The motion of a drifting ship can be considered to be the sum of the following:
- Tidal current, which should move the entire upper part of
the water column. This will reverse on a fairly short time
interval (six hours).
- Wind induced currents, acting on a passive drifting ship.
This will be a fraction of the wind speed, taking some time to
get the water in motion, and will be at an angle to the wind due
to the Coriolis deflection. The magnitude of this current
will depend both on how high up into the wind the object
extends, and how deep into the water the object extends, and the
latitude because the Coriolis force varies with the sine of the
latitude. The Coriolis force results from motion on the
Ebbesmeyer and Ingraham have detailed the difference between
rubber ducks and Nike sneakers, which respond differently to the
wind because of their different drafts. Lifeboats are
designed to move as slowly as possible, with as little leeway,
and sailboats can move either to the right or the left of the
wind without regard to the hemisphere.
- Sailing motions, acting as a result of setting sails and
steering. The course sailed will have
drift to the leeward compared to the course being steered. The
model will include leeway with the steered course into a single
This model uses hourly time steps.
||Possible drift trajectory for the Bonhomme Richard, September
1779. Green is the tide motion, blue the ship motion which
combines the wind acting on the ship any headway made by attempting to
said, and the red is the combined resultant.
This option is experimental, subject to change, and may or may not be of any
use to anyone. The specific conditions for which it was developed may or
may not be generally applicable. We may or may not modify it to make it useful (or more
useful) and could remove it at any time. Given all that, we would be
willing to talk about making this more helpful.
- Open MICRODEM.
- Change the program version
to historic shipwrecks.
- Use File, Historic shipwrecks labs, BHR drift. This will open
automatically several data sets.
& zoom to pick just the area of the model you model to
analyze. Make sure you leave some room, since we will modify
the model and might increase the motion. Unless you do this,
you might not see the results clearly.
Zoom out to show entire data set if you find you
zoomed in too much.
- Stats button, Oceanography, drift
model, from the database form for the model you want to
- Do a drift plot.
- Toggle the wind vector on and off (Vector options
tab). Notice its magnitude compared to the others, and
decide what it adds to the map plot.
- Toggle the ship underway motion, tide current, and wind
current on and off (Vector options
tab). Notice the magnitude of each, and each direction.
- Change the wind motion between +15 and -15 (Oceanography
, and notice the effect on the motion when you rerun. This
changes how far to the right or the left of the wind the surface
current will be. It is a function of the latitude, and how
the object rides in the water.
- Add Uncertainty.
- Just change the value to "Heading and speed", and accept all
Area computation on
map window toolbar allows you to outline the area
within the four endpoints and determine its area.
- When done, change uncertainty back to none.
- Do Monte Carlo
- Accept the default settings, or increase the number of runs
to something like 5000, and run. This may be
slow, and if so, you can abort out.
||Plot of the 5 models and their end points.
Requires a database with the following fields (other fields optional):
- TIDE_DIR: in degrees. This will be used if there is not a
- TIDE_SPEED: in knots
- SHIP_DIR: in degrees
- SHIP_SPEED: in knots
- DURATION_H: duration of time step in hours
- LAT, LONG: one location. The computations will start with this
record and process all records for which the other fields are defined.
Last revision 1/30/2017