# Boat Propeller Calculator

Boat propeller calculator. Size a prop pitch from target boat speed, engine RPM, gear ratio, and slip. Works for outboard, sterndrive, inboard, and pontoon setups with a built-in sizing chart.

## What this calculates

Propping a boat right is the cheapest way to gain speed and fuel economy. The correct propeller lets the engine reach its rated WOT RPM at the rated boat speed without lugging or over-revving. This boat propeller calculator works two ways: sizing mode takes a target boat speed and returns the pitch you should buy; prediction mode takes your current pitch and returns the boat speed you should see. Both modes use engine RPM, gear ratio, and propeller slip as inputs and handle outboard, sterndrive, inboard, pontoon, and heavy-cruiser drive types.

## Inputs

- **Calculation Mode** — options: Size propeller pitch from target speed, Predict boat speed from existing prop — Sizing mode tells you what pitch to buy. Speed mode predicts the boat's WOT speed from current setup.
- **Engine RPM at WOT** (RPM) — min 0 — Wide-open throttle engine RPM. Should land in the manufacturer's recommended range at WOT.
- **Gear Ratio** — min 0.5 — Lower unit gear reduction (outboard / sterndrive) or inboard reduction. Typical outboards 1.83-2.33.
- **Target Boat Speed** (mph) — min 0 — Target WOT boat speed. Used only in sizing mode.
- **Propeller Pitch** (in) — min 0 — Theoretical distance the prop advances in one revolution. Used only in speed-prediction mode.
- **Propeller Slip** (%) — min 0, max 50 — Typical slip: bass boat 8-12%, runabout 10-15%, pontoon 15-25%, inboard boat 20-30%.
- **Hull / Drive Type** — options: Outboard planing hull (10% slip), Sterndrive planing hull (12% slip), Inboard boat (25% slip typical), Pontoon / barge (20% slip), Heavy displacement cruiser (30% slip) — Overrides the slip percent field with a typical value for the hull type. Set slip manually above to use a specific number.

## Outputs

- **Recommended Propeller Pitch** (in) — Propeller pitch (inches) needed to hit target speed.
- **Recommended Propeller Pitch** (mm) — Propeller pitch in millimeters.
- **Boat Speed at WOT** (mph) — Predicted WOT boat speed in miles per hour.
- **Boat Speed** (knots) — Predicted boat speed in knots.
- **Propeller Shaft RPM** (RPM) — Engine RPM divided by gear ratio.
- **Theoretical Speed (zero slip)** (mph) — Speed with no slip, for reference.

## Details

The boat propeller formula

Propeller pitch is the theoretical distance the prop advances in one revolution. Actual advance is less because the prop slips in the water. So:

  - Theoretical speed (mph) = (prop RPM x pitch_in) / 1056 (1056 converts in/min to mph)

  - Actual speed = theoretical speed x (1 - slip)

  - Prop pitch (in) = target speed x 1056 / (prop RPM x (1 - slip)) in sizing mode

Prop RPM = engine RPM / gear ratio. For a 5,400 RPM outboard with 2.0 gear ratio, prop RPM = 2,700. At 21 inch pitch with 10 percent slip: theoretical = 53.7 mph, actual = 48.3 mph. This boat propeller sizing calculator math is unchanged whether you work in inches or millimeters.

Boat propeller size chart

A boat propeller size chart starts with target WOT speed and engine RPM and lists the pitch that lands in the target range. Common combinations:

  
    Boat typeEngine RPM at WOTGear ratioTypical pitchTypical speed
  
  
    Bass boat (200 HP)58001.8723-2562-70 mph
    Pontoon (115 HP)55002.3311-1318-25 mph
    Runabout (150 HP)55002.0717-1938-45 mph
    Inboard ski boat42001.5014-1632-40 mph
    Offshore cruiser42001.6519-2132-38 mph
  

Propeller slip by hull type

Slip is the percentage difference between theoretical and actual boat speed. Light boats with clean props slip less; heavy boats or those pushing a lot of water slip more.

  - Planing outboard boat: 8-12 percent

  - Sterndrive planing hull: 10-15 percent

  - Inboard boat propeller calculator typical: 20-30 percent (shafted inboards have higher slip)

  - Pontoon or barge: 15-25 percent

  - Heavy displacement cruiser: 25-35 percent

  - Model boat propeller size calculator: 15-25 percent for typical RC models

Using the sizing calculator

A boat propeller sizing calculator like this one starts with three numbers you pull from the engine spec sheet: rated WOT RPM range, gear ratio, and hull type. Pick a target speed from the cruise or WOT figure you want. The calculator returns a prop pitch. Subtract 1 inch for the next-smaller stock size if the math lands between whole inches. An inboard boat propeller calculator setup uses 25 percent slip instead of 10 percent, so a 21 inch pitch outboard would be a 16-17 inch pitch inboard for similar speed.

Boat propeller calculator app logic

A boat propeller calculator app usually needs three screens: engine specs, hull slip, and target speed. This one folds all three into a single form. The sizing output is a starting point. Real-world validation requires a WOT test after installing the prop. If engine RPM lands below the rated WOT range, the prop is over-pitched (too tall a pitch) and the engine is lugging. If it lands above, the prop is under-pitched and the engine is over-revving. Change pitch by one inch increments until the engine hits the middle of its WOT RPM band.

## Frequently Asked Questions

**Q: How do I calculate the right propeller for my boat?**

A: Start with the engine WOT RPM range from the manufacturer (e.g., 5,000-6,000 RPM) and target the middle of that range. Pitch (in) = target speed (mph) x 1056 / (prop RPM x (1 - slip)), where prop RPM = engine RPM / gear ratio. This boat propeller calculator does the math. For 5,400 engine RPM, 2.0 gear, 10 percent slip, 45 mph target: pitch = 45 x 1056 / (2700 x 0.9) = 19.6 inches, so buy a 19 or 20 inch pitch prop.

**Q: What is propeller slip?**

A: Slip is the percentage difference between the prop's theoretical advance per revolution and its actual advance through the water. Planing boats slip 8-15 percent, pontoons 15-25 percent, inboards 20-30 percent, and heavy displacement hulls 25-35 percent. Low slip does not always mean a better prop; some slip is normal and driven by hull and load.

**Q: How is an inboard boat propeller different from an outboard?**

A: An inboard boat propeller calculator uses higher slip values (25-30 percent vs 10 percent for outboards) and lower prop RPM because the reduction ratio is smaller (typical 1.5 vs 2.0). Inboard shafts also introduce angle losses. For the same boat speed, an inboard prop is typically 3-5 inches smaller in pitch than an outboard equivalent at the same engine RPM.

**Q: Can I read a boat propeller size chart from the output?**

A: Yes. The body has a boat propeller size chart for common hull types and engine sizes. Compare your target speed and engine RPM against the chart, then fine-tune with this boat propeller sizing calculator. The calculator handles the exact pitch; the chart is for rough category sizing (bass boat, pontoon, inboard, etc.).

**Q: How does this work for a model boat propeller?**

A: The math is identical. A model boat propeller size calculator uses the same pitch-to-speed equation, only with small pitches (1-4 inches) and very high RPM (20,000-40,000 for RC gas or electric). Gear ratio is usually 1:1 direct-drive. Slip for RC models is 15-25 percent. Plug in the numbers; the formula does not care about scale.

**Q: Why does my boat not hit the calculated speed?**

A: Three common reasons. One, actual engine RPM at WOT is below the rated value because the prop is over-pitched (reduce pitch by one inch). Two, slip is higher than the baseline because the boat is overloaded, bottom is dirty, or prop is damaged. Three, hull is not on plane and you need more weight forward or trim tabs adjusted. A working-order boat propeller calculator app result requires all three to be correct.

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Source: https://vastcalc.com/calculators/physics/boat-propeller
Category: Physics
Last updated: 2026-04-08