Small 60 - 90 degree tunnel to get bigger prop viable ?

Hi guys, I did try to ask this question on an old thread,but apparently it was too old for me to reply to so I'll ask it here .
I have a shoal draft yacht [ in construction ] and am shortly to drop the keel to enable me to fit a larger prop - amongst other things - I was wondering about fiting a semi pipe / tunnel [ probably only 60 - 90 degrees ] to enable a larger prop / better, or more tip clearance .
I have a big old slow reving lister [ 1100 rpm max ] with lots of tourque so a large prop = better efficiency , right !?.
Pete.

 

Pete;
Try to find a copy of Dave Gerrs' ropeller Handbook. It contains a wealth of information about application and selection. There is also information about Kort nozzles and the like.

I believe that shrouded props work well for low speed applications but not well for high speeds. With the big Lister you are not likely to be a speed demon so perhaps your idea has some merit. If your Yacht is a sailboat then the big prop will make much drag while under sail. If it is a power boat intended for moderate speeds, then a large diameter prop is probably more efficient than a smaller one.