A propeller that suited a diesel is rarely right for an electric motor. The electric drive turns the propeller at a different speed and delivers its torque differently, and pitch has to follow the propeller's speed. The surest way to waste battery is to keep the old propeller without checking it against the new drive's shaft speed.
Below: how to work out diameter and pitch from your drive's published shaft speed, the four propeller types, and the trade-offs between them.
→ The spec calculator suggests a starting prop diameter and pitch from your hull data →
Start from the drive's propeller speed
Pitch depends on how fast the propeller turns, so start with the speed your new drive turns it at. Electric drive makers publish this. Oceanvolt, for example:
| Drive | Propeller shaft speed |
|---|---|
| SD6 to SD15 saildrives | 1,140 rpm |
| ServoProp 10 and 15 | 1,140 rpm |
| AXC 12, 24 and 36 shaft motors | 950 rpm rated, 950–1,400 rpm suitable range |
That number goes straight into the pitch calculation below. A propeller pitched for a diesel turning its propeller at a different speed will be wrong for the electric drive even if the diameter fits.
Larger, slower propellers are generally more efficient than smaller, faster ones for the same thrust, because they accelerate a bigger mass of water by less. The limit is what fits: the saildrive leg, or the gap to the hull and rudder on a shaft installation.
The four propeller types
| Type | Under sail | In reverse | Hydrogeneration |
|---|---|---|---|
| Fixed pitch | Most drag | Strong | Some (more than folding) |
| Folding | Least drag: the blades fold shut | Weakest of the four | Least; only if the controller holds the blades open |
| Feathering | Low drag: the blades turn edge-on | Strong | Depends on the drive |
| Controllable pitch (Oceanvolt ServoProp) | Low drag in the neutral position | Strong | Highest |
Oceanvolt ranks generation in its hydrogeneration whitepaper. A folding prop makes the least: its controller spins the motor to open the blades and keeps them open. A fixed prop makes more, but "less than half the power of a ServoProp at a given boat speed". Oceanvolt gives the ServoProp's output as more than 1 kW at 7–8 knots and 3 kW at 11–12 knots. A controllable-pitch drive is a paired motor and propeller, not a propeller you can fit to another drive. The hydro regen guide covers the yields in more detail.
Most cruising sailboat repowers end up with a feathering prop (such as Max-Prop or Bruntons Autoprop) or a folding prop (such as Gori or Flexofold).
Diameter
Use the largest diameter that fits with the clearance your drive or propeller maker specifies. Too little tip clearance brings vibration and noise through the hull. On a saildrive, the leg maker sets a maximum propeller size; check it before you buy.
As a starting point, the calculator suggests 0.5–0.6 × waterline length in feet (in inches) for a monohull, and 0.4–0.5 × for each hull of a multihull. For the boats on our repower pages:
| Boat | Waterline | Suggested diameter |
|---|---|---|
| 30 ft sailboat | 8.2 m | 13–16 in |
| 35 ft sailboat | 9.5 m | 16–19 in |
| 40 ft sailboat | 11.0 m | 18–22 in |
These are starting points to discuss with the propeller maker, not a substitute for their calculation.
Pitch
Pitch is how far the propeller would advance in one turn if there were no slip. Displacement boats typically lose a large share of that to slip, and the calculator assumes 30% for a monohull and 20% for a multihull.
Pitch (inches) = (target_speed_kn × 1215) ÷ (prop_rpm × (1 − slip))
The 1215 converts knots to inches per minute. For the 35-footer cruising at 5.5 knots with 30% slip:
- on an Oceanvolt SD saildrive at 1,140 rpm: 5.5 × 1215 ÷ (1,140 × 0.7) = 8.4 in
- on an Oceanvolt AXC shaft motor at 950 rpm: 5.5 × 1215 ÷ (950 × 0.7) = 10.0 in
The same boat at the same speed needs 20% more pitch on the slower-turning drive. That's why the drive's shaft speed comes first. If your propeller's pitch can be adjusted, you can correct it after sea trials; tune for cruising speed first.
Number of blades
Three blades is the usual starting point for a sailboat auxiliary. Two blades fold or feather into the smallest package, for the least drag, but give less thrust and can vibrate more. Four blades give smoother thrust and more blade area for a heavy boat or a diameter-limited installation, at the cost of more drag and weight. Ask the propeller maker to run their sizing for your boat at two and three or three and four blades, and compare.
Drag under sail
The propeller spends most of its life not propelling the boat. A fixed propeller drags all the time. Folding blades close under the flow, and feathering blades turn edge-on to it. For a boat that sails often, fold or feather.
The trade-off is reverse. Feathering blades turn to present the same profile astern, so reverse is strong. Folding blades open astern into a shape designed for going forward, so reverse is weaker. That matters most when you're handling the boat in a marina alone.
Hydrogeneration
For the propeller to generate, three things must be true:
- The controller must support it. Not every motor controller can run the motor as a generator; see the motor and controller guide.
- The blades must present an angle to the flow. Fixed blades always do. Folding blades only do if the system holds them open, as Oceanvolt's does.
- The blade angle should suit generation. A blade pitched for propulsion is a compromise when generating, which is why a controllable-pitch drive generates far more.
Material
Most sailboat propellers are bronze alloys, and makers specify which one. Whatever the alloy, fit and maintain the shaft or saildrive anode the drive maker specifies, and don't mix metals on the shaft without checking compatibility.
A worked example: the 35-footer
Boat: 9.5 m waterline, 7.5 t, cruising at 5.5 knots. The calculator gives it an 8 kW motor. Take an Oceanvolt SD8 saildrive, which Oceanvolt lists for boats up to 12 m and 7,000 kg and which turns the propeller at 1,140 rpm.
| Spec | Reasoning | Starting point |
|---|---|---|
| Diameter | 0.5–0.6 × 31 ft waterline, within the leg's limit | 16–19 in, as the leg allows |
| Pitch | 5.5 × 1215 ÷ (1,140 × 0.7) | About 8.4 in |
| Blades | Sailboat auxiliary | 3 |
| Type | Sails often, often single-handed | Feathering, for low drag and strong reverse |
Take these figures to the propeller maker as the brief. They'll refine them for the blade shape they make.
Note that the SD8's 7,000 kg listing sits just below this boat's 7.5 t. When a boat is at or past a maker's listing, ask the maker before you buy.
Supplier examples for marine propellers
An electric motor delivers flat torque from low RPM, so propellers should typically be larger in diameter and lower in pitch than the diesel original. Folding and feathering props cut drag under sail; some feathering models support regeneration.
Self-pitching feathering propeller with three or four blades. Widely used on electric sailboat repowers because the blades automatically adjust pitch for any RPM — well-matched to a flat-torque electric motor.
Three-blade folding propeller with an overdrive position for efficient cruising. Very low drag under sail and a strong reverse, popular on European cruisers.
Two-, three-, or four-blade folding propellers built for sailboats. Custom-pitched per build and well-supported for saildrive and shaft installations.
Adjustable-pitch feathering propellers with three to five blades. Pitch can be re-set in the water, making them well-suited to electric installations where the optimal pitch differs from the original diesel.
Variable-pitch saildrive that switches between propulsion and regeneration modes via active blade-pitch control. Recovers 200–1000 W under sail above 5 kn.
Fixed-pitch and folding propellers matched to Volvo Penta saildrives, including OEM options for the SD Electric range.
Links go to manufacturer websites for reference.
For the motor and controller that drive these props, see the motor and controller selection guide. For the battery the system runs from, see the LiFePO₄ marine battery guide.
Mistakes to avoid
- Reusing the diesel propeller without checking it. Work out the pitch for the new drive's shaft speed. If it doesn't match, change the propeller.
- Choosing folding for single-handed marina work without trying the reverse. If you back into tight berths alone, feathering is the safer choice.
- Expecting regeneration from any propeller. It needs a controller that supports it and blades that stay open to the flow.
- Ignoring tip clearance. A bigger propeller is more efficient only until it's too close to the hull.
Sources
Checked on 7 November 2026:
- Oceanvolt — SD saildrives, ServoProp 10–15 kW and AXC 12–45 kW: propeller shaft speeds, power and boat ranges
- Oceanvolt — Hydrogeneration whitepaper, part 1: folding, fixed and ServoProp generation compared
- The calculator's sizing model: diameter rule, slip assumptions, motor size for the 35-footer
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