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.
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 engine(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.
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 engine. 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.
The advantages of this approach to installation of trim tabs are:
The disadvantages of this approach to installation of trim tabs are:
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.
Mounting the tab in this area of the hull causes the tab to bridge across a hull strake or runner. This is shown below.
The advantages of this approach to installation of trim tabs are:
The disadvantages of this approach to installation of trim tabs are:
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 engine 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.
Again, the tab surface bridges an area of the hull bottom which contains a strake or runner.
The advantages of this approach to installation of trim tabs are:
The disadvantages of this approach to installation of trim tabs are:
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.
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 effect 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.
Here are some additional examples of installations of trim tabs on classic and post-classic Boston Whaler boat hulls.
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.
Dave Buckalew added trim tabs to GAMBLER, his Outrage 18 with twin 90-HP E-TEC engines. 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.
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:
190 Nantucket with Lenco trim tabs. Photo by Allan Seymour.
When fully retracted the tabs are angled upward. That is a 150-HP Honda four-stroke on the transom. Photo by Allan Seymour.
Detail of the starboard trim tab installation which fits under the swim platform. Photo by Allan Seymour.
The tabs bridge across a hull strake. Photo by Allan Seymour.
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.
The optional LED tab position indicators are shown. Photo by Allan Seymour.
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Author: James W. Hebert
This article first appeared January 2007.