Finding the scale waterline for a new model ship can be a bit of a challenge. Here is one method that will give you a close approximation of the waterline for almost any ship of any particular size. Instead of the traditional putting the scale amount of weight in the ship and floating it in a bathtub, you will be filling the ship itself with water to its scale weight.

Gather your measurements. You need the

- weight of the empty model ship (from your own scale)
- scale weight of the ship (see below), and (optional) +10%, -10%, and -20% of this scale weight
- scale width of the ship (calculated from a reference book)
- amount of water to add to the ship (see below)

For 1:144 scale models, the scale weight of the ship is the full displacement in long tons divided by 1333 to get the weight in pounds. MWCI also has an extensive ship list with the scale weights at http://mwci.org/shiplist.shtml . For other scales, use the following calculation: scale weight = (Full displacement weight in long tons)*2240/(scale^3).

To determine the amount of water to add to the ship, subtract the weight of the empty ship from the scale weight of the ship. That is the amount of water, in pounds, you need to add to the ship. Each pound is about two cups of water (one gallon of water is 16 cups, and weighs about 8.345 pounds).

You need a completely uncut hull, as we will be filling it with water.You will also need a marker or a pencil (but not a grease pencil), and shims or a way to keep the hull level if it is not a flat-bottomed hull.

- Use packaging tape to tape the hull width to the scale width, so the hull does not expand when you fill it with water.
- Find a level slab of concrete for your hull. At the top of the hull, make sure that it is level from side to side. If it is a small ship like a cruiser that does not have a flat bottom, use shims to keep the hull upright and level.
- Add the correct amount of water (from your calculations above) to the hull.
- Mark the water level in the fore, aft, and midships of the hull.
- If desired, change amount of water to +10%, -10%, and -20% of scale weight and mark the water level.

These markings will be very close to the desired water line when the ship is finished, but will be slightly low because the density of the fiberglass is higher than the density of the water. On an Iowa hull, the different in that water level appears to be about the difference of one pound of water added.

**Example:**

An Iowa class battleship has a scale weight of 44.5 pounds and the dry, uncut hull weighs about 4.875 pounds. After taping across the sides to keep the beam width at 9 inches, add 39.625 pounds of water (4 gallons and 3 quarts). The water level will be very close to the desired water line once the ship is finished.

This method of finding the waterline will not work on ships whose keels are not level with the waterline. Some destroyers are like this and there are probably others. Most of the larger ships should be OK as their keels were built on a flat and level surface.