Screw Propeller

Ericsson claims spiral plates on hoops, two of them turning opposite ways at different speeds, and wholly submerged. Write those three points down — you will test each one.

Cut a 60 mm disc from thin aluminium sheet. Mark four equal quarters from the centre — these become the blades.

Cut in along each mark to the centre, then twist all four blades the same way to about 30° off flat. Measure the angle with the protractor — equal on all four.

Push a dowel through the disc centre and fix it. The blades must sit square to the shaft so the screw runs true.

Cut a simple 200 mm hull from balsa. Notch the stern so the shaft can run through it and the screw sits behind and below the hull.

Hook a rubber band from the shaft to a peg at the bow. Winding the screw twists the band; letting go unwinds it and spins the screw.

Float the boat in a basin with the screw completely under water. Wind 30 turns, release, and time the run over 500 mm.

Pack the stern up so the screw runs half in air. Same 30 turns. It slows and throws spray: a half-submerged screw wastes its work. That is why the claim says wholly submerged.

Make a second screw twisted to about 15°. Run both. Note which is quicker — too flat barely bites, too steep stalls the band.

Sprinkle a pinch of pepper on the water behind the running screw. The wake rotates — that spin is energy going round instead of backward.

Hold the hull loosely and release the band. The hull tries to roll the opposite way to the screw. One screw twists the ship as well as pushing it.

Turn a second screw by hand the opposite way just behind the first. Its blades meet the swirling wake head-on. Two counter-rotating screws cancel both the swirl and the roll — Ericsson's central claim.

Tape a flat card paddle to the shaft instead and run it half-submerged. It slaps, sprays and pushes the bow off line — the problem the screw was built to solve.

The patent. US 588, granted 1 February 1838 to John Ericsson, of London, England. Ericsson claims spiral plates carried on metallic hoops, a pair of them running in opposite directions at different velocities, wholly immersed, giving greater propelling force from a smaller propeller than earlier designs.

📌 The title depends where you look, and that is worth knowing. Google Patents records this patent as "Steam" — an artefact of how these very early US patents were catalogued under broad headings. The drawing sheet itself is headed "J. Ericsson — Screw Propeller — No 588 — Patented Feb 1, 1838". When a catalogue record and the document disagree, the document wins. The specification pages are the same story: they describe hoops, spiral plates, contrary directions and unequal velocities, and state the invention is useful "for ships of war or merchant vessels".

Ericsson did not invent the screw propeller, and it matters to say so. The idea of a helix driving a vessel is ancient — the Archimedes screw is its ancestor, and by the 1830s several people were converging on it. Francis Pettit Smith patented a screw propeller in Britain in 1836, weeks before Ericsson's own British patent of the same year. Ericsson's document claims a particular form: the hooped, contra-rotating, fully submerged arrangement. Read the claims, not the legend — a patent tells you what one person claimed, not who was first.

What the counter-rotation is for. A single screw leaves a spinning wake (step 10) and twists the hull the other way (step 11). Both are wasted engine power. A second screw turning the opposite way straightens the wake and cancels the reaction torque. Contra-rotating propellers are still used today on torpedoes and some ships for exactly this reason — though most vessels settle for one screw, because two shafts running through one stern is expensive and awkward to maintain.

What happened next. Ericsson demonstrated a screw-driven launch, the Francis B. Ogden, on the Thames in 1837; the Admiralty was famously unimpressed. He moved to the United States, and his screw drive went into the USS Princeton (1843), the first screw-propelled warship, with the engines below the waterline where shot could not reach them — the naval argument that finally killed the paddle wheel. Ericsson was Swedish by birth, from Värmland, and is better remembered by most people for a later ship entirely: the ironclad Monitor of 1862.

Where it sits. Every propeller-driven ship afloat is doing what your balsa hull did — biting a fixed pitch of water per turn and shoving it astern. The engineering since 1838 has been about blade shape, cavitation and efficiency, not about the principle. The principle you just measured with a protractor and a rubber band.