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Thread: Need to build boat cradle

  1. #1
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    Anyone have any tips on designing and building a cradle for a 22' fin-keeled sailboat?

  2. #2
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    Anyone have any tips on designing and building a cradle for a 22' fin-keeled sailboat?

  3. #3
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    Anyone have any tips on designing and building a cradle for a 22' fin-keeled sailboat?

  4. #4
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    Much depends on the intended use and on how the boat will get on and off the cradle.

    The most robust traditional cradles took a lot of abuse as the boat was floated on and then the cradle dragged, perhaps on roller logs but often just skidded on some planks, out to wherever.

    To withstand this, the base was a square or rectangle with diagonals so the keel could be supported in three places. Usually you need some blocking at the middle and forward station to accomodate the drag of the keel but with a fin keel you may well have the keel only on the point where the diagonals cross.

    From here there's huge variety. Start with uprights at each corner of the base square, well braced fore and aft with diagonals. At the front end the structure can be really permanent with a horizontal timber to recieve the stem and some diagonal blocking to brace her upright. Some cradles actually cut out a hull section in stout plywood but you can create painting problems this way with no real gain in strength. I think the easiest is a V structure starting on the bottom square and going to the uprights at such an angle as to lay against the hull about at the turn of the bilge. The bottom of the V might not be in the center and you end up with a \_/. Have these diagonals also grab that horizontal member that supports the stem.

    The back end needs to be bolt-togetherable as you'll be floating the boat on and off but the theory is the same.

    If you make the space in the V or \_/ a little large, the aft one may be big enough to float the boat in without disturbing that, though you'll need to take down the horizontal member. You can get to the hull by some nice wedges that have plywood gussets to keep them alligned with the V timbers and can slide fat end up down to grab the hull. The advantage of this is you can pop one at a time for sanding and painting.

    For pulling, it's best to yoke from the back corners as then all the stresses translated to the timbers are compression.

    If you're using a travellift or crane and not making a travel cradle, poppets and blocks are the way.

    If you're making a cradle for shipping and are going to load by travellift or crane, then go for plywood hull section forms at the fore and aft ends of the cradle for the extra rigidity. And forget the wedges unless you can toenail them in place.

    G'luck

  5. #5
    Join Date
    Apr 1999
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    31,689

    Post

    Much depends on the intended use and on how the boat will get on and off the cradle.

    The most robust traditional cradles took a lot of abuse as the boat was floated on and then the cradle dragged, perhaps on roller logs but often just skidded on some planks, out to wherever.

    To withstand this, the base was a square or rectangle with diagonals so the keel could be supported in three places. Usually you need some blocking at the middle and forward station to accomodate the drag of the keel but with a fin keel you may well have the keel only on the point where the diagonals cross.

    From here there's huge variety. Start with uprights at each corner of the base square, well braced fore and aft with diagonals. At the front end the structure can be really permanent with a horizontal timber to recieve the stem and some diagonal blocking to brace her upright. Some cradles actually cut out a hull section in stout plywood but you can create painting problems this way with no real gain in strength. I think the easiest is a V structure starting on the bottom square and going to the uprights at such an angle as to lay against the hull about at the turn of the bilge. The bottom of the V might not be in the center and you end up with a \_/. Have these diagonals also grab that horizontal member that supports the stem.

    The back end needs to be bolt-togetherable as you'll be floating the boat on and off but the theory is the same.

    If you make the space in the V or \_/ a little large, the aft one may be big enough to float the boat in without disturbing that, though you'll need to take down the horizontal member. You can get to the hull by some nice wedges that have plywood gussets to keep them alligned with the V timbers and can slide fat end up down to grab the hull. The advantage of this is you can pop one at a time for sanding and painting.

    For pulling, it's best to yoke from the back corners as then all the stresses translated to the timbers are compression.

    If you're using a travellift or crane and not making a travel cradle, poppets and blocks are the way.

    If you're making a cradle for shipping and are going to load by travellift or crane, then go for plywood hull section forms at the fore and aft ends of the cradle for the extra rigidity. And forget the wedges unless you can toenail them in place.

    G'luck

  6. #6
    Join Date
    Apr 1999
    Location
    Hyannis, MA, USA
    Posts
    31,689

    Post

    Much depends on the intended use and on how the boat will get on and off the cradle.

    The most robust traditional cradles took a lot of abuse as the boat was floated on and then the cradle dragged, perhaps on roller logs but often just skidded on some planks, out to wherever.

    To withstand this, the base was a square or rectangle with diagonals so the keel could be supported in three places. Usually you need some blocking at the middle and forward station to accomodate the drag of the keel but with a fin keel you may well have the keel only on the point where the diagonals cross.

    From here there's huge variety. Start with uprights at each corner of the base square, well braced fore and aft with diagonals. At the front end the structure can be really permanent with a horizontal timber to recieve the stem and some diagonal blocking to brace her upright. Some cradles actually cut out a hull section in stout plywood but you can create painting problems this way with no real gain in strength. I think the easiest is a V structure starting on the bottom square and going to the uprights at such an angle as to lay against the hull about at the turn of the bilge. The bottom of the V might not be in the center and you end up with a \_/. Have these diagonals also grab that horizontal member that supports the stem.

    The back end needs to be bolt-togetherable as you'll be floating the boat on and off but the theory is the same.

    If you make the space in the V or \_/ a little large, the aft one may be big enough to float the boat in without disturbing that, though you'll need to take down the horizontal member. You can get to the hull by some nice wedges that have plywood gussets to keep them alligned with the V timbers and can slide fat end up down to grab the hull. The advantage of this is you can pop one at a time for sanding and painting.

    For pulling, it's best to yoke from the back corners as then all the stresses translated to the timbers are compression.

    If you're using a travellift or crane and not making a travel cradle, poppets and blocks are the way.

    If you're making a cradle for shipping and are going to load by travellift or crane, then go for plywood hull section forms at the fore and aft ends of the cradle for the extra rigidity. And forget the wedges unless you can toenail them in place.

    G'luck

  7. #7
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    Oct 2003
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    Bright's Grove, ON, Canada
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    Most people in my neck of the woods buy theres custom amde from "Marine Cradle Shop", but you could easily use an existing cradle for the design template and mofify the design to work for your boat.

    This is a photo of a cradle made by Marine Cradle Shop that is designed for a full keel boat (no cross bar at the stern).


    It is a good idea to add blocks (4x6 steel tube) to the bottom of the cradle at the four corners and under the keel so that the cradle can be lifted for transportation. Many marinas require these blocks.

    Allan

  8. #8
    Join Date
    Oct 2003
    Location
    Bright's Grove, ON, Canada
    Posts
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    Post

    Most people in my neck of the woods buy theres custom amde from "Marine Cradle Shop", but you could easily use an existing cradle for the design template and mofify the design to work for your boat.

    This is a photo of a cradle made by Marine Cradle Shop that is designed for a full keel boat (no cross bar at the stern).


    It is a good idea to add blocks (4x6 steel tube) to the bottom of the cradle at the four corners and under the keel so that the cradle can be lifted for transportation. Many marinas require these blocks.

    Allan

  9. #9
    Join Date
    Oct 2003
    Location
    Bright's Grove, ON, Canada
    Posts
    8,098

    Post

    Most people in my neck of the woods buy theres custom amde from "Marine Cradle Shop", but you could easily use an existing cradle for the design template and mofify the design to work for your boat.

    This is a photo of a cradle made by Marine Cradle Shop that is designed for a full keel boat (no cross bar at the stern).


    It is a good idea to add blocks (4x6 steel tube) to the bottom of the cradle at the four corners and under the keel so that the cradle can be lifted for transportation. Many marinas require these blocks.

    Allan

  10. #10
    Join Date
    May 2001
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    Bainbridge Island, WA
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    You didn't say what the cradle is to be used for. It needs to be more robust for highway use than for long term storage use. In fact, for long term storage, you do not even need a cradle, timbers and wedges alone will do a fine job.

    For highway use, I would assume 2 g's and use a factor of safety of 5 on mudulus of rupture and shear strength of the timber.

  11. #11
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    May 2001
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    Bainbridge Island, WA
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    You didn't say what the cradle is to be used for. It needs to be more robust for highway use than for long term storage use. In fact, for long term storage, you do not even need a cradle, timbers and wedges alone will do a fine job.

    For highway use, I would assume 2 g's and use a factor of safety of 5 on mudulus of rupture and shear strength of the timber.

  12. #12
    Join Date
    May 2001
    Location
    Bainbridge Island, WA
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    Post

    You didn't say what the cradle is to be used for. It needs to be more robust for highway use than for long term storage use. In fact, for long term storage, you do not even need a cradle, timbers and wedges alone will do a fine job.

    For highway use, I would assume 2 g's and use a factor of safety of 5 on mudulus of rupture and shear strength of the timber.

  13. #13
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    It will be for storage while fixing the boat, then transportation 20 miles to the launch site. I was mainly concerned about getting the height of the pads correct. I guess I could make them adjustable by using the pads from jackstands. I was thinking 6 X 6 bottom frame and uprights, with 2X8 diagonal braces.

  14. #14
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    It will be for storage while fixing the boat, then transportation 20 miles to the launch site. I was mainly concerned about getting the height of the pads correct. I guess I could make them adjustable by using the pads from jackstands. I was thinking 6 X 6 bottom frame and uprights, with 2X8 diagonal braces.

  15. #15
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    It will be for storage while fixing the boat, then transportation 20 miles to the launch site. I was mainly concerned about getting the height of the pads correct. I guess I could make them adjustable by using the pads from jackstands. I was thinking 6 X 6 bottom frame and uprights, with 2X8 diagonal braces.

  16. #16
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    So, what's the boat, TimH?

  17. #17
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    So, what's the boat, TimH?

  18. #18
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    So, what's the boat, TimH?

  19. #19
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    Originally posted by True Love:
    So, what's the boat, TimH?
    Its a 22' bleachbottle. I am going to put an interior in it. fix up the outside and put it up for sale. Then buy a bigger boat and do it again....until I can afford a DH17 [img]smile.gif[/img]

  20. #20
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    Originally posted by True Love:
    So, what's the boat, TimH?
    Its a 22' bleachbottle. I am going to put an interior in it. fix up the outside and put it up for sale. Then buy a bigger boat and do it again....until I can afford a DH17 [img]smile.gif[/img]

  21. #21
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    Originally posted by True Love:
    So, what's the boat, TimH?
    Its a 22' bleachbottle. I am going to put an interior in it. fix up the outside and put it up for sale. Then buy a bigger boat and do it again....until I can afford a DH17 [img]smile.gif[/img]

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