Based on the following site:

http://bavaria30cruiser.blogspot.com/2006/04/bullentalie-mon...

this appears to be a 'simple' sort of boom stay arrangement. A boom stay is a line rigged from the aft end of a sailboat to the boom in order to hole the boom in its lateral position.

A more sophisticated boom stay arrangement is shown at:

http://www.mv.com/ipusers/whale/tillerstay/boominfo.html

The translation 'bull tackle' can be found (in a German/English word list), but I can't find the term in connection with sailboats on any English-language sites.

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Note added at 1 hr (2008-02-03 16:19:30 GMT)
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Oops: ... in order to *hold* the boom