Speed and revs relationship

I have a wooden 21 footer with a Perkins inboard. As I increase revs to 1200rpm speed goes up smoothly until I reach a little over 6 knots. Then I struggle to reach higher speeds. ie. I have to really press hard and increase revs to reach 8kts. Any explanation or remadies appreciated.

 

Like as has been expressed more eloquently than I... That is what is commonly called something like "your Hull speed" and it usually means that it is a speed where the bow wave and the stern wave are the same length as your boat waterline. To go any faster you must climb over the bow wave and "onto the plane" and that takes a LOT of POWER (read FUEL) and consequently Dollars and size to get any decent range....

Alternatively buy a cat, something like at http://www.icecat.com.au/objectives.htm - I suggest you read the other pages there too.... "Foreign Affair" was recently in New Caledonia as reported in the free pdf magazine at http://www.thecoastalpassage.com/recent_issues.html - best wishes...

Alternatively, look in my gallery at the "Chamberlin C10" a smaller 10 metre (33 ft approx) version using 2 x 50 hp kubota engines to get 16 knots...