Sometimes when I cook hard boiled eggs they will crack in the saucepan, and some of the egg will seep out into the water.
Am I boiling the water too vigorously, or can I add something to the water to prevent it?
I used the method here before with good success: http://simplyrecipes.com/recipes/how_to_make_perfect_hard_boiled_eggs/
Basically you start with the eggs in the pan with cold water and bring the water to temperature, right when it reaches boiling you pull the heat back and let the eggs cook for 10 minutes.
You don't need a full on boil to hard boil the eggs.
Before you submerge the egg, prick the large end of the shell with a (clean!) thumb tack or safety pin. This will allow the trapped air bubbles - which are normally responsible for cracking - to escape during the boiling process.
Also take care to avoid any impacts:
Don't overcrowd the pot - the eggs should form a single sparse layer;
Use a tongs or spoon to lower the eggs into the boiling water - or start with cold water with the eggs already in the pot.
Make sure you use enough water - cover the eggs by at least an inch.
P.S. Even though the above should always prevent cracking, if you're still having problems, you can add some vinegar to the water. This will not help with cracking but it will make the eggs set quicker if they do crack and mitigate the nasty mess. I don't do this anymore, but something to remember if you're desperate (i.e. if the eggs are very old).
As eggs develop more air as they get older (which is why bad eggs float, fresh eggs sink,and old eggs stand up in water) you are more likely to get cracked eggs with older eggs, and if you plunge them in to hot water.
If you are doing it for hard boiling then you might be better using a technique like ManiacZX describes where you bring the eggs to the boil in cold water, then turn the heat off and leave them with a lid on for 10-12 mins.
For the first time I used a special rack to suspend the egg in the water without it resting on the bottom of the saucepan and for the first time ever it didn't crack.
Any other time I have tried all the above and it didn't help.
I'm now sure it's because of the contact with the hot surface of the pan before the water has a chance to disperse the heat so that it doesn't rise dramatically above 100 deg c
You'll want to fill the pot that you are boiling your egg with to at least an inch higher than the egg layer. This will prevent the egg from bouncing around too much. I've been using the stuffs written here in boiling eggs: http://knowhowtoboileggs.blogspot.com/2012/08/how-to-hard-boil-eggs.html
I trained as a chef; the way to do this is as follows.
The egg cracks for a number of reasons which are mainly due to a change of pressure within the egg itself. You never want to put a hole in the shell, add salt to the water or frankly any of the other suggestions on this page above, and it's not due to eggs banging about in the pan (that one did make me laugh).
Occasionally an egg will crack anyway due to having an uneven thickness of shell but there isn't much you can do about it. It's best to take the eggs out of the fridge (if that’s where you keep them, but that’s about all you can do).
I disagree with comments from Jefromi, for one. Everything should have a logical explanation and root causes. The procedure for boiling up to 30 eggs looks interesting. Similar is the process by Jeff Bullas...
My assessment - It is the pressure buildup inside the shell that causes cracks. If I boil eggs slowly I can see tiny bubbles coming out from the shell at several places, this is the process of relieving the pressure naturally.
My own experience: If I boil cold eggs straight from the frig and raise the temperature of water on high flame very quickly it almost always cracks. If I boil slow after warming up the eggs at 40 deg C, results are much better. Pin hole at the bubble end also helps but may be termed un-hygienic? Now-a-days I warm up the egg to ~40 deg C in warmed water, then put eggs in another pan, pour boiling water halfway, wait 30 sec, pour in more of the same water till submerged, cook for 3 minutes on low flame, switch off, and eggs are ready in another 12 minutes (may be lower time is possible, not checked. Results are quite good, almost no cracks over many attempts.