Sleipner Bow Thruster Tunnel — Installation Manual

Sailboats and Water Deflection

• Many sailboats have a flat, shallow bow section. Fit the tunnel halfway into the underside of the hull and fair it with a deflector/spoiler that directs the flow around it – used by some of the largest sailboat builders, with little to no speed loss under normal cruising.
• A flat back face of the tunnel facing the flow creates drag. Best fix: a recess in the hull at the back of the tunnel, normally angled slightly down.
• Alternative: a deflector/spoiler in front of and under the tunnel – easiest by keeping part of the lower forward tunnel during installation and moulding a soft curve from the hull.
• Without deflection the propeller can freewheel and make noise under sail, and water forced through the tunnel at speed or in waves can damage the thruster. Above about 25 knots the spoiler must protect from the front and from below.
• Rule of thumb: standing in front of the boat looking aft, you should not see the back face of the tunnel.

Rounded Tunnel Ends

• Round the tunnel-to-hull edge as much as possible, minimum radius 10% of the tunnel diameter (R = 0,1 x D). Steel/aluminium hulls: a slope of 10-15% of the diameter works similarly.
• Sharp ends create turbulence/cavitation that blocks the outer tunnel area, cuts thrust and adds noise.
• Rounded ends let the thruster draw water along the hull side, creating a vacuum that pulls the boat sideways: in optimal installations 30 - 40% extra thrust.
• A Sleipner propeller does not cavitate at working speed: cavitation noise in the tunnel comes from the tunnel installation.

Installing a GRP Tunnel

1. Find the position from this manual and your thruster's measurements; mark the tunnel centre on both sides of the hull and drill a horizontal hole through.
2. Mark the circle for the tunnel opening (tunnel outside diameter) and cut the hole (a special tool is available).
3. Grind the gel coat back to real fibreglass in a sufficient area inside and outside the hull.
4. Insert the tunnel and mark its shape to the hull (leave front/underside material if you will make a deflector).
5. Cut the tunnel ends to shape, sand lightly and clean with acetone or similar. Do not add fibreglass where the thruster will sit.
6. Bond the tunnel to the inside of the hull with sufficient layers, preferably alternating mat and roving; fill all gaps completely with resin/fibreglass.
7. Round the outer edges to 10% of the diameter, laminate additional layers outside over a sufficient area, and do the same for any deflector.
8. Apply a waterproof coating on all ground and moulded areas. Original Sleipner tunnels (GRP) are fully waterproof except where cut and bonded.

General guidance only: problems from faulty tunnel installation are the installer's responsibility. Consult a naval architect before modifying the hull, and never modify the tunnel where the motor bracket sits – misfit can lead to gearleg failure.