here's the reply
"Thank you for contacting Stena Line.
Motor bikes are secured by strapping or ropes, you can secure the bike, but our staff will offer assistance where necessary. As this a classic motorcycle, I would advise to bring a piece of fabric or thicker materiel (some drivers have been known to use their gloves) this is placed below the strap or rope and would ensure no movement during the voyage.
We carry hundreds bikes during the North West 200 with very few complaints, if your bike is damaged by us, while on a Stena Line ship, the issue will be investigated, and if our staff are found negligent, we will cover this with our insurance.
We hope you enjoy your forthcoming journey, "