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Motor yacht thrusters: bow, stern or both, sized for the house
You can turn her on the spot on the throttles. The one thing the engines cannot do is push the bow sideways when the crosswind arrives on the approach, so on a motor yacht the question is almost always the bow. The flybridge puts this hull near the top of every sizing table on this site. This guide sizes a thruster for your boat, shows where an external unit still fits, and lists what owners have paid.
The short answer
The bow. Twin screws steer the stern on the throttles, but they cannot push the bow sideways against a crosswind.
A 55 ft flybridge motor yacht needs about 203 lb from a tunnel to steer in a 15 knot breeze. From an external unit right at the bow, she needs about 150 lb.
That is about 54 percent more than the same hull with a low profile, and all of the difference is the house.
By our sizing, a single 90 lb external unit steers her in a 15 knot breeze to about 44 ft, and a unit at each end to about 50 ft. Past that, a tunnel is the usual answer on this hull.
The one itemized invoice this site holds is a 47 ft Bayliner 4788 in 2016, at $13,077.50 for a tunnel. Yard labor has roughly doubled since: a Florida yard's $70 to $90 an hour in January 2021 against $175 in the same state by 2025, in the rate table on our cost page.
Bow, stern, or both on a motor yacht
In one line: the bow. Twin screws already steer the stern at zero speed on the throttles. So a stern unit buys you less than it would on a single screw.
At the bow, one engine ahead and one astern will not push the bow sideways against a crosswind. The section twin screws turn her explains the gap.
On the smallest motor yachts a external unit is the easy fit: it bolts on above the waterline with no haul-out. By our sizing one unit covers to about 44 ft and a unit at each end to about 50 ft. Above that a tunnel is the usual answer.
At the stern, a unit still helps in a tight slip with no room to swing the stern.
At both ends, an external unit at each end is an option on a smaller motor yacht. The section where an external unit still fits works out the lengths, and the bow-and-stern page prices every route.
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.
A 55 ft motor yacht, stopped, turning about her middle. The arrows point the way each unit pushes to turn her: the bow one way, the stern the other.
Twin screws turn her
Put one engine ahead and the other astern, and a Two engines driving two propellers, set apart from each other. Running one forward and one in reverse turns the boat on the spot without any rudder at all. That is why twins need less help at the stern. boat spins about a point between her props. She does it without moving ahead or astern at all. A touch on one throttle walks the stern sideways.
The thruster finder, the tool here that matches a unit to your boat, pre-fills twin engines for this boat type and says what that means on the result. You already steer astern, so a stern unit buys you less than it would on a single screw. The bow is where the gap is.
The gap is real. Differential throttle is the name for what you are doing, with one engine ahead and the other astern. It has two limits, and you meet both of them on the same gusty afternoon:
- It will not push the bow sideways against a crosswind while you bring her alongside.
- It needs room to swing the stern, which a tight slip does not give you.
At approach speed 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. The wind has not eased at all. Our page on why the bow blows off when you slow down has the mechanism.
What the house adds
Every sizing chart asks for your length. On this hull that is the wrong place to start, because a flybridge motor yacht is mostly house.
A saloon, a The open upper helm on top of a motor yacht or trawler. It adds a lot of height for the wind to push on. with a hardtop, an enclosure and a tender on the boat deck give the wind a tall, flat area to push on. From the dock she is a two-story house with a bow on it. A length chart sees none of that The area of the boat the wind can push on: the hull sides, the cabin, the mast and rigging, or the canopy. A tall, boxy boat has a lot of it, and wind force does not care how slowly you are moving.. The first table sizes one boat and changes only how much wind she catches.
| How much the wind gets hold of you | In a tunnel | External, at the bow |
|---|---|---|
| Low: open boat, low profile | 130 lb | 98 lb |
| Medium: hardtop, no flybridge | 163 lb | 120 lb |
| High: flybridge and enclosure (assumed) | 203 lb | 150 lb |
The house alone adds about 54 percent more thruster to the same 55 feet. The next table runs the length.
| Boat | From a tunnel's position | From an external unit at the bow | From an external unit on the transom |
|---|---|---|---|
| 40 ft | 105 lb | 78 lb | 75 lb |
| 45 ft | 135 lb | 100 lb | 95 lb |
| 50 ft | 168 lb | 123 lb | 118 lb |
| 55 ft | 203 lb | 150 lb | 143 lb |
| 60 ft | 240 lb | 178 lb | 168 lb |
| 70 ft | 328 lb | 240 lb | 230 lb |
These figures are for the breeze you dock in. Wind load rises with the square of wind speed. So a unit big enough to hold you alongside in thirty knots needs four times the thrust of one sized for fifteen. On this hull that means a very large motor, sized for a day most owners stay in the slip.
Run it for your own boat. The calculator lets you change the wind exposure and prints every step.
Why the tunnel is the usual answer here
On most hulls 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. at the bow earns its place, because position makes up for power. Turning is thrust multiplied by the distance from the point the boat pivots about.
An external unit sits right at the bow, on the longest 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. the boat has. 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. has to sit 6 feet back on a forty-footer, so it works on a shorter one. Here are the two positions drawn on one boat, with the distances marked.
| Where it sits | Lever arm | 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 |
That 26 percent saving holds on a motor yacht too. What changes is the size of the number it is a saving on.
A 55 ft flybridge boat wants about 150 lb even from an external unit at the bow. The external units on the market are built for smaller boats than that. The tunnel makers build up the range instead.
At 55 ft, 34 tunnel units are strong enough to steer her bow at 15 knots. They publish 176 to 529 lb. Of the external units, only the Sideshift SS350 and the Sleipner Side-Power EX180D qualify at that length.
So on this hull the tunnel is the normal answer. It needs a 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., the glasswork, and a battery bank and cable run sized for the motor.
The wiring calculator sizes that cable from the maker's own current figure. On a big unit fed from a bank aft, the cable is the part of the job owners most often get wrong. The cost page takes the tunnel job line by line.
Where an external unit still fits
Below about 50 feet the arithmetic changes.
On the external side, DockStar publishes 90 lb, captured 23 September 2026. It is a self-contained unit that fixes to a track and carries its own battery.
We size it the way this site sizes every thruster. On a flybridge motor yacht, one unit steers her in a 15 knot breeze to about 44 ft. A unit at each end does it to about 50 ft. Those lengths are our arithmetic from their published thrust.
The pair is two separate purchases at about $8,650 with standard tracks. That total is our addition of their prices.
DockStar’s own manual gives 20 to 50 feet as the intended use, so this site offers the pair only up to 50 ft. The thruster finder shows the breeze each unit steers her in. The figures come from DockStar's site.
DockStar's manual gives 20 to 50 feet as the intended use, with no sizing within it, and has not named motor yachts. The lengths above are this site's arithmetic at 15 knots. No maker has recommended them. Ask DockStar about your boat before you order.
The DockStar review sets out what the manual says and what owners report, including one owner's account of it in wind.
Sideshift's larger external units are the other route. On a big boat its own advice is a pair. The owner of a 56 ft DeFever put it this way on Trawler Forum in September 2024: “Sideshift recommends dual thrusters for a boat of our size/weight.”
Sideshift publishes no thrust figure for any model (the Sideshift review has the detail). So a pair of theirs cannot be set against the table above.
An owner of a 40 ft Sea Ray Sedan Bridge priced an external unit against tunnel quotes in 2015. He wrote on Club Sea Ray that “the external YachtThruster is half the price of a comparable size tunnel system.” Nobody in that thread had used one. Read it as a quote comparison rather than a report.
Below the length where an external unit still fits, the two makers’ kit prices need the same correction before you compare them.
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 fit it by our estimate. A DockStar needs none of that wiring, because its battery and charger are inside, and DockStar puts fitting at 1 to 3 hours.
Our fitted estimate counts each Sideshift kit’s own batteries, heavy cable and a charger, at $601 to $2,136. The fitted estimate adds both to every kit, so compare those figures.
What motor yacht owners paid
The best-documented job this site holds on any hull is on this one, and it is that rare forum post with an invoice in it. An owner of a 47 ft Bayliner 4788, a pilothouse motor yacht, posted his itemized tunnel invoice in May 2016 on the Bayliner Owners Club. Another owner of the same model posted the two quotes he had for a bow and a stern unit.
| Boat | What was priced | Figure | Basis |
|---|---|---|---|
| 47 ft Bayliner 4788, May 2016 | Bow tunnel, itemized invoice $8,577.50 labor and parts across 70 hours, plus $4,500 for the Side-Power SE120/215. The total is our addition of his two figures. | $13,077.50 | installed, paid, 2016 rates |
| 47 ft Bayliner 4788, May 2016 | Bow tunnel, quoted, a second Bayliner owner He was also quoted $10,163.60 for a stern unit. | $11,564.77 | quoted, 2016 rates |
Two things in that table are worth more than the totals:
- The invoice is itemized against the hours the yard billed on a 47-footer. Those hours are the labor figure every tunnel estimate here rests on.
- The two quotes to one owner put the stern unit about 12 percent below the bow unit on the same boat. That is why a stern thruster is not simply the same number again.
One more dated figure belongs here, because adding the thruster to a joystick is a motor yacht owner's question. The owner of a 42 ft Grand Banks fitted a Sleipner tunnel in 2024. He put the joystick at $4,800 of hardware and $5,000 to install.
That is about $9,800 on top of the thruster, posted on Trawler Forum. It is the one dated add-on figure this site holds.
This site's estimate is below. It prices a boatyard job on a 55 ft flybridge motor yacht from published prices and current yard rates. Only units strong enough to steer her bow at 15 knots are counted. That is why the external column is thin.
| External | Tunnel | Retractable |
|---|---|---|
| External thrusters for a 55 ft motor yacht Fitted by a boatyard, least expensive first:
The thruster itself, before fitting: $5,500–$17,800. One of these can be fitted with the boat in the water. One at each end: most jobs cost $24,000–$36,600. Find the right one for your boat | Tunnel thrusters for a 55 ft motor yacht Most jobs cost $17,700–$20,300. The least expensive, fitted by a boatyard:
And 31 more models, up to $28,700 for the VETUS Bow Pro Boosted 180. The thruster itself, before fitting: $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 $28,700–$35,500. Find the right one for your boat | Retractable thrusters for a 55 ft motor yacht Most jobs cost $25,700–$41,300. The least expensive, fitted by a boatyard:
And 9 more models, up to $50,300 for the Sleipner Side-Power ERL170 eVision. The thruster itself, before fitting: $7,400–$29,500. 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 on the approach, that is the gap twin screws leave, and a bow thruster fixes it. With a flybridge and an enclosure, size from the high row of the first sizing table.
If your slip gives you no room to swing the stern, a stern unit still helps. Our page on what a stern thruster costs has the dated prices.
If your boat is under about 50 feet, an external unit at each end covers the 15 knot estimate by our sizing, with no haul-out. Read where an external unit still fits and ask the maker before you order.
If you are near 55 feet, plan for a tunnel. Size the cable with the wiring calculator, because that is the part owners most often get wrong.
What we could not find
An itemized invoice newer than 2016 on this hull. The 2024 Grand Banks job is a trawler's, and it is a ballpark.
Any external unit whose published thrust covers a 55 ft flybridge boat at 15 knots. The two external makers either publish 90 lb or no figure at all. The external unit the cost table counts at that length is sized on its motor, because its maker publishes no thrust figure.
Any owner report of an external pair on a motor yacht over fifty feet, beyond the DeFever owner's account of what Sideshift recommended.
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. Owner figures from the Bayliner Owners Club, May 2016, and Trawler Forum, 2024 and 2025.