Stonehenge and its Environs 134
Image by Chance

This idea of strapping the stone in the water under the boat / pontoon is exactly what was tried with the ill-fated Millennium Stone experiment. As soon as the vessel hit choppy water near the Mouth of Milford Haven the straps chafed through and the stone plunged to the bottom of the sea..... the vessel and its cargo never even reached the open sea.

And I would think that that method just wouldn't be tried with rope made from natural fibre or bast.

mountainman wrote:
This idea of strapping the stone in the water under the boat / pontoon is exactly what was tried with the ill-fated Millennium Stone experiment. As soon as the vessel hit choppy water near the Mouth of Milford Haven the straps chafed through and the stone plunged to the bottom of the sea..... the vessel and its cargo never even reached the open sea.
That's a shame. However, the method was (apparently) still used successfully in Wexford within relatively recent times to transport millstones from a cliff face to a nearby beach*. Not an ocean crossing, but the tides down there are the ones that would also be hitting south Wales.

*Newgrange by Geraldine & Matthew Stout p11