Boating Advice: Trimming Your Boat (Part 2)

USING POWER TRIM
- Positive and negative trim – what’s that?
Modern outboard and sterndrive engines have a power trim and tilt system that allows you to adjust the trim, or fore and aft running angle of your boat while underway. Adjusting this trim or hull running angle to suit changing sea, wind and current conditions will improve your boat’s handling, ride and economy.

You might wonder why changing the trim of your boat is even necessary — assuming your boat has been well designed in the first place. However, even the best of planing hulls will benefit from the use of power trim.

The first thing to learn is that changing the drive angle of your outboard or sterndrive leg will change the trim of your boat when it is running at speed.

Trimming or lifting your outboard leg up will trim the bow of your boat up. This is known as positive trim. Trimming your outboard full down or “in” is known as negative trim. This action will encourage the bow of the boat to push down further into the water.

If you think about the angle of the engine drive leg and the thrust of the propeller it is quite logical.

With the engine drive leg trimmed all the way down, the propeller is pushing or thrusting in a diagonally upward direction, and this lifts the stern of the boat up, which in turn drives the bow of the boat down. When you trim the drive leg up, the opposite occurs.

In nutshell, all you need to remember is that trimming the engine up or “out” will lift the bow up. Trim it down or “in” and the bow goes down. In most sea conditions, modern boats perform best with the outboard set so that the boat has a slight bow up running attitude.

That said, when taking off from displacements speeds, many boats will perform better with the engine and the bow trimmed down. This allows the boat to rise quickly onto the plane. Once the boat is up and running, however, the trim of the boat should be adjusted to suit the specific sea conditions.

Leaving the engine and the bow trimmed full down can cause the boat to handle poorly. If the bow is digging or ploughing through the water, the steering will stiffen up, boat speed will drop, fuel consumption will increase, and a lot of spray will be pushed out from the bow of the boat.

Running the boat with the bow full down is also potentially more dangerous than a bow up attitude, given that when running downwind or before the sea, it can encourage a broaching situation.

Having made these comments, trimming the bow full down can make your boat ride more comfortably through the water, particularly when running upwind or into the sea. Trimming the bow down allows the sharply angled forefoot of the bow to cut, like a knife, through the chop, improving ride comfort.

Conversely, trimming the bow up is important when running downwind or in a following sea to keep from broaching. In normal running conditions, however, it can be inefficient to trim the boat up too high. The boat will have trouble getting onto the plane, it may ride harder, be inclined to leap off waves, and it could also develop a porpoising or bouncing action, even in calm water.

 TWIN ENGINES AND TRIM TABS
- Adjusting lateral balance and lean angle
Nearly all of the above information relates to how to trim your boat on a fore and aft axis. Adjusting lateral or sideways trim is more difficult.

With small, single engine boats the simplest way of compensating for a sideways list underway is to redistribute the weight within the boat. Ask a crew person to sit on the other side of the boat, or move your gear, ice box, whatever across the other side to “square-up” the boat and keep it on an even keel.

Another option, at least with larger trailerboats, is to fit a set of trim tabs. These are metal plates or flaps fitted to each side of the transom of the boat which move up and down (on a horizontal axis) hydraulically at the touch of button at the helm.

You can also now buy the more compact Interceptor-type trim tabs which are generally made from a poly plastic material. Ideally suited to small- to mid-sized trailerboats, the interceptor trim tabs have small plastic blades which influence boat trim by dropping down vertically into the water beneath the stern of the boat.

With both conventional metal-plate-style trim tabs and interceptor vertical trim tabs, you can control fore and aft trim by raising or lowering both trim tabs in tandem, or you can adjust lateral trim by adjusting each trim tab independently.

With trim tabs a lateral or sideways list can be corrected instantly, providing the boat is running at speed. For example, if your boat is listing to starboard while underway, lowering the starboard side trim tab down further into the water will create upward pressure at the stern, thus lifting the starboard side of the boat and correcting the lean or list.

If you happen to be running the boat with both trim tabs half way down or so (to drop the nose into choppy water for example), then you can also correct a starboard list by raising the port side trim tab, because this reduces the upward pressure on the port or left hand side of the boat.

Having noted the above, trim tabs should not be used to correct a list caused by poor weight distribution within the boat. It is intended to correct a list caused by external forces like a strong wind or wave action.

Correcting lateral trim can also be achieved with twin outboards or sterndrives. The principles are identical. In the same starboard listing situation trimming the starboard engine down, will also lift the right or starboard side of the boat up. Trimming the left or port side engine up will do the same thing.

(ED: In respect of trim tabs, be aware that some boat manufacturers wire them differently or back-to-front, so that the trim button may do the opposite to what you expect. Always experiment with a new boat's trim before committing to final trim application so you know how your boat reacts. Changing the operation of your trim tabs is usually no more difficult than swapping over the connectors behind the dash.)

PRACTICE MAKES PERFECT
- Don’t be afraid to experiment
Finding the correct trim angle for your boat is all about experimentation and experience.

There’s more to boat trim and setting your boat up properly than I have explained here, but this article covers the basics to help you get started.

Just remember to do everything slowly and in small increments. The only way you will find out what works and what doesn’t is by trial and error.

Don’t be afraid to try different things, but during the learning process, keep to enclosed bay or harbour waters. Once you are comfortable with your boat, and have become familiar with how it should be trimmed for various sea conditions, you can then venture out offshore.

INSTRUCTIONAL PHOTOS AND CAPTIONS:

PIC 1
AMM’s Matthew Thomas demonstrates a full negative or “down” trim position with this AMM 7000. Note how the bow has dropped down into the water, pushing out a lot of spray. Trimming the bow down helps the boat to plane faster and can soften the ride in choppy water, but it is not economical and can be dangerous in a following sea.

PIC 2
In this photo Matt has the AMM running with the engine trimmed up too high. Trimming the engine up (and thus raising the bow) is recommended when running in a following or “downhill” sea - but in normal conditions if your boat is trimmed too high it may struggle to plane, ride harder, handle poorly and be inclined to “porpoise” or bounce.

PIC 3
Now we’re talking. Matt has the AMM trimmed perfectly here. Most good monohulls handle best and run most economically with the engine trimmed up a little above “middle” so that the bow lifts clear of the water, running cleanly with little spray. Every boat is different however so it may take you a while to find the absolute sweet spot.

PIC 4
The power trim switch is an up/down button located at the top of the throttle lever, where it can be easily toggled with your thumb.

PIC 5
A traditional metal plate or flap style Lenco trim tab that has been cleverly recessed into the transom of this Mako 234CC, so that it does not protrude beyond the transom and risk entangling/cutting off a fishing line.

PIC 6
Trim tabs come in a range of sizes. This Lenco model appears smaller in depth, but wider - and mounted in the regular position against the transom wall.

PIC 7
An interceptor style trim tab with the vertical blade fully deployed. This Volvo Penta branded model suits smaller sized powerboats as it is much more compact than the plate/flap style tab.

PIC 8
Volvo Penta “interceptor”style trim tabs fitted to a Quintrex Yellowfin 6700.

PIC 9
Trailerable, dual engine powered cats are rarely fitted with trim tabs as they can be easily trimmed on a fore and aft and lateral basis using the engines. Lifting one drive leg up - or lowering the other will easily account for a sideways list or lean angle.

PIC 10
Dual engine trailerable monohulls probably don’t need trim tabs, but fitting a set will give you a greater trim range and more versatility. This AMM has a set of Volvo interceptor style tabs to compliment the two Suzuki 140hp four-stroke outboards.