Raised cabin vs flush deck

Discussion in 'Sailboats' started by gmat, Nov 16, 2013.

  1. gmat
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    gmat graham

    A lot of monohulls have side decks for accessing the front of the boat, and a raised cabin in the center of the boat giving head room below.

    If we took the roof of the raised cabin and extended it out to the sides of the boat, making a flush deck, we would get the same head room in the boat, but more actual space inside.

    Is there any reason other than aesthetics why such flush deck designs are not common? They should have roughly the same windage as the standard raised cabin approach ...
     
  2. PAR
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    PAR Yacht Designer/Builder

    As with everything, there's good and bad to consider.

    This is a raised, flush deck:
    http://www.boatdesign.net/gallery/data/500/medium/RYD-39.jpg
    as is this:
    [​IMG]
    and it is the strongest way to build a deck. Right about where the bottom of the ports are (my drawing), is where the sheer would be, if it was a conventional deck line (on the Cal 20 the sheer is more obvious). Of course, you'd need a "trunk" cabin sticking through the deck to offer the same headroom below. A trunk cabin does limit headroom somewhat, though along the sides of the interior are usually cabinets and seating, so you're not really standing there, but aesthetically, it's a much "bigger" space.

    If the truck cabin has it's sides pushed out to the rail, you gain this interior volume too, but it also makes it more difficult to climb over it to get to the foredeck. This type of cabin can make a small boat seem much bigger inside, even if the additional volume doesn't really affect the living spaces in a practical manner. This type of cabin (as well as the raised deck) will improve the stability curve a bit too, though there's a weight and a bit of windage penalty you have to pay for this convenience (more material and surface area). Also the trunk cabin pushed out to the deck edge leaves a place for boarding waves to pile up, particularly if you have deep bulwarks. Good scuppers can fix this issue.
    http://www.boatdesign.net/gallery/data/501/medium/BYYB-223.jpg

    Ultimately, potential buyers seem to like trunk cabins more than flush decks. I personally don't mind flush decks, often and usually requiring a "broken sheer" line, which most do find less aesthetically desirable. On my flush deck above, I use both the back half of a trunk cabin and the raised, flush deck. The idea being a transition to the flush deck, so you climb up to a conventional height sidedeck, before again climbing up onto the flush deck. So, yes some of the points you raise are reasonable, though there's more than meets the eye. There are a few other ways to skin this fish too.
     
  3. michael pierzga
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    michael pierzga Senior Member

    Works ok on a small boat. On bigger boats it is undesirable for many reasons
     
  4. troy2000
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    troy2000 Senior Member

    Not trying to argue; I'm just curious (being an amateur). What are some of the reasons you consider it undesirable, Michael?
     
  5. michael pierzga
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    michael pierzga Senior Member

    Well, imagine what the lifelines would look like on a bigger boat with two deck levels. Not pleasing to the eye. Small boats usually dont have stantions and life lines around the boat.

    Then consider that all the work that needs to be done on a sailboat...its sails ..are in the front of the boat and the best place for the work to take place, the controls , winches, are in the back near the crew and helmsman. It becomes difficult to leed all these lines aft to a level lower than the maindeck.

    It can be done but its complicated.

    Far easier to have normal side decks and a cabin house on a bigger sailing boat.

    Raised deck small sailboats are very nice. The classicis the stonehorse design

    http://[​IMG] sube fotos
     
  6. gmat
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    gmat graham

    I agree that the space is possibly more of a perception than actual useable space. But I have seen the inside of a few flush decks boats and the perception is strong :)

    I was wondering why you consider there to be more windage. In the traditional raised cabin the windage is from the side of the boat plus the side of the cabin. In a flush deck the windage is from the extra high deck side, but the extra height is the same height as the cabin side in the traditional design.

    I take your point about scuppers -- very important.

    About getting to the back deck I am thinking about a 51-52 foot pilot house design with flush decks, and so trying to work out how to get to the deck in such a configuration.

    Also you say there is more than one way to skin this fish. I must admit I am at a loss other than flush deck or traditional.
     
  7. michael pierzga
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    michael pierzga Senior Member

    On some boats it works.. A motorsailor for instance.

    http://[​IMG] imagen
     
  8. PAR
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    PAR Yacht Designer/Builder

    If you look at the evolution of sailboat deck design in the last 40 or 50 years, all sorts of ways have been tried to reduce windage and provide headroom under a deck. The old CCA style of yachts, pretty much all had trunk cabins sticking up through the deck. Some may have had rounded fronts, but they still just pocked through the deck. The side decks were low, crown was increased, bulwarks lowered, extruded toe rails introduced, etc.

    Blister style of decks appeared in the early 70's with the introduction of the IOR racers, again to reduce windage, yet still keep the sheer low. Progressively, the new hull forms of these yachts, required the sheer to be raised and a lot more deck crown employed, often designed with a thick deck flange (several inches), further raising the sheer line, to provide headroom below a reasonably sized deck house structure.

    Again, there are issues with both approaches (raised deck/trunk cabin). It's a mater of scale. The Cal 20 is just too small to have a reasonable head room below, yet the Flicka, using huge roof crown and deep belly can pull it off. These are aesthetic considerations for the most part, as expecting standing headroom in a 20' Flicka is just a wish for a canoe belly design. This is the rub, between say 20' to 35' on deck with modern, shallow hull forms. You can build the hull down and increase the draft, wetted surface and hull volume, for sufficient headroom without a Winnebago parked on the deck or use some other sort of visual trick, to fool the eye into thinking is not a Winnebago, but just a minivan sitting up there (like Flicka does). Once you get over the small and smaller medium size yachts, you have more then enough hull volume to contain the headroom.

    In terms of windage, you want a surfboard with a sail attached, but this doesn't offer much enclosed headroom. Anything bigger does and it's surface area and protrusions that you try to "clean" up. As I mentioned, there's lots of stuff to consider, like stanchion arrangements, which can be ugly on a broken sheer deck line, though they also can be acceptable, if well thought out. Life line arrangements aren't typically given a lot of thought, after they've been sized and spaced appropriately, but the better designers do put some aesthetic consideration into them, some quite clever.

    There's no doubt from a structural stand point, flush decks are much stronger, compared to a trunk cabin arrangement on the same hull. It also does make an interior feel much larger, even if the hull shape and typical accommodation setups do limit the practical nature of this increased volume. In the end, the choice can be a practical one, particularly on a small boat or maybe an blue water vessel, looking for every strength advantage it can get.

    I have a 50' design with a pilothouse and flush deck. I use the pilothouse as the transition from the standard deck height to the raised portion. It helps mitigate the break in the sheer, with the bulk of the pilothouse obscuring the sheer break. On a 50' - 52' yacht, especially if not performance oriented (the pilothouse suggests it's not), it pretty easy to make a pleasing profile. Trying the same thing on a 35'er will be quite tough, but you can still pull it off, though you'll have to make more design concessions in other areas.
     
  9. gmat
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    gmat graham

    Thanks for all the comments -- very interesting.

    The boat I am thinking of isn't supposed to be a speed machine (although I don't want a slug either :)). It's a blue-water design for living on, and for traveling anywhere.

    I started with the Sponberg 45 design (http://www.sponbergyachtdesign.com/Globetrotter45.htm), and have been messing with different ideas, in particular making it a flush deck design.
     
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  10. michael pierzga
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    michael pierzga Senior Member

    You should stay with Mr Sponbergs design and not change it. The boat looks very well thought out.

    Other concepts would be the Freedom 40 type .

    Flush decks at sea are very intimidating for crew with no handholds, no toe grips and a long fall to the leeward side

    Flush deck never provide spray shelter for deck ventilation hatchs, hence they must be permanently dogged down
     
  11. PAR
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    PAR Yacht Designer/Builder

    I disagree, in that flush decks are intimating at sea. Foot holds are usually installed, and if doubt, going forward without a safety lanyard will just show you how foolish you are. A wave or splash tall enough to climb over a raised deck bow, will also climb over a traditional sheered bow and it's trunk cabin, entering it's trunk mounted hatch. In fact the higher freeboard will cause less water to come aboard. Still it's a thing most sailors will have issue with, mostly because it's different than what they're familiar with. When GPS came in, many clung to their Loran units, hoping it was just a fad, but eventually adapted and this is the case with every different or new innovation in yachting. Old dodgers like me and Michael are hard to train for new tricks. Maybe with a shock collar, we can get Michael on board a flush decker. ;)
     
  12. gmat
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    gmat graham

    I agree that the Sponberg 45 is a very nicely thought out boat. And I am not trying to change it necessarily, but to learn from it. For example I learned about free standing rigs from it. The rig is so different, so I wondered what else could be different (but useful, different to be different is pointless and possibly dangerous).
     
  13. Tad
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    Tad Boat Designer

  14. gmat
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    gmat graham

    Thanks for all the links. The Cartwright 44 is actually where I first discovered the idea of the flush deck, and with a pilot house.
     

  15. tom28571
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    tom28571 Senior Member

    Flush deck or trunk cabin is almost always decided based n personal taste with only a little bit of practicality thrown in.

    I happen to like flush deck designs in both large and small boats but the advantages of the extra volume is most appreciated in the smaller ones. That extra volume is not just aesthetics. It provides a lot of righting moment in a big seaway or knockdown and, in many cases, makes it possible to sit on the sleeping berths and lean back comfortably, which may not be possible at all on their trunk cabin cousins where your head will bang the side deck. When there is no other suitable upright seating available on the interior, that is no small thing.

    There is extra windage in a flush deck in spite of an apparently equal height in profile, since negative windage is most apparent when the boat is heeled going to weather.

    There are many very handsome boats in either flush deck or trunk cabin and some that are really in between with only a small bubble of a trunk poking through the broad deck.
     
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