Trimming is usually done to increase a boat’s speed. But that’s not the whole picture. As Graham Lloyd explains, trimming a boat for performance can also make it safer and more economical to run. By Graham Lloyd
The first step in trimming is to get rid of everything on board that you don’t really need. The power-to-weight ratio of your boat holds the key to how well it will accelerate and to how efficiently it will run. Be ruthless, and then distribute the remaining weight so that the boat is properly balanced fore-and-aft as well as side-to-side.
If there’s too much weight forward, the boat will run bow heavy and will be hard to steer and unable to perform to its speed potential. If there’s too much weight aft, the boat will be difficult to lift on plane; it will probably have excessive bowrise as it accelerates from rest; and it will have a tendency to ‘porpoise’, with the bow rising and falling while underway.
To check the fore-aft balance point, look at the boat’s profile as it is carrying its normal load while sitting in calm water and see if it looks ‘right’. That might seem a bit vague, but it does actually work fairly well. A boat more than a little unbalanced will appear to be down at the bow or stern. To check lateral balance, go aft of the transom and examine the waterline across the back of the boat to see if it’s level. If either fore-aft or lateral balance looks wrong, move some weight around until it looks right.
When equipment on board changes or fuel and passenger loads vary, keep that need for optimum balance in mind and reposition movable weights (including passengers) to obtain the best overall balance.
After the balance point of the boat is sorted out, there are one or more methods usually available to trim the angle at which the boat runs across the water. In other words: ‘reduced drag’. The objective of reduced drag is to have the boat running with the least amount of hull in the water consistent with safety and good handling.
Boats with inboard engines may have adjustable ‘cav’ plates that can be used to trim the hull attitude; other boats might have trim tabs, which are a variation on the cav plate theme and which give added flexibility in trimming.
On outboards and sterndrives, apart from quite old or small engines, hydraulic rams are used to trim the prop angle, which also changes the depth of water in which the prop operates. References here will be to outboards, but the same principles apply to sterndrives and are similar for jets.
To help get the boat on plane, trim the outboard right in (or down). This puts the prop as deep in the water as possible for maximum grip, and puts it at an angle that pushes up the stern of the boat to assist in quickly getting ‘over the hump’ and on plane. That prop angle also helps to minimise bow rise and to preserve good forward visibility.
Once on plane, trim the outboard out to about a quarter of the way through its trim range – the exact position will vary depending on the individual boat set-up. The bow of the boat will rise as the prop angle changes, and engine revs will increase as the load on the prop reduces. Because less of the boat is in the water, there is not as much drag. When the trim angle is about right on boats without power steering, only the slightest amount of torque can be felt on the wheel.
To trim for maximum speed, trim the outboard further up (or out). The changed angle of the prop in the water lifts the bow further and takes more hull out of the water for reduced drag and again more speed.
As the boat is trimmed out, torque on the wheel can increase and more effort might be needed to keep the boat on course. Those unfamiliar with their boat should trim slowly and in stages, allowing the boat to settle at each step, which allows the owner to gauge the results incrementally.
If a smaller craft is trimmed out too far, the boat will become unstable because there’s insufficient hull left in the water. This can result in ‘porpoising’ in which the bow of the boat continually rises and falls, or in ‘chine-walking’, where the boat starts rocking from side to side. Trimming too far out can also cause the prop to ventilate and lose thrust as it gets too close to the surface and sucks air rather than water.
If the boat is trimmed too far and there is a sudden loss of thrust or stability, don’t suddenly back off the throttle as this could make things worse. Just trim the outboard back in a bit and, if necessary, gently ease off the throttle until the boat is running flat and stable again.
When a boat is trimmed out for speed, the outboard should be trimmed back in a bit before trying to turn the boat. Doing this drops the bow, which gets more hull in the water for better grip and gives greater turning power and more stability through the turn. As the boat straightens up, trim out again during acceleration to regain speed.
When running before a prevailing chop or swell, trim the engine out a bit to keep the bow up. This will prevent the bow from digging in as the boat runs down the faces of waves. When running into the chop, trim in a bit to keep the boat level and to prevent the bow from flying high when it runs into the waves.
As the boat slows and comes off the plane, get into the habit of immediately trimming the engine right in. This gives maximum grip for manoeuvring at low speeds, and means that you are ready for acceleration at any time. Trying to accelerate with the engine trimmed out will often result in prop ventilation and slippage.
CAV PLATES AND TRIM TABS
Apart from trimming the outboard or stern drive, the skipper can control and vary the attitude of the boat with cav plates or with trim tabs fitted to the transom. For shaft-driven boats that do not have the advantage of trimming with prop angle adjustment through an outboard or sterndrive, cav plates and trim tabs still allow the skipper to adjust the trim of the boat while out on the water, although not to quite the same extent.
Cav plates and trim tabs are fitted to the transom of a boat, and can be raised and lowered either by a mechanical linkage or by hydraulic rams controlled from switches near the wheel. Both devices do essentially the same thing. However, tabs are generally more versatile in operation as they can be used to trim the boat laterally as well as horizontally.
Cav plates are hinged across the bottom of the transom so they can be lowered to deflect downwards the water that is rushing along and past the hull. This raises the back of the boat and pushes down the bow to put more of the boat in the water. When running in a straight line in calm water, the cav plate is left up for easier running and highest speed. In rougher water, or when turning the boat, the cav plate can be lowered for better control and additional stability. With the plate down, more of the hull is in the water for a more stable attitude through rough patches, and for better grip in turns.
For higher speeds, the cav plate is lifted out of the water where it doesn’t create drag. If the boat is balanced correctly, as much of the hull as possible will also lift out of the water for reduced drag and faster running. This can be taken to the extreme, where so little hull is left in the water that the boat becomes unstable. If the boat is kept at that extreme level of ‘high’ trim, it will lose even more stability in a turn and perhaps spin out or roll, and will become dangerous if the water gets rougher.
In such situations, the skipper should lower the cav plate. The bow then drops, the hull grips better, drag washes off some speed, stability increases and so does turning power.
TABS FOR TRIM
Trim tabs are also hinged at the bottom of the transom, but are fitted to each side and can be deflected independently. Using them together has the same effect as a cav plate, but deploying one by itself, or one more than the other, allows control over the lateral trim of a boat. Some tabs are operated manually, some by a manual-hydraulic method, but the best boats have an electric-hydraulic system controlled by rocker switches at the helm.
Lowering both tabs at the same time has the same effect as lowering a cav plate. Water is deflected down to lift the stern and drop the bow for more hull in the water with the usual increase in drag and reduction of speed, but with improved grip and stability. The extra dimension comes from operating the tabs independently. Lowering the tab on just one side will lift the back of the boat on that side, and drop the bow on the other side.
By using tabs one at a time, the boat can be trimmed laterally. Suppose the boat has a heavy passenger who sits on the port side. The boat will naturally run with the port side lower. But by dropping the port tab a bit, the skipper can lift the boat on that side and restore the boat to an even keel.
Tabs are also commonly used for lateral trim on deeper vee hulls, which have a tendency to ‘lay over’ on one side or the other, especially when running with a strong side wind. What happens is that the skipper turns the wheel slightly to counter the effect of the side wind and so holds to his course in a straight line. But that slightly turned wheel banks the boat (it’s actually doing a slow turn to stay straight against the wind) and so makes it run with a list to one side. By dropping the tab on that side of the boat, the skipper can correct the list, causing the boat to run laterally flat for better performance.
Tabs and cav plates can also be used to help get a boat on plane, or to reduce bow rise during initial acceleration. Just lower both tabs or the cav plate before accelerating. The extra lift at the back of the boat puts the hull on plane more quickly and the downward force keeps bow rise to a minimum. As soon as the boat is fully on plane, raise the tabs or plate to reduce drag.
Correctly used, tabs can provide enormous control over a boat which gives more efficient operation, better speed or increased economy. Using tabs wisely for best performance can be very gratifying.
There is some overlap in trimming options when there are both prop trim (with an outboard or sterndrive) and trim tabs available on a boat. When equipped with both of these methods of trim, use prop trim for the fore and aft control of the boat, and the tabs for side to side balance. Every boat is different, so try different combinations of prop, tab, and weight trimming to find what’s best for each particular craft.
Don’t forget that the throttle is an important part of trimming, too. Accelerate hard and the bow will rise. Pull back the throttle quickly and the bow will drop. When confronted with a boat wash or larger wave head on, a quick burst of throttle to lift the bow may be the best and fastest way to make sure the boat goes over, rather than through the crest.
It’s in a skipper’s best interest to learn to use the trim devices available on his boat. Getting to know a boat much better and operate it more economically means it will run faster and safer – and you’ll have more fun being a better skipper.