Gearbox Ratio
What is the Gearbox Ratio and why is it important?
Most marine diesel engines have a reduction gearbox, which makes the shaft turn slower than the engine, at a speed more suitable for the propeller. Engine RPM (revolutions per minute) is the number of full turns or rotations of the engine crankshaft at a given throttle setting. Shaft RPM is the number of rotations the propeller makes in a single minute, called shaft RPM or SRPM. The propeller shaft RPM is reduced down according to a given ratio in the transmission or gearbox. The typical marine diesel is around 2000 RPM, a speed too fast for a propeller to be efficient.
How to Determine the Gearbox Ratio?
The best method to get the actual ratio is to use your cell phone camera and take a photo of the plate on the transmission stamped on the top or side of the gearbox. It is expressed as a ratio or number (example: 2.64:1 or A–2.64).
If there is no tag, it will be necessary to use a digital tach lite – https://www.amazon.com/AGPtek%C2%AE-Professional-Digital-Tachometer-Contact/dp/B004Q8L894/ref=zg_bs_15729791_1. Also available at Autoparts and Harbor Freight.
How do I calculate shaft speed with gearbox ratio?
With the gear box ratio, calculate the rotation speed of the propeller in revolutions per minute (RPM):
Propeller speed (SRPM) = Engine rated RPM ÷ Gearbox reduction ratio
The larger the ratio, the slower the shaft speed, the larger the prop diameter giving more thrust and more speed. The smaller the ratio, the faster the shaft speed the smaller the propeller diameter giving less thrust and less speed.
Marine engines may have several different gearbox ratio options depending on the shaft angle. Choose the best ratio for optimum thrust.
What is the relationship between Diameter, RPM and Pitch?
Diameter, revolutions per minute and pitch are the three most significant factors affecting propeller performance and efficiency.
An increase in pitch allows the propeller to grip a larger amount of water putting more load on the engine which reduces its RPM. Likewise a decrease in pitch reduces grip in the water lessening the load on the engine which increases its RPM.
A small increase in diameter dramatically increases thrust and torque load on the engine and shaft.
Relatively high maximum shaft RPM reduces the optimum diameter propeller due to giving a higher shaft RPM therefore higher blade loading.
It is more important to keep the blade pressure as low as possible and you can sacrifice higher blade tip speeds as long as you have low blade pressure.
Ideally propeller blade pitch should adjust with the forces and load (engine torque and sea state) acting upon them.
A propeller is sized to absorb / transmit the full engine power at a specific engine speed. The maximum power output from an engine will always be at the maximum rated speed. Engine manufacturers require that propellers be properly sized to absorb the engines’ hp to reach its specified RPM. The propeller size must allow the engine to reach this “propping RPM” in order to be approved for use on its engines.
In some cases where customers wish to limit engine speed and noise, propellers are sometimes sized to the engine cruising speeds, meaning that can be slightly ‘over-propped’ and the engines will never achieve their maximum RPM in gear.
What is meant by the propeller “getting the shaft”?
Occurs when there is a mismatch of the propeller size to the output of the engine, and reduction ratio. When sizing a propeller, start with the largest diameter propeller that can fit, see Tip Clearance, then choose the gearbox to suit this diameter. These factors could even determine the choice of engine, based on the engine RPM and available gearbox ratios.