2008 Crownline 21SS LPX - need prop help

[00bullitt]

Hey guys.....first post here. I just picked up a new to me Crownline 21SS LPX pkg. (their semi performance runabout) with 350 Mag MPI (300hp) and 1.47:1 Alpha one outdrive and Corsa Capt. Call exhaust. Currently it has a Stilletto 14.5"x 21p prop (E800321).

I'm not new to boating, just stern drives. Ive had a few high performance bass boats that would all run triple digits (Allison & Bullet) with Mercury Racing outboards. I was very in tune with propping those.

I bought this in lieu of a Caravelle Interceptor 212 that I've always wanted. Its a sharp boat, but I'm still looking for the right Interceptor.

Its a dog out of the hole and the Stilletto has pretty much no bow or stern lift. Trim is almost ineffective.
Thats 40 gallons of fuel, me, my wife and daughter; About 470# for the 3 of us.

Trimmed out, it pegs at 52mph gps at 4700rpm. No trim does 46-48. I see no reason why this boat can't do high 50's loaded.

I need to know if I am expecting too much lift from this rig. Yeah.....its like 3200#, but it always feels stuck to the water. I can trim it and it slightly increases RPM, with no bow rise or stern lift.

I don't expect this thing to drive or run like my Bullet or Allison, but I do feel like it has more potential.

I'm not scared to drop the coin on an Enertia if you guys feel thats the solution.

The reduced diameter worries me as diameter is directly related to the props ability to carry a load, but the Enertia seems to be the exception to the rule.

Thanks in advance for any advice.

-00

 

[alldodge]

I'm finding its 3700 dry and a newer model similar Crownline 215 SS with 260hp tops out at 48 mph, so I think your doing good. To come out of the hole faster you need more torque.

 

[00bullitt]

So the Stilletto has small vent holes behind the blades. I am getting no cavitation to get the engine up into its torque/hp range out of the hole. It labors badly. Once it planes, it will maintain at pretty low speed.

Should the hull be this glued to the water with trim in excess of level?

I would think a 21p prop with better lift characteristics would get this barge up out of the water some more and increase rpm/speed?

 

[alldodge]

Every boat and owner are all different, the only way to find out what works is try several. If you have a local prop shop many allow trials. A prop is paid for and can swap out some to find one you like

I also have a Stiletto (with a cool name it has to be good) 19P and use to use it when pulling 2 or more solemn skiers. A lot of pull but can over rev, but just stick with my 21 anymore

 

[00bullitt]

Every boat and owner are all different, the only way to find out what works is try several. If you have a local prop shop many allow trials. A prop is paid for and can swap out some to find one you like

I also have a Stiletto (with a cool name it has to be good) 19P and use to use it when pulling 2 or more solemn skiers. A lot of pull but can over rev, but just stick with my 21 anymore

I definitely realize that. Just trying to narrow things down based on others' experience for similar craft.

Like I mentioned, not new to boats, just this one.

The Enertia seems to be a popular prop for my application as does the Vengeance, Mirage and the Ballistic.

I don't have access to any local prop shops with loaner programs. Most likely will take advantage of BBlades.

I'm hoping others can offer some suggestion.

 

[alldodge]

The 4 blade guys should be around shortly after church, I'm a 3 blade guy, for the most part

BBlades (Brett) is a sharp good guy, and will swap props with you but also charges 15% restock fee each time. If you want to try Hill Marine 4 or 5 blade prop, talk to them, and they will swap props with you and only cost return ship, no restock fee. Or at least that is the way it was when I bought mine

Your at 4700 if tach is accurate, and can go up to 4800 - 4850 ok. Your currently at 18% slip, so my guess is your trimming a bit to much (which causes extra slip) but does give a bit more speed. If you could hook up better, then you might be able to drop a pitch and keep the same top end, like cupping a prop

 

[porscheguy]

I was going to point out that you’re slip is around 18% which is on the high side. But your max rpm is 4700 so you’re pretty much maxed out at 52mph with your 21p prop. Any 21p prop that further reduces slip is going to cost rpm and I doubt you’ll see an improvement in speed. So your best option is to reduce pitch and use a prop with more bite.

Ive got a mirage plus, an enertia, and a rev4. They all have their pluses and minuses, but the mirage plus gets the most use by a wide margin.

The mirage plus has been in production for 20 years and depending on where you live, you can always find them on Craigslist for a fraction of the price of a new one. Even if you get one a little beat up, a decent prop shop can make it look and perform like brand new and still keep you well below the cost of a new one. Just make sure you get one with PVS vents. The early late 90s mirage plus uses a slightly different hub kit that most don’t stock. Also avoid the early rubber hub quicksilver mirage. The plus is just a lot better. I get a lot of bow lift with mine. Excellent grip. I can maintain grip across the entire trim range. My slip is about 6.5%. In perfect conditions I’ve seen 49mph at 4850rpm. It’s 21p.

The rev4 generates a lot of stern lift. You’re not likely to find it used. I like the stern lift in choppy conditions. It slips a lot off the line but then bites down hard and takes off. It has about 80% of the trim range as the mirage plus before it starts to let go. It’s about 1mph slower than the mirage plus. It’s also 21p so the rule of dropping pitch for an extra blade doesn’t apply in my case. Slip is about 8%.

my enertia is a 22p. I think I hit 50 once at 4800rpm. It has lift on par with the rev4 and needs a little more rpm to stay on plane than the other 2. 8% slip. Hard to find used and new prices are almost $800 at this point. I should also ad that I bought my enertia used and it had some second rate cup added. I haven’t had a chance to have it refurbed. Others give rave reviews that shouldn’t be discounted.

In your application I’d opt for a 19p for any of those props.

 

[00bullitt]

So correct me if I'm wrong, but my boat is plowing water. I have zero bow lift and no stern lift. I would think that a prop that would lift the hull out of the water, would reduce drag and increase speed (lift reduces drag, it shouldn't decrease rpm if its working properly) This is a padded hull and its not riding on the pad at all.

Considering that the 21p Stilletto is not lifting, a 21p that did lift would be faster.

I guess what I need to ultimately determine, is how much this boat will lift hydrodynamically if at all.

I'm very used to 1200# lightweight bass boat hulls with 300+ HP; they lift both aerodynamically and hydrodynamically, with the last 12" of a 12" wide pad riding on the water with a propshaft height 2.5" above the pad.

I'm really trying to validate what this type of boat is capable of.
My expectations and reality may be far departed.

 

[porscheguy]

A 21p prop that lifts and reduces slip means more load on the engine. You already can’t hit redline with the current prop that has a lot of slip. I ran my 21p mirage+ on a 21’ chaparral last year. It had a 5.7 MPI and a bravo drive with a 1.64 ratio. NoW of course some will protest “that’s not the same boat” as yours, but honestly it’s close enough for the purpose of comparison. That boat hit 53/54mph at around 4500rpm with a 1.5 ratio.So yes, he could use a 21p but his max rpm was at the bottom of the range, and the boat was lightly loaded. Any extra weight would have lugged the engine an killed performance.

The thing is, you can reduce pitch, increase rpm, and probably pick up a bit of speed. I’m thinking you could see55-56mph which would result in a 7-8% slip. That’s gonna make you faster than 90% of the boats out there. I think that’s a realistic assessment of what your boat should do.

one of your limiting factors is the alpha drive itself. They tend to run deep and have never been praised for hydrodynamic efficiency. They’re more about a cost effective way to get the job done. Even mercury does not recommend their use in boats that can exceed 60mph. But like I said, I think 55-56 is within reach, and faster than the overwhelming majority of boats out there.

 

[QBhoy]

Hi
I would think you should do slightly better really. Pretty sure it will never be into the 60’s, but the cheap stillehto is not helping your case. Nor will the vengeance (just a shiny version of the standard ally).
As we know, all boats and prop combos show varied results but I’d be almost certain and enertia would help your case. The ballistic is a gamble and a 50/50. Sometimes the are literally ballistic and sometimes they just aren’t suited at all. You really need a load of power and a power to weight ratio (with proper performance hull) favouring power more, to get the best of it. The laser is always usually a safe bet, but hard to come by these days in good condition.
Mother spanner in the works you might have is that the prop you have stated...you mention...you say it’s a 14.5” diameter at 21”p. That’s a bit on the large side for a 21” 3 blade. The equivalent 21” merc prop like a enertia or laser at 21”, would probably be around 13 1/2 to 13 3/4” diameter maybe. Won’t have too much of an influence normally but when fine tuning it’s always worth a thought.
Good luck.

 

[00bullitt]

Took it back out again when we got a break in the weather. So I made a ROOKIE mistake here. I did not study my tach very well. I am only hitting 3800rpms (assuming correct) at 51mph, sometimes bumping 52. Trim is Between 1/2 & 3/4. If I trim more, I see no change and it NEVER porpoises. I just NEVER feel the boat release from the water at bow or stern.
Out of the hole, it hits 2600 and sticks there til the bow breaks and it starts to increase speed. Its a turd getting on plane. 3000rpms is 31mph. Trimmed down the boat will stay level on plane at 15mph.
It handles GREAT at speed.

So, assuming a 19p that should only give me a 400rpm, maybe 500 rpm gain for 4200-4300rpm.

I would have never thought I would need a 17; granted, I do have the 1.47:1 outdrive.

 

[00bullitt]

How can I verify the tach on this thing? If it had an OBD, it would be easy. Is there a similar device (reasonably priced) that will read codes, diagnostics and vitals on a Mercruiser?

 

[alldodge]

Not knowing the serial number I'm unable to say for sure, but being a 2008 its probably has a 555 ECM. If that is the case, then the option to read codes cheep is gone. Would look for Rinda scanner or Diacom software

https://www.rinda.com/marine/marinemenu.htm

 

[porscheguy]

You’re not hitting 51/52 mph at 3800rpm. That would put you at 1% prop slip which is technically possible but rather unlikely on a mass produced boat with an alpha drive.

even overpropped as you are at the moment, your performance shouldn’t be as bad as you’re describing unless there’s an issue with the engine causing it to not make full power. I doubt you need a 17p prop.

 

[00bullitt]

Yeah, I need to get this tach fixed.

 

[00bullitt]

Just dropped the boat at the dealership. Tach is probably THE MOST important gauge on the boat.