Originally Posted by K&S
We had the boat out again today and tried it up at WOT it hits 6100 whith travel left in the throtle speed 34knts
that sounds a bit healthier. and is about 11% slip.
6100rpm is Wot. The rev limiter kicked in if i tried pushing the throtle any harder ( good to know this feature actually works )
so you know the engine can "give" a bit more "oomph"
A quick back of the envelope calculation suggested that with the same slip and still getting 6100 rpm your best possible speed for a 21" prop would be 37.5 knots (is that enough of an increase for you?) and with a 23" would be 41 knots. Those assume the engine has enough power to deliver full revs.
Now I think even if the 23" prop is too big, and the engine manages only 5600 rpm you should still get similar speed to the 21". All very back of the envelope calculations which completely ignore the boat and engine in the calculation - and assume "anything is possible".
Of course what you gain in top end speed you may lose in acceleration/time to plane.
Not sure why someone would suggest a 17" prop for this - unless you were driving a very heavy boat (lots of divers, fuel etc); you would be doing well to get much over 30 knots with that.