There are a few key ideas you have to watch out for when you build your ship.
When constructing or editing a ship, you may notice an icon in the top-left corner with a percentage written on it. This indicator designates how big you can make your ship. This is the biggest limitation laid on the size of your ship. This number can be reduced by using fewer parts, such as large hull blocks/large armored hull blocks; or making hollow structures. Weapons also account for a minor amount of this number, but you should not exceed 80 to 90% before adding weapons. You can place 10 single blocks for each percentage, which is 1,000 total single blocks you can have. The number varies depending if you use blocks that take up more than 1 space.
Speed is affected by weight, water resistance, and number of boilers. Speed can be increased by adding more boilers to your ship. Better or larger boilers have more horsepower(see the boilers page).
A more streamlined ship will move faster than a blocky ship. 1/2, 1/3, 1/6, and 5/6 hull blocks are used often to ensure a more streamlined ship. Streamline the ship by creating channels for the water moving from the propellers to the stern of the vessel to force out more water by compression, similar to a jet engine.
Ultimately, the best designs are difficult to make, and whenever you make a change to your ship, you must make sure to check the info to see the speed. A ship should preferably be above 50 knots in speed, and any less might be too slow. The faster the better, as it becomes easier to avoid enemy shots. The suggested speed is one at or above 70 knots. Due to aircraft, a ship at all slower than 70 knots might also become a sitting duck for large amounts of bombers. However, if you do not have a ship that fast, the problem of enemy aircraft can be remedied by armor and batteries of anti-aircraft weapons.
Strength is the measure of how much damage you can inflict on the enemy, with the combined power of the weapons. The greater the number, the more damage can be inflicted in a given amount of time. Generally, a larger ship should have above 1500 strength; for example, the Yamato, a rather large battleship, has over 2300 strength.
The best method for increasing the strength of a ship would be to install larger guns, such as the 46cm triple gun and the 38cm quadruple gun, since they provide multiple powerful rounds per second. However, since strength measures firepower within one second, certain weapons, like the Type 96 25mm AT/AA, which rapidly fires many rounds per second in the presence of enemy aircraft, can give a false impression of a vessel's true strength.
Many of these more powerful weapons are very large also, and therefore, the easiest way to accommodate them is with a ship with a wide beam or with a fortress, although this might decrease speed and maneuverability. With some effort and a plan, however, it is possible to construct a ship that is both fast and very powerful, with a strength well into the thousands.
Both of these contribute to how long your ship can survive. Toughness is essentially the amount of damage a ship/ship part can take before it is destroyed. The higher toughness level is shown in your info chart, the more damage your ship needs to take before the health bar decreases. (For example, a ship with 0 toughness might take a 50% damage from a shot, and if that ship had a higher toughness, they might only take 25% damage from the same shot.) Armor is how much damage is subtracted from each hit. Obviously, the higher the armor number the less damage enemies can inflict. Both of these are increased primarily by adding armor blocks. Those come in 200mm, 400mm, and 600mm types, with each type having a small variety of shapes and sizes. A good armor level should be pretty high, above 300 for a typical ship. But if possible, get that number even higher. Most large ships need a least 750 armor to be competitive in multiplayer. Find a balance between armor and speed. A good toughness level would be at least 2500 for big ships.
Keep track of this at all times when expanding your ship. Stability is how stable your ship is in the water. Keep this number as high as possible, since it ensures the survivability of your ship. The higher the stability number, the more of your hp bar must be taken away for your ship to capsize. A number above 65 stability and a zero-degree angle is recommended. As you've probably noticed, some ships sink before reaching 0% hp. This is due to low stability.
Stability also keeps your ship flat. When your ship is damaged, it may list, or tilt to one side. Higher stability reduces the listing, and thus keeps your guns above water longer or prevents capsizing, or tipping over, of the vessel. Angle is the tilt of the ship when it's not moving. If this number is anything but zero degrees, your ship will be more likely to capsize. It's not that difficult to keep the angle 0 degrees, but make sure it is before exiting the dock.
Note: tapping a word on the ship's stats screen show information on what that word is (e.g. if you tap Angle a message appears stating that the angle is the degree of the ships after it had sailed forward)
As of version 1.6.1, there are an infinite combination of naval vessels. Some vessels are more common than others. Depending on your play style, you can choose which one you want to build. Stability is always an emphasis on any ship type. The listings only regard performance- armaments are left to their respective pages.
These ships do not care about armor or toughness, since they rely on speed, small size and maneuverability to avoid shots. While armor may be helpful for taking an occasional lucky hit, they generally do not take sustained fire well. Thus, the emphasis should be on speed and maneuverability. Take care to not sacrifice too much speed for stability- some ships of this type are so unstable that a hard turn in any direction could literally sink it. Catamaran hulls are recommended.
These vessels are also the smallest functional combat vessels in the game. This attribute makes them very difficult to hit. However, these vessels are often lightly armored.
Same deal with the Frigates/Fast Boats, only that these ships are slightly larger and trade some speed and maneuverability for some firepower. However, mono-hulls are desired due to their ease of upgrading. Some armoring is provided for increased durability.
Apply a balance between armor and mobility, since cruisers are designed to be able to destroy smaller ships easily through armor and toughness and larger battleship through speed and maneuverability. Their larger size means that hits are more likely, but maneuverability is still an option. Modest armor is necessary for survival.
Emphasize armor and toughness over speed and mobility. However, do not be afraid to sacrifice a little armor for much greater mobility. Large 400mm Lightweight and 3x2x1 400mm Lightweight is recommended for optimum performance.
Forget about mobility in general. Emphasize armor and toughness. Stability will come naturally, since fortresses are inherently stable warships. However, if it is possible, employ some limited mobility for positioning.
Rams will require an unusual combination of armor, speed, maneuverability and toughness. Stability is essential to prevent capsizing after an attack. This is usually achieved by removing weapons entirely.
Armor, toughness, speed and maneuverability should be average. Emphasize stability over speed, since large cannons will be difficult to balance. Monitors are best when decoration is spartan at most.
Average mobility will be necessary to evade heavy guns. Armoring should be modest to preserve mobility. However, in the case of hull conversions, the speed, mobility and toughness would mimic the ship it was built on. The performance statistics are not of concern for carriers, since speed perhaps is the most important.
Since the submarine will often lie literally beneath the damage-dealing guns and ASW weapons are lethal in one-shot, submarine design should focus on speed and maneuverability. However, if your submarine even dives without porpoising or plummeting to the seabed, then it is by all means don't modify the submarine if you do not feel even slightly doubtful about modification.