Self bailing deck

Discussion in 'Boat Design' started by Newhill, Jan 11, 2014.

  1. Newhill
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    Newhill Junior Member

    Take a look at this 21' center console by "Specmar". http://www.specmar.com/adobe/1633views.pdf Would you describe this as a true self bailing deck? The aft edge of the deck seems to be about 80 mm above DWL. Is this good enough?
     
  2. gonzo
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    gonzo Senior Member

    Yes, it is. The whole deck is above the waterline and it drains outboard. However, that is assuming the boat floats as designed and the operator does not overload it.
     
  3. TANSL
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    TANSL Senior Member

    I do not think it's a "self bailing" deck because, among other things, there is not enough height between the deck and the waterline so that the water runs out freely and I do not see openings in the hull, or transom, for this purpose. I think, best water should go by scuppers to the bilge where you slim down with a bomb, automatically triggered by a level alarm.
    A self bailing cockpits, must not only have the holes communicate with the sea, but the total area of these should allow bail in minimum time (or what is the same, the area should be bigger than a certain amount). Therefore, it is not easy to ensure that this boat is self bailing

    The bulwark top is at least 720 mm from waterplane, which is enough, thinking on downflooding points.
     
  4. PAR
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    PAR Yacht Designer/Builder

    With only 3" above the LWL, this is below the minimum I prefer to see. I like to see double this, though some have gotten away with slightly less, you do want enough height that trim differences, from loading, crew and equipment movement can't cause issues. To answer the question, yes, it's self bailing, but some back wash could board, if trim or loading are significantly changed, which is easy to do on small craft.
     
  5. TANSL
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    TANSL Senior Member

    PAR, is possible that my unfamiliarity with the language plays tricks on me but I think what you're saying is that, besides being self bailing, it can be auto filled.
    What the law understand as self bailing, is a minimum size of openings and, of course, that do not allow water entry. To do this effectively, you have to put check valves or any other device that causes the same effect. If not, in my opinion, you can not talk about self bailing.
    Cheers
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Common practice in this country for boats of this kind says it is enough. In fact, much more might be ill-advised, as the centre of gravity will go higher for everything that sits on that floor as well, including any "green" water you take aboard. These floors are usually sloped down a liitle toward the transom at normal trim, so that water runs out better to openings in each corner. I think a non-return flap is a good idea, I have a boat about this size where the water exits through a rectangular channel roughly 4" x 1.5 " each side of the boat, cut at about 45 degrees with a hinged flat that only opens with water pressure from within, and closes tighter with water pressure from without. There is a tiny amount of leakage at times, but it is better than having openings that will allow water to flood in when reversing, or when a bunch of people move to the rear corner of your boat. What concerns me about low floor height is what might occur in the event of a hull failure or holing, if the floor level drops below outside water level, water will flood in, likely slosh to one side and tip your boat over before you know what is happening, since it is all sealed away under the floor. My boat has lots of underfloor foam that would prevent that happening. At the very least, you want a high capacity bilge pump installed that will kick in to head off such a problem, having self draining floor doesn't mean bilge pumps are redundant.
     
  7. PAR
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    PAR Yacht Designer/Builder

    TANSL, if I understand you correctly, the answer is maybe. It depends on how the scuppers are arranged, so it doesn't have to be "self filling". I'm not completely sure what you mean by self filling, but there would be back flooding in some, momentary situations. I can think of a few occasions where underway you'd have water aboard, but given no other issues, it would "self bail" through the likely transom mounted scuppers. Of course we all could think of situations where this arrangement could be problematic, but under the usual circumstances of operation, a reasonable, effective and simple setup.

    I've found at about 20' on deck, you can consider making self bailing decks practical, while below this size, given the general dimensions seen in powerboats, the sole height is too low, at least for this approach.

    The drawing doesn't show the full picture and I'll bet transom mounted scuppers are spec'd, just not shown on this set of drawings, which seem more prospective purchaser oriented, than practical construction drawings.
     
  8. TANSL
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    TANSL Senior Member

    I think we all know what we're talking about and that we're talking about the same thing. I would only note two points to discuss, trying to help Newhill:
    - I think the requirements regulations require a boat to be "self bailing", this boat does not meet them. But of course, with the information that I have I can only guess.
    - I think, too, that no regulation requires the boat to be "self bailing". Therefore not very important to make it or not. What the ship itself should take is a bilge pump.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Incidentally, 225 hp outboard as specified for that boat is overkill imo, 150 would be more than adequate, and you will have plenty of throttle left while that moderate vee bottom hands out a flogging in choppy water.
     
  10. Newhill
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    Newhill Junior Member

    Thank you all.
    Mr Efficiency, your rectangular scuppers, can you describe them more. Are they available in boating stores? Is it an aluminium boat?
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    The scuppers are on a GRP boat, about the same size as yours ( a bit beamier) , and were made up in GRP by me. Initially there were just the rectangular slots. Small quantities of water are a little slow to drain because of the flaps, but any sizeable volume will force them open and exit quickly. The inside of the flaps are lined with rubber to seal better, backing up no water will enter.
     

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  12. Mr Efficiency
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    Mr Efficiency Senior Member

    It is hard to imagine the boat illustrated in the OP link isn't fitted with some kind of external drain off the cockpit floor, though not shown on the drawings.
     
  13. PAR
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    PAR Yacht Designer/Builder

    It's pretty common to have just an open scupper (no flap) on this type of boat.

    [​IMG]
    They come in retangular or:

    [​IMG]
    and round. They also come with check balls instead of flaps. The good ones permit the ball cage to be removed easily so you can clean out junk.
     

  14. JSL
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    JSL Senior Member

    ABYC addresses this matter in H-4. Covers scupper size, freeboard etc and explains self-draining and quick draining.
    Freeboard from cockpit sole (weather deck to water in inches) is L(hull length ft). x 0.22--- so for a 21' boat you should have 4.62". They have a metric (S.I.) formula as well.
    Yes, this might raise the VCG but a 21' boat should not have a problem with raising the deck (and associated items) by 1.62"
     
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