Aluminum "Shoal Sailer"

Discussion in 'Sailboats' started by KVA, Aug 15, 2020.

  1. bajansailor
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    bajansailor Marine Surveyor

    I think that you are trying to find problems for solutions Kyle, and making life unnecessarily complicated.
    No matter what you do, you will have an assymetric waterline when heeled to 15 degrees - unless you have a perfectly symmetrical hull shape fore and aft, like a perfect peapod.
    If you CAD program can plot your waterline at 15 degrees of heel, then you can see what it looks like, but I wouldn't get too worked up about it, so long as it is not outrageously assymetric (eg like a broad stern Open 60 racing machine)
     
  2. Rumars
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    Rumars Senior Member

    Kyle, why that specific hull shape? It does not do anything for you and your SOR. It's not the IOR fashion show, and cods head mackerel tail was a stupid theory. You need a wide stern, with the lines carried aft flat and little rocker. That will give you good form stability and volume to carry the weight. Don't be afraid of an immersed transom, this is a cruising boat the penalty in drag is not significant for you. As for asymmetric lines when heeled, who cares? You have a centreboard and roller furler to play with and balance the boat. Your breadth limitation because of trailering makes sure the boat can not get that "outrageous asymmetry" bajansailor mentions.
    Keep in mind this is a lake boat. You don't need high freeboard, 30-50cm is enough. I linked you the Lucas boats so that you can see how to do a sprayrail to keep the foredeck dry. If you want standing headroom in the cabin, you only need it at anchor, not when sailing. A poptop with gas shocks and it's a matter of seconds to lift the lid and have full space.

    Try redesigning the hull so that it has a straight stem (90°), full width stern, waterline lenght equal to LOD, low freeboard, moderate V forward transitioning into a U shaped underbody, and maybe a sprayrail.
     
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  3. gonzo
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    gonzo Senior Member

    I think that developable surfaces are becoming almost a fetish among boat designs. It is more important to have a good shape whether it will develop into a flat surface or not. Any shipwright worth his salt will make the plate, plywood or whatever, fit properly. At the end, the skin on the hull is not such a huge part of the cost. If fitting plates on some areas increases the total cost of plating by 40%, it will be worth the expense to have a better boat.
     
  4. TANSL
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    TANSL Senior Member

    No part is "a huge part of the cost" but if costs are not reduced in each of the parts of the boat, the total cost can increase disproportionately. Cost control is what makes a business profitable.
     
  5. DCockey
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    DCockey Senior Member

    Generally agree, but for a boat being built from metal and if fairness is important at least close to developable can be a signficant advantage. The tricky part is creating a good shape which is close enough to developable.
     
  6. A II
    Joined: Jun 2020
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    A II no senior member → youtu.be/oNjQXmoxiQ8 → I wish

    Re post #25: The forward canted rudders are called ‘‘fishing’’ rudders and are sliding upward at their set angle, like on Yorkshire Cobles.

    [​IMG]
    drawing by Hans Vandersmissen, info about it on the thread Real Ocean News in post #127.

    [​IMG]
    Pinterest
    ‘‘ A Coble from the NE coast of England. A very deep forefoot and a huge plank rudder* give lateral resistance. The rake of the keel line is said to mirror the slope of the launching beach. They are launched bow first. Broaching by tripping over the forefoot is a 'feature' of the type. ’’

    * underside forward canted ‘‘fishing’’ rudder
     
    Last edited: Aug 18, 2020
  7. KyleA.
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    KyleA. Junior Member

    Yeah, nothing is designed yet.. the previous posts were just introducing some of the tools I'm developing for fun. Probably there are much better and even free ones available online, no problem.

    With the tools in place, can try all of the suggestions so far, that will be fun and hopefully interesting if not at least infuriating.

    Part of the idea with the angled stem was just to be able to overhang the shoreline a bit, as many areas have shallow sloped rocks.. just stepping out of a normal 16' open aluminum fishing boat onto them often leads to a wet foot. That's all, straight stem will be good to try too, perhaps with a bit of a bowsprit for the reasons above (pardon me if this is the wrong term, plank extending forwards in function).

    As for finding problems for solutions, yes definitely !
    The reason specifically I am interested in analyzing the submerged geometry at different orientations is that a friend of a friend of mine has made a fortune optimizing passenger jet aero augmentation (winglets, blisters, etc.) using derivatives of area change.. simply I want to see how that looks here, plotted. Whether it makes any difference or is good or bad is not important, let's see how it looks why not? Indeed a sailboat is not the same as a jetliner, it may be a waste of time and mean nothing, etc. want to try.

    The pop-top idea is very interesting, thank you I never thought about it!

    About the developable surfaces, I mean, if they are getting laser cut to net shape it is important that they will fit with minimal effort and ideally no plastic forming or trimming etc.. I am not a shipwright worth his salt, but will do my best.. specifically to require only as much bending and twisting as is already permitted within floppiness in the flat panels. As for ensuring that they are developable, between passing the cad test of being able to flatten them (not strict I realize), measuring edge lengths, hopefully no numerical methods, and maybe some scale modeling, I think there's a chance. Plus a quick glance online reveals that DCockey here is well versed in the subject (Verifying developable surfaces https://discourse.mcneel.com/t/verifying-developable-surfaces/73594/14), what luck that's awesome! I spend a lot of time making cnc programs for turbomachinery blades, which someone has always designed with ruled surfaces.. this is a disaster, as it does not consider the curvature of the tooling required to generate them in real life.. making these into 'flankable' geometry which exactly fits the tooling is a very nuanced task to say the least.. just saying, I appreciate, am marginally aware of, and am very interested in the finer details of geometry of all kinds and am looking forward to this part.

    The "SOR" I made was incomplete and off the cuff.. for example, how many berths or passengers ? who knows probably two couples or one with two kids sort of thing, or single-handed... not going to court over anything I wrote there, let's not fuss over it... 'shallow draft lake sailer' good enough.

    Please don't stress too much over this, I am doing it for our mutual amusement and would like to enjoy the process, whatever form it takes and however long and winding.

    Thank you gonzo for the input on yaw. What would you expect for steady-state yaw on a relatively smooth water ? It depends on leeway obviously, is 10 deg still reasonable ? We can run through a range of values.

    Thank you all again for your input, you are under no obligation yet take the time, I appreciate it. Take care!
     
  8. A II
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    A II no senior member → youtu.be/oNjQXmoxiQ8 → I wish

    On the thread ‘‘A Small Skiff for Coastal Florida’’ in post #57 I've posted an example of a 15¾' Skiff where, as per the designer's info, bare hull and deck weight for welded sheet aluminium turns out 27.8 % heavier than plywood.

    My guess is this wouldn't be much different for a 25½' Bolger Black Skimmer with cabin conversion from plywood to welded sheet aluminium.
     
    Last edited: Aug 18, 2020
  9. Dolfiman
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    Dolfiman Senior Member

    If a flat sole bottom is your solution (which I share) and rock grounding your daily bread, I suggest you this welding alternative which avoid exposing the weld bead to abrasion. That solution can suit with a sole bottom thicker than usually, the extra weight can be partly deducted from the ballast to be expected for the stability. The continuous outer weld bead does not need to be ground, just slightly round the flat bottom end with a grinding wheel without risking weakening the weld. Interior side, some discontinuous welding lines. The rigidity of the whole will be reinforced.
    Welding alternative.jpg
     
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  10. A II
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    A II no senior member → youtu.be/oNjQXmoxiQ8 → I wish

    Dolfiman, great piece of construction engineering, thanks !

    One question about the posted attachment, do you mean by ‘‘discontinue welding’’ in the top drawing, the inside only needs or must be spot welded ?

    P.S. - Sorry, it was my bad reading, just saw the answer was already given in the last two sentences of post #39, thanks !
     
    Last edited: Aug 20, 2020
  11. Dolfiman
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    Dolfiman Senior Member

    My technical english may be not appropriate : for the inside, I want to mean "dashes of weld" instead of "dots of weld" used to prepare two pieces before the welding process.
     
  12. A II
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    A II no senior member → youtu.be/oNjQXmoxiQ8 → I wish

    It was my reading of the post that was not appropriate, sorry for that. I've added a P.S. about it to my previous post. Far as I can judge your technical english is perfect !
     
  13. KyleA.
    Joined: May 2018
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    KyleA. Junior Member

    Dolfiman, thank you for sharing that! Indeed it looks like a very good way to protect the weld bead and achieve a strong joint.
    It also makes fitting them together a bit easier if the the edges didn't quite line up (the overlap can be trimmed down).

    I think that with laser cut panels, it will make sense to include as many interlocking tabs as can be mustered. This should cut down on fixturing and hold things in place a bit easier during welding.

    In terms of protecting the welds, I was thinking (based on some previous suggestions as well) that chine ribs (term?) could serve to protect the welds and help align the hull panels (themselves fitting into slotted bulkheads).

    This way, if you get the tabs in the slots, the thing is probably pretty straight..

    Here's a rough sketch, same orientation as the cross section you drew:
    sketch.jpg

    One other thing, about the developable surfaces - an easy way to guarantee that they are indeed developable might be to use patches cut from/defined by the surfaces of cones or cylinders only... combined as necessary aligning the rulings.

    Thinking about it, if you start from a rectangular panel wrapped around a cylinder with it's ends meeting (rolled to a tube), and then try to pull one of the seam ends upwards, I'm pretty sure it will want to take the general form of a cone.

    Being able to wrap paper or sheet metal around these shapes easily enough leads me to trust the idea.

    Alternately, the constraint of not being able to stretch, compress, or shear the surface (thickness ignored), evaluation could be done by FEA. with a 2d mesh. flattening/forming the mesh should not lead to any stresses / high spots... alternately with a 3d mesh including thickness, stresses below the elastic limit, or if above then following the rulings indicating a simple bend. This would not be very helpful for making something, rather only as a final check.

    Presently in my hull modeler, start and end lines normal to a guide spline are swept along that spline (but may be at different angles relative to each other, may be bad.); I had not considered it further up until now, thank you !
     
  14. sharpii2
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    sharpii2 Senior Member

    I think you should be giving what happens if you run aground more thought. If it is a calm day, it may not be so bad. But what if it's rough.

    With a wide, flat bottom of thick plating, your more likely to be ground against the rocks right
    Side up. With a "V" bottom, or a narrow, flat bottom, your boat is likely to flop over and get additional damage at the topside/ bottom joint, which would likely put a hole in your boat, or at least make it far more difficult to repair.

    Higher sides are bad for windage, but give more internal volume.
     

  15. TANSL
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    TANSL Senior Member

    What I would do in the case shown in the picture of post # 43.
     

    Attached Files:

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