Foam dinghy or tender plans wanted

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

  1. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Okay, so let's say the boat is ply bottom and the rest is foam. Best of both worlds. Durability of the ply underfoot and lightness of the foam.

    What glass schedule would you use?

    400g 45/45 biax outside
    Same inside or less?
     
  2. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    I recently repaired a standup paddle board for a good client.
    It was ridiculously under glassed.

    • 1/4 " textured neoprene
    • 4 oz twill in epoxy
    • 1/64" bamboo veneer
    • 4 oz twill
    • 4 inches Styrofoam
    • 4 oz twill
    Many holes in one month. The bamboo was delaminating. The foam was crushed with the neoprene pattern.

    Most people worry above beaching abrasion when it is the topside which takes the true beating
     
  3. Chuck Losness
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    Chuck Losness Senior Member

    Probably need to decide how you are going to strengthen the gunnels and attach the interior seating and structure first. That might impact the glass schedule. Also with plywood you will be using epoxy instead of polyester resin. I am somewhat guessing here. The last time I bought biax the store only had 1208. I had never seen it before. I think that it is sufficient for a dinghy. It was easy to work with. I would put 1208 or 1708 on the outside both the bottom and the sides. On the inside I would use 6 or 8 oz cloth on the plywood bottom and maybe 1208 biax or 10 or 12 oz cloth on the foam sides. I would probably do a test panel to see how easy it was to puncture the different types of cloth.

    My next dinghy will be plywood bottom and sides. I will use biax on the bottom and 6 or 8 cloth on the sides and on all of the interior surfaces. All of the interior structure will be tabbed in place with biax as will the chines and transom.

    One thing that hasn't been discussed is the seat arrangement. All of my previous dinghies have had a seat in the stern, one amidship and sometimes one in the bow. This worked ok for one or three people but not so good for two people. My next dinghy will have a seat in the stern and a long center seat that runs from around amidships to the bow. With this arrangement when there are two people in the dinghy the forward person can slide forward to balance the boat.

    Blueknarr that SUP sounds like it was built like a surfboard. I have a little knowledge about surfboards. My son traveled the world on the pro surf tour for over 10 years and was a three time national amateur champion in high school. Surfboards are basically disposable. My son would go through 5 to 10 a year. In order to work properly they have to be able to flex to carve turns on a wave. Typical glass schedule was 4 oz cloth, sometimes 6 oz, over the entire board with one or two 6 oz patches where the feet went on the top. The foam was pretty unique too. The blank had to be very close to the shape of the board. The outer skin of the blank was fairly high density while the interior was softer. If too much foam was removed in shaping the board it would be too soft and not work right. I digress.
     
  4. fallguy
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    fallguy Senior Member

    I am leaning to ultralight vac foam the whole thing, but needs to provide a way for four to be seated in it waiting rescue.

    If anyone has other designs that are suitable, let me know. I think nesting is also best.
     
  5. Chuck Losness
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    Chuck Losness Senior Member

    Are the four people all adults or two adults and two kids? If you ran a relatively narrow full length center seat you might be able to have two people sit on the bottom in the stern and the other two seating on the center seat. One person amidships and the other in the bow. You are also going to need a large dinghy with at least around 800 lbs displacement to safely hold four people. 100 lbs for the weight of the dinghy and 700 lbs for the people.

    If you find a design that you like but it is too small scale it up to the size that you need. Small increases in size can reap large increases in displacement. For example if you increase the size by 10% in all directions the displacement will increase by 33%. A 20% increase will increase displacement by 73%.
     
  6. Deering
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    Deering Senior Member

    If it’s going to be a two piece unit, would it be viable to manhandle each piece separately while taking it up/down from the roof and handle assembly down below? That would effectively halve the weight you had to lift and might change your weight limitations.

    You’re asking a lot of a dinghy to be both ultralight and durable enough to handle the daily abuse that your normal dinghy has to endure. If the main goal is to create a rescue pod for four people to bob around in, then the requirements become much simpler.
     
  7. Chuck Losness
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    Chuck Losness Senior Member

  8. fallguy
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    fallguy Senior Member

    Those are a bit long and not suited for four passengers.
     
  9. DKE
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    DKE New Member

  10. fallguy
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    fallguy Senior Member

  11. Chuck Losness
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    Chuck Losness Senior Member

    A friend of mine had a Portland Pudgy. It is a very small boat. Sadly he was lost at sea several years ago in the south pacific. His sailboat washed up on one of the islands but neither he or his Pudgy were ever found.
     
  12. fallguy
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    fallguy Senior Member

    I really like the Clark Craft 12' dinghy. It is cedar strip, but I think I could strip build in foam. It doesn't nest, but it has a skeg and could track well and it is ultralight and holds four people. I could probably just make a spot on the foredeck for it.

    The only downside is the bilge is round so lot longer build times. And to do in foam would need ribbands probably.
     
  13. Chuck Losness
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    Chuck Losness Senior Member

    The clark craft 12 is a nice looking dinghy. I think that it could be made in foam strips. When Hobie Alter was doing the R&D on the Hobie 33 he made the test boats using foam strip construction. He bent thin wall electrical tubing to shape for the molds and then used strands from woven roving to tie the foam planks to the molds. Glassed the outside then cut the strands to remove the hull from the molds and then glassed the inside. My foggy recollection is that he told me it took about a week to make a bare 33 hull. But then he had a crew working on it.
    Back in the late 70's early 80's Dennis Choate often used strips of balsa to make the plugs for his boat molds and even made some boats out of strips of 2" square balsa.
    Sounds like you are getting through the preliminaries and close to starting construction.
     
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  14. fallguy
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    fallguy Senior Member

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