Trimming your boat

It doesn’t matter what kind of boat you are using; correct trim is always an important factor. Vaughan Marsh, Chief Instructor of the RYA’s Motor Cruising Training scheme shares some pointers to get the most from your boat…

Correct trim

Trim is the fore and aft attitude of the boat. Raising or lowering the bow changes the strike area of the hull hitting the waves.

An extreme example of how trim affects a boat can be seen by picturing yourself rowing a dinghy with extra weight in the bow area. With the stern almost out of the water, you would struggle to make progress, and steering would be very tricky indeed.

Fortunately, many powerboats have trim tabs designed to compensate for this.

Once you have a firm grasp of what they do, their use becomes fairly intuitive, but you do need to understand them first.

Power trim

An outdrive leg is trimmed in or out to change the angle of the propeller’s thrust. Trim affects comfort, fuel consumption and the handling characteristics, especially in heavy weather.

• 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. Adjusted by the correct amount, the revs increase slightly without the throttle being adjusted, therefore speed increases.
• Ease out too much and the prop sucks in air from the surface, making it spin faster but lose grip on the water, slowing the boat. If the boat starts to porpoise, trim the leg-in to regain control, and 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 on trim to maintain the best fuel consumption. Fuel use makes the boat lighter – you will need to trim between full and half tanks.

Watching the difference between engine revs and speed is a good indication of correct trim.

Legs can be helpful getting up on the plane. They should not be needed but may compensate for the crew standing at the rear or a weedy hull.

Trim tabs

Trim tabs are electrical or hydraulically operated flaps fitted either side on the stern and are used together or independently to alter the fore and aft trim and heeling of the boat. They work by deflecting water flow, and the faster the boat is travelling, the greater the effect.

• One tab down – raises one side of the boat. This is useful to compensate for loading irregularities, a beam sea or a strong crosswind.
• Tabs down – lowers the bow and raises the stern.
• Gauges are often fitted to inform you how much trim is used. If they are not, count how long the tabs take to rise, lower and centre while in port, then use time increments for their positioning.
• Tabs up – raises the bow and lowers the stern.

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 them damage.

Tabs make a huge difference to comfort and sea-keeping ability at speed. It is even possible to achieve basic steering using one tab at a time when at speed. Experimentation in different conditions will give the best compromise of comfort, fuel consumption and ride, but two thirds down is a reasonable starting point.

Trim in large waves

In a head sea, tab down, so that the forward vee sections cut into and through the waves. This ensures maximum length of hull in the water to reduce slamming.

If tabbed up, the bow is continually thrown into the air, creating lots of spray and increased slamming.

Outdrives also require the leg-in to push the bow down and to keep the prop in the water.

Following sea

It is important to stop the bow burying when the stern is picked up by a wave.

Trim the tabs up so that the bow is up and adjust your speed to be slower or faster than the wave. Outdrive boats may need the leg trimming out to raise the bow.

Find out more

Correct use of trim tabs can make a huge amount of difference to your boating enjoyment. Once you understand the principles, it only takes a little practice before the whole thing becomes intuitive.

Hopefully these tips will translate easily when you get out on to the water, but for practical training, why not sign up for an RYA Motor Cruising course? Visit www.rya.org.uk/training for more details.

These pointers were taken from the RYA Day Skipper Handbook (Motor), if you would like to read more, pick up a copy by going to the RYA webshop: www.rya.org.uk/shop