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  1. #1
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    More ponies for the family boat

    So Santa was good to me this year and I have a bunch of shiny new parts for the LXR. This is a nice change from last year, which I spent catching up on a decade of deferred maintenance by the previous owner. I documented most of that in this thread:
    http://www.donzi.net/forums/showthread.php?67047-Adventures-in-deferred-maintenance

    To recap, I ended up installing a new SEI outdrive, gimbal bearing, drive shower, cap/rotor/wires/plugs, and GLM aluminum manifolds & risers. Just for fun added a set of 4" Corsa exhaust diverters. The boat sounds pretty good to me (ie obnoxious to my wife) and I think that this is about the least restrictive I can get without going to a super high dollar marine exhaust (which I won't do).

    Once all of that was complete I could get some baseline performance numbers for the boat:

    Boat - 1998 Donzi 21 LXR bowrider
    Weight - approx 3600 wet
    LOA - 21', single step, 8' beam
    Power - Merc 5.7 Vortec 2bbl 250hp. Same roller cam & internals as the 300+ hp 350MPI.
    Drive - SEI 116 (Merc Alpha Gen 2), 1.47 gears, Simrek drive shower
    Prop - Merc Vengeance 19P
    WOT - 51.3 GPS @4400

    Tried every possible combination of trim. Theoretically it shouldn't take much trim with the step as it should run fairly flat/level once the hull is aired out. Ended up settling on 1/3 - 1/4 trim per the gauge as the sweet spot with this setup. Gives 4-5 mph over neutral trim. Any higher and it blows out.
    IMG_0192.jpg
    For any law enforcement professionals perusing this corner of the internet, that is clearly a can of diet Sprite sitting in the cupholder. No beers were harmed in the posting of this thread.

    A couple of issues:

    1. Boat is starving for air and fuel above 3.5k rpm. Sounds like a shop vac under the hatch - the 2bbl and tiny intake and flame arrestor are acting like a NASCAR restrictor plate. This is a roller cam vortec SBC and should want to wind up a little more.

    2. Prop. The Vengeance is a big round elephant eared budget SS prop. It has enough rake that it wants to lift the bow, which is not what this hull needs. Also tends to ventilate/cavitate quite a bit at takeoff and at higher speeds. Not optimum for a stepped hull. See pic below - this is WOT with 4 people and a bunch of gear, but you can definitely see that the stern is digging in a little too much and not taking full advantage of the step.
    Running.jpg

    The fix for issue #1 is some extra power. I have an Edelbrock Performer RPM high-rise dual-plane intake, a 1" phenolic carb spacer, a big stinking flame arrestor, and an Edelbrock 1409 600cfm 4bbl and all the required plumbing in the shop (thanks Mrs. DH!!). I debated going to a 750 vacuum carb based on some discussions on here, but finally settled on the 600 per Randy "BigGrizzly" C. He recommended the 600 for small blocks and ran them on his own fleet, so that was good enough for me.

    There is a place online that sells a kit with the same parts I bought and dyno'd the '98-99 Vortec motor at 307hp after the carb & intake swap. I pieced it together mostly from Jegs and CP Performance and came in $100 cheaper than the kit.

    For issue #2 I am tracking down a 22P Bravo 1 4-blade prop. This was the stock prop from the factory for the Z22 (same hull) and small block 22ZX (similar hull). This prop should provide good stern lift at WOT and excellent mid-range cruise. Also better planing and handling vs. the 3-blade. If the B1 falls through I might take a look at Hydromotive. Bottom line, I want to get the hull running flat and utilize the step to maximum advantage.

    I guess the purpose for this thread is to document the experiment and look for any guidance you guys might have. Not many of these boats out there but maybe someone has attempted the same thing on a similar hull. Also curious to hear about any tuning advice from the collective audience. I have a TB V ignition with the knock sensor and a rev limiter; I need to check the module to see how much advance it has and what RPM the limiter will give me. I'd love to get this thing to turn 4800 on the pins.

    More to follow once I get under the hatch. dh
    "Speed's fine, but accuracy is final."
    - Bill Jordan

    1998 Donzi 21LXR
    1971 Boston Whaler 13

  2. #2
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    Quote Originally Posted by duckhunter
    For any law enforcement professionals perusing this corner of the internet, that is clearly a can of diet Sprite sitting in the cupholder. No beers were harmed in the posting of this thread.
    I love it!
    “Oh right, because you walked into strippers discount warehouse and said ‘Help me showcase my intellect.’” - Archer

    Bill
    Grand Rapids, Michigan
    1985 Donzi Criterion SS
    1967 Donzi 16 Ski Sporter, C16-409, Has a new home!
    38' Carver Aft Cabin
    1968 Sea Ray SRV 180 w/1975 70 hp Evinrude
    10" RIB w/15 hp 1984 Evinrude

  3. #3
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    Alright, I ran my numbers through half a dozen prop slip calculators and kept coming up with ridiculously low numbers - 5-7%. The boat definitely doesn't "feel" like it is performing up to par with the current prop, so I am thinking either I goofed on one of the numbers or my tach is way off. Or maybe the POS elephant ear prop is just too much blade area with the stern dragging in the water, combined with a lack of horsepower. Or all of the above...

    Going to throw on the intake and carb regardless and do a better job collecting data and run the numbers again with the stock prop. I have a shop tach that I can hook up and double check my dash gauge.

    I still believe my theory is fairly sound, ie I need to get the stern up to take advantage of the step and reduce wetted surface. The B1 will do a better job of lifting the stern and running in the subsequent aerated water than a 3 blade. It was the factory recommended prop for these small single-stepped boats 15 years ago, maybe there is something better out there now?

    It would be sweet to hit 60 in this thing somewhere other than on the trailer. If nothing else I will have some bling under the hatch and still be able to scare the snot out of my daughter by hitting the exhaust cutouts while she's on the tube...
    "Speed's fine, but accuracy is final."
    - Bill Jordan

    1998 Donzi 21LXR
    1971 Boston Whaler 13

  4. #4
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    For prop slip, one thing to keep in mind is props with cup add pitch in a way. The rule of thumb is to add an inch on a cupped prop. For example a standard mirage 25 pitch would measure at 25 on a slip calculator while a 25 mirage plus should be entered in at 26. I'm not sure if this factors for you or not just posting in case it does

  5. #5
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    As far as stern lift vs bow lift.... Personally I have not run your hull but on my 28,for top end, I don't think I could get too much bow lift out of the props. Ever. As your stern lifts the bow will drop and to be quite blunt your bow is very wet in that pic. The only way stern lift helps is if you can carry the bow or if the net result is a less wet hull which is unlikely for top end.

  6. #6
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    Vette,

    I really appreciate the input on running angle. I'm not describing things the right way.

    It was clear watching some bigger stepped hull boats run that they were really level in attitude vs. a straight deep v, which carried the nose a lot more. My hull has a lot less surface area to work with than a 27 or 28+, so maybe the effect won't be quite as pronounced.

    Bottom line, after driving the boat for a year it has never seemed to really break the hull free and start hauling ass. I realize that it won't be as exaggerated as a bass boat getting up on the pad (which you can really feel seat of the pants), but she just feels like she is dragging too much hull at WOT. Had some buddies drive it to confirm my numbers as well.

    Since I can't trim the drive very far past neutral at all without the current prop blowing out, that might be part of the issue. If the 4 blade gives better "traction" I would hopefully be able to add positive trim and carry the bow higher while still adding a little lift on the stern.

    An extra 40 horse can't hurt anything, so I'm gonna start there and see what happens.
    "Speed's fine, but accuracy is final."
    - Bill Jordan

    1998 Donzi 21LXR
    1971 Boston Whaler 13

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