umblecumbuz
Lieutenant Junior Grade
- Joined
- Sep 25, 2004
- Messages
- 1,062
Boat Trimming with Foils or Tabs<br /><br />A boat at rest has its weight spread over the area of the hull in the water. When the boat tries to plane, the bows rise. This leaves a smaller hull area to take the same weight, so the stern sinks. This is exactly the opposite of what we want to happen.<br /><br />As the speed rises the stern stays low in the water and a bow wave is created midway along the hull. The boat then assumes a steep angle as it tries to ‘ride’ this bow wave. The helmsman attempts to correct this condition by trimming the outboard leg. Whether he realises it or not, he begins applying ‘negative trim’ - in other words, he pulls the outboard leg as far into the stern as he can, which forces the propellor to push upwards as well as forwards. The stern lifts, the bows lower, and the boat begins to plane. All seems well - but it’s not!<br /><br />The boat still has a marked angle to the water. Whenever the helmsman tries to trim the propellor out for greater speed, the stern sinks again and the boat then planes with the stern deeper in the water. This ‘drags’ the boat, uses more fuel, makes it less responsive to steering inputs, works the engine harder than necessary, and gives an uncomfortable ride, because the propellor is trying both to move the boat forward and to control its angle in the water.<br /><br />FINS<br />One of the ways of minimising this effect is to fit a fin on the anti-cavitation plate of the outboard. This helps, but has several disadvantages. The fin angle is tied to the propellor angle because both are fitted to the same leg. Most fins work better with slight negative trim, and this means that the propellor will still be operating ‘out of square’ with the water. Some fins have an aerofoil cross-section, and with these, the propellor can be trimmed to run ‘square’, allowing the fin to provide needed stern lift. However, regardless of fin type, there is a negative trade-off. Because the fin is central to the hull, it cannot provide lateral stabilisation (port/starboard). As the fin does not follow the deadrise angle of the hull, it is also a permanent additional source of underwater resistance. The enlarged horizontal surface of the fin can also cause the boat to be less responsive in turns.<br /><br />PROPELLOR<br />The function of the propellor is to drive the boat forward, and it can only do this with maximum efficiency when it is at right angles to the water - ie. running ‘square’. THE OBVIOUS AND MOST EFFECTIVE ANSWER IS TO KEEP PROPULSION AND TRIM FUNCTIONS COMPLETELY SEPARATE.<br /><br />TRIM TABS<br />Trim tabs are specifically designed to control the attitude of the boat separately from the propellor, allowing the propellor to do its proper job. In this, they are more effective than a fin, and give positive benefits to boat handling, ride, speed and economy. Trim tabs are fitted to the outer edges of the transom, in line with the deadrise angle of the hull. When the boat is planing, they offer no additional resistance - in fact, many boats are faster with trim tabs fitted, because they keep the stern higher in the water, giving the boat a reduced ‘footprint’. Their position allows them to stabilise the boat on its port/starboard axis. With trim tabs, the boat will lift onto the plane at a lower speed, so the holeshot - the time taken to reach efficient planing speed - is also faster. The ride will be more comfortable and as the attitude of the boat is more level, the helmsman will have a better view. The engine will work more efficiently, so fuel consumption will be reduced. The forward part of the boat’s hull remains in better contact with the water, and this provides the helmsman with sharper handling response. A well-designed tab profile will also help reduce slippage in tight turns, and will have beneficial effects on boats with a tendency to either chine walk at high speed or wander at low speed.<br /><br />Most trim tabs are ‘dead’ - that is, they remain where they are set by the helmsman, and need to be adjusted either manually or with a remote control (often helmstation mounted ). One advantage of such remote controlled tabs is their facility to adjust the lateral attitude of the boat while under way. This permits levelling the boat due to temporary uneven loading, or raising the side of the craft to seaward to minimise spray. Such adjustments require finesse and common-sense if they are not to be overdone. <br /><br />Another trim tab type, for smaller sports and fishing craft, uses a pressure actuator to continually adjust the angle of the tab plate to compensate for the movement of the boat through the water. This is automatic, with no input needed from the helmsman. This type cannot adjust for lateral trim when under way. The geometry of the design permits the tab to offer reducing downforce as the speed of the boat increases.<br /><br />WANDER<br />As any yachtsman knows, the straight line course of a sailing vessel needs correction to compensate for heel. This is because the hull is only symmetrical when sailing upright. As the yacht heels, a cross-section of the hull in the water will show an assymetric profile, port to starboard. The hull is in effect acting as its own secondary rudder. To a lesser degree the same principles apply to a deep V sports hull. As it moves slowly through the water, its lateral attitude is affected by wind, current, shifting load, or other momentary conditions, and this causes the hull to temporarily assume an assymetric underwater profile, which results in the boat ‘self-steering’ - or wandering. Trim tabs will lessen this effect by their stabilising action on the hull.<br /><br />Not a bad result for a coupla metal flaps on the transom!
