---Springs rust, they always rust
---Rubber breaks, quickly in tropical sun
---Elastic lines are silly
Getting bow and stern lines tight is important and the right tension on spring-rigged lines reduces absorbs some of the surge. Inertia of all of the boat weight comes to a halt when the lines stop it...the more the boat travels, the greater the inertia.
I plan to buy some that are made in Italy
that are different than all of the above. I will post a photo
and other info later today or tomorrow. There is a Beneteau
54 here that has these and I am impressed how these "shock absorbers" tame the surge of the boat. I am also impressed that there is no spring to rust and no black rubber to deteriorate.
Hope this will help, although it is mostly advice, followed by a recommendation on a product that I have not tried.