Converting keel to deck step 70's era CC 35-2

Discussion in 'Boat Design' started by alex_sailor, Jan 27, 2013.

  1. alex_sailor
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    alex_sailor Junior Member

    I just noticed a previous post but it has expired, so let me expand. My situation is a bit different from that one and here is why I want to convert from keel to deck.

    I have a 1974 C&C 35 with a keel stepped mast which in this case has double spreaders and the lower is connected to the deck with double lowers, unlike most C&C35's which have only single spreaders as well as the fact that the boat has fore and aft stay.

    Mine was an "offshore" model of which only about 15 were built. So, based on a previous thread, I believe I have the inherent fore-aft/lateral stability in the rigging.

    I want to convert to deck for the following reasons:

    1) The cabin ceiling bends up when the stays are under tension because in that design, the structural bulkheads were not tabbed to the deck, just hull.

    2) Leaking is a pain

    I propose to install a compression post where the mast column entered, modify the collar to accept the mast as a base and possibly swap the mast, unless it could be used. I understand the thinking in the 70's was to use keel stepped to enable the use of a thinner wall.

    Can I convert to deck by simply upping the scantlings on the column?

    Alex
     
  2. Tad
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    Tad Boat Designer

    The short answer is...We don't know.....

    Because rig design is as much personal philosophy as engineering, and we don't know the thoughts behind the original design.....so we're in the dark as to whether the current rig is overbuilt, weak, or just right.......

    Because tubes come in various stock sizes the one currently in the boat may or may not be correct in all dimensions. There's no way to tell without the original rig design data (which may be available from some spar manufacturer) or starting over at zero.

    If you have a rating certificate that states RM it will be helpful.
     
  3. michael pierzga
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    michael pierzga Senior Member

    I dont know what your boat looks like but the typical cause of a deck lifting at the mast partners is from halyard tension.

    Do you have a tie rod connecting the deck, at the halyard turning blocks, to the mast step ?
     
  4. alex_sailor
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    alex_sailor Junior Member

    Converting to Deck Step


    I have a rating certificate and will have to study it to see where the RM (I presume you mean Righting Moment) is. The original spar manufacturer for that series is still in business, so maybe I should ask him.
     
  5. alex_sailor
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    alex_sailor Junior Member

    Michael, the boat does not have such a tie, but there are hooked plates at the partners which accomplish the same thing. What lifts are then the bulkhead sections port and starboard of the entry point, as they are not restrained.

    Apparently, the Rob Ball designed CC's of the 1980's tabbed those bulkheads to the deck to increase stiffness. While theoretically, I could do that, it would make a mess of the ceiling liner at a visible part of the saloon, so I thought the compression post was the way to go to kill two birds.

    Alex
     
  6. michael pierzga
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    michael pierzga Senior Member

    A compression post connects the deck to the mast step. Tie rods do the same.

    I dont understand hooked plates ?

    The deck must be tied down with rods to the mast step or sometimes straps from the deck mast collar to the mast. You should post a picture of your mast partners.

    There is no reason for a mast boot to leak. Some mast collars are poorly designed.

    The only water normally entering the bilge from a mast is rain water entering halyard exits.

    Controlling this water on many boats is via a Mast step Bilge separate from the main bilge.

    Converting to deck stepped is a major project.
     
  7. alex_sailor
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    alex_sailor Junior Member

    OK,

    I will take a picture when installed which won't be till the spring after this thread has expired, so all me to explain;
    The deck is a composite balsa core structure and in the area near the mast aperture, the vertical grain balsa appears to be replaced with horizonatl grain fir ply.

    The mast collar is through bolted with 10 5/16" bolts from the top to the inside, where I manufactured the previously mentioned aluminum collar to spread the load from halyards and stays.

    Now, on the mast, there are 2 5/16" sets of holes and attached to those on either side of the mast, below the deck is a heavy guage metal "hook" which goes up out then down to catch the upper lip of the cast aluminum mast collar. Similar to what your hands would do, if climbing out the same hole.

    When the tension goes up, the aperture with the collar wants to lift, but is restrained by these two hooks.


    The opening between the spar and the collar was filled in with SPARTITE by the previous owner, with no boot being applied outside and some water still penetrates, as well as the water ingressing via halyard and other control line plates. No separate mast bilge here, just spills over into the main bilge...
     
  8. michael pierzga
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    michael pierzga Senior Member

    Spartight is a mast chocking compound...it doesnt form a waterproof boot
     

  9. alex_sailor
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    alex_sailor Junior Member

    Thanks for the reminder, I was aware, and accordingly, I sealed the remaining gap with silicone and rigging tape...
     
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