When designing a propulsion system, one of the most critical determinations is the size of the propeller. Now, for formal quoting, we have our own, proprietary software that performs highly detailed calculations that properly determine the gearbox ratio and the size of the propeller.
However, there is a “quick and dirty” method that can get you a reasonable guess on the spot. This method is related to the Froude Number.
William Froude was an English engineer who got his start in designing early railroads. He later moved on to designing a more efficient ship’s hull. The Froude number is one of many formulas he discovered while working on small-scale tests that could reliably predict hull behaviour in various sea states.
The Froude Number is a formula that calculates the resistance of a partially submerged object (a hull) moving through water:
F=V/LWL^0.5
Generally, the higher F, the smaller the gear box ratio will need to be. This is a starting point in determining propeller revolutions and propeller diameter necessary to achieve a certain speed.
The selection of the correct gearbox ratio is of premium importance.The revolutions at propellers will determine the propeller diameter and the size of the propulsion to be used. The thrust and by consequence the performance of the boat is a direct function of the gearbox ratio selection. The gear ratio to select is based on the following parameters.
•The engine revolutions (rpm)
•The power/weight ratio (hp/tons)
•The speed of advance or the Froude Number ( V/ LWL^0.5)
For V/ LWL^0.5> 1 and SHP > 100
the following formula can be used to start calculation.
•V= Boat speed in Knots
•LWL= length waterline in feet
•P = Total power on board
•W= Weight of the boat full load (tons)
•RPM = Engine revolutions per min.
Optimum Gear Ratio: GEAR RATIO =1.45 e^(0.0034*x)
Where
•X = RPM / ( P / W )
