I have dealt with Perkins
engine for a long time and would suggest that maybe you do not have a prop and engine speed problem - but instead a tachometer problem. Normally the tachometers on Perkins
are electronic working off your alternator
. It is a very unreliable system but very economical to put on a boat. Get or borrow a stroboscoptic RPM meter. This is an instrument with a meter and a stobe light that shines on your front of the engine pulley. Normally you have to adhere a little piece of reflector tape on the rim of the pulley that will reflect the strobe light as the engine rotates. This will give you an authentic RPM of the engine which you can compare to the electric alternator
tachometer at the helm
tachs come with a little rotatable selector on the back that is labeled 2 or 4 or 6 or 8. You rotate this selector switch to progressively increase/decrease the displayed RPM calculated from the pulses that the alternator puts out as it rotates. A faulty alternator will give your Tach wrong information.
If the optical tach and the normal helm
tach are close to each other then you have a different problem - probably a prop problem.
If you are getting 6 kts out of a 40 ft boat at half throttle that is pretty good. If the prop is too big you will also get black diesel
smoke out of the exhaust
as the engine labors to achieve full RPM's but cannot. If you get to maximum book rpm's easily/quickly with the prop then the prop is too small. You would normally find a used prop shop and borrow a variety of pitches in your prop diameter/shaft diameter and test each one to see which one gives you near max speed at 3000 rpm.
For the Perkins 4.108
normal max rpm's is 3,600 and maximum continuous operating rpm's is 3,000 rpms. Normal cruising is about 2200-2400 rpm's. With a little experience you can hear the "sweat" zone for cruising rpm's. There is normally a definite change in tone and the feel of the boat vibration between optimum cruising rpm and the higher rpms.