On Their Own Plane

The cover of the original K-Plane Trim Tabs brochure.
K-Planes continue to prove themselves in offshore powerboat races around the globe. Photo credit: Paul Kemiel Photographics.

I was going through my literature archives the other day and came across a copy of the original Kiekhaefer Aeromarine, Inc., K-Plane Trim Tabs sales brochure. I’ve always respected the quality and functionality of Kiekhaefer’s literature.  I  thought a blog post regarding the history of K-Plane trim tabs would be of interest. More importantly, it will serve as a refresher regarding the fit, form and function of the world’s most durable trim tabs.

Kiekhaefer Aeromarine Motors first introduced K-Plane Trim Tabs in 1970.  They were designed to keep the fastest, hardest running racing boats on an even keel in just about any water condition. US (APBA) and World Offshore (UIM) champions, Doc Magoon and Carlo Bonomi ran nothing else. In the mid 70s, Fred Kiekhaefer upgraded the product for recreational use.

Attitude Adjusters

A pair of 380S K-Planes on the transom if the Cigarette 42X Ducati boat powered by twin Mercury Racing 1100s.
The 380S K-Plane trim tab.

If you have read our Prop School blog series, you know the name of the game is efficiency. You want to select the most efficient prop, with minimal slip combined with optimal outboard/sterndrive trim for maximum hull efficiency. That is all well and good. But larger vee-bottom hulls need assistance in a variety of modes of operation. This is where the trust worthy K-Planes go to work.

 Getting on Plane

This colorful illustration shows the results of trimming the tabs down.

Trim tabs work in-concert with outdrives when powering a boat on plane. Tucking the drives under and the tabs down forces the bow of the boat down. This brings the boat on plane quicker, without a loss of visibility. Once on plane, the operator first trims the tabs up and then trims the drives out until the most efficient running attitude is achieved.

Smooth Operators

Vic Spellberg pilots a Formula 382 FASTech on a rough Lake Michigan. K-Planes help manage the ride. Photo Credit: Paul Kemiel Photographics.

Heavy seas can create an uncomfortable ride. I’ve experience this more than once. One of my most memorable rides was with Vic Spellberg in the 2006 Chicago Poker Run. Lake Michigan was especially rough that day.  The 380S K-Plane trim tabs earned their spot on the transom that day for sure.

Donzi 38 ZR Competition powered by twin 700 SCis. K-Planes make the boat feel even longer and smoother.

Tabs, when put in a level position with the bottom of the hull, act as hull extensions or after planes. As a wave throws the bow up, the tabs dampen the boats reaction, leveling and smoothing the ride.

Tabs and drives should be trimmed up when operating in a follow sea. The raised bow will compensate for the waves lifting the stern. Tabs are also used to fine-tune the handling of a boat such as a catamaran that may have a gentle bounce or porpoise when accelerating to optimum running speed in mild water. In this case, the drives are set at their optimum trim.  Dropping the tabs, a touch at a time, will dissipate the porpoising while maintaining hull efficiency.

At idle, trimming tabs full down will slow boat speed, reduce the wake size and straighten the boat’s track.

Independent Actuation.

A pair of 380S K-Planes balance the load on this Fountain 38 center console.
280S K-Plane trim tab.

Sea conditions or an unbalanced load may cause the boat to lean (or list) to one side or the other. List may occur in a quarter following sea: The waves lift the stern unevenly, causing the boat to pitch forward and roll opposite the lift. Dropping the tab opposite of the side being lifted will correct the listing for a dryer ride. Similar adjustments can be made for boats listing due to an uneven load. Bumping the tab on the heavy load side down will cause the stern on that side to lift, leveling the load.

Seeing is Believing.

Mercury Racing created this electronic drive and tab trim position indicator screen for use with three engines and K-Plane Trim Tabs equipped with Mercury SmartCraft technology. There are twin and single versions, too.

Boats equipped with K-Plane trim tabs require a trim indicator that provides the operator an accurate, real-time reference of both drive and tab positions. Mercury Racing continues to offer a full array of mechanical trim indicators first developed by Kiekhaefer Aeromarine for use with sterndrives, outboards and K-Plane Trim Tabs.

Recent updates include the addition of a SmartCraft electronic trim sender on 150S, 280S,  and 380S K-Planes. The electronic sender provides the option of having drive and tab trim positions displayed digitally on a variety of large, easy-to-read electronic displays available from Mercury’s SmartCraft Vessel View to Garmin and other suppliers.

I hope this helps explain what those black metal plates extending off the transom do and how you can use them to make your next boating excursion more enjoyable.




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15 thoughts on “On Their Own Plane”

  1. I have a triton 2895. I have large Kiekhaefer tabs with only a rocker switch. I need to change to a position indicator switch. Where can I purchase this switch?

  2. I HAVE A MURCURY TRIM TAB IT HAS EM 92193 stamped on outer casing . I need the top boss that screws into the bottom of the ram . Can you assist me with my inquiry please
    many thanks

  3. I own a Velocity VR1 (29′). Great info above. So when I’m running the boat on a rough day at the lake @ 60 mph or faster it’s ok to have the tabs level. If not level they should be full up. I get the bringing the boat up on plane and leveling, was not 100 percent confident on full throttle rough water usage.

    1. Hi David:
      We recommend Quicksilver Power Trim and Steering fluid. It is sold in 8 oz and 32 oz bottles. The part number for the 8 oz bottle is 92-858074Q01 and the part number for the 32 oz bottle is 92-858075Q01.

        1. Hello Everett:
          You will be able to order the cables from your Mercury dealer. You will want to use the Mercury Premium cables, part number 64-820676A__. add the length you need, i.e. if you need a 20-foot cable – order part number 64-820676A20.
          Good luck!

  4. Is there a kit available for older Merc. trim tabs to convert them to digital indicator instead of the old cable indicators? Have a 34′ Fountain CC with tabs with analog cable indicators that are bent and do not indicate the tab’s position correctly. Would like to replace with something that has a LED indicator and/or NMEA 2000 output.

    1. We do not have a kit available to update K-Planes with mechanical trim indicators to the new SmartCraft electronic indicators.


      1. Call Andy at Verado club parts
        He got me all the parts to convert my Mercury tabs to digital.
        They don’t have a dedicated kit but all the parts are available.
        Everything needed was under $1400

  5. Bought a 2000 Checkmate 242 Convincor running 700hp obviously a lot of power for a 24′ boat. At speeds 50mph and above it cant handle a 2′ wave without going airborne and slamming down. Seems to me it would really benefit from a longer K Plane like a 380 for handling and taking waves but wanted to get your feedback before making any purchases.

    1. Hi Bill
      Yes. That is a lot of power for that size of boat. Perhaps too much power. I don’t believe K-Planes are going to prevent the boat from going airborne in big seas. That may be more of a speed and center of gravity – setup issue. What type engine, drive and size/type of propeller are you using?

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