Dory in aluminium

Discussion in 'Boat Design' started by pafurijaz, Mar 14, 2018.

  1. Ad Hoc
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    Ad Hoc Naval Architect

    A neat little summary for those that do not know the difference. Nice find :)
     
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  2. TANSL
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    TANSL Senior Member

    That is a false polemic. The polemic in this thread is the name given to a certain plate :)
     
  3. messabout
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    messabout Senior Member

    The OP found the link to a technical explanation which will suffice. Some of us are well aware that the plate in question is actually an anti ventilation device. Nonetheless, that plate is commonly, even if incorrectly, called a cavitation plate. The semantics are really not important. What is important is that we help the OP as best we can, and leave the nit picking aside.

    The link also explains that a hook in the bottom may cause problems, some of which could be the initial cause of the.... choose your poison, cavitation/ventilation. Now let us tell Pafurijaz what a hook is. ............No not that thing on the end of the fishing line............

    A hook is a curvature in the bottom that can be caused by skin flexure or even built in curvature. To describe a hook, imagine that the bottom surface curves slightly upward and then returns to the designed fore and aft line at some point near the transom. You might think of it as a belly that protrudes upward toward the inside of the boat. On some boats only a tiny amount of "hook" will alter performance. On other boats it does not seem to matter very much. In fact some builders deliberately put a hook in the bottom. In general it is not recommended but it is used to advantage by some of those crazy Alaskan river racers in their purpose build race boats.

    Since this boat is to be fabricated from aluminum sheet metal, it will be an easy mistake to allow a hook to be generated. It can be caused by welding where there is expansion and or contraction of the parent metal while the welding operation takes place. But there are other causes. The builder may make a perfectly straight bottom but when it is exposed to dynamic pressures when planing, the skin may deflect upwards some amount. The obvious remedy is to use some internal or external stringers in the bottom so as to stiffen the bottom at the most vulnerable places. That would be in the afterplane. The afterplane is the area where the the boat most severely encounters the onrushing water while in planing mode. That is usually pretty far aft, at or near the transom. This is just one more admonition to the OP. He has taken the time and trouble to research elsewhere and also to post questions on this forum. It is clear enough that he is a conscientious builder who would like not to make any innocent mistakes...........

    Here is a cheerful salute to you Pafurijaz.
     
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  4. pafurijaz
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    pafurijaz Junior Member

    You are really helping me a lot, with these considerations full of technique, I am very grateful, tonight and weekend I try to reason on the reinforcements to avoid hooks due to the work, I will also check the sheets developed to avoid tensions due to the curvature of the boat .


    The guy, wants to mount a 25hp engine, you say it would be great with a 4HP engine. Considering the fact that overpowering too can be dangerous, I can make changes to the hull for use a 25hp engine? Maybe some kind of chines as you can see in this photo of this forum?
    [​IMG]

    Thanks to all you for the help
     
  5. TANSL
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    TANSL Senior Member

    Your first design could be suitable for 4 hp. The current one will need, perhaps, more power. But that is talking to talk because without knowing what speed you want to reach, or what you want to do with the boat, it is useless to talk about the right hp.
     
  6. pafurijaz
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    pafurijaz Junior Member

    OK, this boat for what I know is for a fishing, but my concern now is for safety, but if you tell me ,with this new shape, the problem does not exist, I continue with the design of the pieces.
     
  7. TANSL
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    TANSL Senior Member

    If you want me to do some concrete calculation, I will be happy to do it, but I would need to have access to the 3D model of your boat or, at least, to her body lines plan and SOR.
     
  8. pafurijaz
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    pafurijaz Junior Member

    Maybe, too kind, you can find the link of the models on this page, if you need something specific tell me that I try to do it ..
     
  9. Angélique
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    Angélique aka Angel (only by name)

  10. TANSL
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    TANSL Senior Member

    Well, with this I can calculate hydrostatic, cross curves of stability and little else, and with not much precision, but I will.
     
  11. Angélique
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    Angélique aka Angel (only by name)

    See also the attached files in post #14, I'll think.
     
  12. gonzo
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    gonzo Senior Member

    My comment on the HP was related to the first design you posted that had a lot of rocker and a narrow stern. This design can definitely use more power. If that is about 15 feet or so, 25HP does not seem overpowered. However, it will pound hard when it gets choppy.
     
  13. TANSL
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    TANSL Senior Member

    @pafurijaz , this is all that I have been able to calculate with the available data. Now you should calculate the weights and centers of gravity of the various elements and state the worst-case load condition to calculate the stability of the boat in that case.
     

    Attached Files:

  14. DCockey
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    DCockey Senior Member

    Link to the formula for maximum "safe" horsepower rating for recreational boats sold in the US. Applies to monohulls under 20 feet in length. Homebuilt boats do not have to meet these rules. This formula gives the maximum power a manufacturer can rate the boat for; manufacturer can rate the boat for lower power.
     

  15. Angélique
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    Angélique aka Angel (only by name)

    I was looking over Jeff Spira's website, and the free PDF study plans of the boats there are really interesting to study....

    ...and, to compare them for their differences suited for their own purpose.....

    Eg from the Midwestern Style Drift Boats section; the 16' Yukon

    [​IMG]

    [​IMG]

    And eg from the V-Bottom Dories section; the 19' Hatteras
    [​IMG]

    [​IMG]

    blog post: July 22, 2010
    [​IMG]

    P.S. - The free PDF study plans are presented at the bottom of each of the above linked plan pages...

    [​IMG] - - [​IMG]
     
    Last edited: Mar 26, 2018
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