The tunnel is the part you cannot easily redo, so buy the thruster maker's own tube in the right diameter and wall thickness. GRP suits fiberglass hulls, aluminum and steel suit metal hulls, and stern tunnel kits bolt a thruster onto a transom. Most performance problems blamed on thrusters come from tunnel position, length and sharp edges.
What we like
- Maker-matched tubes guarantee the motor bracket fits
- GRP tubes arrive watertight and ready for primer
- Long stock lengths cover most hull widths
- Stern tunnel kits avoid cutting the hull bottom
Watch out for
- Wall thickness must match the thruster's limits
- Mixing brands of tube and thruster invites fit problems
- Long tubes cost thrust
- Stern kits add drag and hang off the transom
Nobody gets excited about a fiberglass tube. But the tunnel is the one part of a thruster install you live with for the life of the boat. Motors get replaced, props get replaced, panels get upgraded. The hole in the hull stays where you put it. Get the tube, position and edges right and a modest thruster performs like a bigger one. Get them wrong and no motor upgrade will fix it.
Materials: GRP, aluminum, steel
Match the tube to the hull. Fiberglass boats get GRP tunnels laminated in. Aluminum and steel boats get metal tubes welded in. Sleipner sells all three; its catalog runs from small 125 mm GRP tubes to large reinforced commercial sizes, plus aluminum tubes such as a 250 mm ID, 10 mm wall, 1,000 mm unit. Vetus lists GRP tunnels made from glass-reinforced polyester and offers steel and aluminum versions too. Lewmar sells GRP tubes matched to its TT thrusters.
A note from an older Sleipner manual is worth repeating: original Side-Power GRP tubes are supplied watertight. You do not need to barrier-coat the inside of the tube, only the areas you ground and bonded.
Sizes you will actually order
| Maker | Example tubes |
|---|---|
| Sleipner | 185 mm ID x 6 mm wall x 1,250 mm GRP (item 9003, 7.8 kg); also 1,500 mm and other lengths |
| Vetus | BP150G10 (150 x 1,000 mm), BP185G10 to BP185G30 (185 mm, 1 to 3 m), BP250G10 to BP250G30 |
| Lewmar | 589102 (140 x 5 x 1,000 mm), 589301/589302 (185 x 6 x 1,000/1,500 mm), 589502 (250 x 7.5 x 1,500 mm) |
Two numbers decide compatibility: inside diameter and wall thickness. The thruster's motor bracket clamps to the tube, so the wall has to be inside the maker's window. Sleipner's current SE80-SE150 guide lists 6 to 8 mm for the 185 mm tunnel models and 7 to 9 mm for the 215 and 250 mm ones. A tube that is too thick or thin is a fit problem you find on install day.
Stern tunnel kits
Not every boat can take a tunnel near the stern, and many can take a thruster bolted to the transom instead. Sleipner's 90124I is a 125 mm GRP stern tunnel kit for the SE30, SE40, SEP30 and SEP40. Vetus offers GRP stern tunnels from STERN110P to STERN400P, plus SDKIT extension kits that push the tunnel deeper or clear of transom obstructions when the opening sits too close to the waterline. Lewmar sells a stern tunnel kit (589310) for its 185TT. Sleipner's stern notes add sensible checks: keep the thruster's wash clear of drives and trim tabs, and do not let the tunnel disturb flow under the hull.
The install rules that decide performance
These come from Sleipner's current installation guide, and they match what we see in the field.
- As far forward as you can. More leverage around the pivot point means more turning power from the same thrust.
- As deep as you can. Tunnel centre at least one tunnel diameter below the waterline, so the thruster does not suck air from the surface.
- Not too long. Avoid tunnels longer than about four times their diameter. Friction slows the water and costs thrust. Props must still sit fully inside the tube.
- Round the ends. A radius of at least 10% of the tunnel diameter at the hull joint. Sharp edges create turbulence and cavitation and choke the effective diameter. On steel and aluminum hulls, angled ends work similarly.
- Bond it properly. Grind the gelcoat back about 12 cm around the hole inside and out, then lay up at least eight layers of 300 g glass on the inside, alternating mat and woven roving. Fill every gap.
- Keep the motor seat clean. Do not build up laminate where the motor bracket sits, or the gear leg will not seat correctly.
- Do not gelcoat the bore. Primer and two thin coats of antifouling only; see our antifouling notes.
On sailboats and fast powerboats, Sleipner also describes recessing the hull aft of the tunnel or forming a small deflector ahead of it to reduce drag and protect the props. Its rule of thumb: standing in front of the boat looking aft, you should not see the back face of the tunnel.
Grids and diameter
Owners sometimes ask for bars across the tunnel openings to keep debris out. Vetus's hydraulic thruster manual says grids should only be fitted when absolutely necessary, for example in muddy or polluted water, and then exactly as its instructions describe, because anything in the opening disturbs the flow. On diameter, Sleipner notes that larger tunnels move water at lower speed and lose less to cavitation, but they need more depth below the waterline. Pick the diameter the thruster calls for and do not try to squeeze a larger thruster into a smaller tube.
Weak spots
Stock tubes are generic; the hull is not. Cored hulls need the core removed and sealed around the tube, which adds labor. Very flat or very fine bows end up with long tubes or oval openings, which is where retractable thrusters start to make sense. And stern kits stick out, so they collect growth and add a little drag.
How it compares with retractable and external options
A tunnel is cheaper and simpler than a retractable unit and stronger in a crosswind than most external setups, but it needs the right hull depth up forward. If your bow does not have it, read tunnel vs retractable vs external.
Verdict
Order the thruster maker's own tube, in the diameter the thruster calls for and the shortest length that fits your hull at the right depth. Spend the effort on position, rounded ends and a proper layup. That is where thrust comes from. Our install walkthrough shows how this fits into a 2 to 3 day job, and you can send hull photos if you want a second opinion on tunnel position.
Questions people ask
Can I use a Vetus tunnel with a Sleipner thruster?
Only if the inside diameter and wall thickness fall within the thruster maker's limits, because the motor bracket clamps to the tube. Sleipner, for example, lists a 6 to 8 mm wall for the 185 mm tunnels in its SE80-SE150 guide. Using the thruster maker's own tube removes the guesswork.
Should I gelcoat the inside of the tunnel?
Sleipner's manual says no: there is only room for a thin primer layer and two coats of antifouling between the tunnel wall and the propeller. Its original tubes are supplied watertight, so only the bonded and ground areas need sealing.
How long should the tunnel be?
Long enough that the propellers sit fully inside it, but Sleipner advises against tunnels longer than about four times their diameter because friction cuts water speed and thrust. Very uneven top and bottom wall lengths also cause problems.
What is a stern tunnel kit?
It is a short external tunnel that bolts and bonds to the transom so a thruster can sit at the stern without cutting through the hull bottom. Sleipner, Vetus and Lewmar all sell them for specific thruster sizes.
- Sleipner thruster tunnels
- Sleipner GRP tunnel 185 x 6 x 1250 mm
- Sleipner SE80-SE150 installation guide
- Sleipner SE80/100 IP manual (tunnel finishing)
- Vetus GRP tunnels
- Vetus stern thruster tunnels
- Lewmar thruster tunnels (Defender)
- Lewmar stern tunnel kit 589310 (iMarine)
- Sleipner 90124I stern tunnel kit (Eshopmarine)
Manufacturers update their lines. Confirm current model numbers with us or the maker before you buy.