What is tip clearance and why is it important?

Tip Clearance Measurement

Tip clearance is the distance from the tip of the propeller blades to any part of the hull.  Sailboat tip clearance is approximately 10% of the propeller diameter, depending upon hull shape above and below the propeller blades.

What does tip clearance affect?

Tip clearance enables free water flow to the propeller from ahead and free passage of water aft as it leaves the propeller. This is essential for efficiency and smooth operation.  A propeller blade has a pulse of water coming off it. The pulse dissipates into the water. However, above the blade the pulse hits the hull. It will “slide” up the hull, creating a rhythmic thump/vibration every time the propeller blade passes by the hull. Obstructions to the propeller blades – such as blunt, square edge deadwood, skeg or strut – will create wasteful, turbulent eddies ahead of the propeller. This prevents the blades from having attached water as they pass in front of the deadwood.

Tip clearance, diameter and RPM

The slower the shaft RPM and the lower the boat speed, the lower minimum the tip clearance can be.

In an aperture with minimal tip clearance, the top and bottom of the aperture should be radiused and faired. This ensures that the pulse of water off the blades dissipates easily, i.e. doesn’t bounce back. Where space fore and aft is minimal, the trailing edges should be faired and angled away as thin as practical in a smooth, gently rounded curve from the propeller to help with water flow.

With a typical saildrive leg the maximum diameter is 19-20” depending on the leg placement and the hull shape.

Additional tip clearance is usually obtained at the cost of propeller diameter.  Smaller diameters mean lower efficiency. Increased efficiency and thrust derives from a larger diameter propeller. Improved water flow and reduced vibration derives from greater tip clearance. This is the trade-off. For small sailboats with relatively slow RPM this is less an issue.