high thrust props

I have a Nissan 9.8 4 stroke OB on my Cal 28 and it came with the standard 8.5" pitch prop. It doesn't seem to be reaching 5K rpm, though I need a tach to confirm. Top speed is around 5.1 knots. Has anyone had experience experimenting with high thrust props on a 3 ton vessel? What size/pitch worked best on a 10hp? Nissan makes both a 4 bladed 8.7" x 7" pitch and 8.7" x 5" pitch props. 5" pitch would probably be too small, though if the sailboat is only going 5 or so knots, a small pitch would probably be best. If I'm able to get an additional 1- 1.5K rpm out of the engine, that would in theory produce more power, right? For instance, if the engine is only operating around 4K rpm, it's not producing its full potential for horsepower, right? Under almost full sail in 20 knot winds, I've had it going around 6.4 knots. This was before I learned to reef the main... Thanks for any info anyone has on this topic.