Asymmetrical Cat Hull Design Advice

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

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

    Marshall_79 New Member

    Hi all,
    I bought an asymmetrical 5.2m powered catamaran from a naval architect design in Aus.
    I had issues were the port side bow of the boat would lean or dig into the water and any speed over 20 knots so i spoke to a few naval guys and recommended i change the motors to have a port side counter rotational motors, after doing so i still have the same issues and nearly a year later and these issues are still bugging and after 5 different boat mechanics and a naval engineer no-one can help me diagnose the issue and i am ready to scrap the boat in the junkyard.
    The original motors were 2x evinrude 75hp etecs and now i have 2x mercury 90hp Command thrust counter rotation long shaft.
    If there is anyone that can assist me diagnose the problem i am more than happy to pay for someones service just to fix the issue... 20170721_092818_2.jpeg 20170721_093028.jpeg 20170721_093103.jpeg
  2. UpOnStands
    Joined: Nov 2015
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    Location: Sydney

    UpOnStands Senior Member

    Welcome to the forum - sail area is rather small but never mind ;)
    Could you post photos of the bows looking aft so we can see how asymmetrical this boat is?
    Also, can you indicate where the DWL is with boat afloat & stopped?
  3. Marshall_79
    Joined: Dec 2017
    Posts: 2
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    Location: Sydney, Australia

    Marshall_79 New Member

    Yeah I could I'll take photos tomorrow and repost cheers...
  4. Richard Woods
    Joined: Jun 2006
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    Location: UK, USA and Canada

    Richard Woods Woods Designs

    Is it a one off design/build? If not what are others like? What happens when you run on just one engine? What happens with crew position changes. I assume you have a central wheel not offset? And your tanks are central

    What is the desired max speed. Are you in flat water or at sea?

    Richard Woods of Woods Designs
    But writing on board my Skoota 28 power cat
    Last edited: Dec 25, 2017
  5. UpOnStands
    Joined: Nov 2015
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    Location: Sydney

    UpOnStands Senior Member

    From his initial description it seems as if the plating on the port hull may be locally depressed between frames or the original build was inaccurate. The boat is very short so hull fairness is likely to be a real problem.
  6. HJS
    Joined: Nov 2008
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    Location: 59 45 51 N 019 02 15 E

    HJS Member

    This boat is extremely short and has extremely large engines for its length.
    Check the power trim. Small errors can have a big effect.
  7. IronPrice
    Joined: Jul 2017
    Posts: 110
    Likes: 2, Points: 18
    Location: NZ

    IronPrice Senior Member

    Weight distribution could be an issue? Especially if the problem is manifesting at speed.
  8. IronPrice
    Joined: Jul 2017
    Posts: 110
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    Location: NZ

    IronPrice Senior Member

    Are you 100% sure your props are a matched pair? Obviously they will have opposite rake being counter rotational but should be a mirror image of each other.
  9. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    I recommend seeing if Richard Woods has time to assist you. He is familiar with assymetrical hulls vs symmetrical hulls. I recommend you pay him consultancy if he can help you understand cause and perhaps help correct.

  10. Barry
    Joined: Mar 2002
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    Barry Senior Member

    Either buy or borrow a digital carpenters level and take some transverse readings in the boat when at rest. With normal gear loading, including groceries, ice, beer, water etc but without people on board to see if the boat sits transversely level in the water. An imbalance at rest may exacerbate itself at speed. If you cannot read the level from outside of the boat, ensure that you are standing with your weight over the centerline of the boat. Another contributor asked about tank location as well. Obviously having more weight on one side of the boat can cause the boat to lean and perhaps if you are under 20k, you just don't notice it.

    Then get your normal crew on board, at rest, and take some of the same readings

    Then with crew and equipment on board, take some readings at speeds all the way up to 20 to 25 kts

    Re tank location, do you have any heavy front mounted tanks or tanks say more than half way up the hull from the back?

    When you say the "boat leans or digs" at 20 knots, are you saying that it is a gentle lean to port or a harsh drop in the port side of the boat? To me, "digs in" means a harsh or quick drop of one hull. So which is it.

    I am surprised that with power trim, assuming that you have it, that you cannot just trim up the port side and drop the stbd side to get rid of the issue.
    Do the power trims work off one switch? ie a control for each engine or are they sync'd. If the engines are not sync'd, you might have to recalibrate them as perhaps one engine is losing
    thrust at your 20kt problem speed.

    On those rare mirror smooth days, our 40 foot, monohull, 30,000 pound (wet) at 22kts, trimmed to optimum with trim tabs would lean when one of the crew would
    leave the port seat and walk to the back and opposite side of the cabin (usually to grab a beer) the boat would lean slightly. So small changes in balance can make a difference.

    I don't see trim tabs and not sure if there is room for any. If the boat loaded is horizontal at rest with normal crew and equipment on board AND the boat dips suddenly AND you have heavy
    tanks up front there is a chance that the boat dynamic center of gravity at 20kts moves ahead of the center of lift and the bow drops quickly. In those cases that we have seen, I would not say that there is predisposition to one side or another, just the sudden drop.

    Can you get a picture of the position of the ventilation plate with respect to the hull bottom? Is there any chance that one prop is beginning to ventilate at 2okts most likely the port engine.

    We had a 15 foot - 4.5 meter aluminum floor, inflatable come in with sudden bow drop, quite violent at a certain speed. The outboard motor was around 65 and a jet WITH A TUNNEL in the aft section. So a very light hull. The fuel tank, about 35 gallons, 280 pounds 130 kg was right at the front. As the sudden bow drop was quite unsafe we were asked to seek a remedy.
    We drained the front tank, threw in some protable fuel tanks AT the transom, and the problem went away. Subsequently, we moved all the fuel to the back and never heard from them again.

    Our supposition was that as the center of lift went rearward and approached the area near the tunnel, the boat just lost lift and dropped the nose.

    But in your case, we need some more info. Rather than get rid of the hull, there are a couple of things that you might consider and relatively inexpensive
    Find a way to mount a set of trim tabs.

    As your boat is not painted near the chine, you could tack weld in a lift strake 1 1/2 by 5 inch by 1/4 inch thick angle and tack weld it onto the outer chine. If this works, then you could fully weld it in
    If the strake provides too much lift, you could just taper the front and shorten it.
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