I can get a scale factor by dividing the real boat LOA over the desired lenght.

By dividing sail area over the square of the factor, I can get the required sail area (to stay within scale).

Is there any similar way to calculate the required ballast? following that logic (which I’m sure is wrong) I get a 20 pund ballast (which in my case, with a LOA of 2.13 feet, seems too much).

Lengths scale directly,
areas by the square root
Volumes by the cube root

eg A half scale boat
is half as long as the original,has1/4 the sail area and 1/8 the displacment.
Must post this info somewhere…explained this a few times…
Brett

Hi,
be cautious! Because areas scale with the square root and displacements with the cubic root, stability of a model is drastically reduced! This is why model yachts usually need longer keel fins or more ballast weight than the originals.

That’s precisely we I want to head to… There got to be a way to be able to calculate how much ballast I would need in order to balance a given Sail area.

I would love to make a scale model of our Tartan 30. I can scal everything, but my nonexistant knowledge of rc yachts design won’t allow me to infere, or estimate how much weight I will need to keep it “right”;

My latest target is 30" LOA, 10" beam, 449 SQ inches sail area.

I would really like to cast a keel the more simialr to the original as possible. even if it has to be wider at the bottom to add more weight.