Inboard & shaft drive
Inboard props, solved from the engine curve.
Diesel engine, reduction gear and hull worked out together, so the prop loads the engine to its rated rpm at full throttle. The job Boatdiesel's calculators used to do, with the data built in.
Volvo Penta D2-75 · MS25L 2.74:1 · 34 ft sailboat
Illustrative demo · the recommended prop meets the engine inside the rated rangeCurrent 17 × 12Recommended 17 × 10.5Engine
Inputs and outputs
Everything a prop shop would ask, in one profile.
You enter
Picked from the database
Enginemake, model · rated kW and rpm
Gearboxmodel · reduction ratio · rotation
Hulltype · LWL · displacement · beam
Current propD × P · blades · RH / LH
Sea trialWOT rpm and speed · optional
You get
Free verdict · Pro spec
Verdictover / underpropped · rpm vs rated
Size rangediameter and pitch · free
Exact specD × P · blades · EAR · Pro
Curvesspeed, fuel, range at every rpm · Pro
Checkscavitation · blade loading · shaft · gear · Pro
Database
The engines and gearboxes people actually search for.
Rated power and rpm, torque curves where published, compatible gearboxes and ratios. Built entry by entry from public spec sheets, with the source on every engine page.
EngineVolvo PentaD1-30 · D2-40 · D2-75 · D4 · D6
EngineYanmar3YM30 · 4JH45 · 4JH57 · 6LY
EngineBeta MarineBeta 25 · 43 · 60
EngineNanniN4.38 · N4.60
EngineCumminsQSB 6.7 · QSL9
EnginePerkinsM92B · M135
EngineJohn Deere4045 · 6068 · 6090
EngineCaterpillarC7.1 · C9.3 · C18
GearboxZF / HurthZF 25 · ZF 63 · ZF 85
GearboxTwin DiscMG series
GearboxTechnodriveTM / TMC series
GearboxBorg WarnerVelvet Drive
Repower
New engine, old boat: check the ratio and the prop first.
Repower checkIllustrative
Old40 kW · 3,600 rpm2.0:1 · 17 × 11
New55 kW · 3,000 rpmratio to choose
| Ratio | Prop | WOT rpm | Speed | Note |
|---|---|---|---|---|
| 2.27:1 | 19 × 13 | ≈ 2,920 | 7.3 kn | Smaller prop, more tip clearance |
| 2.74:1 | 21 × 15 | ≈ 2,950 | 7.4 kn | Recommended · best speed |
| 3.00:1 | 22 × 16 | ≈ 2,960 | 7.4 kn | Aperture limit reached |
A new engine with a lower rated rpm, more torque or a different gearbox almost never wants the old prop. The repower calculator compares every available ratio, sizes the prop for each and shows the speed you can expect, before the engine is ordered.
- Every ratio the gearbox offers, side by side
- Prop sized per ratio up to the aperture limit
- Shaft diameter and bearing spacing for the new power
- Saved to the customer's boat, ready for the report
Popular
Propeller size by engine.
Each engine page carries its specs, ratios and the sea trials logged for it, with the calculator prefilled.