more prop talk... jeez...

212rowboat212rowboat Member Posts: 2,591 ✭✭✭✭✭
Just when I think i have a grip on this, and have dialed in what I need, I get whacked with a whole new angle- literally..

I 'had' a 22p 14.25" three blade s/s prop I was going to use, but it turns out it wouldn't fit.. the thing was made for a yami, not a merc, the splines were not 'deep' enough and there was no way to lock it on (shaft too short).. I gave it back to the fella who gave it to me, even though I could have had a new hub pressed in for $50, and it would have been fine..

I found and purchased another 22p 14.25" three blade s/s- a 'turbo' by precision, with a merc 15spline hub all set up and ready to go..  I'm replacing a 4blade Solas 23p 14.25" aluminum, which was slightly below the bottom of the engines comfort range @ WOT.. that is 4300ishRPM, to be precise...

here are my figures, fwiw: 

212 Captiva, 4.3L, LX.. 1.81:1 A1 leg..

23p 4blade 14.25" alum Solas w/rubex hub
4300RPM @ 42~44mph
boat seems to ride low, which is to say the bow rides lower and the stern seems to lift some;
about a five to six second hole shot, @ just under WOT (maybe an inch and a half left on the lever)
attempts to drop off plane around 3100RPM, and at around 23~26mph- which I really don't like.

note- the blade(s) is slightly progressive (I'd guess a two pitch sweep), and has decent cup.. It has a lot of blade surface, and what I'd figure is mid range rake. There are no vents on this prop.

20p 4blade 14" Michigan Wheel alum w/ square hub
hit rev limiter @ 5100~5200RPM, topped under 40mph, which tells me there is some substantial slipping, or that advertised 20p isn't 20p at all..
about a three to four second hole shot @ just under WOT
held plane down to 15~17MPH, and just under 3kRPM- which I kinda liked, at times, but was frustrated with top end at the same time..

note- the blades on this one don't appear to have much progressive sweep, and had minimal cupping, maybe a touch less than the Solas above. There was plenty of blade surface on this one, with less rake than the Solas but likely matching cutting edge.  I was hoping the Solas would be a similar match on geometry when I bounced three inches up, but it turns out this wheel seemed more fit for tugging, while the Solas may have more top end performance in mind. Like I said, I was nailing the rev limiter with this one, bouncing at around 5150RPM, so I 'mathed' out a 600RPM drop to put me around 4600RPM with the move to the Solas 23p.. it dropped me WAY more than that, as it turns out.. 

so- this turbo prop seems to be a popular one.. and I can't wait to slap her on there and cough up some real number for you fellers, but me being me- I'm trying to anticipate those.. Help me if you would, and correct my notions which I readily admit are newbe-ish at best..

moving to a s/s will drop RPM's around 150rpm, correct?

moving from a four blade to a three blade will increase RPM's about the same, or 150RPM's, correct?

dropping pitch one inch ~should~ add about 200RPM's, but I'm also swapping material, as well.. which should make this a push too, right? I keep reading that an aluminum prop will behave like a stainless prop an inch shorter in pitch, due to flex.. Is this true?

the biggest:  Progressive angle of this turbo prop is frying what I have left of a brain.. it 'averages' 22p as I understand it.. which means, according to the tech drawings I've seen, the center of the blade is 22p- the entry is around 20.5p, and the trailing edge (before cup) is nearing 23.5p.. Apparently, it sweeps through over three inches of pitch..

but wait!!! there's more!!! ..... it's vented....

As I understand a NON-vented prop of this deep a progression is designed, it bites into the hole shot panning juice off the blade at the leading edge, which gives the characteristics of using a prop somewhere in the 21p range.. once it spins up and bites deeper, shifting the bite to the trailing edge, it behaves like a higher pitched prop.. when turning the cup comes into play, it forces juice over the bitter trailing edge, which again makes it behave like a shorter pitched prop...

I somewhat grasp that, but I don't grasp the vents interaction.. I understand how the vents work, I just don't understand how they'll effect what I'm going to call 'overall drive pitch', and I don't understand if it happens all of the sudden or if the ventilation slowly disappears providing a smooth transition..

'sposedly, the vents allow exhaust onto the blade surface creating a controlled cavitation, which allows the engine to spool 'mucho' quicker out of the hole... 'sposedly, as speed is achieved the exhaust redirects splitting it's exit point between the center-hub and the vents, and to straight through the hub (as my previous props did) eliminating the controlled cavitation... 'sposedly, this (in theory) allows a person to use a prop higher in pitch than they can otherwise... enjoy decent hole shot and gain more speed.. coupled with the progressive pitch, it's 'sposedly similar to shifting gears..

what are your real world experiences with vented progressive pitched props?  what kind of changes in performance did you see? how much did it shift your RPMs? 

sorry for the wall of text.. it turns out these things are a lot more compli-dang-cated than it appears from a distance.. I don't want to jack up anything until I'm ready to jack it up.. My season is at least another month and a half, down here.. :-)


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    Dream_InnDream_Inn Member, Moderator Posts: 7,594 mod
    Drew, you have way too much time on your hands.  I think you need a bigger boat to take car of! ;)

    Dream 'Inn III -- 2008 400 Express

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    212rowboat212rowboat Member Posts: 2,591 ✭✭✭✭✭
    no kidding, right?

    this crap is fascinating to me... who in the world would have thought there is so much figurin' in a dang prop?

    I read about one prop that has tiny counter weights in the body/hub, which alter the blade angles upon command, and vary across as many as five inched of pitch... dowhahuh? THAT would rate it's own forum, much less thread, no? :-)
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    mvnmvn Member, Moderator Posts: 744 mod


    And to complicate matters further, you need to put a set of tabs on...even a set of the Nauticus ones.  Once that is done, you can throw everything out the window (prop wise) and start again!



    Good,  fast,  cheap.... pick two. 
    2019 MTX20 Extreme

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    212rowboat212rowboat Member Posts: 2,591 ✭✭✭✭✭
    very true, sir... I just thank the stars I can't adjust transom height with the Alpha.. :-)
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    mvnmvn Member, Moderator Posts: 744 mod

    But seriously though, depending on your application, look at a set of inexpensive automatic tabs in combinatin with repropping.  It makes a HUGE performance difference and also gives you something else to tinker with!


    Good,  fast,  cheap.... pick two. 
    2019 MTX20 Extreme

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    nhsdnhsd Member Posts: 182 ✭✭✭

    Drew - once you get those smart tabs on that 212 please take some pictures for me (better yet document the heck out of the install for me). I have seriously considered adding the tabs but I can't find anyone willing to say that they have them on a 212. Alswag said that bhe has installed them or seen thgem installed on a 212 though so I guess it is doable but my limited powers of observation can't see the right placement.  

    Now as for the prop, I have a 14.25 x 21 pitch Precision Propeller Turbo I 3 blade SS prop. I can't say off the top of my head whether it is vented or not, but it runs fairly decent on my 5.0 220 hp 1:1.65 Alpha I. It does well running at 4800 and 51 MPH (GPS)  and a pretty decent holeshot when I am by myself. However it quickly deteriorates on that holeshot and top end when I get even a moderate load in the boat (son, daughter and granson - probably 400 lbs total additional weight). A slightly smaller diameter Quicksilver Vengence 13.75 x 21 pitch SS 3 blade holds up with the load better and only loses maybe 1.5 MPH on top end when by myself compared to the Turbo I. Now to be honest I like my Alpha 4 20 pitch 4 blade aliuminum prop better than either, but I did get to learn the problem with aluminum props. I hit a log (large stick is more like it) that a SS prop would have shook off, but the aluminum one dented one blade and proceeded to shake the boat the rest of the way home.                          


    2002 Captiva 212, 5.0 220 hp, Alpha 1, 1.62 gears

    Moon Township, PA - boating in the Ohio River

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    212rowboat212rowboat Member Posts: 2,591 ✭✭✭✭✭
    you're turning more ponies than I am, but I've got leverage on you- which almost amounts to a push between our rigs~ though you have quite a bit more ability to sustain a load than I would, no matter the leverage evening it up..

    I've looked at tabs, but here is my problem:  I can't get them near the drive on a flat w/o custom fabricating a mount, nor can I get them where I would want them- nearing the outer edges, and because of that same curve... It could be done on the recess the drive comes out of, using a mounting bracket perpendicular to the trim plate, but it would be too close to make a difference- unless the difference I'm looking for is awkwardly balancing the boat on a rail at plane speeds, which I ain't.. :-) ... it would also be inherently weak there, I'm thinking, as the custom plate would have a full value leverage on the mount..  

    I was hoping to talk to some locals and get some true eyes on a successful set up on a similar design before going into something like that..
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    212rowboat212rowboat Member Posts: 2,591 ✭✭✭✭✭
    just a follow up.... the 22p 14.25" 3blade vented s/s prop is a winner... wot is 4700ish, speedo says 45-47mph, hole shot is quicker than the 4blade 20p..... vents make that big a difference? .... i wish i would have gps'd the speed... at any rate, im happy... it held plane at right at 3kRPM, and around 24mph or so... felt like it was going to drop, but didnt.. bow rise is substantially more than any of the other props. 
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    V8192V8192 Member Posts: 18 ✭✭

    Followed the same path! I picked up 2 stainless props from a neighbors garage sale.

    One was a slightly used no name4 blade 20p one small ding I like it, speedo at 45 ish

     at about 5000 rpm at wot the boat handles better and trim is a lot more forgiving. 

    The other is a 3 blade 21p vented Rapture like new haven't tried it yet as the hub is different

    and I don't have the correct washers. But both for $50.00 so not bad and I'm

    happier with the boats performance over the 17p high 5.  

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    212rowboat212rowboat Member Posts: 2,591 ✭✭✭✭✭
    I'm happy for you that you found what you sought!!  Man, the prop game is a lonely journey, huh?  What works for me and my usage in my conditions isn't to say that it will answer for others is what I'm learning..

    plus, it's not only the hard numbers printed on the prop box and the material.. it's the characteristics of the geometry that those guys don't publish that truly screw with my head.. for instance:

    that michigan wheel 20p four blade didn't have a very progressive blade design, and I'm going to bet that it flexed like a mug under power to less than 19p (and that if it was truly a 20p to begin with!!) I'm thinking the geometry may have been fine for a stainless (that didn't flex), but who knows, right?

    the turbo I have on there now is a 22p but the blade is visibly progressive- the 22p is derived as an average pitch.. go figure.. it has a decently moderate rake on it.. but I think the real difference is the vents on the thing..

    those vents allows a much higher RPM off the hole shot.. It's is undeniable.. where the engine (w/23p four blade prop aluminum) would achieve a 3kRPM rev instantly, and then slowly climb to a WOT of 4300ish, the 22p s/s hits no less than 3800RPM within a couple seconds of nailing it- and reaches 4500RPM within a half minute or so, and WOT of 4700ish RPM within likely 45~50seconds of the hole shot..

    ^all that tells me I would likely be better served with a 21p of duplicate geometry, but the punch line is I really don't need top end that much...

    the interesting thing, to me, though: dropping a blade, a pitch, and changing material netted me a much higher RPM than the 'book' would suggest..

    going from aluminum to steel should have dropped me 150rpm or so.. dropping a blade should give me back that same RPM.. dropping a pitch should have gained me 200RPM or so.. it gained me more- and other than the geometry of those blades, the only other thing it could be is the vents- which cold allow the engine to spin through it's weaker range by providing slip (ventilation to the prop) and muscle through where it is inherently weak- and by hitting where its cam profile dumps a lot more power it spins even faster, and pushes the boat with it..

    I'm going to add to my wish list that prop manufacturers add a profile to a prop that shows which prop does better within what power curve.. it makes sense, I think.. but, it may just add more confusion.
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