Help in using SSPA data

[Davide600]

Hello to everybody, I'm a 3°year student of Naval Engineering at University of Naples Federico II. I've a question:I already have self propulsion test data for a ship model, tested with a stock propeller, and now I have to choose for this model a propeller from SSPA series, with 4, 5 or 6 blades. If I want to use values of relative velocity (Va), rotation speed of propeller (n) and torque (Q), coming from experimental test, how shall I do? I don't exactly now how to use SSPA diagrams...please answer me, my teacher won't be back at university until next monday so for a few days I've nobody toask this question.

PS: I've already posted this message, but I was in the wrong section, and for this reason I've decided to post it again.

[RANCHI OTTO]

using the attached data sheet you have to proceed as follows:

(01) from resistance tests you have RBH extrapolated for ship
(02) you calculate the rudder(s) resistance
(03) you estimate the frontal wind resistance
(04) you calculate the total resistance for ship
(05) using open water curves of stock propeller you calculate w,t, eta hull, eta r
(06) you estimate the correction btw. w model and ship
(07) you calculate using final propeller geometry your eta0

[Davide600]

Ranchi Otto thank you for answering me, but what I exactely wanted to know is just last point of your answer- the first 6 points are already clear for me- in fact now that I've calculated for my ship (following ITTC'57 method) with stock propeller values of etaw, etat, etao, etar, Q, Pd, etc for my service speed, I've just to choose my propeller from SSPA series...and here is the trouble. I know that I have to start with a tern of values, for example Q, Va, and n, that I have to combine in a non-dimensional coefficient, and then in correspondence of this coefficient, I've to read on diagrams values of P/D ratio, J, etao, Kt....and then I can calculate effective diameter of screw propeller, and T, and a new value for etad. Here is my problem: I find a value of T bigger than that one I had with stock propeller, and also value of etao now is better. But if I multipy them to get new value for etad, I find it worse than before.....so I think that there is some mistake in my "alghorithm"...
Thanks to everybody wil anwer me

[Davide600]

Ranchi Otto thank you for answering me, but what I exactely wanted to know is just last point of your answer- the first 6 points are already clear for me- in fact now that I've calculated for my ship (following ITTC'57 method) with stock propeller values of etaw, etat, etao, etar, Q, Pd, etc for my service speed, I've just to choose my propeller from SSPA series...and here is the trouble. I know that I have to start with a tern of values, for example Q, Va, and n, that I have to combine in a non-dimensional coefficient, and then in correspondence of this coefficient, I've to read on diagrams values of P/D ratio, J, etao, Kt....and then I can calculate effective diameter of screw propeller, and T, and a new value for etad. Here is my problem: I find a value of T bigger than that one I had with stock propeller, and also value of etao now is better. But if I multipy them to get new value for etad, I find it worse than before.....so I think that there is some mistake in my "alghorithm"...
Thanks to everybody wil anwer me

[Davide600]

I've found an answer to my problem so it's not necessary to reply to my message, anyway thanks to everybody.