By Thomas P.

Many people believe that trim tabs are useful only on large cruisers or hulls with inherent imperfections in their design. There is also the opinion that well-designed hulls do not need the assist of trim tabs. The practice, however, shows us that the smaller the boat is, the greater the need for trim tabs gets, because they are much more affected by weight distribution while under certain circumstances the use of trim tabs will greatly help even the best designed hulls.

χρήση flaps

If we accept that one of the «secrets» for the best performance of each planing boat is the maintenance of the right balance, right weight distribution and the best possible riding angle relative to the surface of the sea in all circumstances and in all the range of engine speed (since these are the major factors that affect the planing, acceleration, cruising speed, fuel consumption, top speed, quality of the ride and generally the handling of the boat in each case) then we reach the conclusion that the use of trim tabs is essential to any boat.

We also reach the same conclusion if we accept that each boat is designed to behave well under certain circumstances. There is no boat that can perform perfect in each condition. Some hulls are designed to achieve high speed, some others to behave very well in rough seas, others to accept less horsepower engines, others to carry large loads, others to ensure a comfortable and functional deck...

What will happen when we meet bad weather and our boat is set-up to achieve high speeds? What will happen if sometimes we change the balance of our boat either because we overloaded it or because we use it for another purpose than that it is intended for by its manufacturer? In these cases it is certain that the performance will «suffer».

Unfortunately the balance of a boat is a very «fragile» parameter that varies each time we go for a ride at sea or we depart for a trip, i.e. different loads, fuel, crew. (Except of course if our boat is set-up for very specific use, such as racing).

Therefore, once the weight distribution changes constantly affecting the boat’s balance and its attitude on the water, and if we consider that all the weights lay from the cockpit and back, and then the use of trim tabs which are operating at the stern is absolutely necessary, playing the role of the regulator of the desired running attitude.

Let’s first clarify some basic functions of trim tabs:

  • Their primary function concerns the stern’s lift. When lowering trim tabs the water pressure on them causes upward trend of the stern. The degree of the stern lift depends on the size of the trim tabs, the angle of their inclination and our speed. However, no matter how well designed our hull is, that function will be very useful in specific weather conditions or excessive loads at the stern.
  • Once trim tabs are independently adjustable we are able to control and correct the boat listing to one side, either when the weather requires or when our loads or our crew is more towards on one side of the boat.

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Let's now see what the practice teaches us:

  • Many hulls, especially these which are characterized as «fast», have their best behaviour and perform excellent after 27 to 30 knots. So what will happen if the weather is really bad? Do we have all the experience to ride fast against large waves? And what about if there are children on board? Obviously the above speeds become very dangerous. We will then need to travel at speeds of 15-18 knots. At these speeds, however, the above hulls can not offer us their best performance. The solution in these cases is the assistance of the trim tabs which would allow us to improve our riding at low speeds.
  • Travelling in rough seas with several ribs we realised that the use of trim tabs improves the ride in almost every direction of the waves. When we are running into a head sea it takes only to adjust the trim tabs lower so as to get the bow down, allowing thus the hull to cut the water with its sharp forward section offering us a more comfortable ride. When we have the weather on our port or starboard bow, the side of the bow that faces the waves receives the greatest stress. By lowering more one of the two trim tabs we can lower the windward side of the bow and prevent its steep flick upwards making our ride steadier. When we have strong wind on port or starboard, we can lower the windward trim tab which will raise the windward side of the boat and thereby reduce the irritating spray carried on the deck. If we travel with tents and the wind comes towards on our starboard we can correct the boat-listing by lowering the portboard trim tab.
    In general we can experiment and improve our ride according to our desires.
  • We know very well that Corsair 31 is one of the best designed ribs with a superb hull. On our trip to Azores, because of our excessive loads which had dramatically changed the centre of gravity of the rib, the use of trim tabs was catalytic. Having big waves permanently on our starboard bow, even with the most negative engine’s trim, the hull cut the water at the level of the cockpit. In order to improve the hull attitude we had to increase too much our speed which was impossible in those conditions because beyond the stress of the hull and the great risks, the fuel consumption would extremely increase, having thus the danger not to reach our destination. With the use of trim tabs we secured the optimum rib’s attitude for these conditions, so as the sharp «V» of the forward part of our hull was cleaving the waves and our riding became softer while we could travel at relatively low speeds.

It is absolutely clear that, in many conditions, the trim tabs can be particularly useful to any boat. Trim tabs are an important «tool» in our hands when the weather is bad or when our loads are heavy.

Their proper adjustment in combination with the use of the power trim can give us the best possible performance.

Benefits of trim tabs:

  • they get the boat on plane easily and faster due to the additional stern lift that they are providing
  • they keep the boat on plane at lower speeds, which is very important when we are travelling in rough seas
  • they correct boat’s porpoising allowing us to handle the power trim in the most effective manner
  • they improve the boat’s attitude giving us total control of the bow
  • they improve the boat handling, offering more stable and comfortable ride when running into a head sea reducing the pounding of the hull and the crew.

Certainly their proper operation requires proper selection and installation.

...keep Ribbing!


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