Crestliner sportfish 2050 recommended prop

[Hankmudd]

Hey everyone. First time poster here. I just recently purchased a 2004 crestliner sportfish 2050 with a 5.0 litre mercruiser I/O alpha one leg. It currently has a 23 pitch 4 blade on it. The previous owner told me he made a mistake installing said prop, and that it should be 21 pitch. The boat runs well however it is sluggish out of the hole, and only tops out at 32 mph at WOT with around 3600 rpms. Both the rpms and speed seem extremely low for the size of the engine. It is apparent I am over propped, but there seems to be so many variables as to how to best prop your boat. (I had no idea this was a thing, and thus have no knowledge on it whatsoever.)

The boats primary use will be fishing, however I want a hole shot and top speed to pop a slalmon skier out of the water. I punished my brother in law yesterday by dragging him a considerable distance before he was able to get up on one ski.

So my question is... What pitch/blade count would best suit a hole shot improvement without coughing up too much stability on turns etc? I am goin to stick with aluminum for now fyi as I'm not ready to drop the money required for SS.

Thanks in advance to any and all replies or advise!

 

[NYBo]

Welcome to iBOATS! :welcome:

While you are probably over-propped, dropping to a 21" won't put the RPMs where they belong. The motor may not be running properly. How heavy is the boat?

 

[Hankmudd]

Thanks for the reply!

The boats dryweight is approx 2500 lbs. One thing I'm curious is if the throttle cable simply needs to be adjusted as it seems to have quite a bit of unresponsive play once put into forward. Aside from that tho the engine runs and idles smooth. I plan on taking it in for an inspection/compression test etc just for piece of mind. That being said, I will be replacing the prop regardless.

 

[fishrdan]

Exactly what prop is on the boat now, numbers off prop? Stainless steel? Pitch and diameter of prop? 1000RPM is a LOT of RPM to make up with a prop change, but there could be other contributing factors, carb not opening all the way as you mentioned, inaccurate tach, drive's gear ratio, excessive weight in boat, etc...

If you want hole-shot, stick with a 4-blade prop.

One thing I'm curious is if the throttle cable simply needs to be adjusted as it seems to have quite a bit of unresponsive play once put into forward.

With the engine turned off, put the throttle into full forward, disconnect the throttle cable at the carb, and see if the carb is opening up all the way. (2bbl carb, 4bbl, EFI?)

 

[Hankmudd]

Sorry fishrdan... I'm in transport back home with the boat, I will post the exact parameters of the of the prop when I get home, as I still have the box there. As for the throttle cable, it is a fuel injected engine with no carb. Thanks again for the reply/advice!

Also, after I posted my original question, I let it rip at WOT for a little longer, and the rpms settled in at 4000. However, the needle bounces a bit at times so I'm kind of questioning how precise the reading is.

Still topped out at 32 mph tho which seems way too slow.

 

[fishrdan]

4000RPM is more in the ballpark for prop swapping to fix the low RPM's. WOT RPM should be somewhere around 4400-4800RPM.

32MPH seems low, was that by the boat's speedo or by GPS. Try getting a GPS reading, you can download a GPS app to your smartphone, if you have one. Are you trimming up the drive for max speed? Try bumping the drive up a little bit at a time until the boat starts to porpoise, then put the drive back down a bit until the porpoising stops.

Same thing goes for the EFI engine, see if the throttle body opens all the way when the shifter is in the full forward WOT setting.

 

[jimmbo]

While you looking around at the linkages, you might as well do a compression check.
A FI 5.0 liter should have a WOT range of around 4600 - 5000, or 4800 - 5200.

What is the gear ratio, it should be on the label/decal just below the upper gearcase on the outdrive? I would be expecting something in the 1.5:1 range

 

[Hankmudd]

Hey guys I've returned home and poked around the boat a bit more. The current prop is an aluminum 4 blade, 14 diameter 23 pitch 4RH. It is quicksilver brand and doesn't have so much as a scratch on it (just to eliminate thoughts of prop damage also affecting performance)

As far as my inexperienced observations go, the throttle cable appears to be fully opening when moving it to WOT while the boat is off.

Jimmbo I can not locate a decal that lists a gear ratio, it is likely sitting right under my nose somewhere but I can't spit it. It does however state on a decal that the max rpms are supposed to be 4600-5000.

The reading of 32 mph is from a Lowrance gps sounder on the boat.

With the little bit more info provided, you guys have any advice or recommendations on what prop to change to?

Thanks again!

 

[fishrdan]

A 19P 4-blade aluminum (Mercury Spitfire?) should bring the RPM's up 800RPM or so, and into the engine's WOT range.

The MPH still sounds off, something doesn't seem right: tach, speedo, gear ratio? (My heavy Crestliner 1800 Superhawk with an anemic 3.0L, 2.0 gear ratio, 18P prop will run - about 30MPH at 4000RPM...) I'd think your rig should be running into the 50's WOT.

FWIW, the Faria tach in my 2001 Crestliner was bad, don't remember what it was doing as this was 7 years ago, but I sent it off to them to have it repaired and they sent me a new replacement tach, $40 or so. I had an old spare SunTune tach that I used to verify the Faria tach was bad.

See what others chime in and say...........

 

[Hankmudd]

Thanks for the input. So stick with the 14 diameter and just drop down a pitch then? Let the shopping begin, I'm itching to make the switch and rest the difference on the water.

 

[jimmbo]

Don't be too concerned if the dia changes when there is a pitch change. That's best left for the prop engineers. I would suggest a 19 as a starting point, however you need to know if the tach is accurate. being an I/O it is almost certain it is getting its signal from the ignition. Check to see what it is set to, a v8 has 4 sparks per rev

 

[Hankmudd]

As I'm sure it is already very apparent, I'm quite green when it comes to the more detailed mechanics of larger boats. Can someone explain in layman's terms what these potential tach problems could be? Would it be just simply reading the rpms incorrectly, or could a malfunction actually prevent the motor from revving as its designed?