Installing optical cables in ducts by Blowing is now common practice. Sometimes this is done with very high speed. Question: what is the maximum allowable speed? Of course paying off from the reel must be well controlled. This can be observed visually, and the experienced installer will learn about its limits, not further discussed in this paper. But what happens inside the duct after a sudden stop? A crash test, where the cable is taken out of the duct afterwards, already exists. But, this test is not suitable for maximum speed. In this paper the compression wave in the cable after a sudden stop is analyzed, as well as the water hammer effect which might occur with installation by Floating. Formulas for the limiting cable speed are given. They can be programmed in intelligent Blowing machines. Exceeding the maximum duct pressure by a water hammer turns out to be irrelevant.