Motor trim tab

[Sloop John B]

Underneath the rear of the cavitation plate on my Yamaha is an adjustable trim tab. I just noticed that mine is cocked just a bit to starboard. Its not loose and cant be turned. Theres a bolt from above that could loosen it.

I examined this trim tab when the motor was new and my recollection is that the tab was straight.

A guy down at the marina repair yard backed my motor into something and bent one of the tips of an aluminum prob blade. I wonder if he caused the trim tab to be moved?

I understand the vertical stabalizer on prop aircraft are kinked a bit to one side to offset the prop's torque. So, I'm confused.

Whats your trim tab look like?

[kmclemore]

Whats your trim tab look like?

Well, mine bends a little bit to the left, and it's kinda old and corroded looking... but thank goodness it's not loose.

[Mark Prouty]

As long as your boat steers straight (not pulling to one side), your trim tab should be ok. If it steers to one side, then you might want to straighten it.

[Jesse Days Pacific Star 2]

It's been my experience that most are slightly off-center.

J.

[Terry]

This is a shot in the dark but here goes: I wonder if it has anything to do with offsetting the rotation of the propeller, If a RH turning prop was steering you more to the left, then adjusting the trim tab the oposite way might counter this effect. Talk about a wild geuss!

While on the topic of trim though, how many have one of those trim meters beside the tach on your pedestal and electric button for adjustment, and it is set up for proper use and does anyone use it. Mine always has the arrow pointing way down even though the bow is lifted when under way. I don't think the dealer really set it up correctly but the engine leg is perpendicular to the hull and the anti-ventilatin plate sits about one inch below the hull so I believe it is installed correctly, I just question the set up of the trim meter. Any ideas out there?

[craiglaforce]

I guess the trim tab has 2 functions. One is that it is a sacrifial Zinc (at least on my Tohatsu) and the other of course is a fine tuning adjustment to make the boat go straight when the helm is straight. If it is getting corroded then maybe it should be replaced to maintain galvanic protection on the motor.
If you are happy with the steering then no need to adjust it. Mine is straight.


My Tohatsu has this and a long bar made of Zinc that sits at the waterline for galvanic protection.

[Jesse Days Pacific Star 2]

Actually Terry, that explaination is correct.

[Chip Hindes]

At least on my Tohatsu 50D it doubles as a sacrificial zinc. That's why it's not painted and except when new usually looks somewhat corroded. That means it's doing its job as a zinc.

I did a google on trim tabs, and IMO there is such a wealth of pseudo scientific, bogus and completely contradictory information, I wouldn't believe any of it.

I think you're on your own. If you think you have a problem, mark it so you can put it back where it is now. Then adjust it, test it and see what happens.

[Kevin]

My trim tab is turned a bit. It's still the way the dealer set it up.

I went through the engine manual and it describes the steps to adjust the tab but also says that it will vary according to weight and speed, so you set up the boat the way you want, get the speed you want, let go of the wheel and see if it pulls. Adjust the tab (which would seem to require pulling it out of the water) repeat.

On the whole, it tracks fine for me with the rear of the tab offset to starboard a bit. I'm not messing with it.

[Bobby T.-26X #4767]

Mine came a bit off center and I've never touched it.
Steers straight for me.

[TonyHouk]

Hey All,
If I remember right to make the boat go right you have to adjust the tab to the left. It is trimming the engine and not the boat. So what it is doing it slightly pushing the motor to the opposite direction of the direction you placed the tab in. Hope that helps and happy sails, Tony

[Moe]

One of the reasons the steering pulls to one side or the other, and must be offset by the trim tab, is that the boat is not balanced fore and aft, and the motor must be trimmed down (stern lift) or up (bow lift).

When the motor isn't vertical, not only are you losing some thrust in the forward direction (and using it to lift the bow or stern), but the prop blade on one side of the boat has more pitch (actually greater angle of attack relative to the forward direction) than that on the other side. This shows what happens with a right-hand drive prop:



This isn't the only reason, and there may still be some pull from motor torque that can be offset with the tab. Don't have time to go into that now.

--
Moe

[Dimitri-2000X-Tampa]

Mine is off center too and for good reason.

Interesting coincidence that when you are in a low speed high power situation in a single engine prop plane, you have to apply right rudder to compensate for the "P" factor. Like when taking off for example.

I wouldn't be surprised if it is the same reason for our outboards, although it implies that the prop is not completely perpendicular with the surface of the water, ie. it is pushing the water down a bit because of angle of attack (this would push the stern up and bow down). The way it is explained with airplanes is that since the air is hitting the prop at an angle, the downward side of the prop rotation has a higher angle of attack and airspeed (props are like spinning wings) than the upward rotating side. On most airplanes, the prop rotates clockwise which makes the downward side the right side. Because there is more thrust on the right side, it yaws the airplane to the left which requires the right rudder to compensate. Seems like fluid dynamics would work the same way as aerodynamics IMO.

I guess that is pretty much the same thing that Moe said too. The only difference is perhaps these motors are not designed to work in a perfectly horizontal thrust angle. This may be verified by the fact that my boat goes slower if I trim the engine so that the thrust is parallel with the horizontal surface of the water. If I trim it so that the thrust is pointing slightly downward (lifting the stern), it goes faster.

[Moe]

If I trim it so that the thrust is pointing slightly downward (lifting the stern), it goes faster.

That means that your boat is stern heavy.

There is an optimum attitude for the hull where it will present the lowest drag.

When the boat is bow heavy, with the bow below the optimum attitude, more of the hull bottom is in the water increasing drag. This can be compensated for by trimming out (up) past the motor being vertical.

When the boat is stern heavy, the attitude is too steep, with the bow above the optimum attitude, this also increases drag because the stern sits lower in the water. This can be compensated for by trimming in (down) past vertical.

The way you find this is to run WOT, trimming up from full down, a little at a time, pausing to let the boat stabilize. As you trim up, RPM and speed will rise as the drag decreases, until it doesn't rise any more, and may eventually start to fall. Note the motor's attitude at the point that RPM and speed just stop rising.

If it isn't vertical, and the propellor shaft parallel with the water, you're sacrificing forward thrust to compensate for an unbalanced boat.

--
Moe

[kmclemore]

For what it's worth, here's what it says in the Tohatsu manual... pretty vague, though.