Trim Tabs and Surface Drives

In every factory installation of SD I have seem, there seems to be the use these large long trim tabs that extend back. They are called a long name that starts with K. Anyway, how important are trim tabs in a SD boat, and why these kind. I guess it is to not disrupt the water going into the prop tht a traditional one would do. I also heard that some people use then to help get boat up on plane quicker. Are they still needed on trimmable drives? Any comments appreciated.

 

K Planes, named for Carl Kiekhaefer the founder of Mercury Marine is the K name you are looking for.

http://en.wikipedia.org/wiki/Carl_Kiekhaefer

They and similar planes like Dana are the tab of choice because surface drive boats are often high performance skinny deep vee's or catamarams with little or no transom left for trim tabs.

Trim tabs are not "needed" at all. They do however absolutely improve performance by trimming the hull and they do help a boat get on plane faster, especially surface drive boats which are typically a little slower to get on plane than boats with other drives.

Trimmable drives trim the drive, not the hull.

There is a reason you see K-Planes on all those high performance boats.

Steve

 

Would those large K-planes effectively add planning area to boat?

 

Some basic information using trim tabs with surface drives.

SD applications tend to run considerably flatter than other forms of propulsion due to the vertical component of thrust associated with a surface propeller. You cannot use the trim feature of a SD to trim the running attitude of the boat.

Many of the older skinny non-stepped deep v hulls running at high speeds surface tended to porpoise. By inducing a slight amount of trim with the 30“ K-plane you could effectively trim out a porpoising tendency.

Many high speed deep v boats require trim tabs for lateral trim.

If trim tabs are needed to get a vessel on plane, the 30” k-plane with their long aspect ratio are not as efficient as a wide aspect ratio trim tab. However, one has to be careful about disrupting water to the propellers.

Typically as a boat goes faster, it is running on less surface or planning area. Lifting strips are used to dry a hull off to decrease this drag. I don’t understand why you would want to use trim tabs to increase planning area. If the boat is experiencing problems planning, you can add lift at the transom though the dynamics of the trim tab, but again, the 30” K-planes are not the most effective for pre-planning conditions.

 

K-Planes or "Trim Tabs" are not exclusive to boats with surface drives nor narrow beamed boats.

They are used to lower the bow upon planning and when on plane they are used to control listing as well as add some stability in an attempt to control chine walking. Some boats drag tab to control a porpoise but a surface drive fixed or trimable does not induce a hull to porpoise. That has to do with the amount of rocker in the boats bottom, (if it has them) step placement and the cg of the boat.

Some are mounted parallel to the deadrise of the boat and others are mounted level to the water. They are often called "Training Wheels" in reference to their ability to balance the boat out when the hull wants to chine walk.