Why so much slip?

i'm running a four winns liberator 261 with twin 351 clevor engines, windsor block with cleveland heads. The power output is 600+hp total. volvo penta 290sp drives are mounted with 1,61 gearratio. The propellers have 14 inch diameter and 24 inch incline.

the boat tops out at 45-47 knots, doesn't mather if you have half or full throttle. The engines easily revs 5500 rpm, trimmed out about 6 deegrees.

slip ratio incline rpm speed [knots]
0,31 1,61 24 5500 46,55
0,15 1,61 24 5500 57,35

as the figures show we have a huge slip, about 31%.
I wonder if someone have experimented with nosecones to these drives? One thing we have considered is that the engines weighs less than ordinary marine engines and that the boat stand to much on the nose!
Any suggestions for a solution are welcome