Keel And Frame ?

Discussion in 'Metal Boat Building' started by MACINAW, Apr 14, 2005.

  1. MACINAW
    Joined: Apr 2005
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    MACINAW New Member

    on a aluminum boat is the keel line and the gunnell parallel or is it common to have a keel line that is deeper in the bow section of the boat?I have noticed that most glass boats are deeper in the bows and alloy boats seem to have level keel lines is there a reason for this?
    also other than the "hungry horse" look is there any structural reasons to not weld the frames to the plate just to the longs?
    I have looked at several aluminum boats and there construction and some of them are claiming to have "box girder" construction.It looks to me like the only differance in "box girders" is they made the longs as tall as the frames.Is there any differance in strength other than having more support under the floor?
    I am new to aluminum boats and there construction and am trying to learn all i can before i decide to tackle building one and have learned a ton just reading on this site any help would be great.
     
  2. Arrowmarine
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    Arrowmarine Senior Member

    Fiberglass can be molded in any shape you want and therefore has more potential to be precisely engineered. So you can cut a little from the side height in the aft, narrow the bottom and make a more streamlined shape. You can add material or take away to change the attitude of the boat at rest, so that it will launch correctly. Having the aft narrower causes it to float deeper, nose higher. This causes the bow to come up at launch, then as it starts planing, the larger, heavier midships section brings the nose back level.(note:my speculations are just examples. I refuse to call myself an engineer! :) Anyway, this is much harder to achieve with aluminum and even harder to reproduce accurately over and over and over. So you dont see that level of engineering on most aluminum hulls. I call it the "pointed shoebox syndrome"The Fiberglass Boat industry is a perfect world were everything is the same until they change the mold. Ask any rigger who has spent a lot of time with both. Aluminum boats are very inconsistent(except of course mine and Thunderhead19's'. Right J.D?) :) Im just playin. If you want to make your sheer line lower in the aft then go for it. Its all aesthetic. You wont hurt a thing. I like that look myself. My best suggestion is to look at other manufacturers basic hull dimensions and use them as a starting point. Stick to what works, and then make it look like you want. Does that make sense? If not say so and I'll try again:)
    I'm a big fan of box girder constuction over traditional angle stringers. They have much more torsional(twist) stability, and they very seldom fail. They have many other advantages as well that i wont get into.
    In my opinion, you should attach the frames to the bottom plate, at least the midship frame, for torsional stability.
    If you are not already proficient in aluminum fab and welding, I suggest you put your focus there. When you are ready, I will help any way I can. Just ask.
    Good luck! Peace, Joey
     
  3. Eric Sponberg
    Joined: Dec 2001
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    Eric Sponberg Senior Member

    Macinaw,

    The reason you may see boxy-like boats built of aluminum is because of construction simplicity. Aluminum comes in flat plates with parallel edges, so builders make use of them with the minimum amount of cutting. There is no reason, however, why you cannot have a more elegantly shaped hull with aluminum, as you might find in fiberglass boats. It just requires more fitting, cutting, and labor.

    As for welding the frames, the general engineering concept is as follows:

    The "panels" of hull or deck plating are assumed to be supported on four edges. That is, most panels, a section of plating defined by its frame boundaries, are typically rectangular or nearly rectangular, and they are stiffest and strongest if all four edges are "fixed", that is, welded. In a longitudinally framed boat, the long edges of the panel are supported by the longitudinal frames, and the short edges are supported by the transverse frames.

    The transverse frames have a second duty to support the longitudinal frames. A longitudinal frame is stronger and stiffer if it is welded to the crossing at the transverse frame. A third duty of the transverse frames is to give overall hull/deck strength and rigidity in hull cross section. Because of these extra tasks, the transverse frames should be bigger (taller) than the longitudinal frames. Scantling rules such as ABS require that in yachts and small craft, the transverse frames must be at least twice as tall as the longitudinal frames.

    The same philosophy applies with the longitudinal girders. You will find these in the lower sections of the hull, and should see them in the deck. Their role is to support the transverse frames and provide "hull girder" strength and stiffness. That is, the hull and deck structure is really a free-floating box beam, and the girders and plating withstand the natural bending (hogging and sagging) of the hull girder. Because of this dual role, at least according to the scantling rules, the longitudinal girders in yachts have to be twice as tall as the transverse frames, just as the transverse frames are twice as tall as the longitudinals.

    In commercial craft, this ratio of 2/1 increases to at least 2.5/1 or greater, in order to meet classification society scantling rules.

    When you see transverse frames and longitudinal frames being the same height, there are a few possible reasons for this: the designer and/or builder does not know what they are doing; the designer and/or builder are not desiring to follow scantling rules (which is OK); or they are applying grillage (all internal structures the same height, by definition) principles to the engineering. This is not unheard of, but it is not common, and usually requires extra engineering analysis for classification society approval, which commercial craft are usually required to have, not necessarily so for yachts and small craft.

    I had the opportunity to review the structure of the aluminum yacht Imagine which was going to be sailed by Mike Carr in the 1994 Around Alone Race. Her longitudinally frames were welded to the hull and deck plating, but the transverse frames were deliberately left about one inch (25 mm) above the plating, not welded thereto. Imagine suffered terrible damage on a voyage from Norfolk, VA bound for New England when she got pounded in the Gulf Stream. The hull plating was dented in so severely that it contacted the transverse frames and buckled a few of them very badly. There was a lot of controversy about this at the time, which I won't get into. I relate this only to say that I think it is a real risk to NOT weld all the frames to the plating.

    On my Moloka'i Strait motoryachts, we have taken great pains to follow the classification society guidelines and use a 2/1 ratio of framing heights throughout the vessels.

    Eric
     
  4. Arrowmarine
    Joined: Jul 2004
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    Arrowmarine Senior Member

    Yeah, what He said:)
    I love when the yacht guys turn loose of some valuble knowledge. Just a note: The boats common to the Pacific Northwest have only 2 longitudinal girders, 2 side stringers and only 2 transverse frames, one at frame 4-1/2 welded to the bottom and one at about frame 2-2/3 welded to the bottom and about half the way up the side. This is the standard , and it commonly leaves an 8' to 12' section from the transom forward supported only by 2 long. girders on the bottom, and 1 longitudinal(stringer) on each side plate. No transverse frames in this section at all. We're not used to the traditional configuration of several longs and a transverse frame every 2 or 3 feet or so.(i will try to post a drawing of the basic configuration we use in the near future) We build the longs and trans. to be the same height to ease the installation of large flat floorboards. Also, most of these boats got to were they are now by trial and error, not engineering:) In fact, other than Alumaweld, I dont know of any major manufacturer using an actual Naval Architect. Thus there's a tendency to be ugly, heavy and overbuilt compared to the rest of the boating world. I myself am trying to get away from this trend, but it's difficult due to the labor/equipment required to shape aluminum into something that's flowing, smooth and appealing. Not cost effective. Just when I think I got it going on, I am humbled by the knowledge and experience of guys like Eric Sponberg and it only inspires me to keep improving at doing the things I love to do, designing and building aluminum boats.
    Anyway, Just my thoughts. Thanks Eric, for sharing.
    Peace, Joey
     
  5. CDBarry
    Joined: Nov 2002
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    CDBarry Senior Member

    The actual rule is that the frame has to be twice the height of the cut out that passes the long thru. If the long is intercostal (there are tonnage reasons for this) or sufficiently clipped (both sides) to the web, (which is often required in Al to get enough shear connection) then the depth isn't controlled by this, though generally 2:1 or better ends up being required for other reasons, like making the required SM.

    It is also woth noting that floting transverse frames put a real hot spot on the longs at the connection, and tend to result is nuisance cracks.

    As to hungry horse, the top ten rules for avoiding welding distortion are:

    1) Don't overweld

    2) Don't overweld

    3) Don't overweld

    and so on.

    More weld is not necessarily better - make sure you calculate and size all the welds, put them on the drawing, and make sure all the welders have gages and know how to use them.
     

  6. MikeJohns
    Joined: Aug 2004
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    MikeJohns Senior Member



    Sounds like the scantling rules were requiring the transverse frame section modulus of a plating welded to the frame and the designer side stepped that bit in favour of fairer plating without recalculating the frames.

    Too many designers do not fully understand engineering concepts.
    I have been involved with a similar fiasco but in a work boat .

    Cheers
     
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