Propellers underwater are shaped differently

How density and cavitation change blade design.

Propellers underwater are shaped differently from propellers in air because pushing against a fluid vastly denser than air lets a much smaller blade produce the same thrust, and that same density is exactly what makes an underwater blade tear the water itself apart if it is ever pushed too hard, so marine propellers end up broader, slower turning, and far more conservatively shaped than an aircraft propeller doing a comparable job.

I once hauled the outboard on a small boat out of the water at the end of a season and found the trailing edges of both blades stippled with tiny craters, pitted like the surface of a golf ball, although the boat had never come anywhere near a rock or a mooring chain. Nothing had physically struck the metal, and yet it looked eaten away in a pattern far too regular to be ordinary corrosion.

Introduction and overview

Water's far greater density than air means a marine propeller does not need to sweep a large area or spin quickly to grab enough fluid to generate useful thrust, unlike an aircraft propeller, which has to be large in diameter and turn fast simply because air offers so little resistance per unit of blade area. That single difference in what the fluid itself is made of drives almost everything else about how the two kinds of propeller end up looking: marine blades are typically fewer in number, broader across their width, and shaped with noticeably thicker leading edges than the thin, efficient sections an aircraft propeller can get away with. Diameter is also limited underwater in a way it rarely is in the air, since a blade tip spinning fast enough eventually reaches a speed at which the water on its low-pressure face can no longer stay liquid, a failure the next article covers in full but which already shapes almost every dimension discussed here. A submarine or an underwater vehicle adds a further constraint an aircraft never has to consider, since the same spinning blade is also a source of noise and vibration travelling through a fluid that carries sound extraordinarily well, so a marine propeller's shape is often as much a negotiation with quietness as it is with raw thrust, a theme the last article in this set returns to directly.

The table-tennis-bat comparison

Pushing a table-tennis bat edge-on through a swimming pool moves barely at all before the whole flat face is shoving a solid slug of water in front of it, while waving the same bat through open air produces almost no resistance at all no matter how fast it is swept. That difference in how hard the two fluids push back per unit of area is precisely why a marine propeller can afford to be small and slow-turning where an aircraft propeller cannot, since a modest blade dragged through water is already doing the work that would take a much larger, faster-spinning blade to accomplish in air. A marine propeller's blade also behaves, in cross section, like a small aerofoil turned on its side, generating thrust from a pressure difference between its two faces under Bernoulli's principle rather than by simply shoving water backward, which is the same mechanism a paddle or an oar exploits on every stroke, only spinning continuously here rather than sweeping back and forth.

One figure worth keeping in mind

A large ship's propeller commonly turns at only a few hundred revolutions per minute, where a light aircraft propeller of a broadly similar diameter spins at ten times that rate or more, and the gap is not a matter of one being better engineered than the other, it is simply how much less speed water needs to generate the same thrust that air only yields at a much higher rate of rotation. Pushed toward aircraft-style speeds, the same marine blade would not produce proportionally more thrust, it would instead start losing thrust past a certain point as the water on its trailing face began to give way rather than keep pushing back, and a wider blade sharing the load across more surface area is one of the simplest ways of holding that limit further off for a given amount of thrust.

What this changes in practice

Because a marine blade's low-pressure face is always closer to the point where the water itself will fail than an aircraft blade's ever needs to be, marine propellers are shaped with that limit built in from the start rather than treated as an afterthought. Thicker leading edges spread the same load across more material and keep the peak low-pressure region less severe than a thin, sharply cambered aircraft-style section would produce at the same thrust. Fewer, broader blades reduce the amount of trailing turbulence one blade leaves for the next one to churn through, which matters far more in a fluid dense enough to carry that disturbance forward with real force. Rake and skew, the blade curving backward and twisting along its own length, are used specifically to stagger the moment each section of a blade reaches its highest local speed, spreading out rather than concentrating the exact conditions the next article explains in detail. Even the number of blades is chosen with this in mind, since a hull's own vibration modes and a blade's own load-shedding both respond to how many pulses of pressure sweep past a given point on the hull every single turn, which is why the count settled on for a given vessel is rarely a round number chosen for convenience.

Where this stops being true

A propeller spun slowly and loaded lightly, on a small vehicle that never approaches the speeds where the water itself becomes the limiting factor, can be shaped much closer to a simple scaled-down aircraft propeller, since the specific problem driving thicker sections, fewer blades, and generous skew only bites once a blade is pushed hard enough to actually risk it. It is the propellers doing real, sustained work against real speed, the ones on a working boat, a submarine, or anything expected to move with purpose rather than drift, where the shape described here stops being an option and becomes the only one that survives.