While you should always read and follow directions found in your documentation, from my experience the FP generator, when running, will operate just fine at any boat speed I can tolerate myself. Whether real or a warranty excuse, FP states some concern with water entering from a high speed pickup while underway (I have never seen some specific speed where this becomes a concern) flowing through an open seacock, into the exhaust and subsequently into the cylinder (or cylinders) of the FP engine when it is NOT RUNNING. So the only "rule" I think applies here is to keep the FP cooling water seacock closed when the FP generator is not running. Otherwise, enjoy the boat and run the generator. The FP's are much more likely to die from sitting unused than they are from running, at any boat speed!