props for 1990 90hp force

[graybird78]

im confused. i have a 1990 18ft bayliner capri. trying to find a prop for it. the prop that came with the boat looks to be oem. part number p669-3, 669265, sure its michigan prop 15p. prop was ok for boat ,top speed was 29 mph with 4400 rpm at wot. i use the boat more more fishing and crusing than anything. had decent hole shot.
i cant for the life of me figure out the gear case size. 4" or 4 1/4 ".
engine is dual exhaust .
ive seen props listed for both .
what exactly do i need .
im lost.any help would be great.thank you

 

[ondarvr]

Your RPM's are way off.

 

[graybird78]

one of many reasons im looking for different prop

 

[graybird78]

current prop is bent has vibration and all around bad shape

 

[graybird78]

my best guess is i have a 4"gear case. i measured current prop at the lip , it measures 3 3/4", ive read were the dual exhaust case is 4 1/4". did i measure wrong.