SeaPerch Remotely Operated Vehicle, Руководство по монтажу

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SeaPerch ROV Construction Manual
– Version 2010-04NW
T-3
The buoyancy of an object is dependent upon the type of water that it is in. An
object floats when it weighs less than the water that it displaces. Denser water
will make a given object relatively lighter, so it floats more easily. Thus, objects
have more buoyancy in saltwater than in fresh water. Saltwater density can
change with depth as well. Therefore, the ballast may need to be adjusted for
different ROV operating locations and depths.
Participating in a
“buoyancy lab” class session (with hands-on experimentation to
better understand buoyancy, the effects of flotation and ballast materials, and
neutral buoyancy in particular) can be helpful prior to attempting to ballast and
trim a SeaPerch ROV.
Ballast and Trim Considerations for SeaPerch ROVs:
The standard cylindrical foam floats or fixed-buoyancy net floats that are often
used for SeaPerch ROVs have a significant amount of buoyancy that must be
counteracted by adding quite a bit of ballast weight to the ROV, typically up to
about ten ounces or more for a basic ROV without sensors or other accessories.
Other types of floats bring different ballast needs, but all SeaPerch ROVs must
have some added ballast in order to achieve neutral buoyancy.
Note that when
using foam floats, their physical size will often decrease with increasing depth
(over about five feet of depth), due to increasing water pressure; this results in
reduced displacement and thus reduced buoyancy at depth. For example, when
the ROV sent to the bottom at the deep portion of a swimming pool (often 12 feet
or more in depth), it may no longer be able to return to the surface easily.
For this
reason, it is best to have the ROV ballasted to be more positively buoyant while at
the surface when using foam floats.
This will also enable the foam-float-equipped
ROV to more easily bring payloads to the surface.
Trim refers to adjustments to the ballast and flotation of the vehicle to make it
level in the water. The ballast and flotation materials should be placed around the
vehicle in such a way that it is not tilted much side-to-side or front-to-rear. Being
level in the water will enable the vehicle to pick things up off the sea (or pool) floor
as well as turn left and right without also changing depth. Standard SeaPerch
floats are in fixed positions on the ROV, so proper trim is best achieved by minor
adjustments to ballast placement, after neutral buoyancy is attained. For ROVs
with PVC tube floats, front to back trim may be adjusted by sliding the floats
forward or backward, but ballast adjustments are still needed for side-to-side trim.
Students should experiment with different types of ballast materials, but always
using items that can be placed in water without damaging them (or the water).
Certain types of metals rust or corrode easily in water (fresh water, pool water, or
salt water), so ballast materials should be chosen wisely. Also, for ballast to do
its job well, it should not move around, which can change the ROV’s trim and
even its dynamics as it accelerates and decelerates. Therefore, bags of materials
(which can also create excess drag) and suspended weights are not good
choices for ballast.