's pair (from $15,526) publishes no thrust, so it gets no mark.
What the pair gives her that one thruster cannot
Turns her in her own length
Bow one way, stern the other, and she pivots about her middle. Every pair above turns her in 19 kt or more.
Backs her where you point her
Astern, a single screw walks the stern one way only. A stern thruster swings that end where you need it, and the bow follows under your control.
Slides the stern over when docking
At slow speed you point the bow where you want her to go. With a pair, you slide the stern in or out as needed.
Moves her sideways: pin her to the dock, or walk her on and off
Both thrusters push the same way. Against the dock, that holds her while you tie up. Off it, she walks sideways with no way on. The wind's whole push on her side is 351 lb at 15 kt, and the bow carries about two thirds of it, because the wind pushes hardest forward of her middle.
Beam wind, engines in neutral: the hard case.
The cheapest pairs that hold her at 13 kt or more, and the wind they hold her in
- 1 BowMax Power CT80 (24 V) 196 lb tunnelSternMax Power CT80 stern pod (24 V) 165 lb pod on transom from $15,564 Holds her at 13 kt
- 2 BowMax Power CT100 231 lb tunnelSternMax Power CT80 stern pod (24 V) 165 lb pod on transom from $16,875 Holds her at 14 kt
- 3 BowMax Power CT125 (24 V) 254 lb tunnelSternMax Power CT80 stern pod (24 V) 165 lb pod on transom from $17,798 Holds her at 15 kt
- 4 BowMax Power CT165 408 lb tunnelSternMax Power CT80 stern pod (24 V) 165 lb pod on transom from $18,306 Holds her at 18 kt
Each pair is from one maker: one remote, and simpler wiring and controls. Where both need a haul-out, one covers both.
Prices are the low end of the installed total, one row per model. A pair is offered only on the boat lengths its maker names.
What that means for what you buy
Makers publish length ranges and thrust ratings. They mostly leave the hull question to you, and on a sailboat the hull question is the one that matters. The useful comparison is position against position.
On the external side, the figure that matters is what a unit at the bow produces. DockStar publishes 90 lb, captured 23 September 2026. It is a self-contained unit that fixes to a track and carries its own battery. So there is no boat wiring and nothing below the waterline.
Set that against the right-hand column above. One unit steers her in a 15 knot breeze at every length in the table to 40 feet, and it runs out at about 44 feet. Past that, one unit still steers her in a lighter breeze, and the finder shows how much. All of that is our arithmetic, not DockStar's.
The bow setup, the unit plus its standard bow track, comes to about $4,380 at the prices on DockStar's site.
DockStar's manual gives 20 to 50 feet as the intended use. Their installation page shows a bow unit on a sailboat.
We sized it on the turning effort a sailboat needs to steer in a 15 knot breeze. One unit covers a sailboat to about 44 ft. Two, one at each end, cover her to about 50 ft. Those lengths are this site's arithmetic from their published thrust and stated distances.
DockStar's selection guide labels both bow tracks "Recommended Boat Type: Sailboat". Ask them about your boat before you order.
A published thrust figure is the maker's own number. The DockStar review sets out what the manual says, what owners report, and what nobody has published. The Sideshift review does the same for the other external maker. Its comparison table lists sailboats, and its owner reports include none.
Maybe your forefoot will take a tunnel, and you want a published thrust rating from a long-established range. That route is open, and the numbers are easy to compare. Our page on what each product drills goes through both routes from the installation manuals.
For a sailboat whose forefoot refuses a tunnel, the choice is between those two external kits. They do not buy the same job.
Compare the whole job. A Sideshift unit runs off the boat’s cranking batteries. Budget $250 to $1,000 for batteries and heavy cable, Sideshift’s own published figure, and about 8 to 20 hours to install it by our estimate. A DockStar needs none of that wiring, because its battery and charger are inside, and DockStar puts installation at 1 to 3 hours.
Our installed estimate counts each Sideshift kit’s own batteries, heavy cable and a charger, at $601 to $2,136. The installed estimate adds both to every kit, so compare those figures.
One cost figure exists
Sailboat owners have published almost nothing about what they paid. The whole record this site could find is one job:
The owner of a 1985 Bayfield 32C installed a tunnel himself for $2,367.90. That is the whole project, the unit included. He wrote it up in Good Old Boat, Mar 2023.
One owner-installed job shows what a determined owner with the right skills can do for the price of the parts. It tells you nothing about what a yard would charge, and nothing about the hours he put in.
He published no professional comparison. Any figure you see set against his total, on this site included, is somebody's estimate.
This site's estimate is below. It prices a boatyard job on a 48 ft sailboat from published prices. The tunnel column assumes your bow has room for one, which is the forefoot question above.
If it does not, read the external column: the external kits named above, with the installation added.
| External | Tunnel | Retractable |
|---|---|---|
| Bow and stern pair: $9,000–$10,200 two DockStar units External thrusters for a 48 ft sailboat Installed by a boatyard, least expensive first:
And one more model, up to $25,200 for the Sleipner EX180D. The thruster itself, before installation: $4,400–$17,800. Two of these can be installed with the boat in the water. One at each end: most jobs cost $18,200–$30,500. Two DockStar units, one at the bow and one at the stern, come to $9,000–$10,200. Find the right one for your boat | Tunnel thrusters for a 48 ft sailboat Most jobs cost $16,000–$19,500. The least expensive, installed by a boatyard:
And 32 more models, up to $29,900 for the Quick BTQ 250. The thruster itself, before installation: $1,800–$10,600. The boat has to come out of the water for these. One at each end, a tunnel at the bow and an external unit at the stern: most jobs cost $24,600–$32,800. Find the right one for your boat | Retractable thrusters for a 48 ft sailboat Most jobs cost $24,700–$29,100. The least expensive, installed by a boatyard:
And 10 more models, up to $48,200 for the Quick BTR 250. The thruster itself, before installation: $7,400–$26,500. The boat has to come out of the water for these. Find the right one for your boat |
A different length, or a different boat? The thruster finder prices your own, and the comparison by boat type puts this row next to every other kind of boat.
What this means when you dock
If the bow blows off as you slow down, that is the end the wind takes. An external unit at the bow fixes it with no haul-out, and a removable one lifts clear of the water on its rail before you sail.
If a yard tells you there is no tunnel location, you still have the external route. At the bow it needs about 26% less thrust than a tunnel for the same turn. The leverage table shows why.
If backing into a slip is the trouble, the transom will take an external unit. A unit at each end turns the boat harder than one tunnel, and the pair section has the numbers.
If you are sizing from your length alone, you are leaving out the rig. Read the external column of the sizing table, or run the thruster finder, the tool here that matches a unit to your boat.
The detail
Why a longer lever does the same turn, the rig as lateral area, and what we could not find.
When a tunnel will not go, a longer lever does the same turn
Almost every article on this subject treats a tunnel as the real answer. An external unit gets treated as the compromise you accept when your boat will not take one. The arithmetic runs the other way.
A thruster is a steering control. Turning is thrust multiplied by distance from the point the boat pivots about.
The hull decides where a tunnel goes. On a forty-footer that is about 6 feet back from the bow, where there is depth for the tube. An external unit hangs right at the bow, on a longer The distance from the point the boat turns about to the place the push is applied. Double the distance and the same push turns the boat twice as hard..
| Where it sits | Distance from the pivot | Thrust for the same turn |
|---|---|---|
| Tunnel, 6 ft back from the bow | 14 ft | 150 lb |
| External, right at the bow | 19 ft | 111 lb |
| External, on the transom | 20 ft | 105 lb |
About 26% less thrust for the same turn, because it sits further from the pivot. A A thruster that bolts to the outside of the hull, up near the bow or on the back of the boat. Its propeller hangs in the water below it. Nothing is cut through the hull, so there is no tunnel. on a sailboat is a different way of buying the same steering, and often a better one. It needs no 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., no glasswork and no hole below the waterline.
The two positions are drawn on one boat here, with the distances marked. The boat’s own rudder hangs at the very end of her for the same reason. Our page on why your steering leaves you at docking speed has the mechanism.
Be clear about what you are buying it for. You are buying the ability to put the bow where you want it as you come alongside. That happens at approach speed, when the The blade behind the propeller that steers the boat, and it only works when water is flowing past it. At docking speed it does almost nothing, and in reverse it does nothing at all if it sits behind the propeller. has all but stopped working.
Pinning the boat against a dock in a gale is a different job. It is rare, and it is the most expensive case to design for. Leaning on the thruster for it when the crew cannot manage without is a bad plan too.
Your rig is lateral area
One sizing point is specific to this hull, and every chart misses it. On a powerboat the wind pushes on the The sides of the hull above the water. and the house.
On a sailboat it also pushes on the mast, the boom, the furled headsail and every wire holding them up. That is a large area a long way above the waterline, and the wind does not care that the sails are down. A How far the deck sits above the water. More freeboard means more of the boat is exposed to the wind. figure misses it entirely.
| Boat | From a tunnel's position | From an external unit at the bow |
|---|---|---|
| 28 ft | 53 lb | 38 lb |
| 32 ft | 68 lb | 50 lb |
| 36 ft | 85 lb | 63 lb |
| 40 ft | 105 lb | 78 lb |
| 45 ft | 133 lb | 98 lb |
Read the right-hand column, because that is the one an external unit has to satisfy. Run it for your own boat.
The calculator assumes a sailboat's rig catches a fair amount of wind. You can change that to match your own rig instead of taking a number worked from your length alone.
What we could not find
Any published sailboat installed price, from a yard or an owner. We found one DIY total, from 2023.
Any published guidance on minimum forefoot depth for a tunnel, which is the question that decides it. Yards answer it boat by boat, and nobody publishes the rule.
Sizing figures are this site's own estimates, checked against the sizing the makers publish. Moment arithmetic is ours, from published thrust figures and stated distances. DockStar figures are their own published numbers, captured 23 September 2026. Where one unit stops and two start is this site's own sizing arithmetic at 15 knots. Cost figure from Good Old Boat, March 2023.