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Bow thruster size calculator, for the bow, the stern or both
Tell it which end needs the help, what kind of boat she is and how long, and it gives you the thrust she needs to keep steering in a 15 knot breeze. You get one figure in pounds and kgf for every place a thruster can go, a bar showing the breeze each amount of push steers her in, and a drawing of where each one sits. The arithmetic for your own boat is laid out line by line, with the makers' published methods beside it.
This is a reference estimate from the numbers you entered, not a substitute for a marine electrician, a surveyor or an ABYC-compliant inspection of your boat. It is provided as guidance only, without warranty, and checking it is yours to do before you rely on it, especially for fuse and breaker sizing. Terms of use.
The short answer
- How much thrust. A 40 ft boat needs up to about 65 lb from a tunnel if she is a planing boat, and up to about 190 lb if she is a houseboat, to keep steering in a 15 knot breeze. Where yours lands depends on what kind of boat she is.
- Boat type counts more than length. At that one length, the top figure is about 2.9 times the bottom one.
- Where it sits. An external unit or a removable unit right at the bow makes the same turn with about 26% less thrust than a tunnel, which has to sit back from the bow.
- The breeze you size for. Every figure here is for 15 knots. Sizing for 30 knots multiplies it by about 4.1.
- What the number is for. It is the thrust that keeps the bow pointing where you want as you come in. At docking speed your 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 almost stopped working, and the thruster takes over that job.
Size yours in four steps
- Pick which end needs the help, your kind of boat and her length. The calculator at the top of the page works out how much wind she catches from the kind of boat. If yours catches more or less, change it right under the result.
- Read the figure for where the unit will sit. An external unit at the bow and a tunnel need different thrust for the same turn. How the calculator works explains why.
- Decide what breeze you need to dock in. Each extra knot costs more thrust than the one before. What the wind number costs you shows how fast it adds up.
- Take the figure to the finder. The thruster finder, our free tool that matches a thruster to your boat, takes these figures and prices each unit fitted.
Thrust by boat length, and why that chart can't be trusted on its own
This is the table most people come looking for. It shows what a boat of each length needs at 15 knots of breeze from 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. at the bow. The caption says how much 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. each column assumes.
Read across a row before you read down a column. Other charts only give you the column.
| Length | Planing | Semi-disp | Trawler | Sailboat | Pontoon | Houseboat |
|---|---|---|---|---|---|---|
| 25 ft | 25 | 33 | 43 | 40 | 68 | — |
| 30 ft | 38 | 48 | 60 | 60 | 98 | 105 |
| 35 ft | 50 | 68 | 83 | 80 | — | 145 |
| 40 ft | 65 | 85 | 105 | 105 | — | 190 |
| 45 ft | 83 | 110 | 135 | 133 | — | 240 |
| 50 ft | 103 | 135 | 168 | 165 | — | 295 |
| 55 ft | 123 | 163 | 203 | 198 | — | 360 |
| 60 ft | 148 | 195 | 240 | 235 | — | 428 |
Take the 40 ft row. Only the kind of boat changes along it. Even so, the figure runs from about 65 lb for a planing boat to about 190 lb for a houseboat. That is 2.9 times the thruster for one length.
A chart that asks only for length has to pick one of those numbers and hope it picked yours. Both Sleipner and VETUS say in their own documentation that sizing on length is not valid. That is why the calculator asks what kind of boat she is.
Every figure above is the tunnel case. A tunnel has to sit back from the bow, where the hull is deep enough to hold the tube. So it works on a shorter 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.. An external unit or a removable unit goes right at the bow. It makes the same turn with about 26% less thrust.
The ratio is the same at every length, because both positions are fractions of the boat. If you are pricing an external bow unit, take that much off the table's figure. A drawing of both positions on one boat shows where the difference comes from.
The pontoon and houseboat columns show wind load only. A pontoon has no hull to put a tube through. A houseboat that can take one pays for the welding to get it. The columns are there to show what the wind does to those boats, which is a good deal more than their length suggests. What to fit is in the pontoon and houseboat guides.
How the calculator works, so you can argue with it
The wind pushes on your The sides of the hull above the water. the way it pushes on anything in its path. The force is half the air density, times the wind speed squared, times a drag coefficient (a number for how well the shape catches the wind), times the area the wind sees.
The thruster does not have to beat that whole force. It works by leverage around the point your boat pivots on. The thruster sits near the bow and the pivot sits near the middle, so the thruster has a long 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. arm. The thrust it needs is a fraction of the total wind load.
thrust × long arm = wind load × short arm
- The wind's push acts near the middle, so it is working with a short lever and most of its force goes into pressing the boat sideways rather than turning it.
- The thruster sits in the bow, as far from the pivot as it can get. A long lever is why a hundred pounds of thrust can answer several hundred pounds of wind, and why sizing off length alone misses the point.
Where the wind pushes, against where the boat pivots, varies from boat to boat: from about a tenth to a fifth of her length. We take the middle of that for the figure, and the colored bar shows what more or less push gets you, in knots of breeze. Anyone quoting you one confident figure from your length alone, with no kind of boat, is guessing.
Steel, aluminum, fiberglass or wood makes no difference to the number. The wind pushes on the topsides and does not care what is under them. No maker's sizing method asks about it. Sleipner puts it plainly: “Thruster power is mainly determined by a boat’s design and not by its weight.” (their sizing page, loaded 12 September 2026).
Where a steel hull shows up is in weight. She is heavier for her length, and a heavier boat takes more to start and stop the swing. So give her a unit that steers her a little past 15 knots. That is why the calculator takes displacement as an optional entry. It adds a line to the arithmetic and does not move the figure.
This number is for steering. It is the thrust that keeps you in control of where the bow points in that much breeze. Pinning the boat against a dock in the same breeze is a different job. It is not the one a thruster does when you dock.
Watch what the last few knots cost you. Wind load rises with the square of speed. Taking this figure from 15 knots to 30 multiplies it by about 4.1. Under-buy and you get a thruster that cannot move the bow in a real breeze.
The usual fix is a bigger single unit, and it is the expensive one. A unit at the other end of the boat buys more turning per dollar than more power at the same end, because it works on a longer lever.
What the wind number costs you
The calculator sizes every boat for 15 knots. That is the most expensive assumption on the page, so the table below shows what a different breeze would cost. It runs a 40 ft trawler through five breezes. She is a semi-displacement boat with a lot of 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..
There are two columns because where the unit sits changes the thrust the same turn takes. 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 go back from the bow to where the hull is deep enough to hold a tube. So it works on a shorter 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. and needs more thrust. An external unit at the bow does not.
| Sized for | External, at the bow | In a tunnel |
|---|---|---|
| 10 knots | 35 lb | 48 lb |
| 15 knots | 78 lb | 105 lb |
| 20 knots | 140 lb | 190 lb |
| 25 knots | 218 lb | 298 lb |
| 30 knots | 315 lb | 428 lb |
Going from 15 knots to 30, the tunnel figure climbs from about 105 lb to about 428 lb. That is roughly 4.1 times the thruster, for a breeze you may never choose to dock in.
It is often the difference between a boat most retrofit products can serve and a boat that needs the biggest tunnel in the catalog. That means 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. and a five-figure quote. The cost page has those numbers.
Why your rudder is not doing this job already. It was, until you slowed down. 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. force falls with the square of boat speed. Wind force does not change at all, which is why a bow blows off in the lane between the docks. Why your boat won't steer at low speed explains that, and it is worth reading before you buy to a number.