Trim Tabs

[MotorYacht]

I’m a recent owner of a 2005 Carver 36 Mariner. I’m looking to replace my trim tab assembly which has been inoperative since my purchase. The current configuration is a Bennett hydraulic with one actuator on each trim tab. The trim tabs are roughly 22x11”. My question is has anyone had experience with putting larger trim tabs on this vessel? Would you recommend that I increase the size for better efficiency?

[rslifkin]

If you can fit wider tabs (such as going from 24x12 to 36x12) it certainly won't hurt. Reducing the tab deployment angle at cruise will usually save a small amount of drag (lift/drag ratio of the tabs is worse at steeper angles). Wider span (higher aspect ratio) tabs are also slightly more efficient by nature.

[MotorYacht]

Thank you. Your response was very helpful. I believe there is enough room to go with the 22x36” tabs. Do you know if that size would require hydraulic actuators or could I go with electric?Are there pros and cons between hydraulic and electric actuators?

[tkeithlu]

I hope that you didn't mean "enough room to go with the 22x36” tabs." That's better than three times the area of your current tabs.

Let's think for a minute. The designer considered this issue and decided that the 11x22 tabs were right for this boat. What are you experiencing that says he/she was not right the first time (which does happen)? Can you not get the bow down at speed? Are the tabs creating a bunch of resistance? I recommend that you come up with a compelling reason before messing with it.

If you do want some more tab area, consider a modest increase in area while leaving the drive mechanism as is. Whether you stayed hydraulic or switched to electric, those actuators are going to cost you. It would be reasonable to assume that the hydraulics have at least a bit of reserve power.

[rslifkin]

Originally Posted by tkeithlu

Let's think for a minute. The designer considered this issue and decided that the 11x22 tabs were right for this boat. What are you experiencing that says he/she was not right the first time (which does happen)? Can you not get the bow down at speed? Are the tabs creating a bunch of resistance? I recommend that you come up with a compelling reason before messing with it.

Boat builders don't always end up installing the optimal tabs from a performance perspective. Undersizing is not uncommon for a variety of reasons. Cost, ease of making them fit (especially on hulls with prop pockets), and sometimes idiotproofing (undersized tabs to prevent people from over-trimming the boat and causing handling problems in following seas).

But the too small tabs often run close to fully deployed at cruise which adds drag and costs fuel and speed compared to a bigger (particularly wider span) tab deployed a lower angle.

As an example, my boat has 48x12 tabs (factory installed) and they're certainly not idiotproof. Fully deployed with low fuel tanks, you can get the bow down so far that the boat will bow steer in flat water. But they're deployed less than half way at normal cruise (even with full tanks), so they're well sized from a drag perspective. And you've got a great range of available trim angle rather than running at one end of the range all the time.