Hatch coaming height?

Discussion in 'Metal Boat Building' started by pdwiley, Sep 19, 2011.

  1. pdwiley
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    pdwiley Senior Member

    Seems that it's all over the place WRT heights.

    Situation: 4mm steel deck. Bulwarks of 175mm height on the foredeck, 100mm height on the quarterdeck.

    Hatch sizes approx 600x500mm. 1 on foredeck, 1 on afterdeck.

    Hatch materials intended to be aluminium, say 5mm, with neoprene or similar gaskets to seal against the coaming lip.

    What's a reasonable height for the coamings? I was going to make them 75mm but that looked too high. 50mm seems more reasonable. I wouldn't go below 25mm.

    Coaming material, if it matters, will be 316 stainless flat bar.

    Also is it worth the hassle of making the coamings match the crown of the deck? It looks better but it's a lot of work and makes getting the hatches to fit snugly a lot more difficult.

    Thanks.
     
    Last edited: Sep 19, 2011
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Since this is a "private" boat not commerical, it is really up to you. Do you want a nice flush deck?.. if so, then there is the answer. If not, use a sill height that is easy to build for parts etc and not too much of a tripping hazzard.
     
  3. pdwiley
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    pdwiley Senior Member

    The problem I have with flush hatches is, I doubt my ability to build them to be watertight and I'm not rich enough to pay the cost for the aluminium ones with the nice lexan (or whatever) inserts at this point in my building budget. Also I'd still need to build the coamings they fitted into and if they were flush, back to issue number 1.

    So while a flush deck hatch would be nice, the chances of building one to meet my standards are low.

    I might mock something up and see how it looks. Thanks.

    Now I've found my camera again I can take some more photos.

    PDW
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    A decent gasket and a bolting pitch of no more than 4 x diamater is good enough for hatches.

    Not so difficult...
     
  5. pdwiley
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    pdwiley Senior Member

    Could you please point me to, or post, some example drawings or photos?

    PDW
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    here is a typ. deck hatch example, this one has mono-bolts:

    Typ flush deck hatch.jpg
     
  7. pdwiley
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    pdwiley Senior Member

    Thanks for that, I've saved the image in my boat stuff folder. It's just the sort of hatch I had in mind for deck access to the machinery spaces. We had just this sort of hatch setup on the aft deck of our ship.

    Would you use this sort of hatch for one which was intended for daily use, say to allow access to/from a fwd cabin or a lazaret locker, for example? It seems to me that it would be vulnerable to any slight amount of water sloshing about getting below decks and that's not something that'd make me popular if the V berth area got wet.

    Sandy & Gena had a very nice folded stainless hatch surround but it looked like a right PITA to get the stainless folded for it. Looking at their Web site I think the coaming was probably around 25mm.

    PDW
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    It all depends how easily your deck washes away water. Also if there are locations of stagnant water then when pitching/rolling it may be a cause of concern. Coupled with if you have a wet boat, ie plenty of water on the foredecks even in modest seas, then this too may dictate your choice.

    So, it really is more a personal issue. But bear in mind, would you open up a watertight below deck hatch in heavy seas or with green water on your decks? Thus hatches like these are usually for emergency and/or fair weather sailing.
     
  9. pdwiley
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    pdwiley Senior Member

    Again, thanks. The decks have a fair amount of crown and there's quite a bit of sheer so I really can't ever see stagnant water lying about. I've been careful in placing the freeing ports to minimise/eliminate any stagnant water along the deck edges as well.

    The answer to your question is, of course, no, I'd only open a deck hatch in such circumstances if the alternative was worse (say an internal fire).

    So I'll build what looks good to my eye and seals well on the foredeck & quarterdeck. I'd really love an access hatch in the bridge deck to allow easy removal of the engine etc and there a flush hatch is the only sensible option. I'd assume there's some small drains located in the bottom of that recessed lip to eliminate stagnant water.

    PDW
     
  10. Landlubber
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    Landlubber Senior Member

    PDW, it is a MUST that you build in a removable section of the sole to allow access to remove the engine(s). You may call them hatches, but in reality it is simply a section of the sole that is screwed (bolted) down, light sealant applied around the surrounding gap, so that it is very easy to knife the edges when required and up she goes, it makes life soooooo much easier.
     
  11. pdwiley
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    pdwiley Senior Member

    It's a sailboat. The engine sits under a bridge deck with 1200mm of headroom and access all round. Typical Tom Colvin design. I'm not talking about the sole, I'm talking about the weather decks. I can take the engine fwd and lift it out the companionway, which is what most people do. Alternatively I can put a hatch in the bridge deck straight above the engine & lift it out vertically if I need to. Engine weight is around 300kg so it's pretty easy to rig & move. I've got bolt holes in the frames already to take shackles or bolt on a rail.

    There's no way I'd build anything in that couldn't be gotten out again. I appreciate you bringing it up though because it's easy to overlook stuff. I passed over a lot of boats before I decided to build one and one of the big reasons I passed on a lot was, I couldn't see any way of servicing or removing some of the equipment without major surgery. I'm pretty determined not to make that mistake myself.

    Question in my mind is whether the work involved in building an access hatch which (hopefully) will be rarely used is worth it when I have alternative access to the space.

    PDW
     

  12. michael pierzga
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    michael pierzga Senior Member

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