What size bow thruster should I choose for my boat?

Choose a bow thruster by its rated thrust in kilogram-force (kgf), using your boat's windage and proposed tunnel position to work out the turning force required. Boat length gives a starting range, but a tall motorboat or a tunnel mounted well aft usually needs more thrust than a low-profile sailing yacht of the same length.

Assess windage and berthing conditions

Check the boat's side profile above the waterline, including the cabin, flybridge, sprayhood and cockpit enclosure. Wind pushes against the whole area. Displacement alone is a poor guide: a relatively light flybridge cruiser can need more thrust than a heavier yacht.

Use these broad ranges to shortlist thrusters for conventional cruising boats with a reasonably effective tunnel position:

• 25 to 30ft: approximately 25 to 40kgf.
• 30 to 35ft: approximately 40 to 60kgf.
• 35 to 40ft: approximately 55 to 80kgf.
• 40 to 45ft: approximately 75 to 100kgf.
• 45 to 50ft: approximately 100 to 140kgf.

These are screening figures, not installation specifications. For a 36ft cruising yacht, I would normally investigate the 55 to 80kgf class before settling on a 40kgf unit, particularly with an enclosed cockpit or an exposed berth.

Ask the supplier to size the thruster for a stated crosswind requirement. Wind force rises approximately with the square of wind speed: holding position in 20 knots requires roughly four times the wind-countering force needed in 10 knots. A recommendation based on length alone cannot establish that capability.

Check the tunnel position

The thruster's turning effect depends on its distance from the boat's effective turning centre. Moving it forward increases that distance, producing more turning effect from the same thrust. For example, 60kgf acting through a 5m lever produces the same turning moment as 75kgf through a 4m lever.

Inspect the forepeak before ordering. Water tanks, bunks, bulkheads, hull shape and access for servicing may prevent the ideal installation.

• Position: fit as far forward as the hull and required immersion allow.
• Immersion: Lewmar's guidance calls for one tunnel diameter below the waterline, with 0.75 diameter as the minimum; use the installation drawing to identify the measurement points.
• Tunnel length: Lewmar suggests at least twice the tunnel diameter.

Insufficient immersion lets air into the tunnel, sharply reducing useful thrust. Poor tunnel entrances and obstructed flow also reduce performance. A larger motor cannot reliably compensate for a tunnel that gulps air.