Karo it depends on the circumstances. Under controlled conditions, like a yard working with a travel lift, sling lifting will be safe and secure. Always the same enviroment, well trained personel and paved level ground should not give any problems. The best and safest method of course is a customized cradle and a syncro lift. But we are talking about different harbours situations and off harbour loading and deloading from a floating ship. Sea state, wind force and worn out slings with unvisible damage and / or corroded lifting gear can be a degrading factor. The worst method is the direct connection of the front and rear sling to a single hook without the integration of load handling attachments. This is prone to sling slippage. The integration of a box type beam traverse (see below) will parallel the load and stabilize the slings. For our own boats, both work boats and smaller yachts like my 50 ft inland waterway cruiser, which are lifted quite often, we have heavy duty threads built into the deck/frame structure for srew in lifting eyes. Together with customized steel cables and shackles, the boats can be lifted safely as often as wanted.