Using the trim tabs
Pat Manley demonstrates how the trim tabs on semi-planing cruisers
can be fine-tuned as a way to increase your cruising speed
Seamanship
S
emi-displacement (or semi-
planing) cruisers are a natural
haven for sailors migrating
from sail to power, and for
some skippers this is the first
time that they will have come
across trim tabs. A ‘semi-planing hull’
ensures good low-speed characteristics
and good all-weather performance, but
doesn’t offer very high cruising speeds.
When accelerating to greater speeds, these
hulls adopt a very tall bows-high attitude
and won’t achieve much above 10-12
knots unless the trim tabs are used. At
speeds up to six knots or so, you can
leave the tabs fully up. At full power
and trimmed correctly, speeds of around
24 knots are possible.
How the trim tab
switches work
Each tab has a separate rocker switch.
Push the front of the switch down and
the tab deflects downwards. Likewise,
press the back of the switch and the
tab deflects upwards. Deflecting the tabs
downwards causes the bow to adopt a
bows-down attitude, while raising the
tabs causes the bow to rise.
So:
To lower the bow, push the front of
the rocker switches.
To raise the bow, push the rear of
the rocker switches.Why are there two sets
of switches?
The hull can suffer from heel: this may be
due to uneven loading or a crosswind,
and it reduces hull efficiency at higher
speeds. Using different settings of the
starboard and port trim tabs allows the
heel to be removed.How do we remove heel?
Once you have set up the tabs for best
boat speed, you can adjust the tabs
separately to remove the heel. Pressing
the front of the port rocker switch will
cause the boat to heel to starboard.Likewise, pressing the front of the
starboard rocker switch will cause the boat
to heel to port. In fact, unless the tabs are
fully extended, reducing the tab angle on
one side while decreasing it on the other
will be more efficient. Some boatbuilders
cross-wire the port and starboard switches
so that the port switch operates the
starboard tab and vice versa. It is believed
by some that this gives a more instinctive
operation for reducing heel.
To find the best setting for the tabs, you
need to monitor the boat speed (not the
GPS ground speed) as you adjust the tabs.An American Fast Fisher at speed. Even when
the hull is properly trimmed, you need to be on
the fly-bridge to see where you are goingPat Manley