Any propeller experts around? Our 61' X 12' X 4' (at the stern) barge weighs about 50 tons. We are considering changing the engine: the existing elderly BMC 5.1-litre is working OK at the moment, but the attached hydraulics and electrics need considerable work and I'm reluctant to spend money on systems for an old engine which we're thinking of changing anyway.
The barge's previous owner rebuilt the stern, moving the prop shaft much further down and replacing the original 48" prop with a three-bladed prop marked as 20 X 17.5 and stamped WR587665. Everyone tells me that the barge is under-propped. However, there is a skeg under the prop (its far end supports the rudder) and only about 2" clearance between prop and skeg. Another rebuild of the stern is really not on. One engine manufacturer has recommended that we go for a 21 X 18 four-bladed prop, thus reducing the clearance by another half an inch. And I can play with PropCalc and with the formulas in Dave Gerr's *Propeller Handbook*. Gerr reckons that the clearance should never be less than 2" and a very minimum of 12% of the prop's diameter (which would be about 2.5" with a 21 X 18). What happens if I ignore Gerr, fit a 21" prop and thus reduce the clearance to 1.5", which fails on both his criteria? I understand there is likely to be extra vibration, but how bad would it be? Would it damage the stern gland? I see what Gerr says, but I haven't really got a feel for the extent of the problem. I may be back later with questions about engine size .... bjg
