Bob,
We have hull #42 fitted with four bladed propellers and are very pleased with the reduction in noise and hull vibrations.
Due to an original "singing" (blades had self induced harmonic vibration in a fixed rpm range)four bladed prop., we had the good luck of driving our boat with a set of three bladed ones, while the original set was adjusted.........so, I could compare the same boat, same loading conditions and similar sea and weather conditions with two different prop. sets. The reduction in vibration is significant.........none felt in the steering wheel,nor the deck on the fly bridge, contrary to the three bladed operation.
Unfortunately, it is very difficult to quantify the total impact of sound and vibration reduction efforts, hence the cost/benefit arguments are somewhat subjective. However, I would suggest that the propeller induced vibrations on the hull above(behaves like hollow drum) are much more bearable from a four than three bladed prop. Theoretically, we are all told of a loss of "efficiency" when going from three to four blades........theoretically I understand and agree...BUT, based on my last summers' experiences, where we ran with other MV-34s in the Rideau Flottilla, our speed (GPS measured) was the same as two other boats with three bladed props at the same engine rpm! Our top rpm is 3650 and 3675 for the 75hp engines rated max. 3800rpm........this is with a "heavily loaded" boat, and three people on board.......PDQ tells me that this is good performance.
With respect to the engine induced vibrations, they drop off as engine rpm is increased........so, an isolated engine (Aquadrive or similar), well cushioned, gives an impressive performance at idle....BUT, that's not where we operate. Bottom line....in my judgement, the dominant vibration on the hull, during normal running conditions, is more from the props than the engines.
I am at times obsessed with noise reduction, (having lost some of my hearing during my working career) and had a second muffler installed in series with the original......this I believe helped also. These two basic modifications (four bladed prop. plus muffler) were relatively in-expensive but did make a significant improvement......I would recommend these for all boats........AND, retrofits are easy.
Almost forgot.......I requested increased clearance between hull and prop. tip by lowering the shaft angle to the maximum.....this gave an extra 1inch clearance....reducing vibrations on the hull.
If you are interested in specifics, suggest you contact PDQ.
One word of caution, when going to four bladed props., insist that they get the "anti-singing" treatment first, as such problems are common with "off-the-shelf" props that have four,five,six blades.