Tuesday, May 24, 2011

Trim Tabs

     I pulled the trim tabs off the boat in order to barrier coat behind them, and also to clean and check them over before re-installing. I did some rough calculations on the size of the tabs while they were off the boat, and started to get that uncomfortable feeling I get every time I look at the design compromises the factory has to make when building a production boat. A bit of searching on some Internet forums confirmed my fears - the tabs on the 340s of this vintage are undersized. The tabs on the diesel version of this boat are 6 inches wider - 12" x 24" instead of the 12" x 18". But if you look at the hull, 24" wide tabs really don't fit that well, since the flat section of the hull, between the lifting strake and the prop tunnel, isn't 24" wide. Don't get me wrong; 24" tabs can be installed, but they won't allow for a smooth flow of water at the edges, and that is NOT how I intend to maximize speed and economy. With the price of gas the way it is these days, every little bit counts.
     So, what to do? If you consider that the tabs main purpose is to provide lift, it is clear that a longer tab is more effective than a wider tab. A longer tab applies the lift farther from the center of rotation of the hull, and therefore has more leverage. It follows that, for a certain amount of lift, a longer tab can do the job with reduced drag. Take a look at Mercury Racing's K-planes - they are long, stiff, and not very wide. Since a longer tab is more efficient, and my hull won't support a wider tab, I decided to modify the existing tabs to maximize performance. (I just can't leave well enough alone, it seems).
     Here is how the tabs looked when I pulled them off the boat:
     That's the cleaner one. First, I ground the 4 rivets that secure the cylinder mount/reinforcement to the trim tab. Then, I separated all four components, and gave them a thorough cleaning with fine rotary wheels on my die grinder. The cleaned parts looked a bit better:
     I then had the local metal supply house shear out some 1/8" thick, 304L stainless steel plates, in an 18" x 22" size. Once I had these home, I layed out my corner radii, corner notches, and bend lines, as seen here:
     The corner notches were cut with my band saw, while the radii were carefully hand shaped with my disc sander (seen in the background). I also carefully de-burred and smoothed all the edges. Then, I returned the plates to the metal supply house, and had them put in the reinforcing bends:
     The four holes are located to allow the plate to be fastened to the old trim tab at the rivet locations, while the single hole is for the trim tab anode. The picture shows the bottom of the tab - the two parallel bends will actually be facing down when the tab is installed, and will act as directional stabilizers. A third bend, facing up, is in the trailing edge of the tab extension, to provide strength and prevent vibration underway. After I bolted the tab and extension together, I TIG welded them at intervals, with approximately 1 inch beads. Finally, I sealed the leading edge of the tab and extension with epoxy, to prevent water from flowing between the two pieces while underway:
     Lastly, I modified the mounting brackets, by slotting the existing mounting holes 1/8" of an inch, so that I could raise the forward edge by the same amount as the additional thickness, thereby maintaining a flush transition from the hull to the tab's running surface (again, to minimize drag). I also added additional mounting holes, between the exiting holes, to increase strength, since the tabs will potentially see additional loading. The finished install looks like this:

    Bottom line - increased directional stability, a flatter running angle, and reduced fuel consumption / higher cruise speed. The surface area of the tabs has been increased by 60% with this modification. I would not increase it any more without adding a second hydraulic cylinder.
     This coming weekend, I am lifting one of the engines in - it's time. The other will follow soon after.

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