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  1. #1
    Petty Officer 2nd Class
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    Default porpusing cavitation bad set need advise

    Ok I have a 1972 larson tri hull about 950lbs dryA 1978 85hp johnson 302lbs13 gallons gas 104lbs2 adults 320lbs2 batteries 110lbsmisc gear,cooler 200lbsSo give or take gw of 2086lbsI have a 14*17 prop runing exactly 5600rpms and top speed of 30.00 mph gps does that sound right?Im happy with speed for me the real problem is getting it up to speed I have serious bow rise even when I put wiegh up front it doesnt change..Then along with the bow rise Im getting bad cavitation or thats what im guessing it is cause its like prop has no grip..I have no crusing ability at mid rpms All that happens is cavitation once it grabs and takes of Im ok..once I get close to top speed it starts bad porpusing wich last trip out I was able to push through and trim most of it out..Can anyone help does this sound like bad setup to you? My av plate is a little below the keel as I ve been reading it should be even with keel right? could this be the source of all me headaches? Thanks in advance..

  2. #2
    Petty Officer 2nd Class
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    Default Re: porpusing cavitation bad set need advise

    Anyone?

  3. #3
    Supreme Mariner rickdb1boat's Avatar
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    Default Re: porpusing cavitation bad set need advise

    I would try raising the motor up 1/2" or so and see how it does, then go from there. Your revs are about 200-400 RPM's low right now. This may help get them up where they should be and help with the handling, too. You may also need to drop the pitch of the prop to get where you need to be, but first things first. What length boat is it and what type prop are you running? The prop and set-up pros will be along to help soon....
    Rick

    "Boating is Life...All the rest is just Details"

    2007 Bentley 200 Cruise
    2007 Merc. 90 4 stroke

  4. #4
    Petty Officer 2nd Class
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    Default Re: porpusing cavitation bad set need advise

    16.5 foot Larson trihull and 14/17 alumin prop..

  5. #5
    Admiral
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    Monroe WA
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    Default Re: porpusing cavitation bad set need advise

    Do you have power tilt and trim on this motor?

  6. #6
    Commander
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    Default Re: porpusing cavitation bad set need advise

    Weres your motor at now on the transome?height wise?am amusuming you dont have a jack plate,like rick said you can try raising up a little to bring your rpms up and it may help getting you out of the hole.
    If it was easy,everbody would be doing it!

  7. #7
    Petty Officer 2nd Class
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    Default Re: porpusing cavitation bad set need advise

    yes have power tilt trim..It the motor was about an inch below the keel that is the av plate was an inch or so below..I raised it today now its about 3/4 inch above, havent been able to test it yet..But do my symtoms sound like set up problems to you or something else..

  8. #8
    Commander
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    Default Re: porpusing cavitation bad set need advise

    No expert but i think thats going to help alot.post the numbers when you get a chance to test it.I think your symtons are set up yes.i think raising that much is going to work wonders. Make sure your motors tucked all the way down to when taking off.
    If it was easy,everbody would be doing it!

  9. #9
    Petty Officer 2nd Class
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    Default Re: porpusing cavitation bad set need advise

    Will do thanks and will report back with #s hopefully by wed..

  10. #10
    Petty Officer 2nd Class
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    Default Re: porpusing cavitation bad set need advise

    Ok went out finnaly today..Didnt catch a thing but was the best day..For the first time my boat ran perfect no cavitation no porposing.Just ran hard at all levels..felt great. Did notice that my rpms had droped from 55-56k to 51k today I have a 13 1/4 * 17 prop that I try next time to get rpms up.. But man it ran like a champ..Trimmed out Top speed of 35.2 gps. Now the operating range listed on the motor is between 4500-5500k do I really want it to turn 6k wouldnt this be over or is this reallly the best operating range for the motor?Thanks a bunch I cant believe how much diference 1 3/4 inch makes..

  11. #11
    Commander
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    Default Re: porpusing cavitation bad set need advise

    I think your in the ball park,were does the 6K number come in??Maybe Dale or Kenny will know about the numbers? IF it jumped up to 6100 that may be a little to much rpms,you may want to lower it 1/2 to 3/4 inch and see what it does .
    If it was easy,everbody would be doing it!

  12. #12
    Petty Officer 2nd Class
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    Default Re: porpusing cavitation bad set need advise

    No right now Im running about 5100 before I was runinng about 5600 and one guy in his responce told me that my motor should be runing at 6000rpms..Somewhere towards the top of my post..

  13. #13
    Commander
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    Default Re: porpusing cavitation bad set need advise

    I think around 5600 -5800 is good for that motor,someone will chime in and tell different if im wrong. Thats odd you raise it up that much and actually lost rpms in theory you should have gained.
    If it was easy,everbody would be doing it!

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