Tim all the above are quite correct in the flat but you try to go on a long trip in force 5 with 50-80cm waves and see what happens
Due to the deep hull forard and shallow V in the middle and with the adverse chine (you know all that so I will not go on and on) they plain very quickly and you can get high speeds with small engines. The 640 SRs are also very stable when you have the weather on your side and also when you have the weather from behind HOWEVER
not that great (they bang a lot and they lift the bow quite high even at 10 knots and also the wind lifts the hull up since the boats are very light) if you try them in force 4-5 and above.
The turning you mentioning well, let me have my doubts. As we found out when you try a tight turn the 640SR in high speeds (say 20 knots +) due to the flat bottom the boat slides quite a lot (like a hoverctaft) and you need to correct it with the steering. The 700SR on the other hand is like a go cart. The 700SR is a very different boat altogether. If you see one of the videos I have posted with the 640SR boats you may be alble to see that effect as I am trying to correct continuasly the steering on tight corners.
Since we are going again out tomorrow with a potential clinet who wants to buy one of the 640SR boats (have explained to him all that but he wants the boat to potter about and to go fishing which is fine) and it blows force 5 I will also take a video and post a few clips to demonstrate exactly what I mean so there will be not doubt, buts or ifs and people can actaully see.