Trim your boat for performance

Mark Rothfield
Trimming is usually done to improve speed, but that’s not the whole picture.

Boat designers and engine manufacturers increasingly strive to make the modern powerboat driving experience as intuitive and homogenous as that of a car, but there’s one thing they can’t replicate – motor vehicles are suspended on four wheels at each corner, atop mostly hard surfaces.

Boats, on the other hand, float on fluid and are generally triangular – vee bow at one end, square transom at the other, separated lengthways by varying rocker and deadrise. That it all comes together so well is a marvel of hydrodynamics.

That said, the driver can’t rest on his or her laurels if they want to keep the craft running with the optimum efficiency, comfort and safety. Trim plays an ever-changing role here, yet it’s frequently under-utilised or even ignored.

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A well-trimmed Signature running efficiently at high speed

Trimming is usually done to increase top speed, but that’s far from the whole story – ride level impacts all aspects of balance and performance.

The first step is to remove all superfluous items on board, as your boat’s power-to-weight ratio holds the key to how it will accelerate and run. Fluctuating fuel loads are a significant factor, but there’s the option of positioning passengers and gear to maintain the best overall balance.

If there’s too much weight consistently forward, the hull will run bow heavy, it will be hard to steer and unable to perform to its speed potential. It may also take spray over the front. With too much weight aft, the boat will struggle to plane; it may have excessive bow rise and a tendency to ‘porpoise’ or ‘chine walk’.

Observing the boat’s profile while resting in calm water under normal load will determine if it looks ‘right’ against its waterline. Lateral balance can be detected from behind the transom, again using the waterline or outer chines as a guide.

Keep your engine trimmed in at trolling speeds for extra prop grip

Starting with the engine trimmed fully down (in) will help reduce bow lift when transitioning the planing hump, however from here on it becomes a matter of reducing hull drag, which means running with the least amount of wetted surface consistent with safety and good handling.

There are several methods available to adjust hull attitude. Boats with inboard engines may have ‘cav’ plates; other boats might have trim tabs, a variation on the cav plate theme that give added flexibility in trimming.

On outboards and sterndrives, hydraulic rams trim the prop angle, which also changes the water depth in which the prop operates. Trimming in, as mentioned, pushes the stern up, minimising bow rise and preserving good forward visibility.

Once on the plane, trim the outboard out to roughly a quarter of the way through its trim range, then to around half – the exact position will depend on the individual set-up, so it pays to look at the engine to confirm the accuracy of the meter.

The bow will rise as the prop angle changes, and revs will increase as the load on the prop reduces. When the trim angle is optimised, only the slightest amount of torque can be felt on the wheel.

Passenger weight is movable ballast when you need to maintain trim

For maximum speed, trim the outboard further up (out). The changed angle of the prop takes more hull out of the water for reduced drag. 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 incrementally, allowing the boat to settle at each step. It’s important to listen to the engine at this point and not just trust the gauges – you can hear when the motor is running well and not labouring or, conversely, over-revving when cavitating.

If a smaller craft is trimmed out too far, it can induce prop ventilation as the blades get too close to the surface and suck air along with water. The hull may also become unstable because there’s insufficient running surface contacting the water.

In this situation, 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.

Don’t forget to trim up when manoeuvring in shallow water

Where a boat is trimmed out for straight-line speed, the opposite applies when turning. Trimming in lowers the propeller and drops the bow for better grip and stability through the turn. As the boat straightens up, trim out again during acceleration to regain speed.

When running with a prevailing chop or swell, trim the engine out slightly 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 to keep the boat level and to prevent the bow from flying high when it runs into waves. On some bowriders, the opposite applies – trimming the bow up may make it smoother for front-seat passengers. All hulls are different, of course.

As the boat comes off the plane, get into the habit of trimming the engine in. This gives maximum grip for manoeuvring at low speeds and means you’re ready for acceleration at any time.

Zipwake tabs work vertically to disrupt water flow

The skipper can also control the boat’s attitude with trim tabs or cavitation plates, where fitted to the transom. For shaft-driven boats that don’t have the advantage of prop angle adjustment, these devices are invaluable.

They 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 as they can 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 the hull. This raises the stern and pushes down the bow for enhanced control in rougher water or when turning.

For higher speeds, the cav plate is lifted out of the water to avoid creating drag. However, if the boat is kept at an extreme level of ‘high’ trim, it will lose even more stability in a turn and perhaps spin out or roll, terrifying your passengers.

Whittley 2180CR running Bennett SLT spring-loaded trim tabs

Trim tabs are fitted to each side of the transom 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 lateral trim.

Some tabs are operated manually, some by a manual-hydraulic method, but the best boats have an electric-hydraulic system controlled by switches at the helm. Lowering either the port or starboard tab will lift the stern on that side and drop the bow on the other side to correct the listing effect.

This is commonly needed when passenger distribution is unbalanced, or when a crosswind or swell impacts a high-sided, deep-vee hull that tends to ‘lay over’. As the skipper turns the wheel slightly to counter the effect of the side wind, this inadvertently banks the boat – it’s actually doing a slow turn to stay straight against the wind.

On outboard and sterndrive boats you want the tabs and propellers to work in harmony. Every boat is different, but as a general rule you should use prop trim for the fore and aft control of the boat, and the tabs for side-to-side balance – tabs are there to assist, not resist.

Gyros keep roll angle in check at all speeds

Don’t forget that the throttle is an important part of trimming. Accelerate hard and the bow will rise. Pull back the throttle quickly and the bow will drop. When confronted bow-on with a boat wash or larger wave, a quick burst of throttle may be the best way to make sure the boat goes over, rather than through, the crest.

Another lesser-known factor in outboard installations is engine height. Ideally, you want the bottom of the keel to align with the outboard’s anti-ventilation plate when the gear casing is trimmed horizontally flat.

That gives you the full benefit of trim range and minimises engine drag, which in turn can help the motor achieve full rev potential. A low outboard, conversely, will almost always impede handling. The tell-tale sign is when your outboard is kicking up too much spray.

Propeller selection can also have a bearing, especially as engine torque increases. Some modern prop designs incorporate cupped tips for bow lift and extended straight barrels for maximum stern lift. Larger diameter propellers naturally push more water, which is good for hole shot, pushing heavy loads, tow sports and slow-speed manoeuvring, but they can impact top speed.

The lower the prop’s pitch (which refers to the distance the propeller would theoretically move through a solid in a single revolution,) the better the hole-shot. The higher the pitch, the more speed that’s achieved unless maximum rpm isn’t achieved.

As a general rule of thumb, each inch of pitch is worth approximately 200rpm. The trick, therefore, is to choose a propeller that delivers both acceleration and top speed to improve your hull’s performance with less reliance on trim.

Mercury’s Active Trim automatically adjusts for cornering

Getting back to that aforementioned car-driving analogy, new technology has automated a lot of trim functions if the skipper prefers to simply point and shoot.

Mercury Marine’s Active Trim is a hands-free, GPS-aligned system that adjusts trim based on changes in boat speed to improve performance, fuel economy and ease of operation. It even responds to boat manoeuvres, however the skipper can also override the system at any time by using the manual trim buttons.

There are myriad automated trim tabs systems on the market, along with gyroscopes and hydrodynamic fins for lateral stability.

After-market hydrofoils that attach to the anti-ventilation plate are found on some smaller boats lacking regular tabs and potentially minimally powered. They help lift the stern during hole shot, however at high speeds they create unwanted lift until eventually they increase drag and add stress on the plate.

Correctly used, trim and tabs can provide enormous control over a boat. Good drivers in rough conditions are constantly varying the speed, trim and steering to give passengers the best possible ride. Using these tools wisely for best performance can be highly gratifying.

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