Re: trim tabs, Jack plate, hydrofoils
Hi Zaxxon,<br /><br />The blurb below has been cribbed from a forthcoming paper discussing trim tabs. It conveys the pro's and con's clearly. Hope it's of some use - <br /><br />A boat at rest has its weight spread over the area of the hull in the water. When the boat tries <br />to plane, the bows rise. This leaves a smaller hull area to take the same weight, so the stern <br />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 <br />the hull. The boat then assumes a steep angle as it tries to ‘ride’ this bow wave. The helmsman <br />attempts to correct this condition by trimming the outboard leg. Whether he realises it or not, <br />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 <br />propellor out for greater speed, the stern sinks again and the boat then planes with the stern <br />deeper in the water. This slows the boat, uses more fuel, makes it less responsive to steering <br />inputs, works the engine harder than necessary, and gives an uncomfortable ride.<br /><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. Additionally, because the fin is central to the hull, it cannot provide lateral stabilisation. 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 />The function of the propellor is to drive the boat forward, and it can only do this with maximum <br />efficiency when it is at right angles to the water - ie. running ‘square’. The obvious and most <br />effective answer is to keep propulsion and trim functions completely separate. <br /><br />Trim tabs are specifically designed to control the attitude of the boat separately from the <br />propellor, allowing the propellor to do its proper job. They are far more effective than a fin, and <br />give positive benefits to boat handling, ride, speed and economy. Some trim tabs are ‘dead’ - <br />that is, they remain where they are set by the helmsman. Others adjust automatically. The geometry of their design permits the tab to offer reducing downforce as the speed of the boat increases.<br /><br /> Trim tabs are fitted to the outer edges of the transom, in line with the deadrise angle of the <br />hull. When the boat is planing, they offer no additional resistance - in fact, many boats are <br />faster with Trim tabs fitted. Their position allows them to stabilise the boat on its port/<br />starboard axis. With Trim tabs, the boat will lift onto the plane at a lower speed, so the <br />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 <br />sharper handling response.<br /><br />Ciao
