A nozzle can boost bollard pull by 30% — but it costs you top speed. Here’s when to fit one.

What a nozzle is

A propeller nozzle (or Kort nozzle) is a short duct around the propeller. By accelerating water onto the blades it increases thrust at low speeds and reduces propeller tip losses. On a trawler or tug, that can mean 20–30% more bollard pull.

The physics

At zero or low speed, the nozzle acts as a lifting aerofoil, generating a forward thrust itself. At high speed, drag on the nozzle exceeds any thrust it adds — so the nozzle slows the vessel.

Who benefits

• Prawn and demersal trawlers at tow.
• Harbour tugs.
• Workboats pulling heavy loads.
• Push-boats on rivers.

Who does not benefit

• Planing hulls at speed.
• Pleasure cruisers.
• Any vessel whose top speed is a priority.

Nozzle fitting

Nozzles are typically welded steel, mounted to a dedicated foundation welded to the hull. The propeller runs with a tip clearance of about 1% of propeller diameter. Install is a shipwright job on a slipway.

Need advice on your boat’s stern gear or propeller? Call Brian directly on 0413 771 090 or use the contact form — we respond the same day.