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Propeller Slip

Propeller slip is the difference between the theoretical distance traveled based on propeller rotation and the actual distance traveled, expressed as a percentage. It is affected by factors like hull and propeller condition, current, wind, waves, draft, and speed. Calculating slip using distance over ground from GPS can indicate combined effects of current and weather, but not isolate hull/propeller condition. Calculating slip using distance through water from the log is a better measure of total effects from hull, propeller, and environment, and provides useful statistics for evaluating hull and propeller condition over time.

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4K views6 pages

Propeller Slip

Propeller slip is the difference between the theoretical distance traveled based on propeller rotation and the actual distance traveled, expressed as a percentage. It is affected by factors like hull and propeller condition, current, wind, waves, draft, and speed. Calculating slip using distance over ground from GPS can indicate combined effects of current and weather, but not isolate hull/propeller condition. Calculating slip using distance through water from the log is a better measure of total effects from hull, propeller, and environment, and provides useful statistics for evaluating hull and propeller condition over time.

Uploaded by

iliescu
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PPSX, PDF, TXT or read online on Scribd
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Propeller Slip

How and why to calculate slip


What is Slip?

Corkscrew in a wine bottle moves a certain Since water is not solid, but «flows away» a
distance per rotation propeller behaves differently

Speed of movement = rotation speed * pitch «Slips» a little bit per rotation
Causes and results of slip
Slip affected by: Slip causes:
– Hull and propeller - Increased rpm needed
condition to uphold speed
– Current, wind, waves
- Higher fuel
– Draft/trim
consumption
– Water depth
– Steering activity
- Heavier propeller
– Speeding up/down running, higher engine
load, temperatures etc.
Should average out over time,
statistically
How to calculate slip

Distance ship would go with no slip aka. (Propeller/Engine distance)

Slip Actual distance

Propeller distance = [movement per rotation] * [number of rotations]

Movement per rotation/propeller pitch is specified in propeller design, (e.g. 8,25m/turn for the VLCC newbuilds)
Number of rotations are read from rev. counter
Slip = [Propeller distance] – [Actual distance]
In % of Propeller distance:

Slip [%] =

dista nce”?
Slip [%] = a ctual
t wh at is “
Slip [%] = Bu
Calculating Slip
Using Distance Over Ground (GPS)

15 kn 15 kn
2 kn 2kn
SOG: 13kn SOG: 17kn

Propeller distance = pitch * [rotations] = 8,25m * [58rpm * 60min * 24 hrs] = 688 568m = 372nm

Against current: With current:

Ship distance = 13kn * 24hrs = 312 nm Ship distance = 17kn * 24hrs = 408 nm

Slip = 1 - (312 / 372) = 16,1% Slip = 1 – (408 / 372) = -9,7% And what is negative slip?

The values indicate the effects of current and weather combined mixed with hull conditions

Not possible to see trends for the condition of the hull and propeller  Not an interesting statistic
Calculating Slip
Using Distance Through Water (Log)

15 kn 15 kn

2 kn 2kn
SOG: 13kn SOG: 17kn

Propeller distance = pitch * [rotations] = 8,25m * [58rpm * 60min * 24 hrs] = 688 568m = 372nm

Against current: With current:

Ship distance = 15kn * 24hrs = 360 nm Ship distance = 15kn * 24hrs = 360 nm

Slip = 1 – 360/372 = 3,2% Slip = 1 – 360/372 = 3,2%

The values indicate the total effect from hull and propeller condition + wind and environmental factors

Will also vary with current and weather, but much less than for slip calculated with SOG  Good statistics

Useful for evaluating hull/propeller condition

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