INCREASING YOUR PROPELLER’S EFFICIENCY

Typically, a minimum space of about 10% of the prop diameter is needed between the hull and the tip of the propeller blade for a displacement boat; for a high-speed planning craft the clearance should be 15 to 20 % or more. Ideally, a boat’s props should be ‘fine-tuned’ to suit the boat’s hull shape and displacement. A quick way to determine prop’s performance is to listen to the engine. If the engine cannot get to its maximum rpm, the prop may be over-pitched or oversized in terms of diameter or aggregate blade area-meaning that it is like trying to start a stopped car using the third gear.
If the engine reaches its maximum rpm easily and may even over revolution, but the boat doesn’t get to its maximum speed, the prop might either be too small or under-pitched. This is like driving down the highway in first gear. Other characteristics indicate other problems to an experienced prop mechanic. If a boat is suspected not running at its optimum, have it checked out by an experienced prop shop, which markets, repairs and tunes propellers.
If decision is made to get a new propeller, first take a number of dimensions from the boat, starting with the distance from the shaft centerline to the hull bottom at the fore-and-aft center of the intended propeller location. This dimension will tell the largest diameter prop that can be fitted to the boat. The displacement of the boat should also be known, ideally, boat condition when it’s in sailing condition, known as half-load condition in naval architecture. This means the tanks are half full, half of the stores are onboard, and all the crew is onboard.
The maximum rated horsepower are also needed, in addition to the maximum rpm speed of the engine, the reduction gear ratio, the diameter of the prop shaft and whether the prop is to run clockwise or counterclockwise.
When a prop comes from the manufacturer, it is pitched reasonably accurately but not fine-tuned. The fine-tuning is done by the prop shop along with any additional needed work such as cupping and balancing. This is to make sure to get the right prop for the boat.