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Skegged boats and vibration.

Discussion in 'Technical Discussion' started by chesapeake46, Jun 23, 2017.

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  1. chesapeake46, Jun 23, 2017
    #1
    chesapeake46

    chesapeake46 Senior Member

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    I've followed the dual skeg/Marlow thread and I appreciate the explanations of the vibration problem.
    Why does a work boat, like a Chesapeake Bay Deadrise, with a skeg that protects the wheel and supports the rudder not have these vibrations ?
    Is it because they are single screw and do not have the turbulence from a second wheel ?

    I would understand a trawler type boat at much slower speeds being vibration free but I've been on work boats that would run at 40 knots that were virtually vibration free.
  2. Marmot, Jun 23, 2017
    #2
    Marmot

    Marmot Senior Member

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    Ooh, that's a tough one, there are so many nuances to underwater hull design and behavior. But maybe not so tough ... I was at the CBMM last week and had a chance to look closely at a few.

    Many of the older deadrise boats are built like bridges with heavy wood structure and are relatively low powered so it's not like they were pushing any structural limits. That type of construction absorbs vibration very well. I have seen some of them with stern frames and horn timber that were not faired into the keelson at all, just a squared off block of wood with a little shaft sticking out. But, that little shaft stuck out a fair distance and was angled to provide a good prop tip clearance.

    I think the newbuild versions are light, faired well, and the props are not in tunnels directly behind a block of deadwood, that and there are so many near identical boats built that each one can be fine tuned and made better than the last or someone elses version ... building those things seems to be like breeding horses, they keep getting better at doing what they are made for and to take advantage of new materials, engines, and techniques. They improved incrementally and weird ideas like Marlow's don't get to breed.
  3. PacBlue, Jun 23, 2017
    #3
    PacBlue

    PacBlue Senior Member

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    There are modern twin skeg designs that do work, but they are designed AND engineered, not just designed for a marketing "advantage".
    The hydrodynamics are well understood by those in the field that make their living engineering hydrodynamic correct apparatus.
  4. Capt J, Jun 23, 2017
    #4
    Capt J

    Capt J Senior Member

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    This, I've run some aluminum crew boats that had twin skegs and no issues. The 43/44' Lagoons have twin skegs if I remember right and no vibration issues.
  5. chesapeake46, Jun 24, 2017
    #5
    chesapeake46

    chesapeake46 Senior Member

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    TIsland011abc (640x347).jpg
    The workboat that I happened to be on was a deadrise style but used more for towing and made of aluminum. it had a 8V92 in it @ about 735 hp.
    It would really leap out of the water.

    This is the only picture I could find of it. Trailing.
  6. olderboater, Jun 24, 2017
    #6
    olderboater

    olderboater Senior Member

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    It seems that you see vibration or cavitation issues more in individual or family owned brands where the owner is or was/is always trying to tinker and do something more. They're people trying to top others, demonstrate they can build a better boat. I know Lazzara had their share of cavitation issues. They've also often been in boats trying to be made lighter for speed and efficiency.

    It goes back to my not wanting to buy the first hull of any boat. Well, with some builders, every boat is a first hull.
  7. Capt Ralph, Jun 24, 2017
    #7
    Capt Ralph

    Capt Ralph Senior Member

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    I still have some brain cells left. Remembering an issue long ago;
    A full keel design motor sailor, rudder rite on the end of the keel, near vertical swing, rudder and keel cut out to accept the propulsion propeller.
    I'm sure the is better terminology for this.
    The issue, under high propeller loads the propeller would make a racket, mostly when he needed the extra power with & against the currents at an inlet.
    Also leaning on it while docking.
    Motor mounts were secure.
    When the boat was fouled, it seemed not be as bad; Huuumm..
    Finally instructions to the diver, do NOT clear the fouling (barnacles and such) along the edge between the rudder and keel.
    Problem solved.
    The wheel was drawing air down the back of the keel, between the keel and the rudder. The fouling acted like a stop to keep the propeller from drawing air along this gap.

    Amazing how simple things can affect an other wise perfect boat.