design of 11ft aluminum weld tender

Discussion in 'Boat Design' started by cehanman, Oct 31, 2014.

  1. cehanman
    Joined: Oct 2014
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    Location: victoria bc

    cehanman New Member

    So this project is real and apologies for the long speech. . I am replacing my 11 ft fiberglass dingy of many years ( whaler style) that tips up on the transom of my old wood Chris Craft 43 ft 12 ft beam.
    I want to replace the fibergrlass with aluminum ( I know tacky on an old Chris ) but there is a practical reason. For years I have had a hinge mechanism on the 25hp merc so that the motor comes off the transom for storage and dogs down in the middle of the boat. The dingy then tips up on the swim grid, motor inside. The engine does not hang off the transom, in this way nor does it pivot. It just ties down inside the dingy on the hull. Puts the weight more central and avoids the use of the seawise type system that clutters up the swim grid.
    I am not a fan of Inflateables with the air bags, loss of internal space and lack of regidity amongst other things.
    The preliminary design which is going to translate into a mock up for cutting panels is based on taking a relatively standard 10.5 Rhib like an AB , but with the use of floatation foam shaped along outer hulls as a D not an O to generate the stability.
    large versions of these exist of course with center consols. it is a given that there is little new under the sun. can anyone point me at some dingy designs that might be useful to consider. I have looked long and hard.
    The goal is to replicate the stability of the RHIB and lose the soft sides allow for the engine bracket to be fitted and the whole thing hinge on the transom. A set of davits effectively lengthens the boat by 6 ft. which has never been an option.

    any direction or thoughts appreciated. Ted.
  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Foam sides will eventually crush. You can simply design a hull with the total beam needed for stability and make air boxes on the sides for flotation reserves.
  3. TANSL
    Joined: Sep 2011
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    Location: Spain

    TANSL Senior Member

    Brilliant idea. What you propose is to replace inflatable rubber tubes (typical of a RIB) by rigid "enclosures" in aluminum.
    Excuse my question, interior or exterior to the hull?
  4. Kevin Morin
    Joined: May 2013
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    Location: Kenai, AK

    Kevin Morin Junior Member

    11foot welded boat

    cehanman, my remarks are about the welding part of the design of this size skiff/dinghy/tender.

    I've found lots of articles/posts/remarks online about the idea to scale down welded aluminum boats below the 16' x 1/8" size range. I like that idea myself, and have designed and built a few of these little(r) welded aluminum boats. So from that experience I'd like to offer a bit of caution as you move ahead.

    Welding aluminum is fairly straight forward between 0.125" (1/8" - 3mm) and 0.25" (1/4" - 6-6.5mm) in both MIG and TIG, but when you go below 1/8" things get harder not just smaller. To put that another way, I found when scaling down the weld's size to be proportional to thinner metal that I wanted to use in smaller boats; it was much more work and effort than to weld the larger skiffs made with heavier scantlings.

    So I purchased a MIG gun that could weld 0.023" wire, and practiced with that gun. I'd welded about 30-some years at the time, almost all in aluminum and thought it would be simple, but it was quite a job to tune all the details down to weld 0.100", 0.090" and 0.080" sheet that I wanted to use in the smaller skiffs. I could 'make it stick' but the contraction issues due to out or proportionally large welds- well the first few experiments turned out scrap that was used for future welding practice- not boats.

    So I figured that I'd try to TIG weld and that was OK but the work to weld that small with a two hand TIG set up was not simple either. I used my TIG gun, and that was the way I ended up doing the majority of the most recent thin skin, small boat builds. Even that (cold wire feed TIG welds of 1/6" to 5/645 bead face welds) is more fatiguing for me than a huge, relaxing 0.035" MIG wire bead on 1/8" to 1/8" sheet parts.

    I hope you'll see my remarks as just a caution: if you find a design for a tender and plan to make the scantlings proportional to the size? it can be a bit more than normal level of effort to weld those thinner hull panels.

    If you make the boat from 1/8" it will be very heavy for its size, and that is why, IMO- the smaller production boats are all pressed of thin material and riveted. I found it to be lots more work to scale welds down for proportional welds on thin aluminum materials than I'd have thought after building hundreds of metal boats using thicker material and proportionally heavier welds

    Good luck with your project, my remarks might be summarized as "When the boat is smaller and the metal thinner; then the welds become more and more difficult to perform."

    Kevin Morin
    Kenai, AK

  5. cehanman
    Joined: Oct 2014
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    Location: victoria bc

    cehanman New Member

    thanks for the comments, the foam is not 'dock foam' but the preformed material that is used on the larger hulls not sure of the material that does not crush but seems to be very resilient .
    I do not propose to 'encase ' it in aluminum. it is attached to the hull by glue and bolts but is chaff protected by a rubber outer band.
    I am not the welder although I do putter in mig and tig, . Luckily I have access to a real artist with a spool gun or tig and it is his marine welding shop which I hope will be able to make use of this design. If you take an AB hull with say a 17deg dead rise, lose the tubes and then flare the sides with extended aluminum the floatation is outside of that. 11 ft long say 5ft wide at the outside of the flotation which is 6in in depth.
    flotation tubes are approximately 15 in wide, no thanks. well thought I would ask , I am sure someone has done this before, but If I cannot find the short cut I will have a go and hopefully be able to report back. any further direction appreciated . thanks. t
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