Lewmar Fiberglass Bow Thruster Tunnels (140mm to 300mm Diameter Range)

Lewmar fiberglass thruster tunnels are designed to be built into the hull of your boat, forming the tube through which the bow thruster operates. Made from high-quality fiberglass composite (glass fiber reinforced resin), these tunnels offer a combination of light weight and exceptional strength, as well as complete resistance to marine corrosion. Unlike metal tunnels, fiberglass will not rust or suffer galvanic corrosion, which eliminates potential compatibility issues between the tunnel and thruster hardware.

The interior surface of each Lewmar tunnel is smooth and fair, reducing water flow turbulence. This means your bow thruster can operate at peak efficiency – generating maximum thrust with minimal hydrodynamic loss. The outer flange or lip of the tunnel is designed to facilitate a strong bond to the hull; during installation it provides ample surface area for fiberglass tabbing or adhesive bonding, ensuring a watertight and structurally sound fit.

Available Variants (Diameter × Length):

• Ø140 mm: Intended for small thruster models (such as 110TT 1.5 kW and 140TT 2.0 kW units). Typical lengths include ~0.75 m, 1.0 m, and 1.5 m, allowing flexibility to suit different hull shapes and thicknesses. A 140 mm tunnel is commonly used on boats around 5–8 meters in length. It provides the proper enclosure for thrusters in the ~30–40 kgf thrust range, ensuring the propeller operates in the ideal environment for thrust generation.
• Ø185 mm: Suited for mid-sized thrusters (e.g., 185TT models of 3 kW or 4 kW). Standard lengths are usually 1.0 m, 1.5 m, up to 2.0 m. This diameter is often found on yachts roughly 9–12 m long. It accommodates the ~185 mm propeller diameter of these thrusters and supports their typical thrust output (approximately 50–70 kgf) effectively.
• Ø250 mm: For larger thruster units (such as 250TT 6 kW to 8 kW models). Lengths vary (1.0 m, 1.5 m, 2.0 m) to match the installation requirements of thicker hulls or deeper placements. Vessels in the ~13–18 m range (45–60 ft) commonly use 250 mm tunnels, as thrusters of this class deliver around 120–160 kgf of thrust and require a robust, correctly-sized tunnel for optimal performance.
• Ø300 mm: The largest standard diameter, made for the powerful 300TT thruster series (10 kW and above) found on superyachts or commercial vessels roughly 18–24 m (60–80 ft) and larger. 300 mm tunnels are often produced in lengths of ~1.5 m or 2.0 m to ensure adequate tunnel length (Lewmar’s guideline is a minimum tunnel length of about 2× the diameter for best performance). This size properly houses big twin-prop thrusters that can produce 200+ kgf of thrust.

Each tunnel can be cut or trimmed to the exact length required during installation (a common practice by boatyards to achieve a flush fit with the hull curvature). The fiberglass material is workable and can be seamlessly integrated into fiberglass or wooden hulls by laminating with resin and glass cloth, achieving a monolithic bond with the hull structure. For steel or aluminum hulls, a fiberglass tunnel can also be used; it is typically bedded and sealed with epoxy or appropriate adhesive, avoiding any direct metal-to-metal contact and thereby preventing electrolytic corrosion issues.

• Installation Tips: The tunnel should be installed transversely through the bow at the lowest possible position that stays fully submerged (usually about 1/3 of the hull depth above the keel line at the bow). Always follow Lewmar’s guidelines for minimum submergence (to avoid air drawing into the tunnel). Lewmar fiberglass tunnels are engineered to handle the forces generated by the thruster’s operation and even impact pressures from waves at the bow – when properly installed, they become a structural part of the bow.
• Compatibility: Each tunnel diameter corresponds perfectly to the propeller size of the respective Lewmar thruster model. This means a precise fit with minimal gap between the propeller tips and the tunnel walls, which is critical for achieving maximum thrust. Using a genuine Lewmar-specified tunnel ensures no thruster power is wasted due to improperly sized or uneven tunnels.
• Longevity: The fiberglass material used (by Osculati/Lewmar) has excellent anti-osmotic properties and is formulated for continuous underwater use. The inside of the tunnel can be coated with anti-fouling paint if desired (to prevent marine growth during long periods in the water) without affecting its strength. Once installed correctly, the tunnel is essentially maintenance-free, aside from occasional cleaning or re-painting of anti-fouling. It is built to last the life of the vessel’s thruster system.