What info is needed to size a propeller?

The optimum size propeller for your boat – whether fixed, folding or feathering – is calculated from the following inputs:

• boat displacement (ton/lb)
• boat length at waterline (LWL) in (ft/m)
• engine horsepower (hp/kw)
• engine crankshaft maximum revolutions per minute (RPM)
• gear box ratio
• shaft rotation LH or RH

Displacement and Waterline

A sailboat’s displacement and waterline determines it’s hull speed (approximately √LWL x 1.34) which is factored into the calculation needed to move the boat through the water. 

Hull Speed

Hull speed is the speed at which a boat begins to climb it’s own bow wave, essentially going uphill. While hull speed can be exceeded, the result is fuel inefficiency and engine wear.  Displacement hull speed, whether under sail or motoring (not planing or light displacement hulls) is calculated by the formula: velocity in knots = 1.35 x the square root of the waterline length in feet. 

Given the engine data and flat calm conditions, a boat will get to hull speed with the correct size and pitched propeller at a specified RPM.  Increase the pitch and thus increased thrust, the boat will get to hull speed at a lower RPM (approx. 400-600 less). When motor sailing, depending on the wind speed, direction and seas, with lower pitch, hull speed or faster should be reached at a lower RPM.

Hp and RPM

To achieve displacement hull speed sailboat manufacturers will specify an engine based upon a rule of thumb “power to weight” ratio of 4 to 6 hp per ton.  The hp and torque produced from the engine crankshaft rotating is expressed as a maximum (or rated) RPM and a suggested RPM to run continuously of between 60 to 80 percent of rated RPM.  The engine’s output goes nowhere until a optimum propeller is installed on the propeller shaft and does the work to absorb and transmit the torque into thrust.  The engine turns the propeller shaft via the transmission or gear box. 

Gearbox (Transmission) Ratio

The gear box in most marine diesel engines is a reduction gearbox, which makes the propshaft turn slower than the engine (crankshaft), reducing the speed of the propeller shaft to produce the appropriate thrust to move your boat through the water.  The gear box ratio determines the rotation speed of the propeller in revolutions per minute (rpm):

          Propeller speed (srpm) = Engine rated rpm ÷ Gearbox reduction ratio

The higher the gear ratio the slower the speed of the propeller shaft (number of propeller’s revolutions), the bigger the propeller. The lower the gear ratio the higher the speed of the propeller shaft (number of propeller’s revolutions), the smaller the propeller.  Engine manufacturers offer several different gear ratios with their engines.  Sailboat propeller shaft speed is usually targeted to be in the 900-1600 shaft RPM range depending on hp and displacement.

How do I determine my shaft rotation?

When standing aft and looking forward, which way does the prop rotate when motoring in forward gear?