Asymmetrical Cat Hull Design Advice

Discussion in 'Powerboats' started by Marshall_79, Dec 24, 2017.

  1. Marshall_79
    Joined: Dec 2017
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    Location: Sydney, Australia

    Marshall_79 Junior Member

    Side view
     

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  2. Marshall_79
    Joined: Dec 2017
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    Location: Sydney, Australia

    Marshall_79 Junior Member

    I had my boat mechanic move the boat motors as high as they could go and no change. Tomorrow we are going to drop them as low as they can go and test again. The fuel tanks are centrally located.
    With me and 2 guys and ice and all fishing gear I would say assume 310 kilograms and the boat is definitely not level with the transom lower in the water. We even put 100 kilos of weight in the form of big water drums to counter the weight and the boat just sat below its water line. I think there is too much boat in the water and it's very sensitive to weight. If this next test doesn't fix it will be on to buy a trihull boat...
     
  3. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    If you intentionally try to make the port hull dig in; how would you go about doing it? Lots of people have trouble with excess bow rise; so they modify angles with wedges, they install hydrofoils. In your case all this is the opposite, but worse! Symptom on one side.

    If you trim up one motor; it is almost inconceivable that the bow dig would not go away unless the motor ends up out of water. Yet I don’t recall you mentioning.

    Then if you adjust the motor tabs obscenely; can you reverse? Have you messed with them; like purposefully trying to err.

    Is it possible the beam/deck bent?

    Have you ever had anyone on the bow to watch the waves? It seems like the design would provide minimal interference, but not zero.

    What is so significant about the 20 number?
     
  4. HJS
    Joined: Nov 2008
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    Location: 59 45 51 N 019 02 15 E

    HJS Member

    This boat is extremely short. With the large engines and the load mentioned above, the boat is also extremely heavy for its waterline length and bottom area. The planing bottom area is far too small for this total weight so that the bottom can not lift into planing mode. The boat plunges through the water. At the slightest imbalance, one hull can go up in planing mode while the other drops into displacement mode.
    JS
     
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  5. Marshall_79
    Joined: Dec 2017
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    Location: Sydney, Australia

    Marshall_79 Junior Member


    Thanks hjs that makes so much sense.
     
  6. Marshall_79
    Joined: Dec 2017
    Posts: 11
    Likes: 0, Points: 1
    Location: Sydney, Australia

    Marshall_79 Junior Member

    I appreciate everyone's help.
     
  7. jorgepease
    Joined: Feb 2012
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    Location: Florida

    jorgepease Senior Member

    Def sounds like the hulls are irregular, rocker or reverse rocker in one of them. Vertical trim tabs might fit that space if you can't figure out a better way to fix the hull. Others I have seen simply affix a small ridge at the edge of the transom, you could test that before doing anything permanent.
     
  8. powerabout
    Joined: Nov 2007
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    Location: Melbourne/Singapore/Italy

    powerabout Senior Member

    Crane it off the trailer and put it on some very flat ground to find out if both hulls are lined up with each other.
    Or lift it up and use a laser
     
  9. d. right
    Joined: Jan 2018
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    Location: kuranda,australia

    d. right Junior Member

    Not much to contribute other than anecdotal evidence. Few years back had a contract for renovations at Double Island, off Palm Cove near Cairns. The resorts workboat was a hull identical to this hull. Usually about 5 or 6 tradies would be run out to the island. First trip out at high displacement speeds the boat would easily broach. After a few months using this boat (I own a 17 ft mono) I could not see any advantages to this hull in terms of handling various sea states. I agree that length to beam ratio may have something to do with it. Extending the W/Line might help.
    Cheers, Gaz
     

  10. d. right
    Joined: Jan 2018
    Posts: 28
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    Location: kuranda,australia

    d. right Junior Member

    Excellent, this would describe the antics of the one that i had been in.
     
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