scale factors

Here’s the thing:

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).


The more I practice, the luckier I get.

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…

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.

Models will sail in the original wind power…

regards from Germany

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.

Any ideas?


The more I practice, the luckier I get.

Hi Marino,

You can use this excel program.



Hugo, Nice program!

However, I couldn’t find the explanation on the IWEMA rating… here are the results of the hull ?'m completing:

boat sailing angle 41.65 degr.
correcttion length 64.9 cm
stability 0.035 kg/m
torque 0.039 kg/m
difference stab/torque 0.00
stability reserve 0.116
force 0.53
maximum wave speed 3.63 km/h
actual speed 3.63 km/h
power performance 2.83
IWEMA rating 78.82

Any info will be deeply appreciated…


The more I practice, the luckier I get.