Bow thruster tunnel placement: check the spot your yard picks

The short answer

1. Forward first. 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. How far forward
2. Deep enough. 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. How deep
3. Then stop going deeper. Once the top of the tunnel is about 30 to 35 cm down, moving it forward matters more than more depth. The depth for your tunnel size
4. Keep the tube short. Keep it under 4 times its diameter. The hull decides the length, because the tube runs from side to side, and the hull is narrower further forward. How long the tube can be
5. If your sailboat’s bow is too shallow, there are two routes. One is a half-tunnel with a deflector ahead of it. The other is an external unit at the bow, which needs no hole at all. Sailboats and a shallow forefoot

The two rules, and why they fight

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 forefootThe 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.

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. The same thruster on a 10 m arm makes 550 kgm. A kgm (kilogram-meter) is the metric form of pound-feetPound-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..

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 her length. 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.

Our arithmetic, on this site’s tunnel position and pivot point.

The smaller the boat, the more each foot costs, because the lever is shorter to begin with.

It is also why an external unitA 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:

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.

From the guide’s product dimensions table. The water depth is to the top of the tunnel.

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.

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.

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

The tube runs straight through the hull from one side to the other, so its length is the width of the hull at the spot you choose. Water loses speed to the tube’s walls, so a long tube pushes less.

The makers give the length in tunnel widths (diameters). For a 185 mm (7.3 in) tunnel, the most common size here, 4 widths is about 29 in. The hull is narrower further forward, which is one more reason the tube goes there.

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 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 keelThe 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.

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 install.

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 fin keelA 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 long keelA 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.

1. Sleipner’s half-tunnel. The tunnel is installed 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.
2. Do not cut the hull. An external unitA 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.

Sleipner offers the half-tunnel for flat-bottomed barges too, 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 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.

1. The better fix: a recess. A recess in the hull behind the tunnel removes the back face, and the water passes it. The recess is usually angled slightly down, following the flow along that part of the hull.
2. The other fix: a deflector. A deflector ahead of and below the tunnel 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.
3. The test. 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

Where the tunnel goes on your boat is decided with the boat out of the water, by someone looking at your hull. The spot is the yard’s call, and a good yard has made it many times. Put these six questions to them.

1. 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 lever table says how much.
2. 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.
3. How long will the tube be, in diameters? Past 4, ask whether a spot further forward would shorten it.
4. Is there a stiffener or a stringer in the way, and what happens to it?
5. 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.
6. 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.