Slipwise: Boat Prop Calculator

Slipwise is a boat prop calculator and GPS speed log. It works out prop slip, tells you whether your full-throttle RPM is in range, and suggests the next pitch to try, then keeps a record so you can compare props on your own boat. Prop charts and web calculators stop at a number. Slipwise measures the speed for you and remembers it. PROP SLIP CALCULATOR • Slip from speed, RPM, gear ratio and pitch, in mph, knots or km/h • A plain reading for your kind of boat, with the usual range shown • Flags figures that can't be right, such as a boat faster than its prop IS YOUR PROP RIGHT? • Enter your engine's full-throttle range and see if RPM lands in it • Over-propped or under-propped, with the pitch to try and the RPM to expect • Once you've run two pitches, advice uses your boat's own RPM per inch instead of a rule of thumb GPS TOP SPEED, DONE PROPERLY • A large, steady speed readout that doesn't jump around • Top speed must hold for 3 seconds, so one lucky reading can't set it • Run both ways and the app averages them, cancelling a steady current or wind • Time to plane, for comparing props on your boat • GPS stops the moment you press Stop PROP TEST LOG • Every boat and prop, with size typed as stamped: 14 1/4 x 21 • Runs with people, fuel, water and notes • Compare props side by side: best speed, RPM, slip, time to plane • RPM sweeps with speed, slip and fuel curves, and your best cruise picked out • PDF report and spreadsheet export, plus backup and restore SEVEN CALCULATORS Prop slip, pitch change, pitch for a target speed, RPM and speed table, gear ratio (find it by counting turns or teeth), power and weight, and fuel range with a reserve. KNOW ITS LIMITS Slipwise reads speed, not RPM: you read that from your tach. Typical slip ranges and RPM per inch are rules of thumb, and sources disagree. The formulas are checked by hand and the GPS maths on simulated runs, but the app hasn't been tried on the water yet, so compare it with your own gauges at first. Your engine maker's RPM range and a good prop shop come first. Watch the water, not the phone. Have a passenger read it, and only run where it's clear and legal. Your boats and runs stay on your phone, with no account. Location is used only during a run you start, and position is never stored. Ads show on the main lists, never on the speed screen, and an optional one-time purchase removes them.