I have difficulties understanding the old Dutch as follows:

In the following CONTRACT VAN BEVRACHTING:

http://resources.huygens.knaw.nl/retroboeken/oostzeehandel/#page=33&accessor=search_in_text&source=8&accessor_href=search_in_text%2Findex_html%3Fsearch_term%253Austring%253Autf-8%3DVacht%26source_id%3D&view=imagePane&size=969

No. 2044

I do not fully understand the last sentence:

„Eyntelijck is bevoorwaert dat den bevrachter gehouden sal wesen 't voorn, schip tot Gevel voorsz. vrij op ende van de reviere te keeren, sonder dat den schipper in eenige oncosten, aldaer op 't schip vallende, gehouden sal sijn".

What is the skipper allowed to do ?

An interesting detail is the word ’voorsz.’ It has been used more that 1000 times in this book allone and in tons of other ancient books, but it has always been put on the short form and is not to be found in any dictionary. I could imagine that it normally means ’abovementioned’ or similar, but in this particular sentence it does not have the typicl position as an attribute to something.  

The word ’reviere’ sounds grandious for the particular stream at the now little town Gävle, but the expression may be generic.

Voorn. = voornoemd, and voorsz. = voorzeid or voorschreven. Both stand for "abovementioned" as you rightly concluded. It's just a bit of legal padding, put in to prevent dissension or legal finagling, but usually quite superfluous as things are clear enough without.

The position of these words is flexible.

Het voorn. schip = the ship that was mentioned before.

Gevel voorsz. = the town of Gävle, mentioned before.

I gather that the sentence means: the skipper is free to come and go up and down the local river, or maybe the harbour of Gävle (if any) without having to pay costs and duties.

It seems to me that it is about the costs of shifting and turning the ship in the (narrow) river, so that it ends up with the bow seaward again.