Pontoon Questions

Discussion in 'Boat Design' started by ddrdan, Sep 23, 2010.

  1. ddrdan
    Joined: Mar 2009
    Posts: 67
    Likes: 2, Points: 0, Legacy Rep: 37
    Location: Raleigh NC

    ddrdan Junior Member

    1. Two pontoons won't hold the load I need, based on the length and width restrictions on the vessel. I'm adding a center pontoon (tritoon), shorter in length of the others. What is the optimum position of the center pontoon. Even at transom? Forward? Trailing? I like the trailing option as it fits the motor design best.

    2. The pontoons are 24"x24" 1/2 ply with 2oz chopped strand & resin. The stern and bow interior compartments will be glassed also. Ignoring deck loading for now, what would your average max spacing be on the bulkheads? Typical type construction in pic below.
    [​IMG]
     
  2. jehardiman
    Joined: Aug 2004
    Posts: 2,799
    Likes: 363, Points: 83, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    If you don't have enough buoyancy, then the 3rd pontoon has to be placed so that the center of buoyancy of all 3 hulls is directly under the center of gravity for it to be of optimum use.

    Just because the boat is heavy, doesn't mean that all you need to do is add buoyancy, that buoyancy must be in the proper place in relation to the CG. Otherwise the boat can take unfavorable pitch angles which may cause the craft to become unstable.

    If you have an existing measurement of the CG and CB, then you could design a 3rd hull for optimum buoyancy, and shape/location for powering. If you have already built a 3rd hull, then you need to place it in the proper loaction for CG/CB consideration and take whatever performance hit there is. If the boat is still building and you are looking for performance and will ballast to level trim, then the better location of the center hull is forward of the two outer hulls a distance dependent on spacing and speed. Optimally, the center hull should be longer than the two outer hulls.

    As for bulkhead spacing, that is dependent on load and length.
     
  3. alan white
    Joined: Mar 2007
    Posts: 3,731
    Likes: 121, Points: 0, Legacy Rep: 1404
    Location: maine

    alan white Senior Member

    A good spacing between bulkheads would be about 24"-30" with 1/2" plywood. They could have a large hole in them except where making watertight compartments.
     
  4. ddrdan
    Joined: Mar 2009
    Posts: 67
    Likes: 2, Points: 0, Legacy Rep: 37
    Location: Raleigh NC

    ddrdan Junior Member

    Thank you for the suggestions.

    I'm at a stage in the design where weights can be moved to accommodate CG & CB. Speed will be 15 knots max.

    I guess longer it is, and forward it goes.:)

    My initial design was to have the center hull sit 4" deeper. My structural deck supports are galv. steel and integrated into the outer hulls, not on top. Any disadvantages or advantages in that design? I'll never power the vessel enough to get it up on plane.
    [​IMG]
     
  5. ddrdan
    Joined: Mar 2009
    Posts: 67
    Likes: 2, Points: 0, Legacy Rep: 37
    Location: Raleigh NC

    ddrdan Junior Member

    Thanks,

    That close huh? Puts a damper on my placing long water tanks in the hulls.
     
  6. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 476, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Depending on the type of plywood, I wouldn't go larger then 24" on center for partitions. I would also fillet each all the way around too, to prevent point loading.
     
  7. ddrdan
    Joined: Mar 2009
    Posts: 67
    Likes: 2, Points: 0, Legacy Rep: 37
    Location: Raleigh NC

    ddrdan Junior Member

    I really appreciate the input. I hope I'm not ruining my welcome in more questions?

    Would increasing the bottom and partitions to 3/4" give me any increase in partition spacing? And if the partitions are 3/4" ply with 1x2 edge all around would I be able to remove a portion of the 3/4" web for pass thru's in the hull compartments?
     
  8. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 476, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    You're going to find it difficult to get answers if you don't ask questions Ddrdan. I'm not sure of the loading you anticipate but 3/4" plywood will be impossible to bend into the entry shape. Use the plywood thickness you need for the loads you expect, going thicker is just a burden the boat has to bare. Don't increase the partition spacing. Plywood doesn't like widely spaced bulkheads in narrow hull shapes like this. Design them as ring frames, rather then full partitions. This will permit some storage options, save a considerable amount of weight and offer good ventilation. I'd use at least a 4" web depth for the ring frames, with a 6" bottom web for slamming loads. This will provide about a 16" square hole in the partitions to pass things through. The inside corners of the cutout should have a healthy radius, say 6" diameter on the top two and 8" on the bottom.
     

  9. ddrdan
    Joined: Mar 2009
    Posts: 67
    Likes: 2, Points: 0, Legacy Rep: 37
    Location: Raleigh NC

    ddrdan Junior Member

    I've tried everything I've learned in here .... but ....... I can't live with 24" partition spacing!!!! :D

    I have a water tank, a cooler, and a livewell that won't squeeze thru an open partition web and they need larger spaces between partitions.

    I will never operate this vessel in seas over 1 to 2' or speeds higher than 15 knots. Here's what I came up with. I doubled the partitions at my deck cross members and added stringers @ 6" o.c.. Hull is 18'-6 o'all. I'll glass the stringers.

    The 40" space will have a 2" rigid foam fiberglassed in place cooler. That should add some strength to that largest space.

    I know I'm adding undue weight to accommodate the spacing.

    Your input, as usual, is greatly appreciated!

    [​IMG]

    [​IMG]
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.