Selecting a propeller for an electric boat

A propeller that worked perfectly behind a 30 hp diesel will almost always be wrong behind a 15 kW electric motor. Electric motors deliver flat torque from near-zero RPM and run most efficiently at lower shaft speeds than a diesel. The prop has to match that — and most stock diesel propellers are too small in diameter and too high in pitch to do so.

Get the propeller right and the boat reaches hull speed at half the catalogue motor power. Get it wrong and you waste 30–40% of your battery on cavitation, slip, and noise.

This guide covers the seven selection criteria that matter for an electric build, the four propeller archetypes you'll consider, and which suppliers credibly cover each option.

→ The spec calculator suggests a starting prop diameter and pitch from your motor and hull data →

Why electric propellers differ from diesel

A small marine diesel turns 2 800–3 600 RPM at the crankshaft and drives the prop through a 2:1 to 3:1 reduction gearbox — so the propeller spins at 900–1 800 RPM. The diesel's torque rises sharply with RPM, so the prop is sized small enough that the engine can reach its sweet spot in the rev range.

An electric motor is the opposite. A typical 15 kW marine PMSM produces full torque from 100 RPM upward and reaches peak power between 1 500 and 3 000 motor RPM. With a 2:1 reduction, the prop turns at 750–1 500 RPM at peak power — but the motor is happy to drive it at 400–600 RPM at cruise. The prop is the limiting factor, not the motor.

The practical consequence: an electric install can swing a noticeably larger, slower prop than a diesel of similar continuous power. This is more efficient because larger, slower props have higher Froude propulsive efficiency at the speeds sailboats and small displacement craft actually run.

Rule of thumb: start with the diesel original, increase diameter by 10–15%, and reduce pitch by 5–15%. Then refine with manufacturer data.

The four propeller archetypes

Most cruising sailboat repowers end up with a feathering prop (Bruntons Autoprop, Max-Prop) or a folding prop (Gori, Flexofold). Variable-pitch (Oceanvolt ServoProp) is a paired motor+prop unit, not a swap-in propeller.

Selection criteria for electric propellers

1. Diameter — the single most important spec

Larger diameter at lower RPM is more efficient than smaller diameter at higher RPM, up to the limit of hull aperture clearance. The geometric limit on a saildrive is the leg's prop aperture; on a shaft drive it's the gap between hull and prop tip plus the rudder.

For a sailboat:

• Maintain at least 15% of prop diameter as tip-to-hull clearance. Less than this causes hull vibration and cavitation noise.
• Use the largest diameter that fits within that clearance.
• Going one size up over the diesel original is almost always correct on an electric install.

For a typical 12 t sailboat with a 16-inch (406 mm) diesel prop, an electric repower usually wants 17–18 inch (430–460 mm). On a saildrive the leg may already cap diameter — check the manufacturer's max prop size for that aperture.

2. Pitch — derive it from motor RPM at design speed

Pitch is how far the prop would advance per rotation if there were no slip. Real props slip 20–40% in displacement service. The basic equation:

Pitch (inches) = (target_speed_kn × 1215) / (prop_RPM × (1 − slip))

For a 12 t sailboat targeting 5.5 kn cruise on a motor that spins the prop at 600 RPM with 30% slip:

Pitch = (5.5 × 1215) / (600 × 0.7) = 15.9 inches

Most electric installs end up with 12–18 inch pitch on shaft installs, and 14–22 inch pitch on saildrives (which run faster prop RPM). This is lower than the diesel original on the same hull because the electric motor's flat torque allows you to harvest more thrust at lower RPM.

If your prop is adjustable in pitch (Max-Prop, Variprofile) you can fine-tune in the water — invaluable when first commissioning, where you tune for cruise speed first and live with reverse and high-throttle behaviour as compromises.

3. Number of blades

For an electric repower of a 30–45 ft sailboat, three blades is the right starting point. Move to four blades if you have a heavy displacement hull (>15 t for a 40-footer) or you specifically want regen — the extra blade improves both smooth thrust and regen output.

4. Drag under sail

The prop spends most of its life not propelling the boat. A fixed three-blade prop costs a sailboat 0.5–0.8 kn at every wind speed — roughly 1 nm per hour wasted on a typical coastal passage. Over a season, that's hundreds of miles of sailing time lost.

Folding and feathering props recover most of that. Folding props collapse against centrifugal-force-resisting springs when the shaft stops; feathering props pivot the blades parallel to the water flow.

Approximate drag rankings (3-blade, 16-inch prop):

• Fixed: 1.0× reference
• Feathering (Max-Prop, Bruntons Autoprop): 0.20–0.30×
• Folding (Gori, Flexofold): 0.10–0.20×

For a serious sailing boat, fold or feather. The cost premium (€2 000–4 000 over a fixed prop) recovers in seasons of additional sailing performance.

5. Regenerative capability

Regen happens when water flow drives the prop, which drives the motor as a generator. Three preconditions must all hold:

1. The motor + controller must be four-quadrant capable (most modern marine motor packages are; see the motor selection guide).
2. The propeller must keep its shape under sail — fixed and feathering props can; folding props can't (they collapse).
3. The pitch should be optimised for generation, not propulsion — a propulsion-pitched prop generates poorly because the blade angle of attack reverses below freewheel speed.

Variable-pitch props (Oceanvolt ServoProp) actively rotate the blades to a regen-optimal angle when the boat sails fast enough. This recovers 200–1 000 W under sail above 5 kn — meaningful on a long passage. Fixed-pitch and feathering props recover much less (50–200 W) because the pitch is a compromise between propulsion and regen.

If long-passage regen is on your spec, plan for either a variable-pitch saildrive (Oceanvolt ServoProp) or accept that regen will be a minor, opportunistic source of power.

6. Reverse thrust

Folding and feathering props differ sharply in reverse:

• Fixed: strongest reverse — the prop is the same shape both ways.
• Feathering (Max-Prop, Bruntons Autoprop): the blades rotate to a mirrored angle in reverse, giving 80–95% of forward thrust in reverse. Excellent.
• Folding (Gori, Flexofold): when reversed, the blades open into a position that's optimised for forward, so reverse is 30–60% of forward. Adequate for marina manoeuvres on a sailboat but not strong.

For single-handed marina handling, prefer feathering. For a fully-crewed boat where someone can step on a fender if the reverse is weak, folding is fine.

7. Material and corrosion

Marine props are typically:

• Manganese bronze — the marine standard. Strong, machinable, good corrosion resistance with sacrificial anodes. Used by Gori, Flexofold, Max-Prop, and most fixed-prop manufacturers.
• Nibral (nickel-aluminium-bronze) — higher strength and corrosion resistance, used on premium props (Bruntons Autoprop).
• Stainless — light and slightly more efficient (smoother surface) but expensive and prone to cavitation pitting if undersized. Used on smaller racing props.
• Composite — emerging, very quiet, limited torque capacity. Suitable for low-power electric outboards (≤6 kW) but not main propulsion at 15 kW+.

For 95% of electric sailboat installs, manganese bronze or Nibral is the right material. Both pair well with a sacrificial zinc or aluminium anode on the shaft.

A worked example: 12 t sailboat repower

System: 12 m sailboat, 12 t displacement, hull speed 7.5 kn, target cruise 5.5 kn. Original diesel prop: 16-inch diameter × 12-inch pitch fixed three-blade. New motor: 20 kW electric saildrive, 600 prop RPM at cruise.

Choice: Bruntons Autoprop H6 (3-blade self-pitching feathering, 17-inch diameter, Nibral). Self-adjusts pitch to motor torque, good reverse, low drag under sail, modest regen capability.

Alternative (lower cost, no regen needed): Gori 3-blade folding 17 × 16 in manganese bronze.

Supplier examples for marine propellers

The propellers below are the ones most commonly specified for electric sailboat repowers in 2026. Selection criteria above should drive your choice — pick the supplier whose product best matches your priorities (drag under sail vs. reverse strength vs. regen).

For the motor and controller that drive these props, see the motor and controller selection guide. For the battery the system runs from, the LiFePO₄ marine battery guide.

Common selection mistakes

A few errors come up repeatedly when sailors pick their first electric prop:

• Reusing the diesel prop. It's the single most common mistake. The diesel prop is undersized in diameter and over-pitched for an electric motor. You'll see 20–30% lower performance until you change it.
• Choosing folding for a single-handed sailboat. Folding props have weak reverse. If you need to back into a tight slip alone, prefer feathering.
• Buying a fixed prop "to save money". The drag penalty under sail (0.5–0.8 kn) costs you sailing performance every passage. The €2 000 premium for a folding or feathering prop recovers in saved time and saved battery.
• Spec'ing regen without a four-quadrant controller. The prop generates electricity that the controller can't accept. The system either doesn't regenerate or trips a fault. Check the controller selection criteria before paying extra for a regen-capable prop.
• Ignoring tip clearance. A larger prop is more efficient — until the tips are too close to the hull and you get vibration, cavitation, and a noisy boat. Maintain ≥15% diameter as clearance.

Putting it together

The shortest path to a working propeller selection:

1. Note the diesel original's diameter and pitch (record from the prop, not the manual — they sometimes differ).
2. Use the calculator to confirm continuous motor power and target cruise speed.
3. Increase diameter by 10–15% (subject to aperture / clearance limits).
4. Compute pitch from (target_kn × 1215) / (prop_RPM × (1 − 0.30)).
5. Pick fixed / folding / feathering / variable based on sailing priorities and reverse needs.
6. Pick blade count: 3 default, 4 for heavy displacement or regen focus.
7. Verify the motor controller is four-quadrant if you want regen.

Get those seven right and the propeller installation is straightforward.