Slipwise: Boat Prop Calculator

A boat prop calculator that measures your top speed by GPS, works out slip, and tells you which pitch to try next.

Coming to Google PlayFinished and waiting for its Google Play listing.

Free
Works offline
No account
No subscription

App languages: English · Español · Français · Deutsch

Slipwise helps you find out whether your prop suits your boat. Enter speed, RPM, gear ratio and pitch and it works out slip. Add your engine's full-throttle range and it says whether RPM is in range, and which pitch to try if it isn't.

It can measure the speed too, by GPS, and keeps every run so you can compare props on your own boat instead of guessing from a chart.

Screenshots

Why we built it

Prop calculators stop at a number. A slip figure alone doesn't say what to do. Slipwise reads it for your kind of boat, checks RPM against your engine's range and suggests the next pitch.
GPS speed apps jump around. The readout is smoothed, and a top speed only counts if it was held for 3 seconds, so one lucky reading can't set it.
A current can flatter a run. GPS reads speed over the ground. Run both ways and Slipwise averages the two, which cancels a steady current or wind, and shows how much it was.
Prop tests get forgotten. Every prop and run is kept with the people, fuel and water that day, so a comparison next season still means something.

Features

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.

How it works

Slip compares how far the prop's pitch would push the boat with how far it really went. Theoretical speed is engine RPM divided by the gear ratio, times pitch, divided by 1056 to give mph. Slip is the shortfall as a percentage.

For a GPS run, press Start before you throttle up, hold full throttle for at least 5 seconds, and press Stop. Read RPM from your tach and enter it. Then run back the other way for a two-direction average.

Pitch advice aims for near the top of your engine's full-throttle range. It starts from a rule of thumb of about 150 to 250 RPM per inch of pitch, and switches to your boat's own figure once you've run two different pitches.

Honest limits

Slipwise hasn't been tried on the water yet. The formulas are checked by hand and the GPS maths on simulated runs, so compare it with your own gauges at first.
It reads speed, not RPM: that comes from your tach. Typical slip ranges and RPM per inch are rules of thumb from retailers and forums, and sources disagree. Your engine maker's RPM range and a good prop shop come first.
Phone GPS gives about one reading a second and lags a little, so time to plane is for comparing props on one boat, not a stopwatch figure.

Questions

What is a good prop slip percentage?

Often about 10 to 20% at top speed, lower on light fast hulls, but sources disagree. Slip alone doesn't make a prop good or bad: full-throttle RPM decides that.

How do I calculate prop slip?

Divide engine RPM by the gear ratio, multiply by pitch and divide by 1056 for theoretical mph. Slip is one minus actual speed over theoretical speed, as a percentage. Slipwise does this for you.

How much does an inch of pitch change RPM?

Sources give about 150 to 250 RPM per inch. After you've run two pitches, Slipwise uses your own boat's figure.

What if I don't know my gear ratio?

It's in the engine's manual. If you can't find it, Slipwise can work it out from counted turns or teeth, or check a ratio against a run.

Does it measure RPM?

No. Read RPM from your tach and enter it. Slipwise measures speed by GPS.

Does it track me?

Location is used only during a run you start, position is never stored, and GPS stops when you press Stop. Your boats and runs stay on your phone, with no account.

Private and offline

Runs entirely on your phone and works offline. No account, no sign-up, and your data stays on your device. The app shows ads through Google AdMob; see the privacy policy. Privacy policy.

Free. An optional one-time purchase removes the ads; nothing is locked and there is no subscription.