It was stolen - I think they simply drove off, and the sudden shock broke the padlock hasp. 

The hasp was hardened steel - which I believe makes it brittle, then easily broken.

THey probably (in the dark) didnt even know the chain was there. 

A chain and padlock is only as strong as its weakest place. 

A wheel clamp might deter, but must be the strongest - probably SAS clamp. I have seen cheaper clamps, that I think could be levered off with a steel bar.

Things that stop the coupling being used, might deter, but I think a prepared and determined thief will get around that by either cutting with a grinder or big bolt cutter, or simply tie the trailer coupling to the vehicle ball.

THe best thing I know of, would be a very strong chain and lock. Which I have - 14mm gauge chain (from Ropes chains and anchors) with a super strong lock LOckwood 29 series Steel Case Monoblock Padlock. 

 This is too small, but I couldnt make it bigger.

See it here http://www.lockweb.com.au/au/en/products/padlocks-and-outdoor-security/maximum-security-range-padlocks/maximum-security-290-series-steel-case-monoblock-padlock

Further - I think the best deterrent by far - is to put the boat away from the road frontage, preferably out of sight from the road. In fact, my current boat is behind our house and out of view from the road; and near to 3 houses. I only have a pin with padlock, stopping the trailer coupling being used; not much, but I dont believe its going to be stolen from there, when there are plenty others in a far easier place.

And obviously need insurance cover. I have had boats stolen twice - and in each case the police did eventually find and recover both. 

 

Do the insurance companies have a list of locks they are happy with?

It is a real shame it gets to this. I am not so worried for my boat but have another trailer I prefer not to get stolen. More whats on the trailer that is likely to make it a target. So have been down all these tracks also. Nothing I have seen is to a point I cant figure a way to defeat it. And the battery angle grinders just make it so much worse. 

All you can do is choose something that works for the way you want to do it, and hope they move on to something easier. It is only a deterrent in the end. If they really want to nick your stuff, they will. They will come prepared. But all the above should be enough for the opportunist creeps out there. I have a trailer hitch lock. But it doesn't make me sleep easy at night.

Alan