Inner tabs - hull extension?

Discussion in 'Hydrodynamics and Aerodynamics' started by HW77, Oct 1, 2014.

  1. HW77
    Joined: Oct 2014
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    Location: Finland

    HW77 New Member

    Hello,

    Please see this 30.5' Bayliner
    http://baylinerownersclub.org/index...rwater-lights-and-thruhull-replacement#244664

    I have been thinking of adding a metal plate in the middle of the hull extending 1' / 30cm further, close to the OMC Cobra drive.

    My problem is that the 21.5' boat is a bit short for any but calm whether. My Ciera SB is also relatively high for its length. Would this 1' / 30cm fixed tabs or metal plate make my boat more stable when planning in waves?

    I have changed a bit oversize trim tabs in the sides - taking of with this short boat and couple of guys always standing right on the back makes the boat very rear heavy. Even thou I have over 300hp the boat runs bow high for a while. With bigger 12x8" tabs its now much better to take off. I think the tabs in the middle would help the take of too?

    Am I way off? Cons? Pros? Speed reduction or gain? Drag?

    -Tuomas
     
  2. Skua
    Joined: Apr 2013
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    Location: Hunt's Pier WW NJ

    Skua Senior Member

    How large where the original tabs??? Mine were 12x9 to start. I now have 12x24's on a 28ft . Less deflection, and less drag. If you can fit them I would suggest larger tabs as opposed to a fixed plate. Also relocation of weight forward would be a good idea as well.
     
  3. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    A bottom extension is a good idea, though tabs are just drag, no matter where they are. Your problem sounds to be a simple static trim issue. Larger tabs, in their usual locations will help the hole shot and acceleration just after the hump, but ultimately are just drag once you're up and scooting good, unless you can retract them. An ideal setup would the use of tabs (with sufficient area) to help get the boat up, then good trim, with the tabs retracted, once at speed. It's all about effective area and angle of incidence, which is why Skua's bigger tabs (150% larger) need less deflection, to be as effective.

    A centerline extension can clean up the flow to the leg, allowing the prop to work in denser water. You can make these adjustable too, but again, anything that's hanging in the flow is drag, so you try to eliminate as much as practical. This is especially true with boats carrying a fair bit of deadrise. These just tend to squat on launch, so tabs are helpful, but if your trim is correct, you can back them off at speed and she should run reasonably level.

    Which specific year and model are we talking about?
     
  4. HW77
    Joined: Oct 2014
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    Location: Finland

    HW77 New Member

    My boat is 1987 Bayliner Ciera Sunbridge.

    Initially I have 8x8" adjustable tabs, now I got 12x10". I also adjusted these new ones to retract higher than the boat hull line. The first ones (8x8") where following exactly the hull line when retracted.

    One more correction in the install of the new ones. They are not 1/2" higher than the bottom of the transom. I think this is how they are intended to be installed.
     

  5. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Is this the 2150?

    This boat has quite a bit of deadrise (over 20 degrees), so you can expect a considerable amount of squat out of the hole and though tabs will help a bit, she'll still drop her stern a fair bit until she "trims" out on plane.

    You can mount them flush or up a little (1/2"), but it depends on the hull and it's tendencies. Some designs tend to drive "over center", so the tabs are mounted a little bit up and retracted they still have some down deflection to counter an inherent high speed longitudinal instability tendency. Most boats will react fine with them flush, assuming good running trim. Having them a little up when retracted, will initiate bow rise.

    Tabs will not really make your boat "more stable in waves". The steep deadrise will insure she's going to dance around, as pressure waves vary under each chine. You can lower the bow a bit and get her to "calm" down a touch, but top speed will suffer and she'll slam a bit more too. A ventilation plate extension will help the hole shot, keep the bow down a bit and will offer some high speed stability gains, but you'll lose a MPH or two off the top end.

    http://www.permatrims.com/V37_Black_Permatrim_Hydrofoil_p/v37_blackomc.htm

    This is the one for your drive and works the best, though cheaper versions can be had, like a DoelFin. Your boat is on the boarder line in regard to tabs or the ventilation plate fin. You can make one up from aluminum plate if you're handy, playing with span and are,a to get it right or just scale it from the images of the Permatrim unit.
     
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