How to use the prop slip calculator
- Enter the engine speed in rpm and the gear ratio of the lower unit or gearbox, for example 2 for a 2.0 to 1 ratio.
- Enter the propeller pitch in inches and choose miles per hour or knots.
- Enter the speed you measured at that engine speed, ideally from a GPS.
- Read the speed with no slip, the slip as a percent and, from the advanced options, the pitch that would give a slip you choose.
Formula
propeller speed = engine speed / gear ratio
speed with no slip = pitch x propeller speed x 60 / inches in a mile (or a nautical mile)
slip = (speed with no slip - measured speed) / speed with no slip
pitch for a target slip = measured speed / (1 - target slip) x inches per mile / (propeller speed x 60)
There are 63,360 inches in a statute mile and about 72,913.4 inches in a nautical mile. Pitch is how far the propeller would move forward in one turn through a solid.
A 24 inch pitch at 6,000 rpm
At 6,000 rpm with a 2 to 1 gear ratio the propeller turns 3,000 times a minute. A 24 inch pitch would move the boat 24 x 3,000 x 60 / 63,360 = 68.18 miles per hour with no slip. If a GPS shows 60 mph, the slip is (68.18 - 60) / 68.18 = 12.0%.
To get the slip down to 10% at the same measured speed, the propeller would need a pitch of 23.47 inches. In knots, the same setup gives 59.25 knots with no slip, and a measured 52.1 knots is 12.1% slip.
What slip tells you
Slip is the gap between how far the propeller would move in a solid and how far the boat really moves. Some slip is normal because water gives way. A change in slip at the same rpm can point to a fouled hull, a damaged blade, a different load or a change in trim, so it is worth tracking over time.
To see the hull speed limit for your boat, use the Hull Speed Calculator. The Gear Ratio Calculator finds ratios from tooth counts, the Boat Fuel Range Calculator turns the speed into range, and the Speedometer Error Calculator helps check a gauge against GPS. Before a long run, the Coolant Mix Calculator helps with the engine's cooling fluid.
Choosing a pitch
A higher pitch means a higher speed at a given rpm but a heavier load on the engine. The right propeller lets the engine reach the top of its rated speed range at full throttle. Use the engine maker's range, and treat the pitch for a target slip as a starting idea to discuss with a propeller shop.
Assumptions and limits
- The gear ratio is the engine speed divided by the propeller speed.
- The measured speed was taken at the engine speed you entered, in calm conditions.
- Pitch is the nominal pitch of the propeller, which can differ from the real figure.
- Rake, cupping and variable pitch are not modeled.
- Results are guides and not engineering advice.
Frequently asked questions
What is a normal amount of slip?
It varies by boat and propeller, so use the guidance of your engine or propeller maker. Tracking your own value over time is more useful than comparing with others.
Can slip be negative?
In theory the boat cannot go faster than the no-slip speed, apart from current and wind. A negative result usually means a wrong input, so the tool shows a note.
Do I enter the gear ratio as 2 or 0.5?
Enter 2 for a 2 to 1 reduction, where the engine turns twice for each turn of the propeller.
Which speed should I measure?
Use a GPS speed over the ground at a steady throttle, and average two runs in opposite directions to cancel the current.