Foam dinghy or tender plans wanted

Discussion in 'Boat Design' started by fallguy, Oct 7, 2018.

  1. fallguy
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    fallguy Senior Member

    Looking for a deck stowable dinghy or tender plan in foam sandwich, 8' max length or nestable, seating for four as a rescue only; three seats with rowing preferred. Nesting and seats oxymoron?

    Google came up empty. Can I convert a tortured ply version if I single skin the foam panels? How does 4mm or 6mm ply convert to foam?

    The boat should be towable to 23 knots.

    Going for ultralight so I can pull it up off the water.

    I have 6mm and 12mm foam and the transom could be 3/4" 28# core, but really not planning a motor.
     
  2. Blueknarr
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    Blueknarr Senior Member

    Nesters can have seats. Some of the ones I've seen incorporate a seat into the union joint.

    Almost any design could be converted to foam sandwich. Switching from plywood sheets to foam sheets is frequently done. Partial skinny may not be required.

    I recall seeing a chart of lamination schedules and how they compare structurally to plywood recently. Unfortunately I don't recall the exact thread.

    It was debated that the thinner fiberglass on plywood would result in overall lighter craft than thicker skinned foam.

    Good luck

    Found this thread:
    Plywood/foam equivalency?
     
    Last edited: Oct 8, 2018
  3. bajansailor
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    bajansailor Marine Surveyor

    Would one of these Catspaw dinghies be suitable for what you have in mind?
    Catspaw | B&B Yacht Designs https://bandbyachtdesigns.com/sail/catspaw/
    They are for plywood construction, but you could laminate sheets of fibreglass and use these instead of plywood(?)
    I built a two part dinghy this way, and it was still stiff / light enough without needing a foam core.

    Alternatively, the Chameleon is a very nice wee tender - I built a smaller (7'10") two part nesting version which works well (I designed it myself, using the plans shown in the link below as a basis to start from).
    Chameleon http://duckworksmagazine.com/04/s/designs/greene/cham/
    However I don't know if Danny Greene is still selling plans, and it doesn't look like Duckworks is selling them (?)

    There are various nesting dinghy designs available from Duckworks -
    Sectional boats http://www.duckworksbbs.com/category-s/382.htm

    I am just wondering why the unusual requirement of being able to be towed at 23 knots?
     
  4. fallguy
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    fallguy Senior Member

    Well, if I wanted to tow it around the Bahamas; I'd be able. This is a tender for a small power vessel.

    I would put it on a hard tow bar.

    I like the duckworks pollywog, but it has no vee; so it may struggle behind the boat or be totally fine, but a bit chancy.

    I spoke with b&b and they habe a nice 9' nesting pram with some vee n rocker that would track nicely I'd say. The plan is ply, but I might be able to convert it. I could vac bag a single side of the panels (inside) and torture then hand lam the outside in a single go.
     
  5. Chuck Losness
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    Chuck Losness Senior Member

    All kinds of nesting dinghy plans on the web to chose from.

    A plywood stitch & glue dinghy will be lighter than a fiberglass dinghy. The problem that I see with fiberglass is that you have to have fairly thick skins inside and out to keep from punching holes in it. And the foam has to be high density. The light weight low density foams will crush and delam from the fiberglass skins under normal use.
    Even as a rescue 4 adults won't fit in an 8' dinghy. 3 might fit if the dinghy has lots of free board. You should also think about whether the people you will be carrying are boaters or landers.
    You will need at least two rowing positions to keep the dinghy balanced depending on how many people are in the dinghy.
    Before you go to the time and expense of making a dinghy I would buy a sabot. They can be had for next to nothing or even free without all the sailing gear. Load it up for your worst case scenario and see if it will do what you want it to. Do your testing in nasty weather.

    I have been through a progression of dinghies. Started with deflatables. Then carried two deflatables and a sabot on summer cruises with my kids. The boys needed a dinghy and the girls needed a dinghy and I needed a dinghy. Next was a small rib. Then I made a 10' hard chine "v" bottomed plywood S&G dinghy based on Sam Devlin's guppy design. That was a great dinghy for boaters but not so good for landers. It would only carry 3 people. I didn't know what I was doing when I made it. Due to my construction mistakes it didn't hold up well. Weighted 90 lbs. P2020027.JPG I replaced it with a 10' fiberglass dinghy which has to weigh over 150 lbs and still needs more re-enforcement to the bottom.
    dinghy 003.jpg
    I am currently thinking about making another dinghy. Probably 9' S&G plywood and still debating whether to have even a small outboard. Or just oars. One problem with plywood and fiberglass dinghies is that they are not really designed to have an outboard hanging off the stern. I solved this problem on the dinghy that I made by moving the transom inboard by 12". Any dinghy that I make will have this feature.

    PB250001.JPG

    One benefit I discovered with the inboard transom was that without the outboard I could easily get into the dinghy from the water over the transom. I could stand back there without the transom going under water.

    Lots to think about.
     
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  6. fallguy
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    fallguy Senior Member

    What about 12mm bottom and 6mm sides and 17oz glass?

    Pretty light. I would build for a motor, but may only row.
     
  7. Chuck Losness
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    Chuck Losness Senior Member

    Just saw your post about using a tow bar to tow a dinghy. I met a guy in Mexico who had bolted a standard 2" trailer hitch to the bow of his aluminum skiff and had the ball on his swim step. Don't know how well it worked.
     
  8. Chuck Losness
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    Chuck Losness Senior Member

    The dinghy that I made was 1/4 plywood sides and bottom. It was scary fast with the 9.9 outboard. I replaced the 9,9 with an 8. It rowed well too.

    I first had one layer of 8 oz glass on the bottom and sides and just epoxy coating on the inside. Big mistake. The fiction traveling through the water at speed wore the thin fiberglass away on the trailing edges of the transom exposing the end grain of the plywood to water. This started rot which I was never able to truly cure. Also just having epoxy on the inside didn't hold up. Next boat will have 1708 biax on the bottom and outer sides. The inside will have 8 oz cloth.

    Another problem was the even the 8 oz cloth did not want to curve around the transom edge even after I put a radius on it. I have a couple of ideas on how to solve this problem.
     
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  9. fallguy
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    fallguy Senior Member

    The houseboats here use a piece of pvc wrapped with a noodle and a couple d hooks tr to bow eyes. The trailing rig follows in prop wash.
     
  10. fallguy
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    fallguy Senior Member

    I wrapped 16 layers of 17oz biax around the transoms, so it won't be an issue.

    Thanks for the glassing tips. If I use foam, 8oz inside will be too light I'd say.
     
  11. philSweet
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    philSweet Senior Member

    Um, you could build a several ton vessel with those scantlings. What you are asking for can be built with 4mm fir ply and a bit of paint. It would weigh about 80 pounds. I used some 6mm in my 9'6" dink, but mostly 4mm. As others have already mentioned, you want to minimize sandwhich construction in this size because it is heavier than single skin. But it does give you some damage tolerance if used judiciously. I used some 1/8 Divinycell 200 density scrap to increase the thickness at the motor clamp because the motor won't clamp to a 6mm transom. I also added 1" low density foam and a skin to part of the bottom of the boat to give a dry foot area and a place to put a grocery bag or laundry bag so it wouldn't get soaked by 1/2 a cup of water in the boat. It also provides enough positive buoyancy to float the engine and anchor and chain.

    Here's a link to some photo's where I replaced the foam in the bottom after several years of hard use.
    Laminating Polystyrene to Ply https://www.boatdesign.net/threads/laminating-polystyrene-to-ply.55053/#post765747
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    I'm tipping it got rusty, quickly.
     
  13. fallguy
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    fallguy Senior Member

    Okay Phil, so I already knew the 17oz was heavy for a dink, but I also know the weight of a 4x8 12mm panel is 17#. So a foam boat with 2' sides and a 4x8 bottom in 6mm foam is only going to be 32 pounds bottom and sides. Reduce the glass to 12oz and you'll save even more.

    I haven't done alll the numbers, but a vac bagged hull would be very light.
     
  14. fallguy
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    fallguy Senior Member

    Chuck-the white n blue boat is sweet, but way too heavy for me.
     

  15. Chuck Losness
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    Chuck Losness Senior Member

    The white and blue dinghy is an old glaspar 10 footer. One of my best friends had one as a kid with a small merc outboard. He grew up in Long Beach, Ca and would often take off for Catalina on a Friday evening after the wind died if his parents were taking their boat to Catalina for the weekend. He was in 5th/6th grade.
    The glaspar has one big problem. In crossing short, steep powerboat wakes the bow rides up and over the first wake and then dives into the second wake sometimes filling the forward section with water. Not a lot of buoyancy in the bow.
    The dark blue dinghy that I built far exceeded all of my expectations for a dinghy. It was light, fast, dry, and stable as long as you didn't step on the gunnel. You could easily get into the dinghy over the transom. It would punch through even the worst chop. Towed well and rowed well. It is based on Sam Devlin's guppy design. The forward half is as per the plans. For the back half I extended the midship section lines straight back creating a deep "V" hull. I might just build another one. Haven't decided yet.

    The problem with lightly skinned foam is it won't hold up to even light use. You can probably get away with a light skin on the outside but the inside will be another story. Make up a test panel and try walking on it. Try dropping a 5 gallon water or fuel jug on it. Or some other heavy object. 1/4 plywood weights around 22/23 lbs give or take for a 4x8 sheet. And a lot of the weight in a dinghy is in all of the internal structure and its attachment points. The transom will have to be plywood cored if you are going to use even a small outboard. The gunnels will need re enforcing also. When all is said and done your lightly skinned foam dinghy may only save you 10 to 15 lbs in weight.

    I'm playing the devil's advocate here. I thought long and hard about a foam cored fiberglass dinghy before building my plywood dinghy. In the end the lower cost and durability of the plywood dinghy won out.

    Weigh all the pros and cons and build what you think is best for your intended usage.
     
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