propping an old juniper hull

Hello. I am trying to figure out the best props for my 42 foot, 45000 pound, sportfishing vessel. I have been told by various people who have looked at the hull that it "appears" to be anything from displacement to planing. I have no idea. My goals are fuel economy and propulsion longevity. Speed is of no importance. Brief history... 40 year old juniper vessel originally powered by 3208s, then ford lehmans, then (finally a little useable data) detroit 871naturals with 2:1 reduction and 24by24 four blade props. I have recently acquired the vessel and was told that it ran around 10 or 11 knots and that any attempt to gain more speed resulted in the "backend squatting" without speed increase. So I guess that I will be starting from scratch. My propulsion will be provided by a pair of Cummins V555M 215hp @3000 and 185 bhp at 2800. Twin disk 502 with 2:1 reductions. I have entered data onto a prop calculator and it appears that 4 blade 21.5x16 without cup or 3 blade 23x16 with no cup would be reasonable starting points. Any ideas would be appreciated.... morning glory ps the boat had a thick sea flex applied at some point in the past which other than weight I don't think will make much difference.

 

My calculations show 23x14, 3 Blade w. 60% D.A.R. min. Max speed 11.7 kts @ 3000 rpm. Calculated Slip of 32% at max speed.

Jango

 

Greeting all
Even my calculations give 23x14.
But even a cavitation of 22%. And 'acceptable or wrong?
Excuse my English.
Bit

 

My calculation shows 32% Slip which is probably Normal for this application.

Revising my original reply to indicate slip of 32%

 

I have 34 to slip. We are ok.
Then I remove the slip from 22% cavitation? And how?
Merci. Bit

 

Thanks for the responses. Gives me confidence that we will be off in the right direction.
morning glory