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Trim Tab Installation

by James W. Hebert

Because of the construction of the transom of classic Boston Whaler hulls from 18 to 27 feet, special consideration must be given to the location for mounting trim tabs. We briefly discuss several of these concerns.

Trim Tab Mounting Location

When choosing a location for a trim tab, the further outboard from the vessel centerline the trim tab is mounted, the greater effect it will have on the boat's lateral trim. However, on a Boston Whaler boat, the transom construction and configuration influence the location.

The transom of most classic Boston Whaler boats is not of uniform thickness, and the stern-facing surface of the transom is generally not flat. In the center section of the transom there is a raised portion which is several inches thicker than the areas located farther from the boat centerline. This is the portion of the transom which is reinforced for mounting of the motor(s). Because of this raised transom center, the location for mounting trim tabs is constrained. The tabs must generally either be located entirely in this raised area, or located outboard of it, unless some spacers are used to bridge the two levels.

Close to Centerline Mounting

The inboard or more centerline mounting location is generally only practical if the boat is powered by a single engine and there is no auxiliary motor. A typical installation is shown below. The trim tabs are mounted as far from boat centerline as possible while still being located entirely on the raised portion of the transom.

Photo: Stern view of Boston Whaler OUTRAGE 20 hull showing trim tabs mounted on raised portion of transom.
  These are LENCO tabs with the actuator mounted in the center of the tab. Photo by Rob Stephens.

The advantages of this approach to installation of trim tabs are:

The disadvantages of this approach to installation of trim tabs are:

Farther From Centerline Mounting

As the tab location is moved farther from the boat centerline, the change in the thickness of the transom tends to force the tab into a new location. The tabs move outboard from the boat centerline until they are located entirely on the non-raised portion of the transom. This is shown below.

Photo: Trib tab installation on Boston Whaler OUTRAGE 20 showing tabs mounted on non-raised portion of transom
  These are LENCO tabs with the actuator mounted in the edge position. Note cable routing is not through-transom. Photo by the author.

Mounting the tab in this area of the hull causes the tab to bridge across a hull strake or runner. This is shown below.

Photo: Tab installation showing change in hull bottom beneath tab surface.
  This close-up view exaggerates the distance the tab is mounted above the hull bottom. Photo by the author.

The advantages of this approach to installation of trim tabs are:

The disadvantages of this approach to installation of trim tabs are:

Alternative Mounting, Distant from Centerline

If the tab is modified by a slight trimming of the hinge, it can be mounted several inches closer to the boat centerline. This also shifts the position of the anchor for the tab actuator so that it lands partially on the raised portion of the transom. This installation is shown below. Note that an auxiliary motor has been mounted to the transom, and there is no interference with the trim tab. Note the addition of a pad or riser to support the base of the tab actuator.

Photo: Trim tab installation on OUTRAGE showing tab moved inboard as far as possible while still located on the non-raised portion of the transom.
  This OUTRAGE 22 has a single main engine and an auxiliary motor mounted to Port. Cable routing is through-transom and is concealed by the actuator base. Photo by John Flook.

Again, the tab surface bridges an area of the hull bottom which contains a strake or runner.

Photo: Trim tab installation showing relationship of tab surface to hull surface.
   The tab has been raised to show the relationship between the tab surface and the hull bottom. Photo by John Flook.

The advantages of this approach to installation of trim tabs are:

The disadvantages of this approach to installation of trim tabs are:

Hinge Mounting

Experience has shown that mounting the trim tab in an un-reinforced portion of the transom is not a problem. The Boston Whaler hull laminate in this area is very thick, and screw fasteners can obtain strong purchase. There are many such installations and no reports of any problems. If the screw fasteners are properly installed and sealed, there is no reason for concern about mounting the trim tab hinges in the un-reinforced area of the transom.

The distance above the hull bottom which the hinge is mounted is generally specified in the manufacturer's installation instructions. This distance can vary depending on the design and shape of the tab. The tabs shown are LENCO tabs, which specify mounting 3/8-inch above the hull bottom. Manufacturer's installation instructions also often call for use of a marine adhesive and sealant to further help secure the hinge to the hull.

Actuator Mounting

The actuator mounting base is an area of high stress. If it is mounted in an unreinforced area of the transom, it may be necessary to resort to use of through-bolts instead of self-tapping screws as fasteners. This technique may be made more difficult if the transom thickness on the inboard side of the transom is not uniform. Reports of real-world experience with success of the mounting of the actuator base on non-reinforced areas of the transom are solicited.

Among brands of trim tabs there are options for the actuator configuration which permit some adjustment of the location of the actuator mounting base position on the transom. Because of the non-uniform surface of the transom on the Boston Whaler boat, some of these options may be useful to move the actuator base location to a position on the transom where mounting will be easier.

The actuator base often specifies that a fairly large diameter hole (in the range of 3/8-inch to 1/2-inch) be drilled through the transom to pass the electrical cable or the hydraulic hose to the actuator mechanism from inside the boat. It is possible to omit this hole and to route the cables or hoses up and over the transom. This may be preferable if you wish to avoid drilling a substantial hole in the transom. There is proper concern for the affect of the hole on the transom's strength. Given the age and condition of some boats, perhaps the fewer holes in the transom the better, although that is a good rule to follow in any case.

More Examples

Here are some additional examples of installations of trim tabs on classic and post-classic Boston Whaler boat hulls.

Outrage 20

The Outrage 20 seen above was later sold. The new owner, Joe Morgan, removed the 12 x 12 Lenco tabs and installed 9 x 12 Lenco Troll-n-Tabs.

Photo: Trim tab installation showing electric trolling motors on tabs.
   The tabs on this OUTRAGE 20 include electric trolling motors. Photo by Joe Morgan.

Outrage 18

Dave Buckalew added trim tabs to GAMBLER, his Outrage 18 with twin 90-HP E-TEC motors. The location of the actuator bases nicely clears the change in transom thickness. The tabs can provide stern lift which helps compensate for the extra weight on the transom, allowing the boat to plane at lower speeds with a nice bow-down attitude.

Photo: Trim tab installation on Outrage 18 with twin engines.
  Outrage 18 with trim tabs. Photo by Dave Buckalew. Photo: Trim tab installation on Outrage 18 with twin engines.
  The tie down points on the trailer have been moved inboard to work around the tabs. Photo by Dave Buckalew. Photo: Trim tab installation on Outrage 18 with twin engines.
  Tab mounting position relative to hull bottom and strake is shown. Photo by Dave Buckalew.

190 NANTUCKET or 190 OUTRAGE

Allan Seymour added trim tabs to his Boston Whaler 190 Nantucket, GRACE, using the Lenco electrically operated tabs. He chose the edge-mounted style of actuator so the actuator would fit under the swim platform; the regular model requires more height on the transom to attach the base. Allan reports, "Lenco was very helpful in this selection and answering after-installation questions." He sent these photographs to show the details:

Photo: Trim tab installation on 190 NANTUCKET with swim platform option.
  190 Nantucket with Lenco trim tabs. Photo by Allan Seymour.

Photo: Trim tab installation on 190 NANTUCKET with swim platform option.
  When fully retracted the tabs are angled upward. That is a 150-HP Honda four-stroke on the transom. Photo by Allan Seymour.

Photo: Trim tab installation on 190 NANTUCKET with swim platform option.
  Detail of the starboard trim tab installation which fits under the swim platform. Photo by Allan Seymour.

Photo: Trim tab installation on 190 NANTUCKET with swim platform option.
  The tabs bridge across a hull strake. Photo by Allan Seymour.

Photo: Trim tab controls installation on helm console of Boston Whaler 190 NANTUCKET.
  Finding a spot for the tab controls often involves something of a compromise when adding them to an existing helm control layout. Here they are located behind the wheel, but easily reached. Photo by Allan Seymour.

Photo: Trim tab controls installation on helm console of Boston Whaler 190 NANTUCKET.
  The optional LED tab position indicators are shown. Photo by Allan Seymour.

Further Discussion

Questions and comments on this article can be posted to a discussion reserved for that purpose.


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Copyright © 2007 by James W. Hebert. Unauthorized reproduction prohibited!

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Last modified: Tuesday, 04-Mar-2008 20:19:35 EST
Author: James W. Hebert
This article first appeared January 2007.