The History of Boat Prop Propellers

When Archimedes used a screw-shaped device to move water uphill, the water moved because there was less water than there was screw. It is unlikely that this Greek engineering genius foresaw that, two millennia later, his water screw would be turned inside out to drive the shipping industry and, a short century-and-a-half later, propel recreational boating from its status as a niche market to a major industry.

History

English inventor Francis P. Smith was testing a model of a screw-drive vessel on a lake when the screw snapped, leaving only a bit of the screw at the stern which drove the boat faster than the hull-length screw. Smith received a patent for his "device for propelling vessels by means of a screw revolving beneath the water at the stern" in 1836 and the basic design, except for materials and a few modern elements like variable blades, has remained unchanged.

Function

The function of a boat propeller is simple: to push against the water and drag the boat along with it, just like a screw driving into wood.

Types

Propellers can have variable-pitch blades or fixed-pitch blades and be made of brass, stainless steel or aluminum. The most common arrangements for recreational boats have three, four or five blades.

Size

Boat propellers are described by the size of the circle through which their blades turn and by their pitch, the distance a single rotation of the propeller will move a vessel forward through the water. A "12/23" propeller will have a 12-inch diameter and move the boat forward 23-inches.

Considerations

Four or five-blade props can be used to modify a boat's speed and capabilities. Most commercial vessels have four-blade propellers for peak fuel efficiency and, on vessels where "quiet" operation is needed (such as submarines), propellers may have eight or more blades.

Resources

• Boat Propellers