Originally Posted by makobuilders
Nozzles work in situations of propellor slip. That is why they have maximum effectiveness at zero speed, working against a tow or trawl net. At free running speed they are only a detriment. My shipyard installs them on every tug they build, clearly told me I would lose 1/2 knot. So even if the Rice provides higher fuel efficiency I don't think that would be a great advantage against the added drag and slower cruise speed. The prop protection would be wonderful of course, but it is easy enough to add ice horns for that purpose.
In any case, do you have any idea how much a typical 44-48 inch Rice nozzle would cost with the special propellor?
A Kort or Rice nozzle will only be a detriment above something like 12 kn. If you yard loses 1/2 kn then they are not implementing the concept very well. It needs a prop matched to the nozzle.
Fuel savings tell the story: fuel saving is better efficiency! Now, my go-to resource for this stuff is Gerr's Propeller Handbook. It seems that the higher loaded the blade, the more benefit. So tugs, and trawlers returning with a full catch, get maximum benefit. For trawlers the payback could be quite fast given their fuel use.
But, for lower blade loadings, the kind pleasure boats will see, the benefits are much reduced, although still present. So payback could be a very long time. I had not appreciated this aspect before. No idea what a Rice nozzle and matching Kaplan prop would cost, but having now picked up an additional nuance as to when they deliver maximum benefit I have to say Ski is right again: not one of the decisions you need to make.