Top tips: Boat trim and trim tabs

Top tips: Boat trim and trim tabs and making sure your powerboat is trimmed in the correct manor.

Speed Boat Face facts: owning a powerboat is all about going as fast as possible and making sure that you show all those unfortunate non planing vessels a clean pair of heels.

But once you are up on the plane there are a number of adjustments you can make to get you going even quicker and these are all to do with trim. Trim is not just about speed either, good trim can ensure you enjoy a smoother ride.

These tips from Paul Glatzel's RYA Powerboat Handbook will help ensure you get the maximum out of your powerboat.

You can trim your boat in two ways; fore and aft – raising and lowering the bow, and from side to side levelling the boat if it is leaning to one side. Don’t forget too that where you position people and kit in the boat will affect the trim of the boat too.

     Leg in to start to keep the prop in the water and help the boat get on the plane.

  On the plane ease the leg out to achieve the best speed. Ease out too much and the prop sucks in air from the surface, making it spin faster but lose grip on the water, therefore slowing the boat. Ease the leg out by the correct amount and the revs increase slightly without the throttle being adjusted, therefore speed increases. 

     If the boat starts to porpoise (the bow bounces up and down), trim the leg in to regain control, then slowly ease out to re-trim.

Trim needs constant adjustment. If crew move around the boat or there is a change in sea conditions or speed, you need to check the trim. Fuel use makes the boat lighter – you may need to trim differently when tanks are full or half empty.

Tip: Even when trim gauges are fitted, they can be unreliable. Practise in various conditions to get the best trim. When there are no gauges, use time increments to know whether the leg is up, down or centred. While in port count how long the tabs take to rise, lower and centre. When trimmed correctly the revs rise slightly and the helm feels lighter without pull to either side.

Trim tabs

Trim tabs are either flat plates or vertical blades fitted either side of the boat and attached to the transom. They are used to trim or level the boat, both fore and aft and side to side. Trim tabs are controlled up and downwards by either a hydraulic, or electric ram system on the transom.

  •  Tabs down – raises the stern which in turn depresses the bow (bow down).
  • Tabs up – lowers the stern which in turn raises the bow (bow up).

  •  As a basic rule you tab down into a head-sea and tab up with a following sea.   
  • Tabs can also be used to compensate for a beam sea or uneven loading.

  • One tab down – raises one side of the boat. This is useful to compensate for loading irregularities or a beam sea. 

Gauges are rarely fitted to advise you how much trim is used. While in port count how long the tabs take to rise, lower and centre, then use time increments for their positioning.

Tabs can be helpful when getting up on the plane. Tab down to lift the stern. Tabs should not be needed, but may help compensate for the crew standing at the rear or weed on the hull.

Although tabs are mainly used at speed, they can also be used tabbed down in marinas to increase grip on the water. Beware of powerful reversing when tabbed down, as this can cause damage to the tabs.

Tabs can make a huge difference to comfort and sea-keeping ability at speed. It is possible to achieve basic steering using one tab at a time.

Experimentation in different conditions will give the best compromise of comfort, fuel consumption and ride, but tab half down is a good starting point.

Tip: If you have trim tabs and the ability to trim the drive leg then try to keep it simple. Use power trim for fore and aft trim and trim tabs for side to side trim.