Installing trim tabs on our dinghy

Trim tabs promise to (this is from the box) …

  • Plane at lower speeds
  • Improve fuel efficiency
  • Reduce pounding
  • Improve your hole shot
  • Eliminate porpoising
  • Reduce engine laboring
  • Eliminate chine walking

All I am hoping is that I’ll get the stern up a bit at slower speeds. Much of the engine is in the water at slower speeds with just one in the boat, unless I sit as forward as I can. Getting up on plane easier would be helpful. With just one in the dinghy, the bow points up quite a bit. It takes some effort to get up on plane. Faster is always good too. ‘Cause that’s fun.

What I ended up with is Bennett SLT6 self leveling trim tabs. These are designed for boats 10′-14’. Ours is right in the middle at 12 feet. There may be other makers, but these are the only ones I could find that would fit our dinghy. These trim tabs came from I’d been holding off on this for a while due to cost. Defender lowered the price $50 recently (or at least since the last time I checked). It was close enough to what I thought was a reasonable price. I was also feeling kind hearted toward our little boat after the last blog post.

mv Archimedes Bennett SLT6 trim tabs for the dinghy

Here’s the stern. In my mind, I had a lot more room than was actually available. Turns out, the trim tabs are going to be a tight fit.

mv Archimedes about to install tdinghy trim tabs

The components of the trim tabs are fairly simple. A flat stainless plate is the tab. It’s mounted to the hull at a hinged point allowing it to move up and down. On top of the tab, a spring loaded strut mounts between the transom and the trim tab. That’s all there is to it.

mv Archimedes test fit port trim tabs 1The left side fits without much difficulty. I would like to mount them as far toward the sides as a I can, but space for the top mount is limited. So that’s about where they will have to go.

The lower plate sits right at the bottom edge of the transom. The trim tab extends straight out from the transom parallel to the underside of the hull. The top of the strut attaches to the transom to complete the installation.

I marked the location for the mounting plate on the port side and transferred those measurements to the starboard side.

mv Archimedes dinghy measured for trim tab install

mv Archimedes test fit starboard trim tabs 2

The starboard side is a bit more difficult. Matching the mounting location of the port side, I end up with a big stainless steel plate on top of a 1/2″ thick starboard spacer behind it between the trim tab mount and the transom.

I assume the geometry of the tabs and the mounting is somewhat important. The angle of the top of the strut is fixed. In order for it to mount flush, everything down below needs to line up.

Since the top mount on the starboard side will be raised 1/2″ from the transom, I decided to mill up 1/2″ spacers for all the mounts. This way I can maintain the proper geometry at all the mounting points.

mv Archimedes making spacers for dinghy trim tabs

Two lower spacers for the lower mounting plates and two spacers for the tops of the struts. These were milled out a piece of 1/2″ starboard I had left over from another project.

mv Archimedes spacers for dinghy trim tab install

Oh! I should be sure to add that I needed to replace all the mounting screws. Since I added the 1/2″ spacers, all the screws need to be 1/2″ longer as well. Don’t forget that step. That’s real important.

Once I drilled the holes, there was no going back. I countersunk the edges of the holes in the hopes of minimizing cracking in the gel coat.

mv Archimedes dinghy trim tab starboard holes drilled

Here is the starboard side tab in place. I taped the area earlier to be sure everything was in the right place, but also to help with cleaning up the squeeze out from the adhesive. I used 4200 between the spacers and the hull. I could have gone with something stronger, but the screws should hold it all together. I just wanted to seal it all up to keep the water out. And of course, it is always possible that I will want or need to remove them at some point.

mv Archimedes right dinghy trim tab installed

mv Archimedes dinghy trim tab mounted to transom

And here is the completed trim tab and strut on the port side.

mv Archimedes completed dinghy trim tab installation

Spacer at the top of the port strut.

mv Archimedes test fit port trim tabs 2

The starboard strut is mounted to the engine mount. This is the reason for the spacers in the first place.

mv Archimedes test fit starboard trim tabs 3

I still needed a small spacer on the starboard side as only one screw went through the metal engine mount. I cut the piece I needed from one of the full size spacers I made.

mv Archimedes satrboard dinghy trim tab spacers cut to fit

It fits under the bottom of the engine mount.

mv Archimedes dinghy starboard trim tab spacer

Here are the completed trim tabs. I zip tied the pins in place so they would not accidentally slip out.

mv Archimedes fancy new trim tabs for the dinghy

This project only took a couple hours. It was pretty straightforward. The hardest part was making the spacers. And I only needed them because of the limited amount of installation space. The installation was easy to do on a trailer in the driveway. It would have been a pain in the butt at the boat. That trailer we bought is turning out to be handy.

Now to take it for a test drive ….