Displacement Skiff

Discussion in 'Boat Design' started by retrosub, Jan 11, 2007.

  1. retrosub
    Joined: Aug 2006
    Posts: 52
    Likes: 2, Points: 0, Legacy Rep: 17
    Location: Ithaca, NY

    retrosub Junior Member

    I've been thinking about building a boat, but have yet to actually do it. I currently have a 14' inflatable that I use for diving, but there isn't much room inside and I don't particularly like the way it handles. I'd rather take a solid hit than bounce up and down, and even though it has an inflatable keel, it sort of skates around.

    I bought the plans for the OD18 because I like the way it looks, but I'm thinking that if I build a planing boat, it ought to have a vee. I had three 7-hour days in row in a panga to consider this.

    Recently I had a different idea. Why not build a boat that only travels at displacement speed? It's rare that I'll dive over 5 miles from shore, so if it takes me a little while to get there, I'll have a sandwich. If I built a displacement boat, it could probably be built lighter. I could get away with a flat bottom, which would be easier to build.

    So what about a stitch and glue dory style boat, say 5 feet on the bottom flaring to 7 feet wide and 18 feet long. Keep the bow pointy for cutting through waves. Is there any reason that wouldn't work for my purposes? It seems like it would be easy to make. I could just build the OD18, but I don't think I need a boat that heavily built if it's just going to putter around.

    Would it be better as a completely flat bottom boat, or should I put some curve in it from bow to stern? Would stringers be necessary? Would it make sense to tuck in the stern or leave it the same width the whole way back? I think I'd use a center thwart because it'll make sense to use that as a midform, would I need other frames? I'm thinking 1/2" ply for the bottom and 3/8" for the sides, but glassed on both sides for maintenance.

    Thoughts? Ideas?
     
  2. lewisboats
    Joined: Oct 2002
    Posts: 2,329
    Likes: 126, Points: 0, Legacy Rep: 1603
    Location: Iowa

    lewisboats Obsessed Member

    A displacement boat seeks to disturb the water as little as possible during its passage. Normal displacement boats have a near verticle stem in order to cut the water and provide for the longest waterline. The aft end normally sweeps up and inward somewhat to allow the water to flow back together as smoothly as possible. You would normally not want to narrow the aft end too much as you would loose too much buoyancy and tend to squat when approaching hull speed. Hull speed for a shape like this is approximately 1.35 times the square root of the waterline so...1.35* (root 16) = 1.35* 4= 5.4 kts or about 6 mph. I took the root of 16 as this would be the approximate waterline lenght with a vertical to near vertical stem and some transom overhang. Doing the Dory shape would cut your waterline length by a couple of feet and your speed along with it. Are you planning on using a tiller motor, if so you will need a seat aft to control it. If not then you could have a center consol, or even a small pilot house at or just forward of midships. 1/2 and 3/8 should be strong enough if some framing is added, especially to the bottom. 3-5 stringers on the bottom and say 5-6 frames ought to do it. There are plans for these types of boats out there...usually of the Semi-dory type but not exclusively.

    Steve
     
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