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cavitation problem

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Old 08-13-2013, 07:02 PM   #1
 
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Default cavitation problem

On my 96 Shabah, 4.3 Alpha One, it wants to porpoise. I have to keep the trim down most all of the time. Doesnt matter how much weight I put in the front. Gets viloent so can run at cruising speed unless water is like glass.Ive read other post and everybody seems to like the trim tabs. Is this problem a design flaw in the boat? Seems to be a problem more than it should. I love my boat, just saying. Ive read about the cavitation anode, Ive seen pics online where they were taking measurements from the hull to the outdrive and using a straight edge. Im a capable mechanic but its a little confusing. I do have a small chunk that was knocked out of the fin where the outdrive bolts to the lower unit. Also about 25% of the fin that protects the prop is missing also.
This was done before i bought it this past spring. Only gave 4k for it, i thought it was a good deal.
Other than that, runs strong, no leaks, got that pre-ignition problem like everyone else it seems, but gonna try some higher octane gas and check timing at end of season.
Anyway, any thoughts on the porpoising problem?
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  • Old 08-13-2013, 09:55 PM   #2

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    Default Re: cavitation problem

    hi Richard. I have the same boat and dont experience the problems your boat has so its not a 'design flaw' and this isn't a "cavitation problem". And uh.. a 'cavitation anode'??? Anodes have nothing to do with the stability of your boat. They are simply installed to provide protection against the electrical currents your boat/motor generate from eating your outdrive alive.

    What prop is on your boat now? It should have a 19" or 20" pitch prop on it. Does your trim button bring your outdrive all the way in? You mentioned that your skeg is damaged (25% is quite a bit) but is that all that is damaged? A pic of your outdrive would help.

    Education is helpful here. There are a couple of 'sticky's in the Prop Talk section that will help you out.

    All that said, the best thing you can do for your boat is to install a pair of Smart Tabs. You can read about their benefits in this thread or view the video at the Nauticus website for more information. Smart Tabs (SX version with 60 lb actuator arms) will cost you around $125 delivered & take about 30-45 mins to install if you know how to properly use a drill.

    Last edited by mikeyt; 08-13-2013 at 11:56 PM.
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    Old 08-13-2013, 11:16 PM   #3
     
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    Default Re: cavitation problem

    Im hearing lots of good things about the smart tabs. I just figured something was out of adjustment. Is there a setting on the outdring where it attaches to the hull? Thought I saw slots in the mounting holes. Its raining pretty hard right now or i would go look.
    Yes, Ive gone over it very well, checking all the functions, the trim makes a full range. I was running a 3 blade 19p, now swithched to a 4 blade 19. Porpoising still exists No longer hit the rev limiter and have a considerable power increase, lost only 5mph top end. still will run 45mph.
    Unless the water is like glass, after about 4 or 5 lunges, you have to power down or you may be launched out of the boat. I can only trim up slightly, not even get the nose out of the water, anymore than that and it starts. Even with two 200lb adults in the front.
    Yeah the skeg, couldnt think of the word when I posted earlier.
    Here are some pics
    Attached Images
    File Type: jpg prop.jpg (150.9 KB, 38 views)
    File Type: jpg prop1.jpg (110.4 KB, 35 views)
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    Old 08-13-2013, 11:28 PM   #4
     
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    Default Re: cavitation problem

    Here is a thread for you to read over.
    When the lower unit has that kind of damage , things aren't going to be right.
    Look it over and see what you think.
    I personally would get a new case , or at least a used one in good shape.
    Your drive appears to have gotten hit awful hard!!
    I'm really surprised it isn't leaking myself. Is it bent at the mount or is that a optical illusion?

    Last edited by ColoradoDiablo; 08-13-2013 at 11:34 PM.
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    Old 08-13-2013, 11:36 PM   #5
     
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    Default Re: cavitation problem

    Tks, You think that could be the problem?
    I dont know how i didnt catch that when I was looking it over to buy it.
    Still a good buy at 4k I think...I hope
    Nothing seems to be bent, everything runs smooth, no vibrations or anything.
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    Last edited by Nubs45; 08-13-2013 at 11:40 PM.
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    Old 08-13-2013, 11:43 PM   #6
     
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    Default Re: cavitation problem

    I would think it could be a issue for sure.
    See what the other members think , but that seems like a lot of damage for that area to me.
    If it isn't leaking you might be able to have it repaired , as stated in the other thread I gave you. Heck , it would be worth a try.
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    Old 08-13-2013, 11:58 PM   #7

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    Default Re: cavitation problem

    With a big chunk out of the lead edge of the cavitation plate and a big chunk out of the skeg, its no wonder you are having those problems.... Get it fixed or replace the lower unit. DO NOT install a hydrofoil or fin. DO BUY a pair of Smart Tabs.
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    Old 08-13-2013, 11:59 PM   #8
     
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    Default Re: cavitation problem

    im pulling my lower unit and having it sent off for repair.....one thing i never mentioned on the thread i started it i remember when i was about halfway back to the docks going about 20mph i noticed my rpms going nuts from 2500 to 4000, the reason being, i found out , on my outdrive anyway, air was getting the prop causing to spin out of water. these outdrives are engineered this way for a reason, and any comprise will cause a ripple affect. an airplane may fly with a chunk of wing missing but its only a matter of time before something else happens. and the 3 years ive learned with my boat and boating in general, its better safe then sorry.
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