Question: In sailing what may cause a bow to dig into the waves?
About the article "Sailing", section "Hull trim".

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Hull trim is the adjustment of a boat's loading so as to change its fore-and-aft attitude in the water.
In small boats, it is done by positioning the crew.
In larger boats the weight of a person has less effect on the hull trim, but it can be adjusted by shifting gear, fuel, water, or supplies.
Different hull trim efforts are required for different kinds of boats and different conditions.
Here are just a few examples: In a lightweight racing dinghy like a Thistle, the hull should be kept level, on its designed water line for best performance in all conditions.
In many small boats, weight too far aft can cause drag by submerging the transom, especially in light to moderate winds.
Weight too far forward can cause the bow to dig into the waves.
In heavy winds, a boat with its bow too low may capsize by pitching forward over its bow (pitch-pole) or dive under the waves (submarine).
On a run in heavy winds, the forces on the sails tend to drive a boat's bow down, so the crew weight is moved far aft.
Slamming is the impact of the bottom structure of a ship onto the sea surface.
It is mainly observed while sailing in waves, when the bow raises from the water and subsequently impacts on it.
Slamming induces extremely high loads to ship structures and is taken into consideration when designing ships. 
Broaching most typically happens when a boat is on a broad reach with spinnaker set and there is an overpowering weather helm.
This may be due to excessive heeling, swell that causes the rudder to be partially lifted out of the water, or because the stem digs into a wave causing a strong lateral force.
The boat will come quickly across the wind with too much sail set.
It can also happen on a close reach where it is typically precipitated by sudden waves.
With an inattentive helmsman it can be a consequence of gybing, which can be a dangerous manoeuvre even when performed under controlled circumstances.
When it happens unexpectedly and the helm isn't corrected, the heading of the boat may veer into the wind causing a broach.
Also when sailing on a dead downwind run an inexperienced or inattentive sailor can easily misjudge the real strength of the wind since the boat speed subtracts directly from the true wind speed and this makes the apparent wind less.
In addition the sea conditions also falsely seem milder on this point of sail as developing white caps are shielded from view by the back of the waves and are less apparent.
When changing course in a brisk wind from a run to a reach or a beat, a sailboat that seemed under control can instantly become over-canvassed and in danger of a sudden broach.
One of the less obvious causes of broaching is an oscillation developed when running downwind.
When running dead downwind, the sails are set with an angle of attack of nearly 90 degrees to the wind.
The high angle of attack causes turbulent bubbles to form on the leeward side of the sail, and when there is insufficient attached flow to keep these bubbles attached to the sail, they separate.
The sudden separation of a turbulent bubble drops the lift of the sail to nearly nothing, which causes the boat to rock slightly.
At angles of attack near 90 degrees, lift can generate on the leech, or trailing edge of the sail.
As the boat rocks due to separation of turbulent bubbles, a constructive interference is formed, as bubbles begin to shed from alternating sides of the sail.
Once started, the oscillation builds quickly, especially in modern ultra-light displacement centerboard boats and sportsboats, which lack the damping effects of a large keel.
Once this oscillation reaches a certain point, the extreme change in heel angle will cause the boat to broach.
Spinnakers generate lift from the top, rather than the side, and thus they are less prone to this form of broaching than other modern sail types.
Square rigged vessels also generate lift from the tops of their sails, which in fact, combined with the large keels they traditionally use, makes them immune to this type of broaching.
Another cause of broaching is encountered in heavy seas and high winds.
If the bow of the yacht is not kept pointed into the waves, then the waves will push the bow aside, turning the boat side-on to the waves.
Once side-on to the waves, the waves will roll the yacht side to side violently, causing severe discomfort to the crew, and the yacht may broach and may even capsize.
Since a sailing ship cannot steer unless the sails are providing forward motion, and since heavy seas are most often from the same direction as the wind, a sea anchor may be used at the bow in extreme weather to keep the bow pointed into the wind and waves.
See heaving to as a storm tactic.
The key to stopping or rescuing a broach is to depower the sails at the first opportunity.
This may be done by releasing the main or jib sheet by a foot or more or by releasing the boom vang (kicking strap) which will spill wind out of the top of the mainsail.
The main means of preventing a broach is to ensure that the amount of sail set is reduced to suit the sailing conditions. 
Poor weather can cause several problems:  high winds  low visibility  cold weather  high waves Wind causes waves which result in other difficulties.
Waves make navigation difficult and dangerous near shallow water.
Also, waves create buoyancy stresses on the structure of a hull.
The weight of breaking waves on the fabric of the ship force the crew to reduce speed or even travel in the same direction as the waves to prevent damage.
Also, wind stresses the rigging of sailing ships.
The force of the wind pushes ships in the direction of the wind.
Vessels with large windage suffer most.
Although powered ships are able to resist the force of the wind, sailing vessels have few defences against strong wind.
When strong winds are imminent, sailing vessels typically have several choices:  try to position themselves so that they cannot be blown into danger  shelter in a harbour  anchor, preferably on the leeward side of a landform Many losses of sailing ships were caused by sailing, with a following wind, so far into a bay that the ship became trapped upwind of a lee shore, being unable to sail into the wind to leave the bay.
Low visibility caused by fog, mist and heavy rain increase the navigator's problems.
Cold can cause metal to become brittle and fail more easily.
A build-up of ice can cause instability by accumulating high on the ship, or in severe cases, crush the hull if the ship becomes trapped in a freezing sea. 
If the boat attempts to tack with a slow initial speed, or otherwise loses forward motion while heading into the wind, the boat will coast to a stop and the lack of water flow over the rudder will cause the sailor to lose the ability to steer the boat.
Stopped head-to-wind, a sailboat is said to be "in irons".
To recover from being in irons, one or more sails can be "backed" by sheeting or pushing them to one side.
A sail backed to one side near the bow of the boat will tend to push the bow to the other side, and the boat may "pay off" in that direction so that the sails can be trimmed again and normal sailing resumed.
A sail backed further aft may make a boat begin to "sternboard" or make way backwards.
If this is possible the rudder can be used to steer the boat out of the no-go zone.