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 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.
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 idea.
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.
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 heavy boat.
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 conditions.
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 your pants.
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 wave.
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.