Propeller School reference
Every term, plainly defined.
Forty-plus words the propeller world throws around, each defined in plain language with the engineering note where it matters. Bookmark it — the rest of the site links back here.
- Anode (sacrificial)
- A block of more-reactive metal (usually zinc or aluminium) that corrodes so your propeller and shaft don't.201 · Different waters want different alloys — zinc for salt, aluminium for brackish, magnesium for fresh.IonGuard makes anodes serviceable afloat →
- Aperture
- The opening between hull, keel and rudder that the propeller lives in. Its size caps the maximum propeller diameter.Why aperture limits sizing →
- Apparent slip
- The difference between how far a propeller would advance through a solid (pitch × revs) and how far the boat actually travels.201 · Some slip is necessary — it's how blades generate thrust. Bruntons engineers prefer to talk about efficiency.
- Blade area ratio (BAR)
- Total blade area as a fraction of the propeller disc. More area grips more water but drags more under sail.
- Blade face / back
- The face is the high-pressure side (facing aft when driving ahead); the back is the suction side — the side that does most of the lifting, like the top of a wing.
- Boss (hub)
- The central body of the propeller that carries the blades and mounts on the shaft taper or saildrive spline.
- Camber
- The curvature of a blade section from leading to trailing edge. Cambered sections generate lift efficiently — flat blades paddle, cambered blades fly.
- Cavitation
- Water boiling into vapour bubbles on the blade at low pressure, which collapse violently — noise, vibration and pitting damage.201 · Not the same as ventilation. Tip unloading and adequate blade area are the design cures.How Varifold's tip unloading fights it →
- Constant-velocity (CV) coupling
- A joint that transmits smooth rotation through an angle. SigmaDrive uses hardened ball races to absorb up to 3° of misalignment.SigmaDrive explained →
- Cup
- A slight lip on a blade's trailing edge that adds effective pitch and bite — common on high-speed props.
- Cutlass bearing
- The water-lubricated rubber bearing supporting the shaft near the propeller. Uneven wear is a classic sign of misalignment.
- Deadwood
- The solid structural timber or hull section ahead of the propeller on traditional long-keel boats. Heavy deadwood disturbs water flow into the prop.201 · The honest screening question before an Autoprop — turbulent inflow upsets self-pitching blades.
- Diameter
- The circle the blade tips draw — the single most important propeller dimension. Set by engine power and shaft speed, capped by aperture and tip clearance.
- Displacement speed
- The speed range where a hull pushes through the water rather than planing over it — capped by hull speed.
- Drive type
- How power reaches the water: conventional shaft, saildrive leg, or sterndrive. Determines which propeller hub fits.
- Feathering propeller
- Blades rotate edge-on to the flow when sailing, cutting drag. Conventional featherers use flat blades and gears; the Autoprop feathers and finds its own pitch.The self-pitching difference →
- Folding propeller
- Blades fold back against the hub under sail — the lowest drag of any type. The Varifold opens ahead or astern using inertia and hydrodynamic force.Varifold folding propellers →
- Fouling
- Marine growth on the blades. Wrecks efficiency on any prop and defeats the geometry of low-drag designs — a fouled self-pitching blade pitches wrong.Prop care in Australian waters →
- Galvanic corrosion
- Electro-chemical attack when dissimilar metals sit in seawater — the less noble metal corrodes. Why anodes exist and bronze alloys matter.
- Gearbox (reduction) ratio
- How many engine revolutions per shaft revolution — 2.5:1 means the shaft turns at 40% of engine speed. The number owners most often guess wrong.201 · It's stamped on the gearbox plate. Sizing uses shaft speed, so a wrong ratio sizes the wrong propeller.
- Hand (rotation)
- Viewed from astern in ahead gear: right-handed props turn clockwise, left-handed anti-clockwise. Twin installations run handed pairs for symmetric thrust.
- Helical pitch
- Pitch that follows a true helix so every blade section advances together — the geometry that separates engineered props from cast approximations.
- Hull speed
- The practical speed limit of a displacement hull: 1.34 × √LWL (in feet, giving knots). Beyond it, the power required climbs almost vertically.Find yours with the explorer →
- LWL (waterline length)
- The hull's length at the waterline — the number that sets hull speed and drives the speed predictions in our tools.
- Motor-sailing
- Sails and engine together. As the sails take load, a fixed prop's pitch becomes wrong; a self-pitching blade coarsens to use the free power.Motor-sailing, explained →
- Nibral / NAB
- Nickel-aluminium-bronze — a premium propeller alloy: stronger and more corrosion-resistant than manganese bronze. Every Varifold is machined from it.
- Over-pitched
- Too much pitch for the boat: the engine can't reach rated RPM, lugs like a car starting uphill in fourth, and may smoke black under load.Diagnose it with the WOT test →
- Pitch
- How far the propeller would screw forward in one revolution through a solid — the boat's gearing. A fixed prop has exactly one; an Autoprop finds its own continuously.
- Pitch distribution
- How pitch varies from root to tip on a well-designed blade. Reducing pitch at the tips (tip unloading) cuts cavitation and pressure pulses.
- Pressure pulse
- The thump each passing blade sends against the hull above. The cause of drumming aft-cabin noise; eased by tip clearance and skewed, unloaded blades.
- Prop walk
- The sideways kick of the stern in gear, strongest astern — caused by asymmetric blade thrust. Identical-face-astern designs like Autoprop reduce it.Using prop walk in a marina →
- Rake
- The fore-aft lean of the blades relative to the hub axis. Aft rake buys tip clearance under a hull.
- Regeneration
- Under sail, a freewheeling propeller can drive an electric motor as a generator — up to 1 kW documented on the Eco*Star.Eco*Star regeneration →
- Saildrive
- An integrated leg through the hull bottom (Volvo, Yanmar and others) instead of a shaft. Needs spline-fit propellers and its own anode discipline.Saildrive vs shaft →
- Self-pitching
- Blades pivot freely and balance engine torque against water pressure, finding the correct pitch continuously and automatically. The Autoprop is the world's only fully self-pitching propeller.
- Shaft taper
- The standardised cone at the propeller end of the shaft (SAE or metric) that the hub seats on. Identifying yours is step one of any refit.
- Skew
- The swept-back curve of a blade outline. High skew lets the blade enter the wake gradually — quieter, smoother running.
- Slip
- See apparent slip — and measure yours with the calculator.The slip & efficiency calculator →
- Stray current corrosion
- Corrosion driven by electrical leakage — from your boat or the marina berth next door. Faster and nastier than galvanic attack; the $40,000 bow-thruster lesson.Monitoring beats guessing →
- Tip clearance
- The gap between blade tip and hull — wants to be 10–15% of diameter. Too tight and every blade pass drums on the hull overhead.
- Ventilation
- Air drawn down from the surface into the propeller — the surging over-rev in a chop or tight turn. Often misdiagnosed as cavitation.
- WOT (wide-open throttle) test
- The one diagnostic every owner should run: full throttle, 60 seconds, note max RPM against rated — reveals over- or under-pitching immediately.Run the WOT test →
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Put the vocabulary to work — the Builder speaks plain English and the engineers translate the rest.