I have to agree with jon. First a 55 gal contractor bag can't hold the weight that you imagine. First, you have to load it, get it up the companion way ladder, off the boat and to whatever you are carrying it in. I'm not at all sure where you could even put 57 bags that big full of stuff filled it to capacity on a 34 foot boat? I guess you never went below. Besides with heavy stuff it would split. (Yes, I'm an engineer who has done calcs so here goes).
Assume:
Each 55 Gallon bag is filled with mixed "stuff" at an average weight fo 60 lbs (probably a high average since some bags would have very light stuff too like soft goods)
57X60lb = 3,420 lbs in bags (most likey a very high estimate)
Since these were anchors you didn't know had, they couldn't have all been big heavy primary anchors so 3 at 40lbs each
3x40 lbs = 120 lbs for anchors
Assume 6 25 pound tool bags (you have to lift the tool bag to use it) A 25 lb bag is pretty big.
6X25lbs = 150 lbs for tools
This totals 3690 lbs for bags of stuff taken off, anchors, and tools
This is still 4,310 lbs short of the 8000lbs taken off? Even if my estimates are off, they are not off by 4000lbs.
By the way, 2 +/- change in draft sounds about right for 3500lbs on a 34 foot boat.
On my 40 foot boat, the CE Certification page lists:
Light Ship Displacement 19,775 #
Full Load Displacement 24,970 # (a full load as 5,190 #)
Sink @ Full Load Disp 2.67"
My numbers are very consistant with the numbers from NIgel Canter's Cruising Handbook which says for a heavily equiped blue water cruising boat, the fully loaded displacement should be about 5000 lbs above the light load displacement. Now I don't consider my boat a Blue Water boat (so lets not argue about that point) but the numbers are consistant with the concept.
If the load cell on the travel lift is lke most OSHA cranes, it is also measureing the lifting equipment itself (hooks, cables, straps, etc) since that is what the cell is seeing and the purpose of the cell is to prevent overloading the crane. It may have some built in bias and also may not have been calibrated in awhile. I just don't believe those travel lift weights.