On a run, bore away...mainsail didnt flip over, but filled more! Pressure increased, boat began a wild curve despite my rudder antics....SPLASH.

What did I do wrong? Looking back at the weather logs there was a F4/5 wind with full F5 gusts. Only tiny waves. 

Kicker was moderately tight. Half centreboard. Boat was flat to begin with. Gentle rudder use.

I must be making some complete novice mistake - and yet the 5 gybes I did in the preceeding 20 minutes went as expected. I yanked the main over the nanosecond it was ready, and straightened the tiller so the power was delivered forwards not sideways. True, there was the odd wild roll, but nothing awful considering the conditions. I found the "boat not wanting to gybe but speeding up" really weird.

Generally novices do the splash with every gybe when gusting F5.  As an expert in capsizing on the gybe with pictures to prove it, get the weight back, go for the gybe when you are travelling fast, if in gusty conditions wait till it eases a bit & you are still travelling quickly, make the course change and pull the main over as the weight comes out of the sheet.  If gybing spinny, don't be in too much of a hurry to gybe after taking pole off, choose the moment, after the gybe you can bear away a bit before going for the new pole.  I think you experienced reverse flow over the main when you started running by the lee.  Expect the way to avoid that is being assertive with the course change so you are not sitting in the 10 degrees by the lee position but changing course 20 degrees so the main will gybe.

Were you flat, or heeled to windward. Latter would cause you to head up and almost no amount of rudder will correct. Was the main too tight as well?

Some hull shapes are less forgiving - I got away (without swimming) with lots of beginner errors in my NSM4, but my newer boat will not let me get away with anything.

"heeled to windward. Latter would cause you to head up"

of course you mean heeled to leeward would cause you to head-up, as heeling to leeward would cause you to bear-away

Mags, which way was the wild-curve? Did the boat bear-away and roll on top of you (capsize to windward), if so, then what has happen is not enough kicker to stop the top batten twisting forward of the mast, solution in that emergency situation is the pull the main sheet in hard to bring to top of the sail back.

See photo to see what happens if the helm doesn't pull hard, fun isn't it!

Hmm, was my rudder movement too small then? I thought too much would cause trouble!
Reverse flow is a possibility - it definitely seemed like a sudden acceleration with the main filling even harder than before.

Boat was flat going into the gybe, but as this mysterious force began to boot us along, we heeled to windward with the boom way up in the air. Then it gybed with a wallop and nothing could stop us going splat into the water.

Never thought about trim before. I suppose with the crew having to be in front of the hoop its obvious that the helm ought to move back to compensate. I can't remember if our bow buried or not.

And the kicker was pretty tight, to avoid the dreaded roll.

After my gybing antics at Salcombe earned a mention in despatches I've followed the Dan Alsop advice from Rutland training.  Go into the gybe with the burgee aiming at the corner of the transom you are sitting on, make a definite course change so the burgee is now aiming at the other corner gybing as the mainsheet lightens by pulling on all the purchases & letting it out on the other gybe so it doesn't absolutely slam, my best gybes in heavy weather are the assertive ones, if fiddling around, shall I, shan't I you are slowing down.  Also in restricted venues, if you are starting to ask the question the answer is virtually always yes, so get on with it.

I have to look UP? Cripes, that seems to go against my self-preservation insticts!

I have heard about this technique of keeping hold of the mainsheet falls in order to release the boom slowly....but doesn't that mean you will then be standing almost amidships....just when you need your butt on the sidedeck?

Mags

The rudder movement will slow the boat (drag) and load up the rig and so exaserbate the situation. A common problem with most sailors both at the front and back of all fleets. Alan is correct, haul in the main and reduce the profile and return the balance to the rig and tighten the shoulders of the kite down. Also rather than try and fully correct the roll, try and antiscipate and adjust weight slightly to bring the hull form back more in line. Also up to 5° of rudder, no much more may be a last resort. One other thing, when it is really windy, just tie the plate down with the trailing edge vertical for upwind and downwind sailing. Any slight drag will be more than compensated by greater stability downwind. 

Have fun

Mainsheet under neutral load and all purchases with mainsheet cleated slightly in is correct as well. A steady gybe, if in seas down a wave, you will have plenty of time to sit down.

Andrew, why do you want to get the weight back?  I thought the point of a gybe was to maintain the maximum boat speed possible in order to reduce load on the rig.  Just like windsurfing?  Weight back, lots of drag, rig loads up, fish worried.

If you look at the Merlin hull form what you don't want to do is dig the sharp bow in when you are changing direction, we are talking about doing this in F5 when I am generally sat on the transom not a gentle pootle round on the river! Look at the "click on the boat" pic for trim, though admit that's a reach not a run.  Difficult to analyse exactly what I do when not doing it but pretty sure I am on my feet crouching well back for the gybe, rudder movements are small as MB says but the point was around the change of course and going for it rather than holding back being anxious.  Gybing in waves can be surprisingly easy if you time it right with the main coming over ever so gently when surfing.

Here's a good example of why to keep the nose out

...just in case anyone missed it, yes I am on my feet but the weight's not back on this one

No-one seems to have mentioned the M-gybe yet ... de rigueur in my RS200 days. Get the boat going as fast as possible, bear away slightly and pull the boom across using the sheet falls ... at the same time move tiller slightly the other way so that boat bears away a tiny bit on the new gybe.

If done correctly the tiller and sheets should feel as light as a feather. If they feel as heavy as lead, scrub the manoeuvre if possible, carry on a bit asnd try again.