We may earn a referral commission from some links on this page. It doesn't change your price or our recommendations.
Install
Bow thruster tunnel placement: check the spot your yard picks
Every installation manual asks for the same two things in 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.: as far forward as possible and as deep as possible. On most hulls those pull against each other, and the spot a yard picks is the compromise. This page shows you what each maker asks for, why forward matters so much, and how deep the tube should sit for every tunnel size.
Where this comes from. The figures are read from the makers’ own installation manuals, on 19 September 2026. There are two of Sleipner’s: the current SE20 to SE40 guide and the 2015 SE80 to SE130 manual, which is the one with numbers in it. The others are Max Power’s CT100 and CT125 manual, Quick’s BTQ250 and BTQ300 manual, Lewmar’s Gen 2 manual and VETUS’s RIMDRIVE instructions. Each is linked where it is quoted.
Where the page does its own arithmetic, it says so. Where the tunnel goes on your boat is decided with the boat out of the water, by someone looking at your hull. The page ends with what to ask that person.
The short answer
A thruster turns the boat about her pivot point, so every foot closer to the bow buys more turn from the same motor. On this site’s figures a tunnel sits about 15 percent of the length back from the bow. On a 40 ft boat, each foot further aft costs about 7.1 percent of the turning.
Measure from the loaded waterline to the top of the tube, in tunnel diameters. Aim for one diameter down if the hull allows it and three quarters if it does not. Never go less than half.
Sleipner says that once the top of the tunnel is about 30 to 35 cm down, moving it forward matters more than more depth.
Sleipner says no more than 4 times its diameter. The hull decides the length, because the tube runs from side to side, and the hull is narrower further forward.
there are two routes. Sleipner describes a half-tunnel with a deflector ahead of it. An external unit at the bow needs no hole at all.
The two rules, and why they fight
Forward, because 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.. The turning it makes is its thrust multiplied by its distance from that point.
Deep, because a propeller close to the surface pulls air down into the tube. Air in the tunnel means lost thrust and a lot of noise.
Sleipner’s current guide gives each rule one sentence. Lewmar, Quick and Max Power say the same, in the same order.
The trouble is the shape of a bow. Go forward and the hull narrows and rises. The spot that is far enough forward is often not deep enough. The spot that is deep enough is often further aft than anyone wants.
A full-bowed trawler or motor yacht has room for both. A sailboat with a cut-away 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. may have neither. That gets a section of its own below.
How far forward
Sleipner’s 2015 manual works an example. A thruster of 55 kg thrust on an 11 m arm from the pivot point makes 605 kgm of turning (kilogram-meters, the metric form of Pound-feet, the unit of turning effort: one pound of push, one foot away from the point the boat turns about. It is how you compare a thruster at the bow against one at the stern, because pounds of thrust alone say nothing about where the push is applied.). The same thruster on a 10 m arm makes 550 kgm.
So the forward position turns the boat 10 percent harder with the same motor. That is the same 10 percent the arm grew by.
Quick makes the same point without numbers. Its manual says the lever effect is proportional to the distance from the boat’s center of mass. Of the two positions it draws, it says to prefer the one farther from that center.
This site sizes every thruster the same way. The tunnel sits about 15 percent of the boat’s length back from the bow, and the boat pivots halfway along the boat. That pivot is the figure for a boat that is not moving. Once she is moving, the pivot moves forward, so a tunnel’s lever gets shorter still.
Here is what each foot further aft costs on that footing. This is our arithmetic.
| Boat | Tunnel back from the bow | Distance to the pivot point | One foot further aft costs |
|---|---|---|---|
| 30 ft | 4.5 ft | 10.5 ft | about 9.5 percent of the turning |
| 40 ft | 6 ft | 14 ft | about 7.1 percent of the turning |
| 50 ft | 7.5 ft | 17.5 ft | about 5.7 percent of the turning |
The smaller the boat, the more each foot costs, because the lever is shorter to begin with.
It is also why 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. right at the bow can do a tunnel’s work with less thrust. It sits on the longest lever the boat has. The size calculator prints both positions for your boat. The pair page takes the lever argument to its end, with a unit at each end of the boat.
How deep
The makers measure depth from the waterline to the top of the tube. Most of them measure it in tunnel diameters, so a bigger tunnel has to sit deeper.
Sleipner’s 2015 manual is the fullest statement. It calls half a diameter “an absolute minimum”. It recommends at least three quarters of a diameter. It calls one full diameter “a really good distance”.
The other makers agree:
Lewmar uses the same two figures: one diameter, with three quarters the minimum.
Quick asks for at least three quarters of a diameter.
Max Power writes it in millimeters for its two tunnel sizes: at least 139 mm and ideally 185 mm to the top of the tube. It says a tunnel shallower than the ideal loses performance the closer it sits to the minimum.
VETUS, for its rim-drive units, wants the tunnel’s centerline at least 250 mm down.
Sleipner adds the rule that settles the compromise. The manual puts the point at about 30 to 35 cm from the surface to the top of the tunnel (about 1 foot). Past that depth, it says:
“other factors should be considered more important, i.e. moving the thruster further forward”.
In other words, spend any depth the hull still offers on moving the tunnel forward.
Sleipner’s current guide for its four smallest units states no ratio. It gives a minimum water depth for each model in its dimensions table instead. On every one of the four, that figure is exactly the tunnel’s inside diameter:
| Model | Inside diameter | Minimum water depth | Motor height above the tunnel |
|---|---|---|---|
| SE20 | 110 mm | 110 mm | 209 mm |
| SE25 | 110 mm | 110 mm | 252 mm |
| SE30 | 125 mm | 125 mm | 263 mm |
| SE40 | 125 mm | 125 mm | 263 mm |
So one diameter is the figure Sleipner’s older manual calls really good and Lewmar calls the target. In Sleipner’s current guide it is the minimum for these models.
Read the makers together and the answer comes out the same. Go a diameter down if the hull allows it. Go three quarters if it does not. Never go less than half.
The depth for every tunnel size
Our product list, the database every table on this site reads, holds 140 tunnel units across 13 tunnel sizes, from 110 mm to 513 mm. The size calculator tells you which sizes the units for your boat use.
This table turns each size into the depth the rules above ask for, measured to the top of the tube.
| Tunnel size | Units in the database | At least | Better |
|---|---|---|---|
| 110 mm (4.3 in) | 8 | 3.2 in | 4.3 in |
| 125 mm (4.9 in) | 18 | 3.7 in | 4.9 in |
| 140 mm (5.5 in) | 9 | 4.1 in | 5.5 in |
| 150 mm (5.9 in) | 6 | 4.4 in | 5.9 in |
| 185 mm (7.3 in) | 42 | 5.5 in | 7.3 in |
| 215 mm (8.5 in) | 5 | 6.3 in | 8.5 in |
| 250 mm (9.8 in) | 24 | 7.4 in | 9.8 in |
| 300 mm (11.8 in) | 16 | 8.9 in | 11.8 in* |
| 315 mm (12.4 in) | 3 | 9.3 in | 12.4 in* |
| 386 mm (15.2 in) | 3 | 11.4 in | 15.2 in* |
| 400 mm (15.7 in) | 3 | 11.8 in | 15.7 in* |
| 412 mm (16.2 in) | 1 | 12.2 in | 16.2 in* |
| 513 mm (20.2 in) | 2 | 15.1 in | 20.2 in* |
* On these sizes, one bore down already reaches the 30 cm (about 11.8 in) at which Sleipner says to put moving forward ahead of going deeper. On the small sizes it does not, and the full diameter is worth having.
There are two things the table leaves out. The first is the tube’s own wall and the rounding of the mouth. The yard measures those on the hull.
The second is where the waterline is. The makers mean the line she floats at when loaded, which may not be the painted line. A boat that sits down at the bow with full tanks and an anchor locker gives the tunnel more depth than one that sits light.
How long the tube can be
A tunnel is a pipe, and water loses speed to the pipe’s walls. Here is what each maker says about length, in tunnel diameters:
Sleipner’s current guide says to avoid a tunnel longer than 4 times its diameter.
Sleipner’s 2015 manual calls 2 to 4 diameters optimal and says to avoid 6 to 7.
Quick calls 3 to 4 diameters optimal and tolerates up to 6. It says outright that a longer tube lowers the driving force.
Lewmar gives a floor only: at least 2 diameters.
The floor matters as much as the ceiling. Sleipner says a tube that is too short lets the water reach the propeller before it has straightened out. That usually happens at the bottom, where the hull curves away. Sleipner blames it for cavitation and noise.
The propeller must sit entirely inside the tube, so the unit itself needs a tube of its own length. Sleipner’s SE20 to SE40 dimensions table gives 111 mm at least and 133 mm recommended on a 110 mm tunnel, about 1.2 times the diameter. That is what the unit needs before the hull adds any length.
The yard does not choose the length. The tube runs from one side of the hull to the other, so it is as long as the hull is wide where it crosses. That is one more reason to go forward, where the hull is narrow.
It is also why a flat-bottomed barge or a full-bowed boat can end up with a tube well past the makers’ ratios. Sleipner’s answer for those hulls is the half-tunnel described under sailboats.
The keel, the stringers and the motor’s room
Keep clear of the keel. Sleipner’s current guide adds a third distance. As a general recommendation, the tunnel sits at least a quarter of its diameter from the The lowest part of the hull, running along the middle from bow to stern.. The guide allows that the installation method can override this. Its drawing marks the clearance on both sides of the keel line.
Leave the stringers alone. The same guide has one rule with no exceptions. Do not put the thruster anywhere that means modifying a hull stiffener or a A long rib bonded inside the hull, running from bow to stern, that stiffens the bottom. Cutting through one weakens the boat, so a tunnel is kept clear of them., because that can weaken the hull.
So the spot is limited from inside the boat as much as from outside. Inside is also where the motor lives.
Leave room for the motor. Lewmar’s manual says to make sure there is room above the tube for the whole assembly, motor and controls included. Sleipner’s table puts the motor height for its four smallest units at 209 to 263 mm above the tunnel. Those units have tunnels of 110 and 125 mm. A bigger unit stands taller.
The motor can lean to fit. The install page has what the makers say about supporting a motor that does. The compartment needs to be dry and ventilated. The cable has to reach it from a battery, which the makers want as close as possible.
Sailboats and a shallow forefoot
Sleipner’s guide names the problem. Some sailboats have a flat bottom and shallow draft at the bow, which makes a tunnel far enough forward hard to fit.
A narrow bow has little width where the tube would cross. Where the hull is finally deep enough, the tunnel is too far aft to be worth much. The sailboat guide puts it the same way.
A A short, deep keel bolted under the middle of a sailboat, with the rudder separate behind it. The boat turns quickly and drifts sideways easily at low speed. and a A keel that runs most of the length of the hull, with the rudder hung off its back edge. The boat holds a straight course well and turns slowly. give different answers on the same length of boat. No chart organized by length can tell you which. The sailboat retrofit page walks the job route by route, with one owner’s account of finding the spot.
On a sailboat whose forefoot refuses a normal tunnel, there are two routes.
The first is Sleipner’s half-tunnel. The tunnel is fitted halfway into the underside of the hull, so part of the tube stands below the hull’s line. A deflector built ahead of it carries the water flow around it.
The guide says some of the world’s largest sailboat builders use the method, with little to no speed loss under normal cruising. It offers the same method for flat-bottomed barges, to avoid a very long tunnel and large oval openings. It is a bigger fiberglass job than a plain tunnel, and it is the yard’s to price.
The second is not to cut the hull. A 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 sits on a longer lever than any tunnel, and it needs no depth at all. The pair page draws the two on one boat and shows the lever. The no-hole page compares that route and prices it.
The back face of the tunnel: recess or deflector
When the boat is moving, the aft edge of the tunnel’s mouth faces the water flow. Sleipner says that flat area causes three problems:
- It makes drag on sailboats and fast powerboats.
- It drives water through the tube on a fast hull.
- It can spin the propeller while you are under way.
The guide gives two fixes and a test.
The better fix is a recess in the hull behind the tunnel. The back face is gone, and the water passes it. The recess is usually angled slightly down, following the flow along that part of the hull.
The other fix is a deflector ahead of and below the tunnel, which pushes the flow out past the mouth. It is made by leaving part of the tube standing out past the hull at the lower forward edge of the hole and molding a curve onto it.
The test is simple. In the guide’s words, you should not see the back face of the tunnel when standing directly in front of the boat looking aft.
On a boat that runs over 25 knots, Sleipner asks for an extended deflector. The bow pounds and forces water into the tube.
Whatever the fix, the mouth of the tunnel is still rounded into the hull, by at least one tenth of the diameter. Sleipner puts the thrust that rounding earns at 30 to 40 percent of the total. Both jobs are step five on the install page.
What to ask the yard
The spot is the yard’s call, and a good yard has made it many times. These questions let you check that call against this page.
- Where does the tunnel go, and how far back from the bow is that? The site’s own figure is about 15 percent of the boat’s length. Further aft is a real cost in turning, and the table above says how much.
- How deep is the top of the tube below the loaded waterline, and what tunnel size is it? Three quarters of the bore is Sleipner’s recommended minimum and the minimum Quick and Lewmar share. One full bore is the target.
- How long will the tube be, in diameters? Past 4, ask whether a spot further forward would shorten it.
- Is there a stiffener or a stringer in the way, and what happens to it?
- Will the mouth be rounded, and will the back face get a recess or a deflector? On a sailboat or a fast boat, ask to see what they have done on others.
- Where does the motor sit, and is that space dry, ventilated and reachable for service?
The answers belong on the quote. The install page walks the rest of the job. The cost page prices it. The quote checker puts your quote against the band for your boat.
Sources: Sleipner’s SE20 to SE40 installation guide (2025) and its SE80 to SE130 manual (2015, a dealer’s copy). Also Max Power’s CT100 and CT125 manual, Quick’s BTQ250 and BTQ300 manual, Lewmar’s Gen 2 manual and VETUS’s RIMDRIVE instructions. All were read 19 September 2026. The lever table is this site’s arithmetic on the tunnel position and pivot point every sizing figure here uses. The depth table is the makers’ ratios applied to the tunnel sizes in the product database.