1. USNS JOHN LENTHALL / USS FORT MCHENRY UNREP 05AUG09
Lessons Learned
USNS JOHN LENTHALL / USS FORT MCHENRY
UNREP 05AUG09

2. “Good shipdrivers prevent themselves from getting into situations where good shipdriving is necessary.”

3. Background: Venturi Effect
In regard to shiphandling, a Venturi effect occurs when water is forced through a restricted space, such as the space between two ships or the space between a ship and a steep channel bank.
When water is forced through a restricted space (bow to bow or bow to bank), pressure increases initially. As the water passes through and out of the restricted space, velocity increases and pressure decreases. This change in pressure causes a suction effect.

4. Background: Rudder position during UNREPs
Helmsmen must account for and compensate for Venturi Effect
Rudder generally TOWARDS the UNREP ship to keep steady 5 10 30

5. “All Conditions Normal”
Inhaul/Outhaul line tensioned forward
Spanwire tentioned aft
Distance fluctuating between 180 and 200 by steering in half degree increments
Speed fluctuating around 14.5 knots 5 10 30

6. “All Conditions Normal”
Inhaul/Outhaul line detensioned forward
Spanwire tentioned aft 5 10 30

7. “Overcompensating”
Helmsman uses too much right rudder when coming to an ordered course (only 0.5° right)
When alongside, this may have only been 5° right, which is already too much with Venturi Effect 5 10 30

8. “Overcorrecting”
Helmsman sees his mistake and comes left abruptly in an attempt to catch the swing of the ship
CONN sees lateral separation fall from 180 to 160 FT and orders “come right steer course 160” (from 159.5) 5 10 30

9. “Overcorrecting”
CONN sees lateral separation fall from 160 to 140…120… 100 FT
Helm notices heading swinging quickly to port and with new rudder order “come right,” puts on a 20 ° Right Rudder 5 10 30

10. Not Good
100 ft

11. “Overcorrecting”
Right rudder causes the lateral separation of the sterns to decrease even more rapidly
CONN “team” sees the Right 20 ° and orders rudder amidships, but sterns continue to close 5 10 30

12. “Overcorrecting”
Right rudder causes the lateral separation of the sterns to decrease even more rapidly
CONN “team” sees the Right 20 ° and orders rudder amidships, but sterns continue to close 5 10 30

13. Too Close
80 ft

14. “Overcorrecting”
Captain orders a Left Standard Rudder, which catches the stern
Simultaneously orders All Ahead Full, which puts wash over the rudder and uses outboard heading as an “out” 5 10 30

15. “Overcorrecting” JLN initiates an emergency breakaway
Captain brings rudder back to amidships (swing has stopped) 5 10 30