Flotation Chambers for Power Cats

Discussion in 'Boat Design' started by Bullshipper, Jan 22, 2022.

  1. Bullshipper
    Joined: May 2008
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    Bullshipper Bullshipper

    Looking for reference material or suggestions to help me design a sealed deck over bulkheads in a 28 x 9'6" FRP power cat.
    Have incorporated 2 1600 gph 1.5" forward and aft bilge pumps in each sponson to handle water, but am wondering if weep pipes passing through the bottom of the bulkheads should be valved to get complete drainage.

    4 Areas have 4 14x24" compression hatches with o-rings with 3m 4000 sealer that I expect will still leak some air, but I hoping hoping pumps will keep up with that using 4 1000 cca batteries above decks in dry area also plumped with 750 gph pump for a day or so.

    Not planning to place bags or foam under decks at this time.

    Any suggestions or links ?
     
  2. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Re the '4 Areas', does this mean that there are 3 bulkheads in each hull, creating 4 water-tight compartments in each hull?
    If so, then re the 2 bilge pumps fore and aft in each sponson, does this mean that you want to fit drains on the two compartments in the middle to get complete drainage?
    This then rather defeats the purpose of having 4 separate watertight compartments.

    What type of 14" x 24" compression hatches are you planning on fitting?
    Are they strong enough for people to walk on?

    I think that I would be inclined to keep 4 W/T compartments under the deck in each hull (no drains between them) and just inspect them occasionally via the hatches (as they have 'O' rings I presume that they are reasonably watertight?).
    And have an electric submersible bilge pump in a sump (in the deck) in each hull to pump out any water that comes on board.
    Or, as you already have 4 pumps, have one (in a sump) fore and aft in each hull?
    So long as the hatches are watertight, then the only way for water to get into the compartment is if that compartment is holed.
    And if it is, then only that compartment should fill with water.
    Will the deck be below the usual load waterline?
    If yes, would it be feasible to build in a clear sight tube (sort of like what fuel tanks sometimes have on the outside, to gauge the level within) for each compartment, with the top of the tube going up to under the gunwhale, and out of the way where it can't be kicked or trodden on?
    Then you would not have to lift the hatch to see if there is (lots of) water in the compartment - if there is water head in the tube, then you have a problem, and you know that the compartment needs to be pumped.
    And maybe you could then have a manual roving bilge pump with a hose that can fit on to this sight tube (or have another spigot for the pump) to pump out any water in the compartments?
    But there should in theory be very little or no water in these compartments to worry about, so long as the compartment is not holed (?)
     
    Last edited: Jan 22, 2022
  3. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    The way this would be done better is for watertights to be made watertight. This can be done by using a subdeck and submersible passive vents. The subdeck is fully watertight and the accesses only allow water into lockers if left open or they leak. Gutters in some hatches can direct water overboard. And, if needed, weeps or thru hulls can be used above the watertights.

    I am building a 32' powercat and the boat has 11 watertight compartments with passive submersible vents. They are all isolated from each other in case of breach.
     
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  4. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    I am a bit confused as to why you are anticipating so much water. Perhaps a drawing is needed as well.

    Here is a similar hatch opening to yours, but smaller. Below this about 16"? is the watertight which is sealed with an an access plate.
    DB5092C3-7090-44C2-AD48-60AA0A3FF7D3.jpeg
    This one is sealed with a pvc female adaptor and a submersible vent; not shown. But you see the hole. Sorry; I can't find a picture of the vent setup. Maybe tomorrow. These are not supposed to get water in them..the holes are for venting air for thermal changes only. I am hoping the hatch lids are fairly watertight. If not, I will need to modify them.

    283BEB96-29CE-4086-B21F-1BA99B89660B.jpeg
     
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  5. Bullshipper
    Joined: May 2008
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    Bullshipper Bullshipper

    Thanks for your comments.
    After taking a closer look at this, It looks like this is not as feasible as I had hoped if I want to use standard rigging tubes for her twin engines and also provide access to al areas under decks.
    So I will divide the sponsons into 3 areas each and rely on my pumps and raft to handle emergencies.
    Bilge pumps are inexpensive, draw very little power, and the difference in price is $20 a pump to go from 3/4 to 1.5" discharges for 4x the flow
     
  6. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Just because you can pump water doesn't mean you ought to.

    The Skoota has 11 watertight compartments. Ironically, they were not closed and the top deck was sealed with plastic and not deck lids and the plastic broke and water got in after about 8" of August rains.

    I had to handpump them and ever since I have been considering ingress.

    But I would only plan for the boat to be wet in certain places; not thru n thru.

    If a locker leaks; staying on top of the issues is key.
     
  7. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    I put a valve on all the sealed compartments in my boat, after discovering that the forward one had quite a lot of water in it!
    Dividing the hulls into watertight sections with their own pumps and alarms is about as safe as it’s going to get, but valves between them could save your boat in the event of a pump failure.
    I found a whole bag of 2”sch80 PVC valves on Craigslist, and am in the process of isolating the sections of my boat, glassing PVC nipples into the bulkheads where the limber holes were, and threading a valve onto the most easily accessible side.
     

  8. Bullshipper
    Joined: May 2008
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    Bullshipper Bullshipper

    Thanks, very useful input. FYI, Amazon also sells PVC gate vales for around $7. Great Quality.

    My hull is self bailing, so the only water I pump is the little bit the hatches may leak if dirty during wash ups, or in the rare event of a breach.
     
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