IMHO There's so many mistaken ideas idea's about trim tabs and their correct use in this thread. A few people have the right idea with prop torque being a MAJOR culprit in causing a hull to lean, esp. to port.
The little trim tab on the underside of the anti ventilation plate plays a MAJOR role in getting your boat to perform best. If it's not set right, everything else is out and Trim Tabs are a band aid solution.
Before you start messing with the trim tab on the AV plate, make sure your boat floats perfectly level when normally loaded with fuel etc. and no one on board.
Basically the trim tab needs to be set, so that at WOT running speed with the motor trimmed to max rpm without venting (no cavitation as its commonly termed), you should feel no bias in the steering force to turn either way. Best tested and set in nill or very light wind situations. At this point the motor is at its most efficient. Anywhere from this point trimming up, you would hear ventilation. Trimming down, will bring the bow down, decrease in RPM and speed, an increase in force req'd to hold the steering straight in a tiller steer or an increase in force to turn to port and easier to turn to starboard in hydraulic steering, older cable etc. types should be easier to feel. With hydraulic steering it can be hard but not impossible to feel the torque. You will also notice the port side lowering as you increase down trim and hold a straight line. This is the prop torque. Once we have that sorted you'll that evenly loaded in light winds the boat will run true.
Once the AV trim tab is set at WOT, you'll be able to find the sweet spot at ANY speed.
Wind from the side of a boat tries to push the boat sideways, the rear end is pretty well fixed to its track in the water by the motor leg, so the bow moves easier. As wind pushes on the starboard side, the bow moves to port, so you turn more to stb. to counter that, effectively your boat is now in a slight stb. turn. What happens in a mono hull when you turn stb.? It rolls to stb. as well. Why? Because the prop not only turns the boats bow by changing the thrust line, but that thrust line to the stb. is below the centre of gravity of the boat, so it pushes it over or rolls to the starboard.
Here's where Trim Tabs like Bennets or Lencos etc. come into play.
When the boat is rolling into wind, its not cutting the water as the hull was designed to do, it's partially bashing a much shallower angled hull side making the ride uncomfortable and not very efficient. Use trim tabs to level the hull.
When pushing into chop or a sea, it's often more comfortable to have more of the front of the hull working, cutting the slop and smoothing the ride. As we saw, trimming down or in will lower the nose and smooth the ride, at the expense of a roll due to prop torque. Use the Trim Tabs to lower the bow but leave the engine trimmed at its most efficient spot.
As mentioned already, in a following sea you want Trim Tabs up and the motor trimmed at its most efficient spot for the speed, keeping the bow higher.
Of course trim tabs can also even up a poorly loaded boat, but at the expense of fuel consumption.
I hope this helps someone. I learnt so much from my first PTT equipped tinny.
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Originally Posted by Chimo
