Outboard Bracket Hydrodynamics Help

Discussion in 'Hydrodynamics and Aerodynamics' started by flboats, Aug 7, 2016.

  1. flboats
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    flboats Junior Member

    I have a 33' power planing catamaran hull, came with two aluminum Armstrong outboard brackets from the factory for 30" outboard motors, bottom of the outboard bracket about 5" above the keel line. I have to repower my boat, so I have two choices, I can put back the bracket to the original position, or I can lower the bracket even with keel line and use a 25" shaft motor. What I thing the second option give me little bit more buoyancy because the bracket lower in the water, maybe little bit less drag because the bracket even with the boat bottom, so no whirlpool or drag behind the transom. Please help because I don't know anything about this things
    Thank you
    George
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    OK, so what is the idea, to switch to 25" motors ? I prefer the idea of sticking with 30", and leave things the same, if it was working properly, why change ? You have the possibility of putting the powerheads too close to the waterline, which is something of a known problem with cats. And without seeing the shape of the brackets and the hull ahead of it, it is difficult to comment on any issues of drag from dropping them down.
     
  3. flboats
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    flboats Junior Member

    I got the boat without motors, so I can buy 25 or 30" motors. This is planning hull, very shallow 20" draft so I don't think I will have problem to get too close to the water.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    20" Hull draft seems low for a 33' planing cat. But I would not chance 25" legs on a cat of that length, even if it has a hull draft of just 20". Especially if you are going to to have the cav plate close to the level of the hull bottom. You have to remember that the engines are sitting out there on the corner of a boat with slim hulls to support it, swells can push water up the exhaust of (stopped) low-slung engines. You would want a minimum shaft length of 25" for a 20' cat, imo. In any case, if the brackets are made for 30", I don't see how 25" will fit them. Unless cut down.
     
  5. flboats
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    flboats Junior Member

    The aluminum brackets for 30" motors 5" above the keel, so if I lower the bracket with 5" the cav plate will be the same position. I have a Prokat 3200, draft is 20" only, same size cat like Glacier Bay with narrow pontoons much more draft. Also if I lower my brackets, maybe lift my transom little bit because more buoyancy, this is what I think, but maybe I'm wrong, this is why I'm here to get advice from someone know more than I. Thank you for any help
     
  6. flboats
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    flboats Junior Member

  7. flboats
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    flboats Junior Member

  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Right, well the tunnel there is pretty narrow, so I guess much of what I said is not so applicable. It is hard to get a clear picture with that black bottom though.
     
  9. flboats
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    flboats Junior Member

    That is the picture when I got the boat, after I removed both brackets sandblasted and powder coated again. This is the time to put them back, so I can bolt it back to the original position 5" above the bottom or lower with 5" even with the bottom, I just really want to advantage and disadvantages to lower the brackets. Here is picture now without brackets
     
  10. flboats
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    flboats Junior Member

  11. Mr Efficiency
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    Mr Efficiency Senior Member

    You might be guided by how the boat handled previously. Was it inclined to porpoise, or did it dig the bows in running down-sea ? These things would give you a hint as to how it might change, particularly if the underside of the brackets would come into play underway.
     
  12. flboats
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    flboats Junior Member

    I don't know that, I never drive this boat, when I got it motors was already removed. When I removed bottom paint I noticed it was trim tabs on the boat before, so maybe also I can get some lift at the transom if I lower the brackets
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    I can't tell from the pix, but I'd say the bottom of the brackets would have a negative angle to the boat bottom, so won't affect lift underway to any great extent. I think I would be guided by whether the original set-up worked well, if you can come by that info.
     
  14. TANSL
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    TANSL Senior Member

    Imo, you should place the brackets as low as possible, as long as there are no clogged hull drain holes aft.
     

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  15. PAR
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    PAR Yacht Designer/Builder

    Why would you place the bracket pods "as low as possible", TANSL? Ideally, you'll want a significant step here and as MR. E. suggests a bit of rise to the buttocks as well. It does appear there's some convolution to the bottom of the pods, with tiny fixed tabs. It also looks as though there's transom drains there too. I'd mount 30's, if the ventilation plate is appropriately located. I'd also lose those tabs and see where you stand on a "baseline" run, before making major changes to anything. In other words, you're not going to gain much volume lowering the bracket to 25", so why bother.
     
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