A Tab Question: 2520XL hull.....

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Well-known member
Mar 22, 2006
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Franktown, Virginia 23354
As I prepare mentally for the job of drilling and filling holes in my hull I am faced with this situation.

The Lenco 12x12's on the 2520XL fit nicely in a cuttout in the hull..... designed I assume to accomadate this size tab.

When removing the old tabs and replacing with Bennett's 24X9's (they will mount directly on the transom) there will now be this cuttout in the hull that will do nothing except make for some turbulant waterflow.

Should this be filled in to make a smooth surface? How would you fill this in?

I don't have any pictures at this time....

Tough question...I'm not sure how to approach this. One side of me says leave it alone, but I believe you would get a lot of spray where the water running across the channels would run into the hinges of the new tabs.

Instinct tells me to roughen up the depressions, fill it in with several layers of resin soaked roving and top it off with fairing compound made from resin and West 407 sandable fairing filler. Last thing to do would be to spray on a layer of gelcoat, sand, buff, and polish.
If you can "hold on" for a few days, the project I am trying to work with Bennett will be to use a Bennett 24x9 hinge that is a direct bolt-up to the existing Lenco holes, less the new ones you need to drill ...
I don't like the idea of not filling it in..... It would certainly disturb water flow and it wouldn't look proper.

What would be the temperature requirements to work with these materials? If I had to I could rig a tent with duct tape, tarps and a heater. I'll probably want to do this in February and I could find some days that might get up to 50.

DaleH":1wffkam4 said:
If you can "hold on" for a few days, the project I am trying to work with Bennett will be to use a Bennett 24x9 hinge that is a direct bolt-up to the existing Lenco holes, less the new ones you need to drill ...

That won't work for my hull as the Lencos are hinged to the bottom of the hull in a recess that is 12x12... the back edge of the tab is flush with the transom.

I'll go get a quick picture.
Well the back of the tab is not flush.... but I think you can see by these pics what I'm dealing with.



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Hooo Boy....Now I'm really confused.... :?

I've spent a few days digging on the 'Net for everything I can find out about Trim Tab Theory and installation. There's not much out there except for what the manufacturers have on their websites. :(

Airplane Wing design is vaguely similar, and sites such as NASA's are interesting, NASA Wing Geometry Definitions
but boat trim tabs don't use the top of the tab plane, and therefore the true wing design equations don't really apply.

I am particularly interested in chord vs. span (aspect ratio) sizing. Given the same tab surface area, depending upon how you load and use your Parker,, is it preferrable to use a 9 inch chord or a 12 inch chord?

So now,, here comes Richard with a photo of his new recessed tabs that fly in the face of what I thought I knew about how/why tabs work. :(

I thought that the primary functions of tabs were to provide strategically controlled "lift" and "drag" to the hull. When mounted across the transom extended beyond the stern, they provide extra extended surface area of the hull (tabs level) and provide extra "lift". You get the benefit of extra hull surface area (and lift) without the penalty additional weight of longer boat length. By pushing the tabs down, you get drag and drop the bow, slow the troll, etc.

So how does recessing the tabs into the hull provide extra lift? I can see the "drag" function when you drop them down, but explain the "lift" component to me? :?

From the picture above.. there is only an inch or two of additional chord to provide lift beyond what is already present on the fiberglass hull surface itself. So where does the extra "lift" come from?
Here are my observasions and thoughts of what goes on with tabs.

Compared to the total surface of the hull the tabs are quite small. They do provide drag.... I don't think there is enough surface area to provide any type of "lift" for the boat but by redirecting the angle of the hull it helps the hull find the most efficient angle through the water.

These tabs do a good job of providing a better fuel efficient planing than without.

When the boat is up on plane with tabs up and motor trimmed properly the hull has a center of gravity that is "distance A" forward of the transom. Deploying the tabs causes drag and an upward pressure is applied at the stern. Now the hull has a new attitude and the center of gravity has moved foward. For any given speed and weight distribution inside the boat there is a corresponding hull angle that will provide the most efficiency. The operator of the boat has to constantly adjust the tabs if speed is changed to maintain this effecient state.

As the tabs are deployed further this process continues to move the center of gravity farther forward so now the same amount of tab deployment produces less total movement of the hull and the hull is no longer at its most effecient planing position so there is more drag on the total system..... but the quality of the ride improves to where we want it......

I would think that tabs mounted on the transom extending behind the transom should act slightly differently..... they are actually extending the length of the boat in a way. There should be more lift at a given speed than without tabs. They should also have a beter mechanical advantage as they are now further away from the boats center of gravity.... or fulcrum exerting the same amount of upward pressure on the stern.

Then there are all the issues of turbulant and laminar flow...

But we don't care about that so much as we do.... how comfortable can I get running into this 2 foot chop.
To better demonstrate the cut out in the hull here is a picture of the tab fully deployed. There is not as much travel as one might think there is.

I've already decided this cut out will get filled in.

I believe the Bennet suggestion is to have the lateral edge of the tab several inches from the chine. As you can see the current tab is flush with the chine.

Another decision......


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esfishdoc":2hnuhaai said:
Here are my observasions and thoughts of what goes on with tabs.

Well put Richard, your analysis makes sense to me.

From what I have seen on other boats, the installation of tabs within the fiberglass hull footprint in a recessed "pocket" is a common method of manufacture, which provides a "cleaner" appearance, and also makes it easier when trailering. Are there any other control benefits?

Mounted behind the transom, even the 12X12 OEM tabs provide a couple of extra square feet of hull planing surface without weight burden,, "free lift" if you will. CP members report "size matters" and 24X9 tabs greatly improve performance, presumably lift is a factor.

When did Parker move to these recessed tabs? Are they recessed on -all- hull models? They made the decision that any benefit from "free" lift on the XL hull with tabs mounted behind the transom was offset by the gains in appearance and control with the tabs tucked into and under the stern. If the tabs were larger to begin with, providing greater lift, would they still have recessed them?

Bennett's website reminds us that boats with a bracket, twins, or other stern-heavy conditions benefit from more lift. We will all be interested to know if you notice any improvement you can attribute to "lift" when you move your tabs out from underneath the hull.

I think the only way to really compare how much "lift" the new tabs have is to do the following:

Mount the tabs so when fully retracted they are actually above the centerline and out of the way if you will.....

Then, carefully adjust the tabs so they are in "neutral" with the hull. Get the boat up to a normal cruising speed and trim the motor. Then, retract the tabs up and out of the way to see what happens. My guess is there will be a noticeable difference.

So this brings up a question: How is everyone mounting their tabs when they are fully retracted? In a neutral position or slightly up......

Dale... I think we need a tab section! :D