Bow shape of WW1 warships

Take a look at this article. The tragedy in St Lawrence may have been the watershed that created the shift to "7" shape flared bow designs. The Empress of Ireland sank in 14 minutes as she was cut open by SS Solstad C-Bow.

Different ships are built with different bow shapes for different purposes. This isn't really a history question so much as engineering, though it relates to history as knowledge developed and design goals changed.

The pure "7" shaped bow is uncommon nowadays - it is usually combined with an extended bulbous bow on modern cargo ships, for the reason given in Peter Erwin's answer - increasing the waterline length increases the ship's speed, by increasing the wavelength at which water piles up at both bow and stern, sucking energy away from the ship's propulsion. Adding more power simply creates a bigger wave, rather than more speed.

What the "7" gives you - either on its own, or above a bulbous nose - is a buoyant volume that rapidly increases as a wave rises around it. This lifts the ship, keeping it above the wave, making her more seaworthy in heavy weather, less likely to be swamped by large waves. But by the same token, it upsets the carefully aimed guns, making gunnery inaccurate in any kind of weather.

So a straight, "ram" or bulbous bow gives a better gunnery platform than a "7", eliminating that as a contender for a gunnery platform (though it could sail through weather that might sink other ships). As guns have given way to missiles, that's less important now.

Why did the Dreadnought abandon the straight stem (Titanic) and ram (pre-dreadnought)?

Two developments : (1) the steam turbine, allowing much higher power, and (2) the test tank, pioneered by William Froude at Denny's shipyard of Dumbarton allowing hull shapes to be developed to exploit that power as efficiently as possible.

For what it's worth, the Wikipedia article on inverted bows notes that

Inverted bows maximize the length of waterline and hence the hull speed.

Inverted bows were popular on battleships and large cruisers in the early 20th century. They fell out of favour, as they were very wet on high speeds and heavy seas, but have made a comeback on modern ship design.

I do know of cases where individual warships built in the 1920s and 1930s with rather vertical bows were refitted with more forward-slanting "Atlantic bows" because they shipped too much water over the bow in their trials (e.g., the two ships of the German Scharnhorst class), which supports what the Wikipedia article says. Possibly the trend towards faster and faster warships in the early 20th Century (typical battleship top speeds going from less than 20 knots at the beginning of the century to around 30 knots in the 1930s) contributed to this.

The same Wikipedia page includes some other examples of modern inverted bows, in addition to the Zumwalt class.

I can't source this at all but I believe that early armor-plated ships were shaped this way after the design of the ACW-era ironclads like the Merrimack and the Monitor. The idea was to present the enemy with convex angles which cannonballs would bounce off of at oblique angles rather than concave ones that would take the brunt of the blow. As time went by, the relative advantage of this set-up was lost as solid shot was replaced by explosive shot, and the significant disadvantages of this set-up (chiefly, I think, the fact that you had so much more of the ship below the waterline, making them both slow and vulnerable to torpedo fire) meant that they soon went back to the old-fashioned V-shaped fronts.

As an example of US Civil War era technology, here is the USS Merrimack, converted into the CSS Virginia:

What was the reason of C-shaped bows in 19th century and WW1? Was it the same as a ram in ancient galleys? Why did everybody expect to ram enemy's ship? Were there any successful attempts in the age of heavy naval artillery?

1. It actually depends on which ship you are talking about from that era. In the British naval world the HMS Dreadnought actually marked the transition from ram bow to bulbous bow. Take a look at the images below.

Here is a Swiftsure Class pre-dreadnought battleship and the last British battleship class completed before the Dreadnought herself. Note the pointed ram bow. Image Source

Here is Dreadnought with a bulbous bow. Image Source

Here is the Lord Nelson class battleships completed right after the Dreadnought. Even though these ships are pre-dreadnoughts, since they were built after the Dreadnought they used bulbous bows instead of ram bows Image Source

2. Why keep the ram as modified to a bulb? Well, as @Alias72's answer says, it is to have better stability for gunnery. Passenger liners don't really need this portion as much because they don't have to shoot guns at sea (Although it would also help with keeping seasickness down among passengers too). And yes, as stated above, Dreadnought did ram and sink SM U-29 by ramming as sort of an ironic statement to her "father's", John Fisher, decision not to give her a true ram.

3. Speaking of "modern" ships, the Zumwalt class guided missile destroyers are actually designed with a bow shape very similar to those of the 19th century. There is nothing really "modern" about any of the hull types, the designers just use what seems fit for what the ship needs to do (and for looks to a lesser extent). I also do want to point out that the Zumwalt also uses a tumblehome (wider hull at waterline tapering inwards as it goes up) which has not been used on naval ships for quite some time. Take a look:

Image Source

All in all, ship designers find what type of bow is the best fit for a certain model, so the designs will bounce back and forth over time.

There are a number of reasons for the appearance of the ram bow in the 19th century. The ram bow is a more natural bow form for tumblehome hull forms. Tumblehome was used on most ships during this period, hence the proliferation of the bow type. Shot deflection was not a major concern when adopting this form as the bow section is not an easy target, or a vital area of the ship. Ramming of enemy vessels was a concern until in the 1890's advancements in torpedoes made close approaches of enemy vessels dangerous (battleships had submerged broadside torpedoes back then). The final reason is stability. The ram bow cuts under waves and reduces the pitching motion present in rough seas. This was very important during this period as gunnery was relatively imprecise and every advantage helped. By 1905, bow forms begin to reach vertical and take on more traditional angles by 1914. This coincides with the development of accurate gunsights and techniques.