SUP/flats-skiff hybrid in foam

Discussion in 'Boat Design' started by FlyLight, Jan 1, 2017.

  1. FlyLight
    Joined: Sep 2015
    Posts: 1
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Hobart, Australia

    FlyLight New Member

    Hi everyone. Newbie here, so be gentle :)

    I have a lot of experience in designing building high-performance RC model gliders in foam/glass/carbon, and like the idea of building a small flats boat for sheltered water using the same construction.

    I have little boat design experience, but whether water or air, it all seems like fluid dynamics to me... I have a bit of experience sailing dinghies and cruising yachts.

    I can't find the boat I want in Australia, as not a lot of people fish the sort of water I want to fish. There are a few boats from the USA that suit, but would cost $5000AUD by the time I get one here. I can make one for about $700 in materials!

    I kind of want a hybrid between a 12ft sailing skiff, flats boat, SUP-board, kayak. Easy then! (????)

    My design brief is:
    -Light enough for me to lift onto rooftop alone (I weigh 62kg) and plane with a <4hp motor (so I don't need to register the boat) = <25kg (60lbs) (should be easy with intended building methods)

    -Fast enough on the plane to cover a few kilometres (10-15kts would be plenty)

    -Slick enough at rest to pole or paddle standing up, or use with oars and rowlocks.

    -Stable enough at rest to stand and cast a fly rod (with a thigh-height lean-bar).

    Max size 3.6m LOA x 1.2m beam (12ft x 3.8ft)

    I got hold of a big slab of 170mm (5") white insulation foam (EPS) and made a rough "concept boat" the right size but completely flat bottomed. I just covered it in packing tape to cut surface drag. Weight of hull was 10kg. It planed no problem with me and my 25kg son on board using a 2.5hp 4-stroke at less than half power (My knock-together timber transom mount wouldn't handle any higher power). Plenty stable even standing on the gunwhale. A bit slow to paddle with completely flat bottom, but was a bit better with weight a little forward and the hard-edge transom higher in the water.


    So I started playing with models. See pics.

    I intend to have a flat bottom foam slab (oversize surfboard) above the chine, and a thin plywood shallow-vee bottom below, with the back open. The idea is that the vee will flood at rest giving the stability of a flat-bottom punt, and empty out underway to give the stability and manoeuvrability of a Vee-hull in the intended maximum 1ft chop.

    The red lines on the deck show where stiffening vertical webs will be to bond the top and bottom surfaces (4mm ply)

    I have a few questions (for now...) :
    1. Am I way off target here?
    2. Will it be too light (I think total weight with just me on board will be 100kg (220lbs)). I'm worried it might drift all over at the front if it's too light or start bouncing up and down over waves.
    3. The sponsons at the rear are designed to only be in the water at rest to help support the motor. Will rounding their waterline edges help reduce drag when paddling?
    4. There are two hull-shapes shown, one has more volume forward. Any ideas about which is better?
    5. Will having the transom step just forward of the real transom help reduce drag, if the real transom in slightly rounded? Will the submerged hard edge have the same drag regardless?

    I look forward to any advice or input.
  2. sailhand
    Joined: Jan 2017
    Posts: 141
    Likes: 43, Points: 28, Legacy Rep: 29
    Location: australia

    sailhand Senior Member

    I have had a look at the solo skiff in the flesh and the hull is simply lightly rounded tapering to a sharp entry in the bow. The keel that you have made all the way through the models from bow to stern doesn't exist once you get about 700 mm from the bow. Redcliffe marine in qld sell them in australia check out for more info. Also they are fairly heavy which no doubt helps with their stability.
  3. KJL38
    Joined: Jun 2008
    Posts: 107
    Likes: 16, Points: 18, Legacy Rep: 75
    Location: Tasmania

    KJL38 Senior Member

    Your construction method is similar to the boats they race in Western Australia in the Avon Descent. You could ask the club if they can help with details.
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