Brian J Goggin was thinking very hard :
> 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
>
>
> ------------------------------------
The clearance on Harnser is far less than recommended and I have not
noticed any problem. I have not repacked the stern gland in 6000 hours.
------------------------------------
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