As we have seen in previous articles, the application of trim tabs is particularly useful to any boat. They improve hole shot, boat’s attitude and handling, making the ride more stable and comfortable in choppy seas.
However, in order to get the maximum from their use we should choose the appropriate size for our boat and install it in the right way.
It is known that the outer surface of a hull is covered by a special resin called gelcoat, which mainly protects it.
Thus, this gelcoat is constantly exposed to the sea-water and sun, leading to its erosion. Over the years the gelcoat loses its shine, becomes dull and porous. The bigger the pores become, the more permeable the gelcoat gets. It also stains more easily and then of course it is harder to clean it putting the hull in a great risk.
By the term of deadrise we mean the angle which is formed between the horizontal plane and the side of the hull, at its any point.
As it can easily be understood, this angle is not constant but varies gradually over the whole length of the hull. In other words, the deadrise angle starts with its lower value at the transom and gradually increases as the hull goes forward until it takes its maximum value at the bow, where the hull cuts the wave.
It is a fact that every manufacturer of outboard engines installs different gear ratios while we see that as the horsepower decreases the gear ratio gets higher (numerically), which means the revolutions per minute of the propeller decrease.
As we have seen in previous articles, everything is a matter of Torque and combination of propeller’s diameter and pitch.
The torque that is produced by the engine is converted to thrust by the combined effect of the gear ratio with our propeller. Physics teaches us that the Torque is proportional to the horsepower (Hp) and inversely proportional to the rotation speed (Rpm). Thus, the higher the horsepower or the lower the rpm of the propshaft is, the greater the torque that is produced gets.
The propeller rotates at the same speed with the propeller shaft, while the propeller shaft usually rotates much slower than the engine crankshaft turns, at the same throttle setting. This is due to the reduction gear which exists on all the outboard engines among their crankshaft and propeller shaft.
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.