A brief overview of Marine Transmission and Propeller Ratios

Marine engines are powerful, constant load engines. However, they need more than just power to get your boat moving. Unlike car engines, they do not have a number of gears to cycle through. Additionally, boats require a marine transmission, shaft, and propeller. These critical parts transfer power from the engine. This combination provides the thrust that propels the vessel through the water. The term gearbox ratio describes the relationship between these parts. Propeller pitch describes the distance a propeller can travel in one revolution.

The marine transmission or “gearbox” is the critical link between the engine, running gear, and propeller. Most marine transmissions are of a reduction type. This means they convert the engine’s quick rotations into slower rotations that are better suited for driving the propeller. Engine “RPM” (Revolutions Per Minute), is how many revolutions the engine is spinning over per minute.

Gearbox Ratio

The gearbox ratio refers to the ratio between input and output revolutions. For example, a gearbox ratio of 2.51:1 means that for every 2.51 input revolutions from the engine, there will be 1 output revolution to the propeller.

The following formula depicts this relationship nicely:

Gearbox ratio = propeller RPM divided by engine RPM

Lastly, propellers are specified by diameter and pitch. Propeller pitch is the theoretical distance a propeller can travel through the water in one revolution.

Another factor to consider is propeller “slip”. Propeller slip is the water slipping through the blades of the propeller instead of moving forward. The easiest analogy is to think of screwing a self taper in to a piece of wood. As the screw grabs, it moves forward and secures itself into the piece of wood. If the self taper slips, the screw becomes useless. This is the same with a propeller. Ideally the propeller grips the water, and propels the boat forward. Because water is not completely solid, there is always some level of “slip” and hence has to be accounted for in propeller and gearbox ratio calculations.

Slip can be managed by different propeller styles. Common designs include 3, 4 and 5 blade propellers (although not limited to). Each extra blade means more surface area, producing less slip and more “grab”.

Duo-props again increase propulsive surface area, and being counter-rotating have additional benefits such as negating transverse thrust or “propeller walk” in single engine/driveline configurations. The only negative to more propeller blades is generally you have a lower top speed as there is more drag.

Practical application

Let’s look at a basic example: if the engine is running at 2000 RPM and the gearbox ratio is 2:1, the output revolutions to the propeller will be 1000 RPM. Add in the pitch of the propeller (the distance the propeller travels through the water in one revolution of the shaft) – let’s say it’s 20″ (less 10% factor for slippage = 18”) – then the total distance/speed travelled would be 18,000” or 0.247 nautical mile per minute meaning we would be travelling at 14.82 knots.

By changing the propeller pitch by just one inch, you can increase or decrease the top speed and the total theoretical distance travelled.

• Increased pitch means slower acceleration and higher top speed. 
• Decreased pitch means increased acceleration and lower top speed. 

Decreasing the propeller pitch by just 1” to 19” the total theoretical distance travelled would be 0.235 nautical mile at a speed of 14.1 knots, giving a faster top speed. By decreasing the propeller pitch the top speed of the boat lessens however it would achieve the new top speed slightly faster and with increased torque. For instance, increasing the pitch to 21″ would increase the theoretical distance travelled to 0.259 nautical miles at a speed of 15.54 knots, 

The goal is to find the right balance. Torque, acceleration, and speed need to be suited to the application. This allows the engine to operate in its optimal range and provides for a comfortable and safe boating experience.

Some Final Considerations

It’s also important to note that the engine’s rated RPM is a critical factor in measuring propeller pitch. Propeller pitch adjustments can be made to cater to different loads and vessel purposes, but the engine must still be able to reach its rated RPM (for diesels) or recommended RPM range (for petrols). Operating within these guidelines reduces undue stresses and internal wear to your engine.

If you’ve made changes to your boat, like adding extra fuel or water tanks, installing a tender to the stern, or planning to use the boat at maximum capacity for most of the time, you may need to consider adjusting the propeller pitch to improve performance. It almost goes without saying but critical to optimisation is matching engine and propeller size. Consider any changes carefully. You can use an online calculator to calculate ratios, pitch and speed here.

Keep in mind that wind, currents, and tides can also impact the calculated distance travelled by the propeller, and clutch or propeller slip can affect performance as well.

About the Author

Brendan is a dual certified Marine Surveyor & Engineer, who completed his trade at well respected marine engineering company based on the Hawkesbury River. After relocating his family to the Gold Coast, he joined the BoatBuy team. Brendan is an expert in his field, and completes over 200 inspections per year. Liked this article? Feel free to email me with any boating related questions you might have here. You can also contact the team at BoatBuy here.