The thrust of the matter

The thrust of the matter

Like many of the best inventions it all started with Archimedes, the famous mathematician and engineer, had designed the largest ship ever, the 360-foot Syracusia. But he wasn’t looking for propulsion. No, with a ship of that size he wanted an effective bilge pump. So he designed the Archimedes screw, a screw-like blade in a cylinder that could be turned by hand to propel the bilge water up and over the gunwale

On twin-screw vessels, when cruising or in transit, you can set the blades on one propeller to slice through the water with as little resistance as possible. This means fuel consumption is cut in half, while only 20% of the speed is lost.

THE PROPELLER REVOLUTION

But it wasn’t until the Industrial Revolution and the invention of the steam engine that people successfully linked the Archimedes propulsion technique to this new power source. And that was almost entirely down to Francis Pettit Smith and John Ericsson, two self-taught engineers working independently. In 1836 they obtained separate patents for screw propellers, heralding the start of modern propeller development. However, both contemporary as well as later day experts have recognized John Ericsson’s solution for the propeller’s characteristic values, such as diameter, number of blades, area and shape, pitch and shaft speed. For the purposes of identifying a good design, these conditions have a very intimate connection on both empirical and theoretical grounds.

WINNING THE WAR

Ericsson favored two bladed wheels turning in different directions while Pettit Smith worked on a single screw-blade approach like Archimedes. In the American Civil War the final victory by the North over the South could partly be attributed to Ericsson’s invention, with the USS Monitor successfully upholding the blockade at Norfolk against the CCS Virginia. This was the world’s first battle between two ”ironclad” warships.

THE PROPELLER VERSUS THE PADDLE

Of course, even with a superior design to the paddle wheels of the day, it took a while to convince ship owners and navy admirals of the benefits – not even a tug-of-war between a paddle steamer and screw-propelled ship provided the ultimate proof (the screw won, pulling the other ship back at over 2 knots). Despite this success, it wasn’t until the end of the 19th century that screw propellers overtook paddle wheels for seagoing use.

This is how to express the thrust of a propeller. Or you can just take look over the stern.

MODERN HYDRODYNAMICS

So where does that leave us today? While still recognizable from the early designs, modern propellers have been tweaked and optimized over the years to squeeze the most efficiency and reliability out of them.

That starts with the blades, which through advanced hydrodynamic design can be optimized to increase power and fuel efficiency while reducing wear and downtime (good design can reduce the bubbles that damage the blade surface, also known as cavitation).

Nowadays you can also combine and position propellers to give the best performance and meet changing needs out at sea: from single and multiple screws to horizontally rotating azimuth thrusters and transverse thrusters that can maneuver the ship with the grace and precision of a dancer.

Transverse Thruster

 

 

 

 

 

Azimuth Thruster

CONTROLLABLE PITCH AND FEATHERING

There are also some pretty nifty tricks you can perform with controllable pitch propellers, where the pitch or angle of the blades can be altered to create certain effects. From the traditional full ahead position, you can angle the blades and switch seamlessly into full astern, without changing the direction of the propeller shaft. And on twin and multiple-screw vessels, you can take engines and their propellers offline, setting the blades to slice through the water with as little resistance as possible. This “feathering” means that you can maximize the efficiency of the active engine and propeller when you’re cruising or in transit. This may mean a speed reduction of say 20%, but on the other hand it would mean you’d use half as much fuel!

CUSTOMIZATION IS KEY

The roots of Caterpillar Propulsion go back over a century – not far from those early pioneers and their Archimedes designs. But it’s clear that things have come a long way since then: The science of hydrodynamics and marine engineering certainly, but mostly the needs of operators out at sea. Great demands are placed on modern-day propulsion solutions, from raw power and performance to high uptime and low cost.

The key nowadays is to get a solution that is truly customized, right down to a micron of tolerance in each blade.

The principle of feathering

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