Hi:
First and Foremost you have to realize that over 80% of them are resin. So, this only applies to those which are Vac-U-Formed or Injection Molded. I am doing one for a friend and it is an H.O (1/87) version of a Great Lakes Freighter ( Ore Boat). It is reasonably well done as a Vac Kit goes, Some parts missing due to age, but all in all a good model. Waterline of course!
Now,because of the scale and length( slightly over 60") she is long ,narrow and Flimsy( Even as a waterline ship Model!). This indicates needing internal structure even if she's anchored to a table-top! Thisti keep her from developing age twisting and oil-canning. Tup this kind of damage would be multiplied because of her length and lack of thicker plastic for ridigity.
There two ways to achieve this. One is the geosesic framing liike that used in the Famous "Wellington " Bomber. Or the standard Keel, Frame Type of support! The only thing in the second type is the necessary increase in support For the areas in between the frames!
The Choice is Simple. How do you build light and strong? You use the lightest Keel and Frame you can, Coupled with Minor Geodesic where needed. "Hmmm" How? You kake the Keel basically a triangle in cross section and a squared looking tube llengthwise. It's the same type of structure used for the Frame and Keel for the 1/200 Andrea Doria in Paper!
After doing that Put a diamond in between the frames,Well actually two small ones that extent to the middle of the space and another from the middle of the frame space ( Point to Point.) The Bottom point will be close to the Bilge line and the top just below the deck.
The nice thing? These supports can actually be coffee stirrers (straw Like) that you cut to fit and Hot Glue in place. Yes! I said Hot Glue! It won't show, but give the support needed! Try it on any model! You'll like it!