2 or 3 ?
Just to be different (almost) and give you a straight answer, We have a stock 3 bladed prop married up to a M25 (21 hp) diesel, that provides everything we need. In this area it is not unusual to encounter a 5 to 6 knot current while transiting some of the local channels/narrows. The additional power of a 3 vice 2 blade prop is more important to us than any possible added drag caused by an additional blade. I guess what I'm saying is that you need to examine your cruising grounds and the type of sailing (ie: racing) you will be engaged in prior to choosing a prop. Just as a side note, the last 2 surveyors we used warned us against the use of folding props. It is their opinion that the vibrations caused by the prop will result in damage to the strut to hull mount. I have no way of verifying this, but the surveyors seemed to be well qualified.EG