Boat Repower Converter

The Boat Repower Converter converts horsepower, torque, and fuel consumption between metric and imperial, estimating range, running costs, and installation feasibility.

What Is a Boat Repower Converter?

A Boat Repower Converter is a planning tool that estimates the engine power and propeller setup needed to reach your speed and range goals. It uses proven naval architecture relationships to relate weight, hull form, speed, and propeller slip to horsepower. The tool helps you decide between engine options, check propeller pitch and gear ratio, and forecast fuel use at cruise.

“Repower” covers both replacing a worn engine and upgrading for better performance or efficiency. The converter bridges specs from engine brochures with what your hull can actually do on the water. It also provides confidence checks, so you can see when the numbers are unrealistic or when more data is needed.

How to Use Boat Repower (Step by Step)

Start with your hull type, weight on the water, and a realistic cruise speed. Then add engine and propeller details such as maximum RPM, gear ratio, and pitch. You can fine-tune environmental and efficiency factors if you have them.

• Choose hull type: planing, semi-displacement, or displacement.
• Enter wet weight (boat, fuel, gear, crew) and length at waterline.
• Set target cruise and top speed, in knots or miles per hour.
• Provide current or proposed engine data: max RPM, gear ratio, fuel type, and rated power.
• Enter propeller pitch and expected slip, or let the tool estimate slip by hull type.
• Pick options for altitude, propeller efficiency, and a safety margin.

When you calculate, the converter returns required shaft horsepower, suggested engine size, prop pitch checks, and fuel burn. Review the notes section for cautions about sea state, fouling, and load swings. Adjust your inputs until the numbers align with real-world expectations for your boat class.

Boat Repower Formulas & Derivations

The converter combines simple, industry-tested relationships to estimate power, speed, thrust, and fuel burn. These are approximations meant for decision support. They are calibrated by hull type and usage, not exact hydrodynamic models.

• Planing speed and power (Crouch’s formula): Speed (mph) = C × sqrt(HPs/W), where HPs is shaft horsepower and W is boat weight in pounds. Rearranged: HPs = W × (V/C)². Typical C values: 150–170 for heavy deep-V, 170–190 for efficient planing hulls.
• Displacement hull speed: Hull speed (knots) ≈ 1.34 × sqrt(LWL in feet). Near hull speed, power often rises sharply with speed due to wave-making drag.
• Admiralty coefficient for displacement boats: AC = Δ^(2/3) × V³ / HPs, where Δ is displacement in long tons and V is knots. Rearranged: HPs = Δ^(2/3) × V³ / AC. For small trawlers and cruisers, AC ≈ 15–25 is common.
• Propeller speed check with slip: Speed (mph) ≈ (RPM_engine / Gear) × Pitch_in × (1 − Slip) / 1056. Convert to knots by dividing mph by 1.15078. Slip is expressed as a fraction (for example, 0.12 for 12%).
• Thrust and shaft power: P_shaft ≈ T × V_water / ηp, so thrust T ≈ P_shaft × ηp / V_water. Here ηp is propeller efficiency (often 0.55–0.70 for small craft).
• Fuel burn from power: GPH ≈ (HP_engine × BSFC) / Fuel_density. Typical BSFC values: gas ~0.50 lb/hp·hr, diesel ~0.40 lb/hp·hr. Fuel density: gasoline ~6.1 lb/gal, diesel ~7.1 lb/gal.

For planing hulls, the Crouch constant C is the main calibration knob. For displacement hulls, the Admiralty coefficient AC serves that role. Propeller RPM and slip provide a cross-check to ensure the pitch and gear ratio produce the requested speed within the engine’s RPM band.

Inputs and Assumptions for Boat Repower

Good inputs produce reliable estimates. The converter works best when you know your real wet weight, your intended speed, and your engine’s rated RPM and gear ratio. The following inputs drive the core calculations.

• Hull type and length at waterline (LWL), used for hull speed and calibration.
• Wet weight (boat, fuel, water, crew, and gear) in pounds or kilograms.
• Target cruise speed and intended top speed, in knots or mph.
• Engine data: rated power, max RPM, fuel type, and gearbox ratio.
• Propeller parameters: pitch, diameter limits, and expected slip.
• Efficiency factors and conditions: propeller efficiency, altitude, and a safety margin.

The converter assumes clean hull, correct engine tune, and moderate sea state. Very heavy load, strong headwinds, heavy seas, or marine growth can raise power demand sharply. If you are near hull speed in a displacement boat, small speed increases can require much more power.

Step-by-Step: Use the Boat Repower Converter

Here’s a concise overview before we dive into the key points:

1. Select your hull type and enter LWL.
2. Enter wet weight and confirm units.
3. Set target cruise and top speed.
4. Add engine max RPM, gear ratio, and rated power.
5. Enter propeller pitch and expected slip, or choose estimate.
6. Pick fuel type, BSFC, and any altitude or efficiency options.

These points provide quick orientation—use them alongside the full explanations in this page.

Real-World Examples

Planing center console, 22 ft, wet weight 3,000 lb, target cruise 25 knots. Choose Crouch constant C = 160 for a moderate deep-V. Convert cruise to mph: 25 kn ≈ 28.8 mph. Required shaft HP: HPs = 3,000 × (28.8/160)² ≈ 97 hp. With a 5% transmission loss, engine power ≈ 102 hp at cruise. Add a 20–30% reserve for weather and aging, suggesting a 130–150 hp outboard. Prop check: 4,200 RPM, 2.00 gear, 19 in pitch, 12% slip gives speed ≈ 33.3 mph (29.0 kn), so 25 kn cruise occurs near 3,600–3,800 RPM. Fuel burn (gas, BSFC 0.50): 102 × 0.50 / 6.1 ≈ 8.4 gph, giving about 3.0 nmpg at cruise. What this means: a 140–150 hp repower will comfortably meet the goal with reasonable economy and headroom.

Displacement trawler, 30 ft, LWL 28 ft, wet weight 10,000 lb. Hull speed ≈ 1.34 × sqrt(28) ≈ 7.1 kn. Target cruise 6.5 kn is near hull speed. Displacement in long tons: Δ = 10,000/2,240 ≈ 4.46 LT. Δ^(2/3) ≈ 2.71. Using AC = 20: HPs = 2.71 × 6.5³ / 20 ≈ 37 hp. Diesel fuel burn (BSFC 0.40): 37 × 0.40 / 7.1 ≈ 2.1 gph. A 60–80 hp diesel offers reserve for current, weather, and alternator loads while cruising around 40–60% power. What this means: a modest diesel in the 60–80 hp range is appropriate, with efficient cruise at 6–7 knots.

Limits of the Boat Repower Approach

These estimates guide decisions, but they cannot capture every factor on the water. Hull fairness, trim angle, sea state, and loading can change results. Use sea trials and manufacturer data to validate final selections.

• Planing constants and slip vary widely between hulls and props.
• Displacement power near hull speed is sensitive to small errors in drag.
• Altitude, temperature, and fuel quality affect available power and BSFC.
• Cavitation and ventilation limits may cap usable pitch or RPM.
• No allowance is made for auxiliary loads like hydraulics or large alternators unless added.

Treat the converter as a first-pass estimate. Confirm with a marine technician, propeller shop, or a sea-trial log before you buy.

Units & Conversions

Units matter because speed, power, and propeller pitch appear in different systems. Consistent units prevent large errors. The table below covers common conversions used in repower planning.

Pick your source unit and multiply by the factor to get the target unit. Keep your inputs consistent. For example, if you enter speed in knots, keep all speed targets in knots.

Tips If Results Look Off

If the estimates appear too high or low, start with the basics. Check your wet weight, hull type selection, and whether pitch and slip are reasonable for your setup. Then revisit efficiency and altitude.

• Try C = 150–170 for heavy deep-V planing hulls, 170–190 for lighter planing hulls.
• Use slip 10–15% for well-matched planing props, 15–25% for semi-displacement, higher at low speeds.
• Apply a 20–30% safety margin for real seas, growth, and aging.
• Confirm gear ratio and max RPM from the engine plate, not memory.

Finally, compare with similar boats’ published performance. If you are far outside those ranges, recheck inputs or consult a propeller specialist.

FAQ about Boat Repower Converter

How accurate are these horsepower estimates?

They are ballpark figures for planning, usually within 10–25% if inputs are solid and the hull type is correctly selected. Always verify with sea trials.

Should I size the engine to my cruise or top speed?

Size the engine so cruise occurs near the middle of the power band with reserve for weather. Top speed is secondary in most repowers.

What is propeller slip and why does it matter?

Slip is the percentage the prop “falls short” of its theoretical advance per revolution. It ties RPM, pitch, and speed together and helps catch mismatches.

Do I need different numbers for diesel vs gasoline?

The power estimates are similar, but diesel usually has lower BSFC and better torque at low RPM. Use the correct fuel density and BSFC in the tool.

Key Terms in Boat Repower

Wet Weight

Total running weight, including hull, engine, fuel, water, crew, and gear. It is the most important driver of required power.

Hull Speed

The speed where a displacement hull’s wave-making resistance rises sharply. Approximated by 1.34 × sqrt(LWL in feet), measured in knots.

Gear Ratio

The reduction between engine and propeller RPM. A 2.00 ratio means the prop turns once for every two engine revolutions.

Propeller Pitch

The theoretical forward distance a propeller moves in one revolution, with no slip. Actual speed is lower due to slip.

Slip

The difference between theoretical and actual propeller advance, expressed as a fraction or percent. It varies with hull, speed, and load.

Shaft Horsepower (HPs)

Power delivered to the propeller shaft, after transmission losses. It is lower than engine brake horsepower at the flywheel.

BSFC

Brake specific fuel consumption, the fuel mass used per horsepower-hour. Lower BSFC means better engine efficiency.

Admiralty Coefficient

An empirical constant relating displacement, speed, and horsepower for displacement hulls. Useful for estimating power near hull speed.

References

Here’s a concise overview before we dive into the key points:

• Hull speed explained (Wikipedia)
• Brake specific fuel consumption overview (Wikipedia)
• Discussion of Crouch’s constant and values (BoatDesign.net)
• Propeller basics and slip (BoatUS)
• ABYC marine safety standards overview
• Admiralty coefficient and powering basics (U.S. Army training text)

These points provide quick orientation—use them alongside the full explanations in this page.