Judging by the number of badly trimmed
planing powerboats you see on the water, lots of people don’t
understand what the word ‘trim’ means and, apparently,
don’t even realise when their boat is and when it is not trimmed
Warren Steptoe explains the techniques for trimming
Ultimately, trim is a safety issue; it’s certainly
possible to swamp or overturn a boat simply because it’s badly
trimmed. Fortunately, common sense and the high standards of today’s
boating industry make such extremes unlikely.
However, the more open the water you’re boating
on the more important it is, it’s very important indeed to
understand trim and how to adjust it to suit different conditions.
The difference between a well trimmed boat and one
which isn’t is quite dramatic – in stark contrast to
how easy it is to adjust an outboard or sterndrive powered boat
from one to the other at the literal touch of a switch. But before
getting into the effects of the dreaded trim button, it’s
necessary to understand trim – and that begins at the same
place in any boat – weight distribution.
||It’s common sense not to load
a boat with all the passengers on one side, it has to and will
steer badly and will ride much less comfortably than it would
if weight is distributed evenly.
It’s a skipper’s
responsibility to ensure weight is distributed appropriately before
a boat ever moves from a ramp or mooring. Positioning of passengers,
iceboxes, portable fuel tanks and, in fact, any gear heavy enough
to affect weight distribution and in turn static (stationary might
be a better word) trim should be reviewed EVERY time a boat is loaded.
Passengers can be asked to move and anything that’s
not bolted down changed around to redistribute weight as necessary.
Remember too that you may need to move a fuel tank or an icebox
as fuel is consumed or an icebox emptied.
Common sense suggests that a boat needs to sit level,
‘square’ if you prefer, in the water, yet while that’s
essentially true, it’s not always the case with planing powerboats.
Bows down is never a good idea, a bows heavy boat
won’t steer well, and bows heavy planing hulls take longer
than they should to get onto the plane; and will use excessive amounts
of fuel doing so.
On the other hand, in certain circumstances it’s
good to be stern heavy. In an emergency with an anchor or sea anchor
deployed, moving weight aft keeps the bows high to deal with oncoming
seas. Similarly when under tow, redistributing weight aft minimises
any tendency of tension on the anchor/tow rope to bury the bows
into the towing boat’s wake.
Generally speaking then, static, (stationary) trim
is about common sense; about simply balancing a boat by distributing
weight to keep the hull floating level in the water. Except that
sometimes it may be better to bias some weight aft.
|Trimmed and terrific, and the boat’s well
set up too, Ryan Williams and Lavinia Gorse-Flint show ideal
trim for calm water.
Once a hull moves, none of this
changes, but speed does introduce the effects of the hull’s
hydrodynamics and planing speeds magnify both these and the interaction
between hull and surface conditions, wind chop, waves etc. At speed,
trimming becomes a little more complex because appropriate trim
for travel in one direction relative to a prevailing sea or even
a substantial wind chop may not be appropriate for another.
This is why easy trim adjustment at the literal touch
of a switch is provided on planing powerboats – because it’s
necessary to adjust trim to suit as and when conditions or direction
of travel change. That oft cursed trim button is actually a wonderful
There are two common means of adjusting at speed
trim in planing powerboats. One of these is trim tabs, a pair of
adjustable tabs or flaps mounted on the transom (some larger inboard
powered boats incorporate them into a recess in the hull bottom),
which work much like the elevators on an aircraft. The other is
by adjusting the angle of an outboard or sterndrive drive leg relative
to the transom.
Small outboards adjust manually by moving a pin along
a series of holes in the mounting bracket. The so-called power trim
‘n’ tilt fitted to larger outboards and sterndrives
adjusts the angle of the drive leg electric/hydraulically at the
touch of a switch. Either alters the angle thrust from the propeller
is delivered, relative to the water surface and the hull, and the
attitude (or trim,) of the boat to the water accordingly.
|With the outboard trimmed too far in, the Stacer’s
bows are visibly pushing water and the whole hull is throwing
lots of unnecessary spray.
|With the outboard trimmed too far out, the bows
are lifted high and the stern buried. If the water was rougher
you can see here that the hull would meet oncoming chop well
back along the hull instead of using its bows to cut through.
At speed with an outboard or
sterndrive, adjusting the propeller closer to the transom, commonly
referred to as trimming in, raises the transom and pushes the bows
down. Trimming out, adjusting the leg away from the transom, lowers
the stern and raises the bows.
Trim tabs work by adjusting (usually electric/hydraulically
at the touch of a switch) the tab up away from contact with the
passing water; or down (sometimes also referred to as trimming in,
or digging the tabs in) further into the water. Adjust the tabs
down, into harder contact with the water if you like, and the aft
end lifts and the bows lower. Adjust them up, easing contact with
the water, and the transom squats lower and the bows rise.
Trim tabs are fitted to all kinds of powerboats,
inboard shaftdrive, inboard sterndrive, and outboards. Both tabs
and power trim ‘n’ tilt can be fitted to the same boat
and often to boats driven by big outboards or sterndrives.
Trim tabs are usually independently adjustable making
it possible to adjust both fore and aft (longitudinal) trim and
lateral trim. With either independent controls or some kind of two-way
switch arrangement, one tab may be set harder than the other to
correct a lean to one side or the other.
It’s important to stress at this point that
while at speed trim adjustments will to some extent compensate for
uneven weight distribution, they should never be used for that.
Side-to-side (lateral) trim adjustment is really
there to compensate for the effects of strong winds on one side
of the hull, and not to straighten up a boat leaning over from all
the people aboard sitting on one side. Similarly, a bows heavy boat
re-trimmed with a set of tabs and/or the drive leg is still a bows
The problem is that being bows heavy the boat is
still waiting patiently for an opportunity to bite. In fact, it
may turn out to be like the cattle dog who lets you in the gate
with a wag of its tail then turns into a rabid monster when you
try to leave. An encounter with a bigger than average wave, or even
a bigger than average boat wake and suddenly you find out how violently
a bows heavy boat can broach…
Getting static trim right is always the starting
point before the motor is ever put into gear. From there, at speed
trim changes to a planing hull are about trimming to suit changing
||As Haines Hunter’s John Haber
demonstrates, too much out trim when running into a sea has
a tendency to lift the boat right out of the water as it crests
even small waves.
Quite fine adjustments can make
dramatic differences to how a boat handles, especially at sea. It
takes awhile to attune yourself to this and when not au fait with
a particular boat’s individual character, the rougher the
water, the harder it is to discern actual trim angles.
Although grossly inappropriate trim angles are obvious
enough, even experienced skippers habitually familiarise themselves
with an unfamiliar boat. For the less experienced, a few simple
exercises will reveal a lot about an individual boat and will meanwhile
generate a much better understanding of trim and the adjustment
of it to suit different conditions.
In any case, if a trim gauge or trim angle indicator
is fitted (they usually are to boats with power trim ‘n’
tilt, and sometimes aren’t with trim tabs,) they’re
a great help. Trim gauges are especially valuable in difficult circumstances
where it’s not easy to perceive trim angles by the seat of
To accelerate to planing speed, nearly all hulls
like their tabs down, and/or their drive leg trimmed in. So, to
start our familiarisation process, trim the leg all the way in,
and/or apply maximum downward angle of the tabs, then give the throttle
a burst to boost the hull onto the plane.
Some boats will prefer a little less than all the
way in trim, or less tab than this. The best way to find out is
to try it a few times, progressively trimming the leg out and/or
the tabs up some more each time.
Keep trying until you find you’ve gone too
far. That’s when the hull pokes its bows in the air and takes
longer than it has been to reach planing speed. If you have a trim
angle indicator you should be using it. Experiment until you’re
familiar with the best trim angle for acceleration to planing speeds.
Once comfortable with that, go to planing speed and progressively
trim the leg out, and/or the tabs up. This will lift some more of
the hull off the water, reducing drag and increasing speed.
As a rule of thumb, an indication of good calm water
trim angles is when the steering goes light. Try it a few times.
Trim in/down and out/up until you feel this freeing or lightening
in the steering.
If you keep on trimming out, the propeller will eventually
reach an angle where it goes too far and loses grip, revs climb
and speed decreases. Knowing where this happens is important when
adjusting trim for different sea conditions.
Then, find somewhere with plenty of room and not
many other boats about and try some reasonably tight turns. You’ll
find that trimming in prior to a turn allows tighter turns before
the propeller loses grip. And trimming out for straight running
gives more speed without applying any more throttle.
Need we remind you to be careful during this entire
process and to be ready to ease off if things start getting out
of hand. The idea is to discover for yourself what that dreaded
trim button does, not to end up on Club Marine’s claim book!
Now to the advanced course.
Each side of the lightened steering effect, there’s
a range of trim angles used to deal with changing sea conditions.
When encountering surface chop, particularly smaller
boats will deliver an unnecessarily bumpy ride if left trimmed at
calm water angles. Ride quality is improved significantly by trimming
in, lowering the bows and using the sharpest part of the hull to
slice through rather than crashing into the bumps.
When travelling upwind, trimming in has other benefits
too. It helps counteract the lifting effect of wind under the bows,
which can be quite noticeable in lighter boats. At sea, in trim
also minimises the tendency of the bows to loft into the air as
the boat crests after climbing the steeper downwind side of a swell.
In big swell conditions, it may become necessary
to actually accelerate up the face of a swell and ease off for the
crest, allowing the bows to drop gently onto the backside of the
A down swell direction of travel is just the opposite.
Travelling down swell, trimming out raises the bows to help the
hull recover as it encounters the back of one swell after descending
the previous one.
Without out trim, the bows are more likely to bury
and may veer uncontrollably to one side. This is known as broaching
and it’s extremely dangerous. Broaching places the boat side
on to the sea in a vulnerable position and the sudden change of
direction when the broach occurs can be violent enough to throw
the person at the wheel aside.
Correcting a broach is the same as correcting a slide
in a car; that is by steering towards the original direction of
travel or steering into the slide. The place to learn about trimming
out to improve downsea handling and reduce the chance of broaching
is obviously and most definitely NOT in big seas.
Take the advanced course with common sense and while
you’re at it try out all the angles as well as directly up
and down sea. Familiarise yourself with using out trim to reduce
the tendency of the bows to bury when travelling down or down and
across a sea, and how much in trim to use when travelling into or
across and into a sea.
It goes without saying, but is worth saying anyway.
You should be completely au fait about loading your boat to trim
it correctly and be comfortable using the trim adjustments provided
on every planing powerboat to cater for changing conditions before
even thinking about going to sea.
We would like to express thanks to Lavinia Gorse-Flint,
Ryan Williams, Cliff Antees and Damian Hoyle of Telwater (Quintrex
and Stacer;) and John Haber of Haines Hunter for their help with
photography for this feature.