Skin-on-frame scow

Discussion in 'Boat Design' started by hospadar, Feb 21, 2012.

  1. hospadar
    Joined: Apr 2011
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    Location: Michigan

    hospadar Junior Member

    After building a skin-on-framke kayak, I'd like to try building my next boat skin-on-frame. I enjoy the construction and it's nice to not have to use any epoxy. I'd like to build a scow-like boat somewhere in the 12-foot range.

    I'm wondering what extra measures I need to take to keep the bottom from buckling in since it will have a fairly flat hull section. All the skin-on-frames I've seen are either kayaks or have a decidedly v-shaped hull section.

    I'm thinking right now:

    - 6-8" rocker
    - Replace one or two ribs with rigid frames made from 2-by
    - Extra thick keel and stringers on the bottom (1" or 1.5" square). Possibly laminated.
    - Possibly extra stringers on the bottom

    Does this sound like enough to keep the bottom from buckling in? Or possibly it's overkill? I don't want to go way overboard as I'd of course like to keep the weight to a minimum.

    The general plan is:
    ~12' LOA
    ~4' max beam
    Vertical solid wood/plywood transom
    Pram bow of solid wood/plywood (possibly framed 1/4 inch)
    6"-8" rocker
    6"-8" radius chines
    Sides bowed perhaps 6"-12" from max beam to bow/stern
    Sides tilted outwards ~15 degrees or so
    ~16" from deepest part of the hull to gunwhale

    I realize there are plenty of plans for pointy skin-on-frame rowboats around this size, but I'd prefer something a little more square so I can pack more people into for partying, and I'm in the mood to experiment.
     
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  2. Petros
    Joined: Oct 2007
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    Petros Senior Member

    I have built a number of skin on frame hulls, from kayaks to small sailboats. Your plan is good except for one thing, you can not have large flat panels with skin on frame; the fabric bows inward and will contact the frame members. It will float okay it is not a very efficient shape.

    You can go with the flat bow and stern using plywood as you suggest, but I would make a round bilge dingy or row boat. You have to have the stringers no further than about 6 inches apart and have no hollows or even large flat sections. The fabric must bend an angle over each stringer. You can go with larger stringer spacing but the frame has to be much heavier to take the stress of a much heavy fabric than you would normally use on a kayak.

    I built a 14' sailboat using 1/4" x 1" white oak frames steam bent every six inches, with a heavy built-up frame about every 24". All of the stringers were 3/4" square doug-fir, with a 3/4 x 1-1/2" gunwales. All stringers and frames lashed together at each joint, with screws at the built-up frames. Stringers and keel were screwed to the plywood transom using triangular blocks at each joint.

    Here is what the frame looks like, feel free to copy the construction method. You can put a front transom where I have the mast step and daggar board box (I do not have plans, I made it up as I went along). Sails great;

    [​IMG]
     
  3. mydauphin
    Joined: Apr 2007
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    mydauphin Senior Member

    You can always epoxy the skin on...
     
  4. hospadar
    Joined: Apr 2011
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    hospadar Junior Member

    That's almost exactly what I was thinking, thanks for the photos!

    Couple questions:
    -Do you flare the gunwales out or are they just vertical?
    -Did you mortise the ribs into the gunwale? I'm thinking to just use an inwale instead of cutting the mortises. The thick frames would either be thinned at the top or slit on the end to accept the inwale
    -I assume you stapled the skin on? I'm planning on just stapling to the gunwales/transom/bow and maybe putting rubstrips over the staples
    -I presume you build the heavy frames, put the stringers and gunwales on, then bend in the ribs?

    I'm thinking I'll probably use some kind of shoal keel or bilge keels as opposed to the daggerboard chest, but other than that it'll probably be just about the same construction.

    Do you do anything special for the oarlock sockets? I'm thinking to just thicken up the inwale a little and use a side-mount socket.
     
  5. hospadar
    Joined: Apr 2011
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    hospadar Junior Member

    I'm also wondering if the heavy built-up frames are even necesary, I've been doing some more research and I've seen a number of skin-on-frames with more or less 4' beam that appear to have very slight deadrise/lateral curvature between the bilges.

    For example, some of the aerolite boats like this one:
    http://gaboats.com/graphics/westportdinghy_445.jpg

    I'm wondering if using slightly thicker ribs (3/8 let's say) would allow me to forgo using the rigid frames. If I went without the rigid frames I'd be able to more easily lash in floorboards and I imagine the interior would be a little cleaner in general. Also I could put in the diagonal airolite-style lashings to add some extra stiffness (although that wouldn't help with floor buckling, which is my main concern).

    Perhaps I can get away with just one frame in the middle? Or two frames 1/3 of the way from the stern?

    Also I read somewhere that for aluminum johnboats, most of the buckle resistance while planing comes from the stringers (not the framing).

    Luke
     
  6. Outlaw45
    Joined: Jan 2012
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    Outlaw45 Senior Member

    Luke, I would suggest you get the book UMIAK by Skip Snaith. it has a great understanding of how to build a SOF boat. I am currently in the planning stages of a 24ft modified power Umiak using polyester skin. am now trying to figure out a cabin for it to sleep and stay dry in. what fun.lol. good luck on your build.

    Outlaw
     
  7. Outlaw45
    Joined: Jan 2012
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    Outlaw45 Senior Member

    petros, do you have any pic's of that sail boat in it's finnished stage. would be fun to see what it looks like. thanks.

    Outlaw
     
  8. hospadar
    Joined: Apr 2011
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    hospadar Junior Member

    @outlaw: $250 on amazon! yikes! Looks handy though - want to send me a copy? ;)

    I'm thinking I'll just go at it without any hard frames, if the boat ends up having a tendency to buckle I see no reason I can't add frames/extra stringers in after the fact. Possibly running extra stringers on the inside of the ribs along the floor if I need more floor stiffness (similar to the doubling of the gunwale, I've also seen this one stringer down from the gunwale to provide extra support for thwarts and seats).

    It might even be better to lash the hard frames on after the fact anyways, since I can strap the floorboards directly to the ribs, then put the hard frames on top of the floorboards. Floorboards lay flat and I still can have hard frames.
     
  9. cor
    Joined: May 2008
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    cor Senior Member

  10. Petros
    Joined: Oct 2007
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    Location: Arlington, WA-USA

    Petros Senior Member

    You wrote:

    >That's almost exactly what I was thinking, thanks for the photos!


    >-Do you flare the gunwales out or are they just vertical?

    the flare of the gunwales determines the amount of shear or up-turn of the gunwale, vertical gunwales give you zero shear, the more flared out gives you more shear. I think I used zero flair, on kayaks I have built I have found that 15 deg flair gives a good amount of shear.


    >-Did you mortise the ribs into the gunwale? I'm thinking to just use an inwale instead of cutting the mortises. The thick frames would either be thinned at the top or slit on the end to accept the inwale

    I did mortise the ribs into the gunwale like you do on a kayak, using wood dowles to hole them in place. I have thought of also using screws with an inwale, but that adds more weight.


    >-I assume you stapled the skin on? I'm planning on just stapling to the gunwales/transom/bow and maybe putting rubstrips over the staples

    Yes, that is exactly what I did, the skin was wrapped over the deck and stapled to the inside edges of where the coaming would go. Than I screwed 1/4" wood coming over it to secure it. I have used this method on two man kayaks with an open cock pit.

    >-I presume you build the heavy frames, put the stringers and gunwales on, then bend in the ribs?

    I first build the heavier frames and than clamped on the stringers, and than used a plastic strip to determine rib length for each location. Cut the wood rib to exact length for each location and than boiled each end in a large turkey sized pan on a hot plate, working the ends alternatively into the approximate shape. than with it still wet and warm forced them into position and clamped/bungied into place. this results in a more fair hull and less trouble with fit (though sometimes I have to trim a bit off the end to make it fair). With the stringers in place over the heavy frames I have the hull shape defined better than if I just used light steam bent frames. I have found btw that boiling works much better than steam bending, fewer broken ribs.

    >Do you do anything special for the oarlock sockets? I'm thinking to just thicken up the inwale a little and use a side-mount socket.

    Yes, that would work fine. The key in design for this light of a hull is to spread the concentrated loads out over a larger area, the doubler to thicken a length of the gunwale is a good solution.

    I used the built-up frames to give me a better control over the shape since I wanted an open cockpit without thwarts. Unlike a typical kayak where you have deck beams to hold the gunwales to shape. If your design can tolerate thwarts than using all the light steam bent ribs would work fine. Note that I also used fairly heavy floor boards, 3/4" x 1-1/2" that I screwed to the heavier frames/floor beams. It was faster than lashing and gave more stiffness to the hull, plus it puts the floor a little higher off of the bottom skin to keep you dry from water splashing in and sloshing around in the bilge. I would use at least two heavy frames at about the one third points on along the length of the hull if you intend to put a sail and keel on it, consider that all of the force that drives the boat forward is pulling on the rigging and opposed by the keel.

    Since I had a fairly large sail and a spinnaker for it I was worried about the stiffness of the hull, especially around where the dagger board and mast step were located, so these frames are a bit heavier, and use a full width deck beam. The diagonal lashing would likely add some torsional stiffness as with the Aerolite hulls, which use much lighter fabric IIRC. The 10 oz polyester I used will also add torsional stiffness. Torsional stiffness has not been an issue, the most noticeable flex is when it rides up on a wave top and slams down into the next wave there is a small but noticeable amount of flex along the length of the hull. That is one of the advantages I think of skin-on-frame, it can be made very light and flex will not cause it to leak! The small amount of flex actually is more comfortable in slamming sea conditions, like a suspension on a car, very unlike a rigid hull. Not too much flex is good however, you could loose control of the boat if it is too soft, just like a car with a soft suspension on a rough road.

    Here is a picture of the completed boat on its maiden voyage at Port Townsend WA. That is my daughter and I in it, the Tyvek main sail worked great, the jib is down because it was cut badly (was in too much of a hurry to get it done) and actually made control difficult and it pointed higher with just the main. I eventually re-cut the jib, and also made a spinnaker. I also simplified the rigging (which is why were are looking down in the picture, trying sort out the mess of lines). With a fun boat, KISS is the rule.

    [​IMG]
     
  11. Outlaw45
    Joined: Jan 2012
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    Outlaw45 Senior Member

    WOW, way too much. keep searching, I saw it for way less then that.

    very nice lookin boat. thanks.

    Outlaw
     
  12. Collin
    Joined: Nov 2011
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    Collin Senior Member

    Flare doesn't work with open boats. To get sheer you have to either steam them or laminate them and depending on how heavy the stringers are, you have to over do it a and let them straighten out from the stringers and later the floor. Getting sheer in an open SOF boat is one of the hardest things about building SOF (it's not that hard tho) and the strangest.

    Mortising takes a long of time and means you have to have the rib lengths EXACTLY right or they won't work. an inwale system makes building a lot simpler and you can use any rib length you want-just clamp it in and trim it later.

    Using plywood frames is hard if you're just going at it as you build. Just use bent ribs. You may want to space them closer together if you're going to be carrying a lot of people. You shouldn't need to modify that much.

    Building with 3/8 vs 1/4 will increase the breakage rate substantially. If you get good wood and a good steam setup, it's fine---but 1/4 works fine and is easier to work with. If you use 1'' 1/2 wide ribs, just place them 4'' apart.

    In SOF, the floor will help stiffen the boat-which is easy to overlook for people that haven't built this way. You don't need as many ribs as long as you have decently thick floorboards to spread the weight.
     
  13. Outlaw45
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    Outlaw45 Senior Member

    hospader, whats happening with this?

    Outlaw
     
  14. ancient kayaker
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    ancient kayaker aka Terry Haines


  15. Outlaw45
    Joined: Jan 2012
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    Location: Olympia,WA

    Outlaw45 Senior Member

    there is alot of info and some great sof builders over there.

    Outlaw
     
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