1976 Chris Craft 36' Commander SF Props (Cummins VT555-M)

Hello, I'm trying to "maximize" my boat as much as I can until I can repower. Currently have the original Cummins 555VT-M 270hp engines. The are working well. I bought the boat with the current props. Last time I had them checked they were 22x22 4 blades. Seems to me the boat is slow for the power. At 2300rpms I'm seeing 13.5knots and at 2550rpms I'm seeing 15.5knots SOW. I would guess I'm missing around two knots from my mentioned speeds. This was done on a clean hull and around 3/4 full tanks. I found the original owner had some 3 blades stowed away which seem to be the original props. No tsure on the specs but I'll have them inspected. Any input on props you've guys have used with the original diesels? Maybe some speed numbers so I can compare? If I do prop slip calculations, I'm at 49% which seems high.