Speed of the boat

[isidrom]

Hi everybody .

I have a DF 150 (2008) ,with a propeller 3x15 1/4x19R .

The Engine reach 5800-5900 rpm , and even 6000 if the sea is good and you leave it .
But the speed of the boat is not good . Up to 4000 rpm is normal .4000 rpm is about 19 Knots . But you need to increase a lot the rpm to speed up the boat . 5800 rpm about 29 Knots .
The hull is clean and i trim the engine .
I think maybe the prop is not correct and i should go to 3x15x21R , next pitch .
The boat is a 6 meters with cabin .

What do you think ???

Thank you very much .

[mitch]

is the hull clean

[isidrom]

Yes it is clean .

The fact is that you get just a few knots of speed increasing very much the rpmīs ( when you go over 20 knots ).