Why are transom trim tabs flat?

Why are all adjustable or non adjustable transom trim tabs (or all the ones I have seen) flat plates?

They are there to provide additional lift at the stern, but they project from the stern so are easily damaged so the smaller they are the better, surely?

If they curved slightly in the fore and aft plane, wouldn't they provide more lift for their surface area than a flat plate so could be made smaller for a given lift?

 

With the "interceptors" (also "Gurney flaps") that is taken to the extreme; zero nominal chord (although part of the hull acts as a "virtual chord") and 90 degree (or even more) deflection. Better lift/drag ratio than flat tabs.

 

Flat plates are simple and effective, cheap to produce.
As long as manufacturers insist on marketing hull designs that perform poorly, trim tabs will continue to sell.
Their behavior is smooth and predictable, which is an important characteristic in terms of safe operation.
Can you imagine backing up a boat with long curved tabs.

 

-Reduced ram loading.
-Reduced object strike loads.

Simplicity itself goes a long way.

 

Well, I'm laying up my own out of plywood and epoxy, so no great issue with cost of manufacture...
It's for a small sailing boat that has a lot of tuck at the stern.
The boat itself (Swift 18), was later sold with a massive sugar scoop bolted on (Swift 23) in an attempt to stop it rounding up so readily. I think it worked reasonably well, but I don't want to build or bolt on a massive, ugly, semi permanent sugar scoop.

I can try multiple variants quite easily, including vertical plate interceptors. I'm also thinking about twisted tabs. So the inboard side of the tab is a flat plate, but the outboard side curves downwards. So the more the boat heels, when accelerating in a gust, the more that wallowing, bogging bottom corner of the hull develops lift.

I guess my question is: will curving the plates downwards fore ant aft allow them to provide much more lift than flat plates would? Is the difference worth bothering with?

RE reversing with them. They will be actuated by rods and lines to cleats in the boat. Two lines to pull and they will lift up flat against the yransom out of the way. They need to fold up anyway as there's a boarding ladder one side of the ( transom hung) rudder.

 

If your Swift 18 is rounding up, you just need more sail forward, or centreboard / keel back a little, to balance it better. If not racing, a bowsprit holding the jib further forward may help. Or / and a bigger jib, genoa, or assymetric up front.
I had a bearing away problem, and made it better by leaning the mast back a few degrees, bringing sail area rearward; could you lean your mast a little forward and try it? Eg an extra short shackle on the side stays, and shorten (temporarily) the head stay. Easier than messing with the hull, and reversible if it doesn't work.

 

I am not sure on your explanation
So viewed from the side, the trailing edge of the tabs turn down? If so, they would generate more lift BUT it would also generate more drag.
If your tab was say a constant arc, ie leading edge at the bottom stern of the boat and the trailing edge curled down. Joining the lead edge and trailing edge with line, the lift (normally) is
90 degrees to this line and would be tilted rearward at some angle off vertical from the bottom of the boat. This rearward lift would have a horizontal component drag and a vertical
component lift.

If you want to build tabs that minimize drag, you want as wide as tabs as you have room for, to increase the length of the stagnation, high pressure line. And the longer the fore to aft length,
the more square inches you are having the resulting force act on them.

If you want to maximize the lift, you could, when viewed from the rear, turn the tips of the tab downward, to minimize curl at the tips. As you see on most airplane wings to minimize tip vortices

Additionally, have the hinge point of the tab, say an inch above the bottom of the boat to create a new high pressure stagnation point as compared to just extending the tab that would act as
an extension of the hull.

 

Sailboats tend to heel over at varying angles, so where would you place the tab so that it could provide lift in different angles of heel?

 

I’m with @Barry, here, and only generally comment that hook is rarely desirable because it causes drag. I’m not a sailor, so my remarks are limited to my concerns your results will cause braking on the side in the water and the boat will likely want to turn that direction, so it seems wholly undesirable to me, enough to stick my neck out, but only on this aspect. I’ll go back to my corner snd observe the balance of comments.