Another Trim Tab Question-With a Twist

[Porkchunker]

The only surprise in the equation is why a 2120 has the same tabs as a 2520......

What size does the 2820 have?... don't tell me they are the 12 X 12's.

To support the American way of bigger is better I'm installing the 30 X 9.

To answer your question, yes, I believe the 28 has 12x12. I believe Parker buys them in bulk and gets a good deal, so they put them on everything from 21 to 28. Before we had heavy 4-stroke engines on a bracket, 12x12 probably sufficed. Now that is no longer true, because so many of us feel that the center of gravity is pulled rearward with the 4-strokes, giving you that stern-heavy feeling/response.

This area is probably the only sore point the Parker Nation has with the the way our boats are built/rigged. Fortunately, it is easily corrected. Some replace the tabs. I corrected it with a pair of Cobra plates, one bolted to the bottom of the anti-ventilation plate of each motor.

See the pic.

 

[cbigma]

Here's an explanation that Tom McGow from Bennett Trim Tabs crafted in response to my question about the pressure wave that rides out in front of trim tabs, and why a 24X9 tab doesn't have a greater amount of drag related to this pressure wave than an 18X12 tab.

From Tom:

So, what about long span, short chord Tabs, why do we feel they are more effective than longer chord, short span ones? First, history, in the 47 years Bennett marine has been manufacturing Trim Tabs we have had lots of experience with them and have seen the results on many boats. When a builder like Viking sets up Tabs on one of their boats they will install three (two on the sides one in the middle) to four (two Tabs per side on either side of the prop pockets) to get the most span. They have tried many combinations of span and chord. They both tank test and extensively trial their boats.

The issue of water pressure lost out the sides of the Trim Tabs is the primary reason that longer span Tabs are more efficient. The ratio of pressure lost out the sides is much higher on a long chord Tab then on a long span Tab, and pressure is where we get our lift.

You are correct about the pressure set up under the Tab in relation to its chord. However at the speeds we are talking about it is likely not as great as you think. The flow over the Tabs at these speeds is quite laminar. Also the “depth” of the wave is shallower on the longer span Tab and then on the longer chord one, therefore the increased length of it does not really add up to too much more.

The pressure lost out the sides more then negates any gain from the wave difference between the two types. A good example of this is that on many boats where transom space is limited and we are forced to use a longer chord Tab in employ drop fins on the sides. These fins extend down about 3” on either side of the Tab. They capture the pressure normally lost out the sides and cannel it aft, keeping it “under the Tab”. The increase in lift on an 18” x 12” Tab with drop fins is in the neighborhood of 20 -25%.

From a practical stand point the longer span Tabs have several advantages as well. They are less likely to get in the way when fishing, will normally fit between the transom and a swim ladder if one is fitted, and are less likely to be damaged by a fork lift.

P.S. I am a great fan of Parker Boats!

So there you have it. An explanation as to why long span/short chord is better than short span/long chord.

Thanks to Tom for taking the time to present this explanation. It's easy to see why so many Parker Owners rave about Bennett Customer Service.