Flat spot, pad at the Transon on Cig Boats?

Discussion in 'Boat Design' started by big-boss, May 30, 2008.

  1. big-boss
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    big-boss Junior Member

    I see alot of pictures of Cig Style boats with a flat pad on the bottom of the transom. It simply looks like someone picked up the boat and set it on a grinder. What is this for? THanks
     
  2. Bullshipper
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    Bullshipper Bullshipper

    Better fuel economy. Delta keel pad.
     
  3. big-boss
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    big-boss Junior Member

    Do you know why? Can it be retrofitted? Thanks
     
  4. Bullshipper
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    Bullshipper Bullshipper

    Flat planning surfaces provide more lift than sloped ones. Sloped hulls with a lot of entry and deadrise angle in the stern (Contender, Regulator) cut through head seas with a softer ride, but consume more horsepower riding lower in the water.

    Delta pads on cig boats with a lot of deadrise angle attempt to still get the good ride and gain back speed and fuel economy with the pad lift and stepped hulls (ei Reggie Fountain) to create a thin layer of bubbles along hull bottom to reduce laminar flow drag. Now you even see the pads on pangas designed by Mercury and Yamaha for third word countries.

    But modifying the keel on any boat is like changing out your backbone. Difficult to maintant strength in a sensitive area as your ribs (stringers) also are cut down and made weaker.
     
  5. big-boss
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    big-boss Junior Member

    Do designers design this pad? Or is it just trial & error?
     
  6. tom28571
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    tom28571 Senior Member

    A pad like this does not seem to be the best way to go. Geometry says that the buttock lines of the pad would have to be at a lesser trim angle than the rest of the V bottom. This looks like lesser lift per bottom area, not more, the degree depending on the deadrise angle of the V bottom. If the pad were built-down to the same trim angle, then the lift would be greater but that looks like a lot of drag at the edges.

    Sometimes such design "features" get a popularity because others copy them and the buyer wants the latest gimmick. A long pad extending all the way to the planing water entry would certainly add lift though. Ken Bassett's Rascal comes to mind.

    I suspect that the greatest benefit of such pads may be to reduce planing draft and allow an outboard prop to clear the hull (like a tunnel). Makes the transom and powerhead safer from following waves when at the dock or checking-up also.
     
  7. big-boss
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    big-boss Junior Member

    What about on some of the fast boats they cut back the lower bottom of the transom? Sort of a notch?
     
  8. Bullshipper
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    Bullshipper Bullshipper

    Notches as you call them allow the water to lift sooner, relieving drag on high powered vee hulls. This also allows you to lift the motor higher reducing drag from the lesser amount of lower unit in the water, and as the water comes up at a greater angle behind the transom, this again helps to produce lift and reduce ventilation on the cavitation plate above the props.
     
  9. big-boss
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    big-boss Junior Member

    Are these kind of things ever incorperated into a planning work boat? THe "notch" or "Pad"? Thanks alot guys this helps.
     
  10. Bullshipper
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    Bullshipper Bullshipper

    I believe you are talking about something else, if I understand the question completly.

    Reggie Fountain has set enough speed records and won enough races with this design to prove out the value of this "theory". The Yamaha/Mercruiser 27' boats with soft rides and 4 mpg outboard fuel economy at 30 mph are also head and shoulders above other designs getting 2-2.8 mpg. Even the smaller 23' seacrafts with the infamous Mosely bottoms do not approach these efficiencies.

    http://anglerboats.com/performancetests/Angler_26_Panga.pdf

    The pad is completely flat down the keel, but tilts up to provide lift on plane at the deepest part of the hull (keel) where there is more upwards pressure when it tilts up on plane. The delta terminology refers to it tapering from 10-15" wide at the stern where the engines hang to a point at the bow where less lift is needed.
     
  11. Bullshipper
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    Bullshipper Bullshipper

    A panga is a third world fishing-netting-dive boat for that fishermen that want to carry a lot of weight with small engines .
     
  12. Ike
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    Ike Senior Member

    Pads are very common on fast bass boats
     
  13. big-boss
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    big-boss Junior Member

    I want to design on for a 30 ft aluminium boat. Any ideas on how I would do it? Any instructions? Thanks.
     
  14. CTMD
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    CTMD Naval Architect

    Big boss,

    its not rocket science but does need to be done from day one (can't be retro fitted). The easy way to do it is to model your boat with the keel line sloping down all the way to the transom and then add another surface that cuts it off on your actual keel line. This lets you control your buttocks while you do it. Once you're happy remodel the hull bottom panels to join at what was the intersection.

    Good luck.
     

  15. Jimboat
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    Jimboat Senior Member

    Vee Pad Design

    big-boss - A pad design like the one you describe will generate some more efficient liftand possible performance gains. A deep vee planing surface generates Lift with less efficiency than a shallower vee or flat pad. As with all design, however, there are compromises, as a shallow vee has a "harder ride" in heavier water conditions whereas a deep vee can be more foregiving.

    A pad-vee hull is normally designed at the outset for the hull, since there are many design factors to consider. The "trial and error" approach is never the best way to design a performance hull - it's expensive, slow and unpredictable. There are analysis techniques that can be used to optimize your hull design including pad design.

    A proper pad design provides a much improved hydrodynamic lifting capability, resulting in more speed for less drag. But it's a compromise, as dynamic stability is often sacrificed. A safe performance ride will depend on, among other factors, the speed that you want/expect to be going while on the pad, total weight of complete fitted hull (with engine, fuel, passengers, payload, etc), deadrise of pad, planned angle of attack of the pad and AofA of the hull itself. From analysis of these and other design factors, the dimensions of the pad can be optimized to support the hull and to maintain a safe dynamic balance of the ride. A flat pad at low (3 degree) angle of attack is most efficient; and wider pad is more efficient, although rougher ride in heavier waves.

    Note that with heavier boats the pad area available becomes more limited in its ability to provide a significant portion of the hulls required Lift. In these cases, lifting strakes are often used also, providing additional lift as well as uncompromised stability. In smaller, lighter boats, the pad can provide up to 100% of the lift...this is harder to do with heavier boats, and so the design trade-offs are often not as beneficial.

    You may find my article on "Pad Vee Design" helpful.


     
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