#QuickTips - The Ups and Downs of Trim Tabs
By Bill Jennings
Most boaters know that adjustable “trim tabs” on boats are those metal extensions that stick out from the base of a boat transom. Fewer boaters understand how to make the best use of them. Here are a few considerations to be sure your tabs work well for you or, if you don’t already have them, why should you consider buying a set.
Trim tabs are designed to perform four main functions. These are: extending the planing surface of your boat by adding length to the bottom, lifting one side of your boat when running to correct a lean or list to one side, tilting your boat away from waves that could splash into your boat, and helping to keep your bow down in circumstances where it could improve ride quality. If your boat could use some help in any of these functions you are a candidate for adjustable tabs.
The easiest way for you to be an expert on the subject is to understand water flow characteristics as they pass under trim tabs. Try this: Hold a soup spoon with two fingers at the end of handle. Turn on a water tap in your kitchen sink. With the face (or scoop side) of the spoon facing the tap, slowly move it until it touches the water stream. As the spoon touches the water stream, the water will push it back, moving the spoon away from the water. Likewise, in your boat when the tab is pushed down into the water, the water will push back, raising the tab along with that side of your boat. Now here is the tricky part. Try turning the spoon around and slowly move the rounded side of the spoon into the tap water stream. The spoon is attracted to the water stream and pulls itself into it. Likewise, when your trim tab is raised above neutral, or above a position parallel to your boat bottom, the tab is attracted to the water flowing past and pulls down into it, along with that side of your boat. Few boaters realize that lifting an external tab above neutral will suck that side of their boat down into the water. Because this phenomenon means that a single tab can both raise and lower one side of your boat, tabs can be confusing to operate effectively. Here is the secret to simple tab operation:
Set both of your tabs to neutral. To do this, run a straight edge along the bottom of your boat and move the tabs individually until they are parallel with the bottom. If you have tab position indicators, mark this neutral position on the indicator with a dot of 'white out.' If you don’t have an indicator, count the seconds it takes to run the tab from neutral to a full up position. This will allow you to find neutral when running, by moving your tabs full up, then moving them down for the same number of seconds you counted between neutral and full up. Once in this neutral position, the tabs are delivering the advantage of a lengthened planing surface. But what about the other benefits?
Most tab controls consist of a pair of toggle switches. The toggle on the right generally controls the left or portside tab. Stated differently, each toggle switch operates the tab on its opposite side. Manufacturers will give you all sorts of reasons for doing this, none of which I have figured out. Especially now that we know they only need to move down from the neutral position. To look at an example, let’s say your boat is leaning left (to port). You figure that if you raise your portside the boat will be level. You know that if you lower your port tab, it will raise the port side. To lower the port tab, you press “down” on the right-hand toggle switch. Presto - that’s it. Two important notes: 1) Tab switches react quickly to change the attitude of your boat so don’t be caught off guard and consider making small adjustments at a time. 2) When your tabs are not needed keep them in the neutral position.
Tabs that come as original equipment are usually notched into the bottom of the boat. This eliminates a disturbance of flow over the tab and the drag that is created if there is a space between the hull and the front of the tab. Aftermarket tabs are not flush mounted but are easily mounted to your boat transom and can represent a useful driving tool that is as important as your trim.
If you decide you are in the market for tabs, where do you find them and what is the price? Since the invention of adjustable tabs in 1959 by a man named Bennett, I am aware of over twelve manufacturers that produce them. Each offers tabs in various sizes and materials to match your requirements. Some are electric and others hydraulic. Your budget may be the deciding factor, but for boats up to 25 feet they generally run north from $600 for a pair, including controls and connections. Over that length, contact your dealer and a tab maker for their recommendations. For high performance boats contact specialized builders like Mercury Marine with their K-Plane brand.
Remember that all small boats will lean like a three-legged stool if the load is not properly distributed. Before departing, seat your passengers carefully. But asking that overweight friend to change seats could evoke hard feelings - so tabs could be an option. If your wife won’t go in the boat anymore because she always gets soaked - tabs could be an option. If your boat leans so much you always come home with fewer people than you left with – tabs could be an option. If you can’t see anything but sky for two minutes, whenever you come on plane – tabs could be an option. When you have your tabs figured out and friends compliment you on your boat driving, you can just tell them to ‘put it on your tab.'
We should note that there is another boat appendage also called a “tab." This is the “torque trim tab” found on outboards and stern drives. Please don’t confuse the two. The “torque tab” is the small aluminum triangle attached to the bottom of the cavitation plate. Its function is to help counter the effect of prop torque, created by water pressure differences at the top and bottom of the propeller. If you notice your boat consistently leaning slightly, even when your fat friend is not onboard, a simple adjustment of this tab may straighten things out. Once you have rotated and fixed this tab to its optimum position by trial and error, it does not move. Always play with your torque trim tab, before investing in full set of adjustable trim tabs.
For a visual tutorial and to watch trim tabs functioning at speed check out the video below (*begins around 11:40)