Hi Graham,
Intriguing - I would have thought winching-on exerts a lot more stress than winching-off, especially if the bows stick on the first roller crossbeam. I'm no metallurgist but if the crack spread progressively over time (not necessarily true fatigue) then there should be some evidence of corrosion history on the crack face.
Also, how tightly connected was the post at the bows? Was the strop long enough to allow the bows to float up and down against the face of the restraining bracket? This is worth understanding because if the stress is from the boat moving horizontally (from winching-on/of or deceleration on tow) then I can see ways to simply reinforce the post. If the stress is vertical from the bows moving up and down relative to the trailer then in my view it should be allowed to float vertically so that the boat hull is not fighting flexing in the trailer frame. I think I originally learned this from Klaus. On the other hand the boat still needs to be attached so it cannot slide off backwards.
Up to now I have relied on the winch strop and ratchet mechanism to stop the boat sliding off backwards. Not shown is that I use a rubber bungy to hold the winch handle down against the ratchet but following your experience I'll think about a better way to arrange this. Maybe I'll attach a strong strap to the draw-bar well in front of the post and lead it back over the post to the bow U bolt at an oblique angle. Has anyone else any thoughts on this?
BTW the photo shows the jockey wheel attached but after long experience of the naughtiness of jockey wheels and their enthusiasm for loosening their brackets I remove mine altogether and carry it in the car. Less trailer weight too.
The other photo attached shows my current attachment system. The main strap passes between the cockpit coming and the mast and in front of the cleats. The logic for this is that this is the strongest part of the boat and the hull is supported close-by underneath. Also this strap is lightly inclined forwards which helps restrain the boat from deceleration forces. The rear strap is simply around the boat, firmly but not too tight, because it would be easy to crush the sides together. BTW I put one twist in the strap to stop it thrumming in the wind and the buckles on the driver's side so I can watch them loosen and detach in my wing mirror. But this has not yet happened.
I notice an interesting neighbouring thread about repairing the gel-coat mentions strap buckle damage. While on the topic Matt Newland suggested using the cutup windsurfer mast-sleeves to protect the hull from buckle scratches and pad the straps over the gunnels and although untidy this has worked well.
Not shown in the photos is a long strap I now pass around the hull (with one twist of course) to support the centre board so it is not straining the CB up-haul.
All above are unqualified suggestions, not recommendations and owners must use their own judgements as to safe operation of their boat and trailer or get qualified professional advice etc. etc.
I seem to have got in rant. Sorry.
Matthew
BR20 Gladys