Trim tab position ?

[Vronsky]

The trim tab above the prop on my DF115 is turned a few degrees off the center line to starboard >
Engine has been delivered this way, I believe.

Is this perhaps the default position position of the trim tab ?

I notice that the stern of my boat moves to starboard when I shift to reverse:
never understood why this is happening (stern should move to port when prop rotates anti-clockwise in reverse), but could the position of the trim tab be the cause of this?


BIG THANKS,
V.

[Solarman]

That little trim tab is not what causes your motor to turn very much. It may affect steering a small amount at higher speeds, but not by much. A couple degrees is not much out of line.

Perception is the key to Dynamics. You say your boat moves starboard. Are you in a lake, with absolutely NO currents, perfectly still waters, and NO winds that may be acting against the hull (causing boat to turn - even slightly?).

For example, what causes a jet/plane to fly is positive and negative pressures created by the wings plus the ailerons / flaps / rudder.

So water is more of a solid than air (air compresses, water doesn't). This means when the prop turns it increases pressure on one side of the blade and creates a negative pressure on the other side. Add into all of these thoughts, the torque factor of the motor, and then is it possible the motor turns slightly as all these effects are occuring?

I don't know. You tell me what you come up with. I think I would be concerned if the motor sounds good, or if I'm having a good day catching some nice fish. Pulling slightly starboard, in reverse, doesn't sound too critical, in the overall scope of things. Lol

Happy boating! Enjoy!

[Vronsky]

So the trim tab being a few degrees off set is the default/factory setting ?

This is what happens when reversing: for instance when I'm approaching my dock, no wind/current effects >
first I bring RPM down to idle, then shift to neutral for a few seconds, then shift to reverse and maintain idle rpm: as the boat slows down, the stern slowly turns starboard, and I correct this by rotating the wheel counter clockwise (still in reverse) to keep boat's course straight; if I keep the wheel rotated too long, the stern goes to port, so I rotate the wheel back before that happens.
This is strange to me as the prop walk (+anti clock wise in reverse) -if at all present- is supposed to move the stern to port in the first place.
Whatever causes the stern to move to starboard is apparently stronger than any prop walk.

[Solarman]

You can check with your dealer on what they set the trim tab at, I cannot see it, unless you post a pic. But I can tell you there are many actions that can affect exactly what you're saying. Yes, you can change the trim tab slightly, 1-2 degrees, but I have experienced exceptionally fast deterioration of the trim tabs (mine are zinc tabs) on both of my motors when they were more than 1-2 degrees from inline, or centerline position. With respect to the rest of the lower unit/leg.

I was told adjusting them slightly would help my stiff steering to port at cruising or faster speeds, but it didn't help at all. After running about 80 miles at 3 degrees (1 trip to the gulfstream and back plus trolling), both tabs had disappeared/dissolved. After replacing and putting back straight, 3 more trips to the gulfstream, and they still look new, also steering had not changed. Both of my motors are clockwise rotation, so I'm attributing the steering stiffness to that. Hopeful to correct that as I'm replacing both motors in the next month, and they will be counter rotating this time (not offered on the short-shaft motors in the past).

Good luck.

[Harper]

The motor comes with tab centered (what you are referring to as "default"). As Solarman has said, it should not cause any appreciable yaw when placed off center. It is just not large enough to do that. The purpose of moving the tab is to balance the effort required to turn the motor. If it is more difficult to turn the wheel in one direction than the other, then moving the tab in the direction of the more difficult steering direction should help to balance or equalize the steering effort.

[Moonlighter]

And by the way, the tabs on the 4 strokes arent anodes, they are purely trim tabs and are painted. The motors have a large bock style anode bolted to the bottom of the bracket.