How do I know which thruster my boat needs?

Start with what your boat is and what goes wrong at the dock. A thrust figure comes later. Seven to nine questions settle it: boat type, length and weight, what drives her, what is going wrong, whether you will cut a hole below the waterline, and whether you can haul out. Those answers rule out whole families of product before any number is compared. Who installs it and what you had in mind to spend are sort tiles on the answer instead of questions: each changes what leads and how the install line reads, and neither hides an option.

Is a bow and stern pair better than one bigger thruster?

For turning the boat, usually yes, and by more than people expect. Two units at opposite ends make a couple: 90 lb across the length between them, 3,510 lb·ft on a 40 ft boat, against 2,100 lb·ft for a 150 lb tunnel. What it does not buy is more push at either end, because each end still gets 90 lb, so it will not pin a boat to a dock in a hard crosswind.

Does it matter whether my boat has one engine or two?

More than almost anything else. A single screw with a rudder loses nearly all its steering at docking speed and gets prop walk in one direction only. Twin screws steer on differential throttle at zero speed, and an outboard or sterndrive swivels the whole drive and backs down accurately. A twin-screw boat usually needs authority only at the bow; a single-screw boat is the strongest case for a unit at each end.

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Decision

Which thruster does your boat need? Find out in seven to nine questions

The bow has ideas of its own in a crosswind, and the fix depends on your boat and how you dock her. Answer a few questions about both. You get the thrusters that fit your hull, what each costs in money and time, what you give up either way, and where to buy from the maker or find a dealer.

The guide for your kind of boat has the same answer at length. Any word here you do not know is on the glossary page.

The list covers electric thrusters. Hydraulic ones, for big boats and long running, are sold on a dealer’s quote with no published price, so they have their own page.

The guided version needs JavaScript. The whole decision is written out below, with the same questions, the same arithmetic and the same outcomes.

The short answer

Start with the boat. Which thruster a boat needs depends on the boat and on how she is docked. A thrust figure alone does not settle it.
One engine. A boat with one engine and a rudder loses almost all her steering at docking speed. She also has prop walk in one direction only, so she usually wants a unit at each end.
Two engines or an outboard. A twin-screw or outboard boat already steers astern. She usually needs help only at the bow.
Holes and haul-outs. If the boat will not be hauled out, or you will not cut below the waterline, a tunnel is ruled out before any number is compared.
What the finder gives you. The thruster finder on this page asks seven to nine questions. It returns the one or two kinds of thruster that fit the boat, drawn from four families: tunnel, external, retractable and bow-mount trolling motor, with every brand that makes one, and ranks them against the budget you give without hiding any. Each one comes with its brands, the cost, the hours, marine growth and the trade-offs.

The whole decision, written out

Why this is a hard question

The thrust figure the industry publishes answers a question you did not ask. Thrusters are sized on the force needed to hold a bow against a beam wind, and nobody docks that way. You come in with the boat still moving, the engine working ahead and astern, and somebody on the finger pier pretending not to watch. What you are short of is the ability to put the bow where you want it.

By then your has mostly stopped working. Its side force falls with the square of boat speed. Count 6 knots of as full steering. You have about 44% of it left at four knots, and roughly 11% at two knots.

Wind force does not care how fast you are going. So its advantage over your steering grows about 9 times over as you slow from 6 knots to two. Our page on why your boat won't steer at low speed works that through.

So the finder compares products on turning, the job a thruster does while your engines drive the boat. Pounds of push come second. That is why question three asks what drives your boat before it asks anything about thrust.

The seven to nine questions

Two more are asked only when they can still change the answer. You can answer “not sure” to any of them. The finder then uses a typical value and shows you what it used.

1. What kind of boat?

Pontoon & tritoon · Sailboat · Houseboat · Catamaran · Trawler · Motor yacht · Bass & flats · Wake & ski boats · Express cruiser · Center console

Decides whether a tunnel is even possible before anything else is asked. A pontoon has no hull up forward to put a tunnel through, so a tunnel is never on her list.

2. How long is she, and roughly how heavy?

Length overall · Displacement if you know it · Otherwise estimated from type and length, shown so you can correct it

Sets the thrust your boat needs, which decides which recommendations fit.

3. What drives her?

One engine, with a rudder · Twin engines · Outboard or outboards · Sterndrive · Not sure

This decides how much steering you already have at docking speed. A single screw with a rudder loses almost all of it, and gets prop walk one way only. Twin engines steer with one ahead and one astern, even stopped. An outboard swivels the whole drive and backs down accurately. The answer is pre-filled from your boat type, so it is one tap.

4. What goes wrong at the dock?

The bow will not swing where I want it · The stern will not tuck in · I get too close to the boat next to me · I cannot line her up without a go-around · She will not stay put while I wait · Nothing yet

Decides bow, stern or both. Each answer is a docking move that is not working. That is the job a thruster does.

5. A hole below the waterline?

Absolutely not · Rather not, but I would if it were clearly right · Fine, if it is the right answer

This is the question that rules out the most. Read what each answer drills, because several products that avoid a tunnel still put fasteners through the hull.

6. Can you haul out?

She is hauling anyway this season · I could, it is money and time · No, she stays in the water

A haul-out already scheduled is the single biggest thing that makes a tunnel affordable, because the yard cost is already spent. On a boat that may live on a trailer (a center console, bass or ski boat, or an express cruiser to 30 ft) it asks where she lives instead, and a trailer boat has no haul-out bill at all.

7. Room forward for a battery bank? (only if a tunnel is still in the running)

Yes, space and a charger · Tight · No idea · Nothing forward of the V-berth

Batteries and wiring are the hidden line on a tunnel quote. One houseboat owner itemized his job on a forum in December 2016, and the battery bank, charger and cables came to about three quarters of the money.

8. Do you already have a thruster at either end?

No, starting from scratch · Yes, at the bow · Yes, at the stern · Yes, at both ends

Narrows a both-ends need down to just the end still missing, and says so plainly when both ends are already covered and there is nothing left to recommend.

9. Who will do the work? (not asked when both ends already have a thruster)

I will do it all myself · I will do some, and a yard does the hull work · A yard does all of it

Decides whose hours the prices show. A yard job is priced as a yard bills it, with nothing about work you do not want to do. If you are doing some or all of it, each model shows how much is yours and how long it takes.

One more you answer on the results page

Who installs it and what you had in mind to spend only ever reorder the options. That is why they sit above the answer as sort tiles, and why nothing drops off the list when you press one.

What did you have in mind to spend?

Under $2,000 · $2,000–5,000 · $5,000–10,000 · $10,000–18,000 · Whatever it takes

This is what you had in mind, and it is not a limit. Budget never hides an option here. If the right answer costs more than you picked, you still see it, with the real number on it.

The three bands, and the arithmetic

Once the site has worked out what your boat needs, one comparison decides which recommendations you get. That comparison is about turning .

Thrust is where most people go wrong. Two 90 lb units at opposite ends do not give you 180 lb. Each end still gets 90 lb. Two units buy you more turning. They do not buy more push.

What the pair gives you is a . That means two equal and opposite pushes some distance apart, and their turning effect is the same measured from any point on the boat. On a 40 ft hull the pair makes 3,510 lb·ft. A 150 lb set 6 ft back from the bow makes 2,100 lb·ft.

So the test is whether the pair's couple matches the moment the sized tunnel would make. Our page on a thruster at each end explains that setup and prices each way of getting it.

The turning moment a bow-and-stern pair produces, by boat length.
Boat length	Pair produces
24 ft	2,106 lb·ft
32 ft	2,808 lb·ft
40 ft	3,510 lb·ft
50 ft	4,388 lb·ft
60 ft	5,265 lb·ft

At every one of those lengths, the pair's couple matches any tunnel rated up to about 251 lb. That ceiling barely moves with length. A longer hull lengthens the tunnel's and the pair's together.

What each band means for the recommendation you get.
Band	When	What we recommend
A	One 90 lb unit, working on its own lever, makes the turning moment your boat needs	One external unit covers it outright. Recommended at full strength, with a tunnel offered only if you are hauling anyway.
B	One unit is short, but a bow-and-stern pair’s couple covers it	Either answer works. The tunnel leads when you can have one. If you have ruled out holes and haul-outs, the pair leads, and the tunnel moves down to the trade-up section, with the reason beside it.
C	Even the pair is short of the moment she needs	A tunnel is the answer. External units still appear. They are what owners who will not cut a hole fit instead, and the row says what they give up.

Where this stops being true. Matching the moment does not match the force. Say your problem is holding the boat off something while stopped, with the wind on the beam and no useful engine option. Then one end still has only 90 lb. That case comes up less often than the sales pitch suggests. It is where a pair falls short.

The figures behind the sizing are estimates, checked against the sizing the makers themselves publish. So a result close to a band edge means either answer could be right.

The six outcomes

Each outcome below tells you what it does and what it costs you.

A tunnel thruster

A tube glassed or welded straight through the hull with a propeller inside it. The one most people picture, and what most quotes are for.

Marine growth. Bad. The water in a tunnel sits still, so growth takes hold, and it is the hardest place on the boat to clean.

What you give up. A hole through the boat below the waterline, and a tunnel that collects marine growth and slowly loses thrust. You also work to the yard’s calendar and need a battery bank forward. Choosing one end or both does not change the product.

Check this yourself

An external thruster

Mounts on the outside of the hull, so there is no tunnel to bore. Also sold as pod or bolt-on thrusters. The widest category and the one that varies most: some stay put, some drop down when you need them, one lifts clear of the water on its rail.

Marine growth. A fixed mount thruster stays in the water and fouls on the body and the prop, like any other underwater fitting. A removable thruster comes out of the water when not in use and does not foul at all.

What you give up. It hangs off the hull, so it needs care at haul-out, launch and storage, and it sits in plain view on the hull. This site lists no external bow unit for a boat that planes. External does not always mean no holes. Sideshift’s SS350 keeps every fastener above the waterline, while Yacht Thruster wants a 50 mm through-hull.

Check this yourself

A retractable thruster

It lives inside the hull and drops through a hatch when you need it. It suits a boat where a permanent tunnel mouth would cost speed or fuel, which is why sailboats get them.

Marine growth. On a mechanism that has to move. Less growth than a tunnel, worse consequences: VETUS builds anti-jam protection in for exactly this.

What you give up. This is the most expensive answer on this page. Fisheries Supply says installation costs significantly more than a tunnel. It also adds a moving part that can jam on marine growth.

Check this yourself

A bow-mount trolling motor

Pontoons, tritoons, houseboats and light aluminum only. On those, owners report it genuinely working.

Marine growth. None. It lifts out of the water.

What you give up. It has to be deployed, aimed and given several seconds. Nobody sells one as a docking thruster. On a 30 ft-plus displacement boat one owner put it plainly: “I might as well have been trying to push a freight train uphill.”

Check this yourself

Not a thruster, at least not yet

Sometimes the right answer. A spring line and some practice solve more docking problems than people expect.

Marine growth. None.

What you give up. Nothing, today. But it stops being the right answer as the boat gets bigger or the crew gets older. One Trawler Forum owner, January 2024: “If thrusters help enable us to boat longer then so be it.”

Check this yourself

Check the quote you are holding first

If a quote already arrived and frightened you, take it apart before you decide anything else.

Marine growth. Not applicable.

What you give up. Nothing. If you are holding a quote, the quote-check page shows you where it sits against the figures here and where you can pay less.

Check this yourself

Trading up and trading down

Whatever you land on, the results show a step up and a step down from it, each with real numbers on it. Two things catch people out.

More money does not always mean more turning. Moving from a pair of external units to a single bow costs several thousand dollars more. On a 40 ft boat the 150 lb tunnel in the example above also produces about 40% less turning than the pair. The move can still make sense for other reasons, but it buys no extra turning. Any comparison that ranks these options on a single line from worse to better hides that from you.

Time and effort are different things. A tunnel is 32 to 70 hours of billed labor, almost none of it yours. Those hours start whenever the yard's next slot opens. The removable external unit is 1 to 3 hours, all of them yours, and you can use it the afternoon it arrives.

The bolt-on brackets in the same style run to 20 billed hours, so check the row for the unit you are looking at. A quote that blends the two will surprise you.

Work out what your boat needs from what kind of boat she is and how long, sized to keep steering in a 15 knot breeze.
Read why your steering left in the first place before you buy to a number.
Take a quote apart line by line if you are already holding one.

Where to go next

Bow thruster cost Installed prices by boat length, yard rates, and why labor is the expensive half.
Types of bow thrusters Tunnel, external, retractable and water jet: how each works and which boats suit it.
Is your bow thruster quote reasonable? Take your quote apart against published parts prices and current yard rates.

More in this section: Boat won’t steer at low speed? Is a bow thruster worth it? Bow or stern thruster first? Docking alone or short-handed Docking as you get older

The sizes and prices on this page are estimates for planning. They are not an inspection of your boat. Check any thruster, cable or battery against the maker's manual and with your yard before you buy or fit anything. The full wording is in our terms.