What is tip clearance and why is it important?

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

Tip clearance enables free water flow to the propeller from ahead and free passage of water aft as it leaves the propeller which is essential for efficiency and smooth operation.  A propeller blade has a pulse of water coming off it. The pulse dissipates into the water but above the blade the pulse hits the hull shape, usually “sliding” up the hull creating a rhythmic thumping (a 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 and therefore the blades may not have attached water as they pass in front of the deadwood.

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

In an aperture where there may be minimal tip clearance, it is important that the top and bottom of the aperture are radiused and faired so that the pulse of water off the blades dissipates easily, i.e. doesn’t bounce back. Where space fore and aft is minimal, it is important that the trailing edges be faired and angled/cut back and 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.  Since smaller diameters mean lower efficiency, you are faced with a trade-off between the increase in efficiency and thrust from a larger diameter propeller and the improved water flow to the propeller and the reduced vibration from a greater tip clearance. For small sailboats with relatively slow RPM this is less an issue.