Repower Calculator

Repower Calculator

Uses the old engine's sea trial to learn how the hull and the old prop behave, then predicts top speed with the new engine and sizes the gear ratio and prop to let it reach rated rpm. These are estimates to plan a repower. Confirm with a sea trial and adjust pitch.

Start from Examples only. Replace every number with the boat's own.
Boat
As it was for the sea trial: fuel, water, gear, people
Old engine & sea trial
Fills in power and rpm. Not listed? Type them below.
Engine rpm at full throttle on the trial
GPS, averaged both ways
New engine
Fills in power and rpm from the maker's spec sheet. Not listed? Type them below.
Crankshaft hp at rated rpm
Usually rated rpm, or a little over when the boat is light
Or tap a row in the gear table
New engines and gears minus old. Lighter is negative.
With proper hull clearance. Blank = no limit
Blank = let it pick
Assumptions
0.96 typical inline gear; 0.94 V-drive
Only for blade loading. 0.50–0.60
150 cruisers, 190 fast runabouts
How this is calculated
  • Power: shaft hp = rated hp × gear and shaft efficiency. If the old engine didn't reach rated rpm, its power at the rpm it did reach is taken as rated × (rpm reached ÷ rated rpm), assuming roughly flat torque.
  • Speed, planing: Crouch, knots = C ÷ √(lb per shaft hp). With a sea trial, C is worked out from the old boat's actual speed, so the hull, bottom and running gear are built into it.
  • Speed, displacement: knots = K × √LWL ÷ ∛(lb per shaft hp), capped at Gerr's maximum speed-length ratio 8.26 ÷ (D/L)0.311. Past that cap, more power doesn't buy speed.
  • Slip: Gerr, slip = 1.4 ÷ knots0.57, scaled by how far the old prop's real slip was from that formula.
  • Pitch: pitch = 1215.2 × knots ÷ (shaft rpm × (1 − slip)).
  • Diameter, calibrated: the old prop shows how much power this hull takes at a given rpm, diameter and pitch. Power absorbed is taken as ∝ rpm³ × diameter³ × pitch² (torque rises roughly with the square of pitch-to-diameter). Diameter is solved so the new prop takes exactly the new engine's power at the target rpm. More blades absorb a little more (by blade area, square root).
  • Diameter, Gerr: 632.7 × shp0.2 ÷ shaft rpm0.6 for three blades, ×0.94 for four and ×0.89 for five. This is what an unrestricted, efficient prop would be. Real installs usually run smaller because of hull clearance.
  • Smaller than ideal: when the prop has to be smaller than calculated, pitch goes up to keep the same load: pitch × (ideal dia ÷ actual dia)1.5. Expect more slip and a little less speed.
  • Old prop on the new engine: holding the old prop's load curve, it finds where the new engine's full-throttle power meets what the prop absorbs through the new gear.
  • Blade loading: thrust ≈ 326 × prop efficiency × shp ÷ knots, spread over the blade area. It's compared with the old prop, which already ran on this hull: more than 15% heavier gets a caution, more than 35% a warning. With no old prop, over 10 psi gets a caution and over 12 a warning. Blade area uses typical ratios: 0.40, 0.55, 0.69 and 0.80 for two to five blades.
  • Not checked: shaft diameter, struts, cutless bearings, coupling and engine-bed strength for the new torque. Check those separately on every repower.
Predicted top speed
0.0kts
Prop for the new engine
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Old prop on the new engine

Gear ratio options

Same boat and engine, other gears. The star marks the slowest shaft (biggest prop) that still fits with a normal pitch. Tap a row to use that gear.

Old vs. new

Summary for the job

Top speed0.0 kts—