Is there a general rule of thumb for determining how strong your dock lines
should be?

I have a 34 foot boat that weighs somewhere in the neighborhood of 19,000
lbs. I want to buy some dock lines to carry onboard for tying up at fuel
docks or the occasional restaurant etc. when I'm not in my slip. I really
don't want to have to untie the lines at my slip every time I leave, since
I'd have to find just the right adjustment again every time I return.

There are LOTS of sizes and strengths of line out there. What should I
really be looking at?

Thanks!

Gary