OceanFlyer Grand Banks 49' Classic Operating Manual page 24

Here is a sketch of a properly anchored boat with a shore line (In this drawing, S=Scope, which should
be at least 4 x DL, the Depth at Low Tide):
3B5: Trim Tabs
The boat is fitted with a set of Bennett Trim Tabs. These are wide
"flaps" attached to the aft end of the boat, under the swim step at the trailing
edge of the hull, operated hydraulically under the control of the skipper by
rocker switches with indicators at each helm station.
At low speeds, up to approximately six knots, the tabs do little, and
should be left in the "Bow Up" position (see below). But at speeds over this
range, the tabs begin to take effect and will help the operator lower the bow for
more efficient cruising.
The best way to adjust the tabs is to lower them while watching the "Speed" indicator to get the
highest speed at a given throttle setting by adjusting "Bow Down". If the tabs are "Bow Down" too
much, the steering will get mushy and speed may drop off a little, and the tabs should be adjusted "Bow
Up" a little. Note that it will take time to make these adjustments; when the buttons are depressed, they
need to be held 2-5 seconds each time for change to be felt and observed (the best way to see the
effect of the tabs is by the knot meter and by observing the height of the bow relative to the horizon,
most easily seen from the lower helm station).
Because the trim tabs are so large, THEY MUST BE IN THE FULLY-BOW-UP
POSITION WHENEVER THE BOAT IS TO BE OPERATED IN REVERSE,
otherwise the great water forces against the tabs may damage them
severely, even tearing them off the hull!
Section 3B: Maneuvering Suggestions 3.8
There are Trim Tab controls at
each helm.