Well 1.81 @4000 38mph is 5% slip quite good. But not enough to really now how that engine pushes a prop on that hull. So here's a WAG a 22 would put you at 52 with the same slip..very close to the posted test speed...And you had a 24 at 4400.

23 would get 4600
22 would get 4800

If the slip remain's constant which is should Solas Hr titan makes a very good 22p 3 blade SS for that combo...http://boatpropellers.iboats.com/Mer...t_id=138527619 Top end prop



Or a Stilletto or Turbo would well in a 21 work well also...same money virtually same prop with one difference the Precsion props are big lifting prop..shallow v could be a little bumpy or porpise a bit

Midrange Stilletto 20p 4blade baypro 2 http://www.boatownersworld.com/Props...pro_series.htm Great midrange prop it will act like a 21 3blade in terms of slip...maybe a 50 mph prop on that boat. I use a 20 on a 20' 4.3 mpi engine with 1.66 gearing actually makes you feel ike your in a small hotrod.