Prop recommendation to correct tendency to cavitate

[SkiGuy1980]

My 2016 Crownline 21SS has a tendency to cavitate during slight turns when running at top speeds. Unlike my previous boats the cavitation doesn't greatly improve with throttle/trim adjustments once it starts. I've had to come off plane and take off again. Kind of wondered if the max trim setting might be too high. Also thought that the aluminum prop flex could be contributing and I'm considering going to a stainless prop.

The boat is a '16 Crown (20 hours), Merc 350 MAG MPI, 300 HP, Alpha drive. The current prop is a BlackMax 3 blade, 14.25 x 21 P.

I've had the boat out twice this season. Running with a half tank and 2 persons and WOT is 4800-4900 RPM at 55 MPH. RPM spec for the engine is 4800-5200 at WOT. Bow rise is about 3-4 seconds. I'll usually have the kids with us and I want a strong pull for watersports.

Am I properly sized? Would stainless help? What impact would a 4 blade have?

Thanks for any thoughts you might share.

 

[high&dry]

"Aluminum" prop, well, that is likely the issue or a major contributing factor. Try something similar to the Mercury Revolution 4.

I had to run an aluminum prop on one trip due to my paranoia of hitting bottom (using my boat in the flats and around oyster beds). Well, we did not ever hit anything due to my extreme caution but the aluminum prop constantly blew out much as you experienced. This does not occur with the Mirage Plus (lake use) or Revolution 4 (ocean use) that I normally run.

 

[jimmbo]

First of all, if your current prop is not damaged in any way, including the tiniest nicks or chips what you are experiencing is likely Ventilation not Cavitation. Some props never recover once they lose their bite. Reducing trim prior to turning may help reduce the chances of blowing out. A prop with a better blade design should help. I have had very good results with Mercurys Enertia. If you want to stay with Aluminum, I’ve heard good things about Turning Point’s Hustler model. You mention ‘Prop Flex’. That is the one of the biggest con jobs ever pulled by Madison Ave, and one that has rationalized the purchase of millions of Stainless Steel props. I doubt that the “flex” in your 21” prop, if any, is even 1/100”. You are at the low end of the WOT rpm range for your engine so dropping down to a 19” will improve the hole shot. Check to make sure the hull in front of the outdrive is free from anything that could increase the chance of ventilation, such as depth gauge sensor. Of course there is a chance that the boat manufacturer mounted the engine too high. An extra blade will kill your top end, as to any advantage, I’ve never seen any

 

[SkiGuy1980]

jimmbo - mounting position was one of the other things I worried about (then decided that didn't matter now... just gotta figure out the next step).

 

[Scott Danforth]

have you tried trimming down a bit?

 

[jimmbo]

I/O are often mounted with the AV plate higher than even with the hull bottom, just like an outboard that is mounted higher, and for the same reason. If so, one would think the boat would have been equipped with a higher performance prop by the manufacturer. Of course, a less than ethical dealer could swap props so as to be able upsell at some point.

 

[SkiGuy1980]

Scott - I had trimmed till the boat planed out and was running smoothly. On a smooth surface this was at the top end of the trim setting. I've had it blow out at slightly lower trim settings as well.

 

[floater212]

my boat is similar to yours, 21' 350mpi, 300hp ALPHA, rinker, i use a ron hill 5 blade 19" s/s have been happy with it since i 1st bought it. the prop that came with the boat was the high five 21" we use it mostly for skiing, tubing and kneeboarding. the high five sucked, so i was checking all options and found ron hill's props, they are 14.25" dia. the high five was 13 3/8" dia. the blade width was normal width as other props, the high five was thinner by about 3/4" and with our crew ( boat being full) i dropped to a 19". the top end is around 45- 50ish...too fast to ski behind... but all around great prop, if you are using the boat like mine, i would recommend the ron hill pleasure prop in a 19" and i think you would be happy doing anything with the boat. ...my 2cents.

 

[Scott Danforth]

Scott - I had trimmed till the boat planed out and was running smoothly. On a smooth surface this was at the top end of the trim setting. I've had it blow out at slightly lower trim settings as well.

back off on your trim. as you throttle up and down your trim should be going up and down too.

 

[SkiGuy1980]

Scott - I adjust trim based on speed/rpm... that isn't what I'm trying to address. The issue is the cavitation that occurs if I make a slight course correction or a wave alters my path. This is the first boat I've had that revs out so easily. I'd think a 14.25 x 21 P should hold it, but looking for feedback relative to different pitch or maybe 4 blade.

 

[jimmbo]

Most props will ventilate to some degree on a 'course correction' or when the boat encounters waves and swells, when trimmed out. As mention at least a couple of times, some trimming in prior to deviating from a straight flat path may be necessary

 

[Faztbullet]

You got a tranducer on hull or a "whale tail"?

 

[porscheguy]

I love how crownline sales videos constantly talk about “high end this, premium that” and yet they ship their rather pricey boats with aluminum props.

Back to the question at hand. I don’t blame you if you don’t want to fart around with trim adjustments every time you go to turn the steering wheel. That would annoy me too. But if you want a prop with grip, go stainless. The mercury mirage+ has been a mainstay prop on V8 sterndrives for years. They’re also sold as the Quicksilver thunderbolt. I’ve got one. I can trim all the way out and turn sharply and it does not let go at all.

 

[SkiGuy1980]

You got a tranducer on hull or a "whale tail"?

Faztbullet - No whale tail, not sure what a tranducer is?

 

[porscheguy]

A transducer is for your depthfinder if you’ve got one.

 

[QBhoy]

Must admit...I haven’t checked...but I thought your engine rpm range was 4600-5000 rpm like mine. Where did you get the details from ?
if you are running pretty quickly and trimmed out...and make anything more than a slight turn...you will likely blow out and cavitate. Usually need to trim down a touch. Especially with a standard black max. Sounds like you are right on the money with the 19”. I would try one of the go to stainless props, like a 19” laser 2, Enertia, mirage etc and see how you go.

 

[SkiGuy1980]

porscheguy - I do have a depth sounder but no transducer hanging off the boat. I was a little surprised not to find one... but that may be why the darned thing is so hard to set (goes off constantly ).
I took the boat out this evening and ran lower speeds (30-40)... would still blow out and over-rev with moderate course corrections. I tried several runs with lower and lower trim eventually no blow out once trim down was extreme enough to bog the speed and RPM.

 

[high&dry]

My boat came with a stainless Mirage Plus. It is a great prop but running into an inlet on the back side of a roller or during aggressive maneuvering it would ventilate and lose traction. I replaced the prop with a Revolution 4. The Rev 4 does NOT ventilate at all and holds the boat solid on an aerated roller. Did lose some speed though, around 4 MPH. I am considering going to the Enertia three blade in a 14.5X17.

The steel props have different blade deigns typically with more rake and area and more cup than an aluminum prop can hold. These features allow a higher engine position and will likely solve your problem of ventilation during maneuvering.

I am going to sell my Mirage Plus at some point to finance the Enertia. Darn things are expensive.

 

[porscheguy]

High&dry, what kind of boat? Are you looking for an enertia or enertia eco? The enertia is also sold as the Quicksilver q3 for a few hundred less. If you can find one.

 

[jimmbo]

My boat came with a stainless Mirage Plus. It is a great prop but running into an inlet on the back side of a roller or during aggressive maneuvering it would ventilate and lose traction. I replaced the prop with a Revolution 4. The Rev 4 does NOT ventilate at all and holds the boat solid on an aerated roller. Did lose some speed though, around 4 MPH. I am considering going to the Enertia three blade in a 14.5X17.

The steel props have different blade deigns typically with more rake and area and more cup than an aluminum prop can hold. These features allow a higher engine position and will likely solve your problem of ventilation during maneuvering.

I am going to sell my Mirage Plus at some point to finance the Enertia. Darn things are expensive.

The Enertia is not cheap, Mercury just raised the Wholesale and MSRP in Canada recently. But, it is a very good prop, and well worth the money. Porsh**** claims the Q3 is the same prop. Well when I looked at the two of them, I saw subtle differences between them. How much difference that will make I don't know as I have not run the Q3, only the Enertia, for the past 12 yrs. Great prop and well worth the price. Of course, YMMV