Choosing
Bow or stern thruster first? Pick the end that fixes your docking
Most owners buy one thruster, and the brochure puts it at the bow. On a lot of single-screw boats the stern is the end that fixes the docking.
- The finder’s own rules
- Single screw, twins, outboard and sterndrive
- One end now, the other later
- Symptoms that point at one end 2 of 6 › The others: 2 for both ends, 2 for neither.
- Drives with the gap at the stern 1 of 4 › One engine, with a rudder. Every other drive already steers the stern.
- A stern unit needs 75 lb › on a 40 ft trawler in 15 knots.
- A bow tunnel needs 105 lb › That is 40% more than the stern unit for the same turn, because it sits back from the bow on a shorter lever.
- One 90 lb unit steers a trawler to 44 ft › in 15 knots. With one at each end, 50 ft.
- External units for the stern only 31 of 46 › About 67% of the external listings on this site.
The short answer
The thruster finder, the tool here that matches a thruster to your boat, reads your docking problem the same way. The pair page has the case for both ends.
- Fix the end that goes wrong If the bow won’t swing where you want it on the approach, start at the bow. If the stern won’t tuck in as you back into the slip, start at the stern. A bow thruster does nothing for that. Each symptom, and its end
- Your drive decides the stern On a single screw, the rudder sits in slack water astern and prop walk turns her one way only. Twins, an outboard or a An engine inside the boat driving a steerable leg out through the back of the boat. It steers like an outboard with the motor inboard. It is common on express cruisers, and it leaves no room for a stern tunnel. already steer the stern, so for them the gap is at the bow. Your drive
- The stern needs the least thrust On a 40 ft trawler in 15 knots, an external stern unit needs about 75 lb to steer her. A bow tunnel needs about 105 lb for the same turn, 40 percent more than the stern unit. The case for the stern
- One unit goes a long way On a trawler, one 90 lb external unit steers her in a 15 knot breeze to about 44 ft. With a second at the other end, it is about 50 ft. How far one unit goes
- Neither end holds her alongside A thruster steers the boat at docking speed. Holding her against the dock is a job for a spring line and the engine. What neither end fixes
Read the symptom
The finder asks “What goes wrong at the dock?” and reads your answer as an end. Here is every choice it offers, where each one points, and why.
| What goes wrong | Which end | Why |
|---|---|---|
| The bow won't swing where I want it | The bow | The bow has the windage, and the pivot moves forward as she slows. So the bow is the end that pays off first on the approach. |
| The stern won't tuck in | The stern | Astern, the rudder sits in slack water and what turns her is prop walk, one way. A bow thruster does nothing for it. |
| I get too close to the boat next to me | Both ends on a single screw, the bow otherwise | Sliding her sideways in a slot takes a push at both ends at once, and one unit cannot do that. On a single screw the finder reads this as both ends. On twins, an outboard or a sterndrive, which already move the stern, it reads it as the bow. |
| I can't line her up without a go-around | Both ends on a single screw, the bow otherwise | Sliding her sideways in a slot takes a push at both ends at once, and one unit cannot do that. On a single screw the finder reads this as both ends. On twins, an outboard or a sterndrive, which already move the stern, it reads it as the bow. |
| She won't stay put while I wait | Neither | A thruster is a steering control. Nothing on this site is sized to pin a boat alongside. |
| Nothing yet, buying ahead of it | Neither | With no symptom yet, the drive decides: the end the boat cannot steer is the end to buy first. |
The finder’s 6 docking symptoms and the end each one points at.
Two things follow from that table. First, a boat with one symptom usually has one end to fix. The finder prices that end alone.
Second, “she won’t stay put” is on the list so the finder can turn it away. A thruster is a steering control. It is judged on whether it turns the boat at the speeds where 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 quit. The steering page says what holds a boat instead.
What drives her decides the stern
The finder’s second question is “What drives her?”. Its help line says why it asks: your drive decides how much steering you already have at docking speed.
Going ahead, a rudder behind a propeller steers only while the engine is in gear and blowing water over it. Astern, the rudder sits in slack water. What turns the boat then is The sideways kick a propeller gives when you put the boat in reverse. A single propeller pushes the stern one way and only that way. That is why a single-screw boat backs predictably in one direction and badly in the other., and it pulls one way only.
That is the whole stern case on a One engine driving one propeller. It steers with a rudder, so it loses steering at low speed, and in reverse the propeller kicks the stern one way only.. It does not exist on a boat whose drive already points the thrust.
| Drive | What you already have | Where the gap is | Typically |
|---|---|---|---|
| One engine, with a rudder | Almost no steering at docking speed, and prop walk one way only astern. | The stern | houseboat, ski or wake boat, trawler, sailboat |
| Twin engines | You already steer at zero speed by running one engine ahead and the other astern. | The bow | motor yacht, catamaran |
| Outboard or outboards | The whole drive swivels, so you back down accurately already. | The bow | pontoon, bass boat, center console |
| Sterndrive | Same as an outboard. The drive itself turns, so it points the push. | The bow | express cruiser |
The finder’s drive choices and its own hint for each. The last column lists the boat types the finder starts on that drive.
The finder prints its reason on the card. On a single screw with no thruster yet, it says:
“on a single screw you have prop walk one way astern and little else, so a stern unit is the other direction”
On twins, it says:
“you already steer astern on twin engines, so a stern unit buys you less than it would on a single screw and the bow is where the gap is”
A houseboat, a ski or wake boat, a trawler and a sailboat all start as single screw in the finder. Change the answer if yours is not.
The case for the stern first
The stern first suits a single-screw boat that backs into her slip. Turning her stern-first in a crosswind is the hard part, and one unit at the stern does that job.
Why the stern needs less thrust
The arithmetic favors the stern. A unit on the The flat surface across the very back of the boat. works 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. on the boat. It sits half her length from the point she pivots about.
That is the stopped boat, turning about her middle, and every lever figure on this site is worked that way. Under way the The spot the boat turns about when she swings. With the boat moving ahead it is usually about a third of the way back from the bow. In reverse, it moves further back. moves forward. That shortens a bow unit’s lever and lengthens a stern unit’s.
Take this site’s 40 ft trawler in 15 knots. The calculator sizes each position like this:
On the transom, the longest lever on the boat.
Right at the bow, on nearly as long a lever.
It sits back from the bow on a shorter lever. That is about 40 percent more than the stern unit for the same turn.
The size calculator prints all three for your own boat.
The stern is where the bolt-on units live
This site lists 46 external units, counting DockStar’s one unit once for each of its 4 mounts. Of those, 31 are sold for the stern alone, about 67 percent. Only 6 are for the bow alone, 7 for either end, and 2 as a bow-and-stern pair.
The stern-only listings come from DockStar, ezDrive, Max Power, Sideshift, Sleipner and Yacht Thruster. Sideshift’s are its stern kits, Max Power’s and Sleipner’s are pods, and DockStar’s is the same unit as its bow one, on the stern track.
A stern 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. is the harder job, because the stern is where the machinery lives. The stern cost page has every figure this site holds for both routes, dated.
The case for the bow first
The bow wins when the trouble starts on the approach. She blows off as you slow, before the stern is anywhere near the dock, and you are a boat length out turning the wheel for nothing.
The bow carries the 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.. As she slows, the The spot the boat turns about when she swings. With the boat moving ahead it is usually about a third of the way back from the bow. In reverse, it moves further back. moves forward under it. That makes the bow the end the breeze takes first.
On twins, an outboard or a sterndrive, the stern already steers. The bow is the only end with a gap. On a sailboat the bow also raises the question of whether a tunnel fits 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. at all. The sailboat guide works that through.
How far one unit goes
This table answers one question: how long can your boat be before one small 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. stops steering her in a 15 knot breeze?
We worked it for a 90 lb unit mounted right at the bow, the size DockStar sells. Read across a row. The first figure is the longest boat of that kind one unit steers in that breeze. The second is the longest she can be with a second unit at the stern.
Take the trawler. One unit steers a trawler up to 44 ft long. With one at each end, it is up to 50 ft. A longer boat needs more thrust: a bigger unit or a tunnel.
| Kind of boat | One unit at the bow: up to | One at each end: up to |
|---|---|---|
| Pontoon | 30 ft | See the pontoon guide |
| Motor yacht | 44 ft | 50 ft |
| Trawler | 44 ft | 50 ft |
| Sailboat | 44 ft | 50 ft |
This site’s own arithmetic, from the size calculator; no maker publishes a table like it. A pair stops at DockStar’s own 50 ft. Kinds of boat this unit is not listed for are left out.
A tunnel has no such line. The thruster finder sizes one to the boat at any length the hull will take it. The band it prints for your boat is the same band the cost page prints.
One now, the other later
Buying one end does not close off the other. DockStar’s bow and stern units are two separate purchases, one at each end, with no pair discount.
The finder’s last question asks whether you already have a thruster at either end. If you already have one end covered, it prices the other alone.
Let the symptom set the order. Fit the end that fails you now. Add the other end if one unit is enough on calm days and not on the days you remember.
What two units add is a Two equal pushes in opposite directions, one at each end of the boat. A couple spins the boat about her middle without shoving her sideways. The further apart the two pushes sit, the harder the boat spins, which is why moving a thruster further from the middle does more than making it stronger.: more turning, with no more push. The pair page has that argument in full, with the routes priced from the same cells as the cost tables.
What neither end fixes
Some jobs belong to neither end. Holding her alongside while you wait for the bridge is one. Pinning her to the dock in twenty knots while the lines go on is another. Both are jobs for a A dock line run forward or aft at an angle rather than straight across. Held against the engine, it pulls one end of the boat in to the dock. It is the cheapest docking aid there is. and the engine, and no joystick changes that.
The steering page puts them ahead of any thruster, because the first three fixes on its list cost nothing. Nothing on this site is sized to hold a boat, at either end.
Where this comes from: the thruster finder’s own rules, its symptom and drive questions and what it does with each answer, as of 20 September 2026. Then the size calculator’s per-position figures for one boat. No maker publishes a which-end rule, and none is quoted.