Re: 23 foot Doral with Mercruiser 5.0
Indeed if you are only able to go 3,900-4,000 at WOT, you're under-revving. Was that absolutely full speed and trimmed out?
If yes, you're quite shy of where you should be. A 1998 5.0 is most likely a Vortec, 220 hp 2bbl version, will most likely have a sticker on top of the motor that recommends 4,400 to 4,800 operating range. You are currently 400 to 500 shy of the MINIMUM. This means your current propping will make the boat both slow out of the hole, and slow on top speed as well. It also creates unnecessary added strain on the motor and accellerates wear & tear.
So yes a 17 will help on all counts probably. Particularly on holeshot. And I wouldn't be surprised if you'd get even better results and end up needing to use a 16 or a 15...I always feel that targeting the top end of your range is best for overall performance and responsiveness...so you should strive for 4,700 to 4,800 at full speed in a perfect world. This may vary a little bit depending on your intended usage...
Assuming your motor is well tuned, running at its peak, proper ignition timing, strong compression, your tach is correct, and you are using your trim properly to get up to full speed/rpm, then indeed you need to move to a smaller pitch.
If you are able to share with us your outdrive ratio, we can answer even more of these questions. Should be stamped somewhere on your outdrive. Something tells me it's probably a 1.62. If yes, the numbers line up and indicate that you probably have valid data (good tach, accurate speed). It does NOT tell me if your motor is tuned properly however...only you can do that. Often times a solid tune-up is worth many RPMS and MPH's...and results in no need for a re-prop....
Not sure if this is a cuddy or a bowrider...but in any event 23 ft is a rather big boat for a motor that size. A little underpowered IMO. So I wouldn't expect miracles on top speed, but I would think 40 to 45 mph would be a reasonably expected top-end if set up properly.
