Look at the images of built Tenderlysn none have a mast retention system.

I just put a simple one on my Skerry two days ago, it's not a bad idea. It is just a line through one of the open base cleats on the mast down to the mast partner. KISS, then simplify it more.

Any wind that could blow the rig out of the boat would cause so many other problems that losing the mast would be the least of your worries.

Serioualy, the forces are all wrong for that. The wind blows perpendicular to the mast, forcing the end against the side of the mast step with the mast partner as a fulcrum, The sideways force of the wind is amplified by the lever that forms to tightly jam the mast against the step and partner. The actual lift generated by the sail rig is directed perpendicular to the wind, not upwards. That's why the boat moves forward along the water instead of up into the air. The stronger the wind, the greater the sideways force until either the mast snaps or the boat turns over. The wind is not going to pull the mast out of the boat unless you go sailing in a tornado and maybe not even then.

In a capsize you've got much the same situation. The boat is trying to turn over but the wooden mast is acting as an outrigger. The flotation generated by the wooden mast combined with the force of the hull rolling set up the same leverage that pins the mast against the step and partner. In addition, you've the got the sail resisting  the rolling motion through the water, especially if the mainsheet is tight. That force also pins the mast in place. For that matter, if the mainsheet is tight, it will hold the mast in place.

Be happy, don't worry,

Laszlo

I added a mast retaining line to my GIS.  It is the blue/white line forward of the mast.  The same line/cleat/eye serves as a tie down for the mast when it is horizontal for trailering.

On my boat, the line is not needed as long as there is any tension on the halyard or vanghaul (two lines running aft).  My problem is that I am barely heavy enough to right the Goat during a capsize.  If I am slow about it and any water gets on top of the main, I have to blow the halyard before I can right the boat.  In that scenario, there was nothing holding the mast in place.  I agree with Lazlo's analysis that in a static scenario with the boat laying on its side, the forces should act to hold the mast in place, but when there are any waves and/or boat wakes where I sail and things get rattled around pretty good.  On the GIS, there would be significant damage if the mast base happened to get out of the step while the boat was being righted.  For me, the mast retainer is cheap insurance.   

To Laszlo's wise words, I might reinforce two things and add another:

My experience with the lug rig (I assume that's what we're discussing here) on my PMD is that there is no tendency for the wind to blow the mast out of the boat, even heeled over.  Simply too much pressure on the sail being transmitted to the partners and step, increating creating too much friction to be overcome what little, if any, upward thrust might be a component of the trust the sail is transmitting to the hull.  Now, if a turbulent gust of wind gets hold of your sail before the halyard is fully hoisted and the downhaul secured, it might be able to turn it into a mad kite from hell and pull upward enough to lift the rig out of the boat, but that would probably be a good thing if it happened and better than what is more likely to happen, which is a capsize with the mast remaining stepped.  I have had, shall we say, some moments of extreme concern like that when failing to hoist the sail smartly and getting it properly flattened out quickly.

I've capsized my PMD, and the rig remained in the boat the entire time, as Laszlo describes, showing no tendency to come out.

The thing I'd add is that being able to ease the sheet and quickly toss the whole rig, mast, sail, and all, overboard in a really hard chance has always been thought as a sort of safety valve in a sudden squall in boats with simple rigs like this.  If you've got the sheet secured to the yard and a good stopper knot (I like Ashley's better than the more usual figure-8 for this) in the other end to keep it from pulling through, you won't loose the rig.

.....Michael

   Sigh.  Excellent points men, both for and against.  Like everything else in sailing, it's a judgement call.  Thanks for giving it your attention.