There's a lot of good information in this thread. The actual static testing done by the coastguard shows that breaking waves of 1/3 the boat's lenght, which is nearly always about equal to the beam width, are likely to capsize
a boat when they are coming from abaft 100 degrees to about 150 degrees.
This happens because the moving water
of a breaking wave pushes the highly buoyant aft quarter of the boat over the lateral resistance of the keel
and is not sufficiently opposed by the opposite forward quarter of the bow, which has far less buoyancy than the aft quarter. Essentially, the boat is pushed up over it's keel
and the bow lacks the buoyancy of the stern to resist it.
This is especially likely to happen in a broach, where following seas pick up the stern and push it sideways, rotating the boat until this angle of highest vulnerability is reached, which then rolls the boat.
Fin keeled boats are more likely to be able surf sideways in a breaking waves because their lateral resistance is dependent upon forward speed (which is good) but they're also far more likely to be broached because they rotate on their keels easily (which is bad). There's not enough data to determine whether those effects are better or worse in combination than a full keel, but my guess is that surfing doesn't mitigate as much risk as broaching causes, so I'd presume that a full keel under a competent skipper
In non-breaking waves, there's very little risk of roll-over at any wave height because the water
itself isn't moving, and so the boat will rock with the wave surface (which can be extreme and can build momentum with sympathetic harmonics), but is never pushed over it's keel. This is what the OP experienced.
Drogues function by preventing the broaching rotation and keeping the most buoyant part of the boat (the stern) presented head-on to the waves. This increases the risk of being pooped, but that's a lot safer than being rolled. A robust storm hatch
for the companionway
and an open transom can completely mitigate the danger
Many new production boats come with lexan
or polycarbonate french-door companionway
hatches, which are fine for most sailing, but these should be complimented with a set of fitted 1x8 hardwood slats that can be strapped in place over the closed companionway. Use flat straps that can be cut away by a rigging
knife in the event of an emergency
, and you've got a strong hatch
that can take being pooped without risk. It needn't look good as nobody will ever see it; just boards cut to fit will do.
A breaking wave greater than the length of the boat can roll it no matter what you do.
Mid-ocean storm waves commonly reach 25 feet in height, and in extreme conditions can reach 50 feet. Singlet rogue waves are exceptionally rare but can reach 100 feet in height. This is why boats under 30 feet are unsafe in typical ocean storms, boats to 50 feet are safe except in extreme conditions, and boats over 100 feet are at very little risk under any circumstances (although rogue waves can cause significant damage even when they don't roll a boat).