HALF-INCH UP

QUESTION: Help! I purchased a set of Eddie Marine E-Plane trim tabs. The shop I had install them said they are supposed to be mounted flush with the bottom of the hull. I was told they should be mounted a half-inch to an inch up from the running surface to prevent them from interfering with the water flow while in the up position.

Which is the correct mounting position? They have already been mounted by the shop flush with the bottom, which is bothering me!

ANSWER: It is customary for all hinged-style trim tabs such as Mercury Racing K-Planes, Dana Marine tabs and Bennett Marine trim tabs to be mounted on the transom about a half-inch above the bottom running surface. It is best not to have the tabs contacting the water when they are in the full up position.

If they are mounted flush with the bottom, it is common for the water to be forced through the hinge area, which causes unnecessary drag. And if they're spring-loaded tabs, they can be sucked down by the water passing under the plane while underway, and that can cause undesirable handling problems.

The bottom line is that you should take the tabs off, repair the transom and reinstall them in the correct location.

WHAT IS NEUTRAL?

QUESTION: I am just getting into boating. I would like to know how to read drive and tab indicators. What are the neutral positions on the indicators?

ANSWER: When it comes to mechanical indicators on drives and tabs, there are at least a half-dozen brands, and that many more indicator cards. Some start at zero and go up, and others count down from about three to zero, and then back up to eight or so.

Considering the varying systems and the human element of cable adjustments, the “neutral” position could be just about anywhere on your indicator. So how do you know what the neutral position is on your drives and tabs? You will have to find out for yourself and mark your indicator panels.

With the boat out of the water and on the trailer, use the trailer jack to level the boat. In most cases, using a level on the keel or chine near the transom will give you a good indication of when the boat is level. Then, have someone trim your drives to a point where the cavitation plates or prop shafts are level (parallel) to the keel. The indicator reading at the helm with the drives parallel to the keel would be “neutral” for your boat.

The procedure is similar for the tabs except a straightedge is used to determine the “neutral” position of the tabs. The tabs are considered at a neutral position when the bottom surface of the tab is parallel with the running surface of the bottom just in front of where the tab is mounted on the transom.

Most trim tabs are mounted about a half-inch higher on the transom than the running surface. If you place the straightedge on the running surface of the bottom just in front of the tab and there is a half-inch gap between the straightedge and the tab near the transom, the neutral point will be when there is also a half-inch gap at the trailing edge of the tab straight aft. Once this position is attained, note the indicator reading at the helm. That is the neutral position of the tabs.

This procedure will also alert you if the indicator cables need to be adjusted. If there is a difference from side to side on the indicator panel when you have the trim at a level position or the tabs set the same, the indicator cables should be adjusted to result in the same reading at the helm. Be advised that there is a certain amount of slack in the indicator cables and that longer cables have more slack than shorter ones.

That being said, it is always best to approach a setting from the same direction. By this, I mean that you should trim both drives up to the desired checking point without having to adjust one back down. A minor adjustment in the opposite direction could result in no movement of the indicator because of the slack in the cables.

—Need some guidance from Bob Teague, Powerboat's lead test driver and owner of Teague Custom Marine? To send him an email click here.

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