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Finding one
External vs tunnel bow thruster: where each one wins
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. is the thruster the yard quotes and the one you picture. An external unit bolts to the outside of the hull instead of going through it.
This page sets the two side by side, one question at a time, using this site’s own figures. Neither wins outright, so what follows is a table, and the verdict is yours. Once you know which questions matter on your boat, the thruster finder, our free tool that matches a thruster to your boat, prices the units that fit.
The short answer
The tunnel units here run to 2425 lb. The biggest external unit is 430 lb.
Right at the bow it works on a longer lever, so the same turn takes about 26 percent less thrust. On a 42 ft trawler that is 88 lb against 118 lb from a tunnel.
On a trawler, our arithmetic has the biggest external unit sold for the bow covering the need up to about 77 ft. Its maker offers it to 66 ft. Above that, look at a tunnel or a unit at each end.
Every tunnel is a tube through the hull and a haul-out. Of the 43 external models in the database here (DockStar’s one unit counted once), 19 go on with the boat afloat, by the maker’s own claim.
At every length, the cheapest external unit installed costs less than the bottom of the tunnel band, mostly because of the yard hours. On a boat that planes, this site lists no external bow unit, so the bow answer there is a tunnel or a A thruster that lives inside the hull and drops down through a hatch when you press the button, then pulls back up. It leaves no permanent hole in the water flow..
The trade in one table
| Question | Tunnel | External | Better choice |
|---|---|---|---|
| Thrust on offer | Up to 2425 lb in the database, across 5 makers | Up to 430 lb, across 6 makers | Tunnel |
| Thrust needed for the same turn | 118 lb on a 42 ft trawler, from a tube about 15 percent of the length back from the bow | 88 lb for the same turn from an external unit right at the bow, about 26 percent less | External |
| What it drills | A tube bored right through the hull | Varies by model, from nothing below the waterline to a 50 mm through-hull | External |
| Haul-out | Always: the tube goes through the hull below the waterline | Of 43 rows, 19 go on with the boat afloat by the maker’s own claim. The rest are marked usually or unknown | External |
| Who can fit it | A yard, for the hole, the glasswork that ties the tube into the hull, and the smoothing of the mouths into the hull. The electrical can be yours | An owner, on most models. Sideshift publishes an installation time per thruster | External |
| Permanence | Part of the boat for good | Bolts off. Of these, one model, DockStar’s, rides on a rail and is lifted clear of the water on it underway | Neither |
| At speed | A tunnel mouth in the water flow. The makers ask for a rounded mouth, and on a fast hull a recess or deflector | A unit in the water. Sleipner offers its pods for “a practical thruster solution for displacement and semi-planing boats up to 60 feet, independently of hull form, hull-material, propulsion and depth”. Sideshift caps rough water at 15 mph. This site lists no external bow unit for a boat that planes | Tunnel |
| Fouling | Bad. The water in a tunnel sits still, so growth takes hold, and it is the hardest place on the boat to clean. | 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. | External |
| Run time | Minutes, then a cool-down: 2 to 3 on a brushed Sleipner | Seconds per burst on Sideshift (never more than 30). About 10 minutes per charge on the battery unit | Neither |
| Looks and snags | Two tunnel mouths in the hull below the waterline, faired in | A leg and propeller outboard of the hull, where lines and debris are, and in plain view | Tunnel |
The sections below take each row in turn. If you already know which rows matter on your boat, the finder weighs them for her and prices the units that fit.
Where these figures come from. The thrust figures come from our size calculator, run on one boat, and from the range of units listed here. The hours and the bill are the ones every cost page here prints. The speed lines are the makers’.
The no-hole page has the product-by-product detail on what each one drills.
Thrust, and the lever it works on
On raw thrust the tunnel wins by a wide margin. The tunnel units here run to 2425 lb, the external ones to 430 lb.
Raw thrust is not what turns the boat, though. Leverage is. A thruster turns the boat about her 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.. What it delivers is its thrust times its distance from that point.
A tunnel has to sit back from the bow, where the hull is deep enough to hold a tube. On this site’s figures that is about 15 percent of the length back.
An external unit sits right at the bow, on the longest lever the boat has. So the same turn takes less thrust from up there. Both positions are drawn on the same boat, with every distance marked.
Take a 42 ft trawler, sized as every trawler is here: enough to keep steering the bow in a 15 knot breeze while the engines drive her.
From a tunnel’s position she needs 118 lb.
From an external unit at the bow she needs 88 lb, about 26 percent less for the same result.
The size calculator prints both positions for your own boat. The placement page has the arithmetic behind the lever.
The lever only goes so far. On the same trawler profile, our arithmetic has the biggest external unit sold for the bow, the Yacht Thruster Duplex 600D at 397 lb, meeting the bow position’s need up to about 77 ft. At 78 ft the need passes it.
That is our arithmetic. Yacht Thruster itself offers the unit for boats of 56 to 66 ft, and the finder lists a unit no more than 10 percent past its maker’s top length.
Above that, the answer is a tunnel, or a unit at each end. The pair page prices each route.
The hull: what each drills and what stays
The card in the finder for a tunnel says what it drills: “a tube bored right through the hull”.
For an external unit, the external card reads “varies by model, from nothing below the waterline to a 50 mm through-hull”.
Read that second line twice. “External” tells you where the unit mounts. It does not promise an undrilled hull. The no-hole page lists what every product actually drills, from its own installation manual.
Of the 43 external models in the database here, DockStar’s one unit counted once, 19 go on with the boat afloat, by the maker’s own claim. Every tunnel is 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..
Permanence cuts both ways. A tunnel is part of the boat for good. A surveyor will inspect the fiberglass work, the mouths are blended into the hull, and nobody takes it out again.
An external unit bolts off. Here, one model, DockStar’s, rides on a rail and is lifted clear of the water on it when underway.
Whether either one adds anything at resale is a question nobody documents. The worth-it page says what we could and could not find.
The job and the bill
The hours are the gap. On the itemized owner invoices this site holds, a tunnel job runs from 32 billed hours on a boat around 35 ft, to 70 on a boat around 47 ft.
The table below prices each length band at one length near its middle. So its 40–50 ft row reads 64 hours, the figure at 45 ft.
Add the haul-out, and the days on the hard while the fiberglass cures. The install page walks the eight steps.
An external unit is 8 to 20 hours on this site’s arithmetic. Sideshift publishes 6 to 8 hours per thruster. The DIY page says which of those hours can be yours.
| Boat length | Tunnel, installed | Yard hours in the tunnel price | External, installed from |
|---|---|---|---|
| 25–30 ft | $9,000 to $11,100 | 32 | $4,540 |
| 35–40 ft | $10,100 to $14,200 | 40 | $4,540 |
| 40–50 ft | $11,800 to $19,700 | 64 | $4,540 |
| 55–60 ft | $17,400 to $20,800 | 70 | $7,763 |
The external floor sits below the tunnel band at every length. The cost page has each column’s full band, with the unit that sets its ceiling.
The electrical line is the other difference. A tunnel wants a battery bank forward, cable and a charger. That comes to $1,000 to $1,400 on the worked bill.
The external family runs from $0 to $2,136, counted unit by unit on each product page. A self-contained battery unit needs no boat wiring at all.
Under way and at rest
At speed, the tunnel wins
A tunnel’s mouths are holes in the water flow. The makers ask for them to be rounded, and on a fast hull for a recess or a deflector behind them. The placement page has what the manuals say.
An external unit is a leg and a propeller in the water, and the makers are careful about where they sell one. Sleipner offers its pods as “a practical thruster solution for displacement and semi-planing boats up to 60 feet, independently of hull form, hull-material, propulsion and depth” (read 16 September 2026).
Sideshift’s manual caps rough water at 15 mph.
Fast boats. This site lists no 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. bow unit for a boat that planes. At planing speed a hard wave lands on a unit hanging off the bow.
On a fast hull the bow routes are a tunnel or a A thruster that lives inside the hull and drops down through a hatch when you press the button, then pulls back up. It leaves no permanent hole in the water flow.. The stern kits that mount on the drive or the transom stay on the list.
At rest, the external unit wins
The card in the finder describes a tunnel’s Weed, slime and barnacles growing on anything that sits in the water. Inside a thruster tunnel it quietly costs thrust, and on a retractable it can stop the unit coming down. this way: “Bad. The water in a tunnel sits still, so growth takes hold, and it is the hardest place on the boat to clean.” The barnacles page is about exactly that.
An external unit’s propeller is on the outside of the hull, where you can reach it. A removable one is lifted clear of the water on its rail.
What you pay for that is a leg outside the hull, where the lines and the debris are, and where a stray dinghy painter will find it. Then there are the looks. The leg is in plain view, and on a boat whose lines you care about that is a fair objection.
Run time is a draw
A Small carbon blocks inside a motor that press on the spinning part and carry the current into it. They wear down, they shed dust, and they are the usual reason an old thruster motor stops. tunnel unit runs for minutes and then has to cool.
Sideshift’s units work in bursts of seconds.
A battery unit runs about 10 minutes per charge and then recharges.
A docking uses seconds of any of them. Nobody needs ten minutes of thrust to get a line on. The run-time page has every maker’s limit.
Which one, for whom
A tunnel suits a boat big enough that the external family runs out of thrust, or one fast enough that nothing should hang off the bow. It also makes sense when the haul-out is happening anyway, or when the hull has a deep enough place forward for a tube and you want nothing outside it.
An external unit suits a boat that stays in the water, or an owner who wants to fit it. It is the answer when the hull refuses a tunnel forward: a fine sailboat bow, a pontoon, a houseboat. It is also the answer when the price of a tunnel is what stopped the project. On a displacement boat around 42 ft it does the steering job on a longer lever with less thrust.
Either, at each end, suits a boat whose stern blows off as well as her bow. Two units at the ends out-turn one bigger tunnel at the bow. The pair page prices every route.
The thruster finder asks the questions behind those three: what kind of boat, how she is driven, and whether a hole or a haul-out is acceptable. Then it shows the units that fit, with what each one costs installed.