Boaty Beginners Guide – Understanding trimming your outboard engine

Okay, so I’ve nailed my lefts and my rights, and therefore I’ve mastered my ports from my starboards. However, I have a problem. I have now discovered my orientation kryptonite – its UP and DOWN.

With hope that I’m not the only directionally challenged among us, Christian has promised to help me write a boaty beginners guide to understanding using the trim of your outboard motor. Using Rebel as a model of course. Mostly to make me feel less confused. Because when you’re starting out it’s an important skill to have locked down, one that soon becomes second nature with practice and getting to know your boat.

So by the end of HomeintheBay’s “Boaty Beginners Guide – Understanding trimming your outboard engine” we should all be that little bit more confident with up vs down, (including me!).

What is ‘trimming your engine’?

For regular boaters, it’s easy to forget that we use boaty lingo all the time! So for beginners, ‘Trimming’ is the word used when we adjust the angle of the outboard engine on the back of the boat, thereby adjusting the angle of the propeller in the water and changing your boats position on the surface. This can be done, either at stationary or whilst moving through the water.

When we start learning to trim the engine, it’s important to understand the effect the angle of the propeller will have on the boat hull in the water. This sounds a little bit like geometry class all over again we know! We’ll show you what we mean in a minute, but first…

Which buttons do we use to control and adjust the engine trim?

Okay, Rebel’s outboard engine is a Suzuki 60HP 4 Stroke and like most outboard set ups, there are 2 places on Rebel we can adjust the trim.

  • Firstly, there is a button located on the throttle lever, perfectly positioned so you can adjust trim up and down with the use of your thumb or index finger depending on the orientation of your control box, whilst maintaining grip on the throttle. This is the primary button used in trimming, and the one we will discuss using.
  • The second button operation is located directly on the side of the engine itself requiring you to be at the back of the boat in order to operate it.
Throttle Trim Button
Outboard Trim Button

And it’s not so much a button, but there is dial we use to monitor the trim of the engine, it sits on the dashboard in front of you and looks something like below. This stops us looking behind us to visually check where the engine is positioned. Just be careful, not all boats have one of these gauges, particularly the smaller ones! We have fitted a shiny new trim gauge, pictured below, during Rebel’s refurbishment as her old one had long stopped working.

What happens when we ‘trim up’?

When we trim up, we tilt the engine up towards the boat. This lifts the propeller attached to the engine in an upward motion making the propeller closer-to and facing towards the surface of the water. As the propeller spins, the forward force produced presses downwards into the body of water.

By pushing in a downwards motion on the rear end of the boat, this causes an opposite action in the bow and lifts the boats nose higher out of the water.

What happens when we ‘trim down’?

By trimming down the opposite actions occur. As we trim down we tilt the engine down and away from the boat. This pushes the propeller downwards causing it to face away from the surface and into the water. As the propeller spins, the forward force pushes in upwards direction and towards the bottom of the boat instead of the body of water below.

By pushing upwards on the rear of the boat, the bow of the boat is forced further down into the water. Completely the opposite of the above.

Remember

Trim UP = Engine UP = Bow UP

Trim DOWN = Engine DOWN = Bow DOWN

UP, UP, UP & DOWN, DOWN, DOWN

What does this mean as we travel through the water?

Imagine for a moment that our body of water is calm and we are travelling at a steady cruising speed – we’ll talk about waves, slow speeds and will cover best practice when setting off and stopping on a separate guide.

If you trim up, lifting the engine up and also lifting the bow up, there is less hull in the water to carve through the water in front of you. As you travel the mid-section of the boats hull now becomes the first point of contact with the water as you go along. Trim too high and the boat can become unstable and uncomfortable as it looses the ability to carve through the water, and skates around with reduced steering capacity due to less water resistance by not slicing through the water at all.

If you trim down, pushing the engine down, which bring the nose down. Suddenly the very front of the hull becomes the first point of contact with the water. Trim too low and there is much greater resistance as the hull of the boat you’re in digs in deep, causing drag and overworking your outboard.

The art here is to find the balance between these two points.

Video time! – Notice below as Christian cruises along, he will first trim Rebel down, then trim her right up, and will then settle a bring her to her ‘sweet spot’ in-between the two. Observe the point at which the wake starts as he does this, you can clearly see it move from further forward to very far back and then to somewhere in the middle as he goes.

Lovely stuff, so we’ve nailed the up and down bit, how do I work out the ‘sweet spot’?

This is where it gets a little trickier to say. The more you use and get to know you’re own boat, the easier this will become. You need to factor in waves/sea state vs your own boat and outboard set up and weight carried on board. Because of these factors changing, and in particular the water being different every single time, you will most likely be making small adjustments throughout almost all your journeys on the water.

You should be sat on the plane (keep a look out for a guide on planing!), sitting with the boat hull roughly level travelling across the surface of the water. The engine should not feel sluggish and if you trim too low you will feel the resistance to moving forward increase and your engine may feel laboured letting you know it’s working too hard.

If you trim too high, your boat may begin to do one of the following three things

  • “Porpoise” – this is an uncomfortable bouncing motion as the water slaps against the front of the hull instead of you carving through it, it’s a bit like ‘bunny hopping’ in a moving car. Take a look again at the video and see if you can spot the point at which Rebel begins to do this.
  • “Chine walk” – this is where the boat becomes unstable and begins to bounce from side to side, this can be dangerous as it will become more and more unstable. To rectify this problem just pull the throttle back and trim down until it stops.
  • “Cavitate” – this is where the propeller comes too close to the surface. This can cause it draw air through the blades instead of water, the engine will then effectively ‘loose grip’, and it’s a bit like wheel spinning in a car. You will notice the engine suddenly rev hard and the boat will slow down. Again to rectify this, throttle back, trim down and try again.

If conditions allow, and only when it is safe to do so, it is useful to practice adjusting your trim to understand the unique trim operation of your own set up.

Why should I learn to control my trim level?

There are many reasons why mastering the trim level on your own boat can be beneficial. Here are a few:

  • It will improve your fuel economy by pushing less of your boat through the water and unburdening some of the load on your engine.
  • You will experience a more comfortable ride for both yourself and your passengers.
  • It helps you maintain control of your vessel, for safer handling on all your journeys.

We hope this has helped get you started with learning your ups vs downs. By writing this guide out (with lots of diagrams at home here with Christian!) I certainly feel more confident with trim.

Remember all boat set ups are slightly different, some boats require a large amount of trim where others need very little adjustment in order to find that ‘sweet spot’. Sea conditions vary constantly as will your own boat, depending on passengers, fuel on board and more. For thorough training on trim, you can always learn from the experts at your local RYA training centre by working through the Powerboating levels.

Further reading on trimming your outboard engine…

Starting off: We all have to start somewhere, and understanding where your trim should be as you set off is really useful as a boaty beginner. We’ll help you get up and onto the plane in no time.

Slow speed: Manoeuvring your vessel at slow speed is whole lot easier when you understand which level to trim your engine at.

Stopping: Best practice when coming to a stop regarding the use of your engines trim function.

All these can be found on our next Boaty Beginners Guide: Outboard engine trim best practice, starting off, slow speed and stopping.

Likewise, all of the above is great if you have a power trim and tilt system, for smaller outboards you might be operating using a manual trim. This is a little more work, and we will talk in the future about using your manual trim on a smaller outboard motor too.

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