How Big a THrough Hull Do I Need?

Discussion in 'Boat Design' started by Excalibur, Nov 7, 2006.

  1. Excalibur
    Joined: Sep 2006
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    Excalibur Junior Member

    Ok, I looked through the archives, and found nothing to help, maybe someone out there can. I need a raw water supply for the genset I'm installing. I would like to tee off of an existing supply rather than put another hole in the boat. I have a 1" line that currently supplies the AC/Heat with cooling water. The maker (Mermaid) says I need 100 gph for that 9000 btu unit. I'm trying to figure out if I can tee off the seacock (before the pump supplying the AC), and run a second line to the genset (A 4kw Kohler). No word from Kohler on how much the genset pumps. Does anyone know what the capacity of a 1" through hull/sea cock is? Since all of the units in question are at/near waterline, there is little rise. The hose runs are about 6' to the AC, and about 10' to the genset. Anyone tried this?
  2. Tim B
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    Tim B Senior Member

    For simplicity, (though I don't know how practicle this is) it is convenient to have both ends of the pipe going to fittings that are the same diameter.

    What is more important from a flow-rate point of view is to make sure that the pipes are run neatly and without kinks.

    There are various rules of thumb to give pressure differences over a pipe of certain length and diameter. I'm sure a bit of googling will find them.

    Tim B.
  3. Greybarn
    Joined: Apr 2006
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    Greybarn Junior Member

    Be careful about teeing off another intake line. Whichever intake pump has more suction will cause the other one to be starved for flow. This is a case of simple hydraulics. Our practice is to have a separate intake for every piece of machinery. You can "cheat" a little bit with air conditioning intakes but even that case requires attention to the sizes.
  4. Crag Cay
    Joined: May 2006
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    Crag Cay Senior Member

    I totally endorse Greybarn's comments. Shared intakes are a constant source of coolant starvation problems, unless the tappings are off a large water chest fed by a 1 1/2 inch through hull. And even then....
  5. Excalibur
    Joined: Sep 2006
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    Excalibur Junior Member

    I was concerned about that, since genset cooling is critical. Will using a Y instead of a T (to reduce turbulance) help prevent the problem, or should I just go ahead and buy and install a manifold (sea chest)? I was planning on putting the T (or Y) directly on the seacock, so as to cause as little interaction as possible from line friction. I also have seperate strainers.
  6. Frosty

    Frosty Previous Member

    And even then.... its not the right way to do it. Your gen needs its own supply.
  7. Willallison
    Joined: Oct 2001
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    Willallison Senior Member

    Yep. Nobody likes drilling holes in their boat. But I'd bet you'd dislike rebuilding a cooked genset more!
    Surely the genset cam with installation instructions regarding the size thru-hull that you need...?
  8. Frosty

    Frosty Previous Member

    A good thread would be 'how to cut a hole in my boat for a seacock'

    This is not as easy as it sounds, getting it in the right place, can you put the floor boards back after fitting, get it as close to the gen set as possible , fitting the water strainer, Raising the strainer with a stand pipe to raise to the water line, what material is the boat, if balsacore wwhhhooaa, never cut from outside in-- you might hit a stringer, drill a pilot hole from inside first, dont put a seacock down water of another1 foot min, especially the toilet outlet.

    And thats only the start of it. What caulking should you use??? Well!!!!
    Should I insulate the bronze from steel?

    And then should the cock be bonded,-- WOW thats a big one.

  9. Excalibur
    Joined: Sep 2006
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    Excalibur Junior Member

    I'm beginning to see the light. Since I intend to wet store Thetis, she won't be hauled until spring and I was hoping to get the genset in this fall. But, if I can't, I can't. The genset is used, so no directions. I do have a maintainance/repair manual, and it does show a "typical" installation. The normal raw water line size is 5/8". The manual states that the flow rate of the raw water impeller is 7.5 gph, which I don't believe. I think it's a misprint, it's just not enought cooling water for a 26 hp gas engine. Now 7.5 gpm, I might believe...
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