hull extension as trim tab?

You should ask the people who did that patchwork.
Generally speaking, a boat gets modified during its lifetime because it didn't meet some expectations during the service, or the original (design) goals are not sufficient for the person who has bought the boat. If a person knows what he needs from the very beginning, then he should look for a vessel which has all that stuff already provided when it sails out of the boatyard, rather than patching the hull afterwards.

However, by reading your previous posts, I got like an idea about how generally the process of these swim-platform additions start. See this:

Get my point?

Cheers

 

Assuming a suitable boat is available within the financial constraints, or there are no financial constraints. But it is also very easy to undersestimate the cost and effort which alterations will require if done properly.

And then there is the very big "If a person knows what he needs from the very beginning" and the implication that those requirements do not change.

 

you might call it patchwork but 60% of all motorboats( older models and some new ones like the foto) have it built like that, most newer boats have the whole thing enclosed. As far as I can see it aint an addon, the hull was just left extended when built. The water birds love to build their nests in them because they are just under the water.

 

Pars comments apply particularly to the last photo you referred to. The platforms looked particularly slab sided.

By having curved and properly extended sides on the platform, you would avoid losing maneuverability as the prop and rudder move further from the stern, thereby making it harder to push the platform sideways on turns.

 

nearly done

swim platform/hull extension is finished, just windows and antifoul and back in the water.

http://troppo1.mine.nu/photo/photo_one.php?dir=766572686f656620382e33&name=44534346393832332e4a5047

 

looking good