Cowls for Ø250mm stern tunnel

We recommend installing the thruster as deep as possible and at least 3/4x the tunnel diameter below the waterline (eg 300mm x 0,75 = 225mm minimum).

On some boats this might be impossible due to the positioning of other equipment onboard and for the lack of better options, it might only be possible to get the top of the tunnel ½ x the tunnel diameter below the waterline. Notice though that this increase the risk of cavitation.

For some boat types, external or retractable thruster models might be a better solution if it is difficult to place the tunnel deep enough. 

We always recommend to seek the advise of experienced installers for optimal results. 

A solution is adding cowls (sold separately) to your stern tunnel to prevent water cavitation. The cowls are perfect for deflecting water away from obstacles such as stern drives, bathing platform stanchions, trim-tabs, or other transom fittings.

Please note that adding cowls reduces the actual thrust by a few percent, so it is not recommended to add them if not needed. 

The thruster should be positioned as far forward as possible. This is important as the relative distance from the boats pivot point to the thruster installation point will change the actual thrust of the boat. The further away from the pivot point of the boat – the larger the thrust.

The thruster shall be positioned as deep as possible for two reasons;

• So that it does not suck air from the surface, causing cavitation which will significantly reduce thrust and create noise
• To increase the water pressure so that the propeller has a firm substance to work with for maximum performance. We recommend at least ¾ x the tunnel diameter below the surface and more is better. It is never recommended to go less than ½ x the tunnel diameter below the water surface.

In reality it is often a compromise between how far forward you want the thruster to maximise the force arm from the pivot point of the boat – and how deep you can position the thruster.

It is also a good advice on high speed boats to consider that the tunnels can come out of the water at high speed, but as slamming loads can be overcome by working with the tunnel installation to deflect the water.