25' Double Eagle aluminum build (placing stringers)

Discussion in 'Metal Boat Building' started by Northeaster, Mar 10, 2014.

  1. Northeaster
    Joined: Sep 2011
    Posts: 265
    Likes: 8, Points: 18, Legacy Rep: 45
    Location: Eastern Canada

    Northeaster Senior Member

    Hi Folks,

    Have most longitudinal stringers (3/16" x 1 1/2" x 20' 6061 flatbar) in place on hull bottom, and am now bending the ones on the topsides, which have more of a bend near the bow. In one pic you can see I broke one stringer ( it had been tacked/heated, then ground off, tacked again/reheated and it broke while bending a second time)
    I am having some trouble deciding how much to curve the stringers upwards (really downwards as boat is upside down) as I bend them in to meet the more narrow frames 7 and 8 near bow, and finally to touch stem. The plans say (on forward stations) to try to position the stringers / side stiffeners roughly evenly spaced between chine and sheer. It seems though, that as I bend the flatbar against it's strong side- it "likes" to sweep up some near the bow. But, this would mean that near the bow the spacing would be closer to the chine and not evenly spaced between the chine and sheer. You can see in one pic that I have cut a couple of slots in the side frame, in order to try bending while changing the position a couple of inches.

    I have only tacked enough stringers to keep frames level and at the proper distances from each other. The plan is that one the hull sheet is in place, I will look under and see if some stringers have high / low spots, and then I would cut the tacks and adjust appropriately. Then weld all/ most stringers to the hull sheet before welding the stringers to the frames.

    As always, I would appreciate any help / advice!
     

    Attached Files:

  2. Barry
    Joined: Mar 2002
    Posts: 1,428
    Likes: 229, Points: 63, Legacy Rep: 158

    Barry Senior Member

    Everyone does things different but if I were at this point in trying to put the stringers in as you are doing, I would first give your hull a plating test with 2 sheets of 1/4 inch plywood butt joined, on the 4 foot length, together with a thin batten and screwed together
    Get some help and drop a sheet of plywood over what you have done so far to ensure that the plate will make contact with the stringers. From the picts that you have sent, it appears that you have some curvature between the keel and the chine. This means that in order for the aluminum to make a curve between the keel and chine and then the curve around the bow, it has to bend in two directions at once. Aluminum does not do this very well.

    Did you get a set of frame plans for this build or did you develop the frames yourself?
    The reason that I ask is that if you got the plans from a supplier who has had several builds with either plywood or aluminum, things might work out.

    Normally the way we would move forward from here if you cannot get the plywood to lay flat on the chines and the keel, is to lay the plate out and let it follow a natural curve making contact on the keel and the chine and do not worry about the stringers at all at this time. They could be scribed to fit the resulting curve when you flip the boat.
     
  3. Barry
    Joined: Mar 2002
    Posts: 1,428
    Likes: 229, Points: 63, Legacy Rep: 158

    Barry Senior Member

    I just happened to run across your thread in the welding forum. If others have recommended a thicker bottom sheet, maybe give it another thought

    Our 21 foot jet boats used 1/4 bottoms. We originally started with 3/16 but when we were welding the stringers in, we would get a lot of heat distortion from welding

    1/8 will flex a lot and could cause problems

    Ryerson metals must have a branch out your way, and will have longer lengths than 8 or 10 feet.

    With your plan stating only 1/8 inch for bottom plate with only 3/16 inch stringers, perhaps let us know how they intend to provide a solid base for which to mount your diesel engine. If it is a direct or vdrive, I would be very interested in knowing how they will provide enough strength around the shaft packing area

    Our 21 to 25 footers we used 1/4 inch bottoms, 1/8 sides, stringers would be 6 x 3 inch angle spaced on 14 inch centers, progressively getting down to say 4 1/2 by 3 as we moved toward the chine.
    1/8th .125 inches thick thin when you consider that many riveted boats are .100
     
  4. Ad Hoc
    Joined: Oct 2008
    Posts: 6,973
    Likes: 917, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Without knowing too much details, it looks as though the person that ran the stringer lines didn't have any real appreciation for the change in curvature of the hull up fwd. Stringers should naturally get closer and run out at the bow as they rise too. The stringers must also remain normal to the hull plating.

    2 simple quick ways, one Barry has pointed you too...scribe in the FBs once the plating is in place...but that can be time consuming. The other is up fwd, especially where there is so much change of shape I run the stringers out and add 1/2 or 1/3 mini frames from the chine to the keel; that is half or 1/3 of the frame spacing as mini frames to maintain the small panel aspect ratio for the hull plating.. This way no more long.ts in tight spaces, nor forcing FBs to bend to a curve they don't want to ( a no-no). You just have simple mini transverse frames, which are also easier to shape exactly to the hull shape too, and easier access for welding too.
     
  5. SukiSolo
    Joined: Dec 2012
    Posts: 1,270
    Likes: 25, Points: 0, Legacy Rep: 271
    Location: Hampshire UK

    SukiSolo Senior Member

    Although I have not built a boat in metal, my timber boats have given some experience of this problem. Mainly it is the stress in the stringer as it bends into the desired shape. In wood this is relatively easy to resolve, you laminate. In metal which has different tension/compression properties it is hard to get rid of the tension without reducing the size to useless. On internal stringers I always run a transverse batten (like a loose straight edge) along the contact points to check for fit. This shows up any unwanted distortion, mostly before it becomes inbuilt!. The ply sheet suggested does this as well. I would surmise that unless you constantly anneal the stringer it might not go where you want it, and possibly it will be too soft when it is there!.

    The modes AdHoc and Barry suggest eliminates the problems. Probably the best way of resolving the bow area and keeping it clean. It is surprising how much tension and compression occur when trying to bend stringers. I think after fighting with one boat I decided never again, let's resolve the problem. The less stress in the initial shape the more accurate the result will be.
     
  6. Northeaster
    Joined: Sep 2011
    Posts: 265
    Likes: 8, Points: 18, Legacy Rep: 45
    Location: Eastern Canada

    Northeaster Senior Member

    Thanks to all for the detailed observations and advice. I will answer or clarify each point, and would appreciate if you guys have the time to clarify further or provide additional advice.

    Barry – I can definitely follow your advice and use sheets of thin plywood to see how the fit is. I did this already, in some areas, but only with a 4’x4’ sheet that I had on hand.
    Although certainly a novice, I do under the concept of developable surfaces and understand that the aluminum can only bend on one axis at a time.
    It is a professional / purchased design – and I did follow the patterns for the frames.
    This boat has been built numerous times in plywood and at least a half dozen times in aluminum, and it is not a design intended to require an English Wheel or similar to shape the skin. It is an older design, originally for building in wood, adapted to aluminum by the designer.
    Correct me if I am wrong though- but if the plywood test sheets (and eventually real aluminum sheet) fits the general curves, but is not tightly fitting all stringers - I could then move the loose fitting stringers out a bit within the frame slots so that they touch the hull skin before welding to the skin. Then the stringer would be welded to frame slot permanently.
    Re: hull thickness and strength- The stringers are 3/16” thick by 1 ½” flatbar, which is of course, welded perpendicular to the hull sheet. The plans call for 3/16” thick engine girders (I will use ¼” as I have more of that and will add little weight) that are several inches tall, with ¼” x 3” flatbar flanges welded at 90 degrees on top. These girders go between each frame and are welded to the hull skin, acting as stringers as well in this area. The plans call for an inboard up to 875lbs, which I am close to with the Cummins.( but that was for a 23’ boat – I have stretched aft 6 sections the allowable 10%)
    Re: strength at shaft packing / stern tube – it will be shaft drive
    Re: strength at prop strut (where the two strut legs join the hull) – plans call for 4 x 10 doubler or ¼\’ plate insert where the strut legs enter hull and are welded to engine girders (or tank bulkheads if internal tanks)
    Re: stringer strength – my stringers are smaller / weaker than what you guys use but are placed, on avg at 5” – 8” centers.
    I am concerned about harder welding of 1/8” vs thicker and heat distortion. One guy used 3/16” or maybe even ¼” hull, but he also stretched the boat to over 28’ and increased beam.

    Adhoc – I don’t disagree with your statement – I am admittedly new at this and likely don’t understand how the hull is to curve, and what that means in running / placing stringers.
    Bottom stringers - I think my bottom stringers are OK (correct me if I am wrong). The plans show measurements on transom and frame 4 only, so I just marked and cut fwd frames where the seemed to want to go.
    Side stringers on aft frames - The plans state to place side stringers approx at equal distances apart, between cockpit sole and sheer – this worked out ok as there is not a lot of bend here.
    Side stringers on fwd frames - The plans state to place side stringers approx at equal distances apart, between chine and sheer. I marked at these locations, but is appears to me that as I bend the stringers in, nearer the bow, they “want” to rise up (toward the stem as boat is upside down)
    When you say “ Stringers should naturally get closer and run out at the bow as they rise too.” I think they do get somewhat closer nearer the bow, even it doesn’t come out in the pictures –
    Dumb question But, when you say that they rise near the bow – do you mean they rise as it sits on my jig – upside down? (therefore actually moving toward the stem/centerline) Or, as it sits upside down, should they be sweeping down towards the sheer?

    I understand your suggestion about using mini-transverse frames, if required, rather than bending the stringers too much. Plans also say you can cut stringers from sheet, where the bend is too great, rather than try to over-bend flat stringers. I like the idea of mini-frames.

    Sukisolo- I agree with what you are saying. I can lay a flatbar on its loose / bendable edge, and run that easily along the frames – but would that then be the proper position if \i cut the slots there, and turned the flatbar 90 degress to place it? I would think tha tit would want to sweep up or down, as it curves around the frames, trying to maintain 90 degrees to the frames and hull skin.
    I agree with the other suggested options likely being superior.
    Unless you guys suggest differently, I think I may just bend my stringers where it is easy, tacking occasionally. And then use tie-down/ratchet straps to hold them in close – out of the way- near the bow. Then use plywood to see how they fit. If they can be moved a bit to fit well, great. If not, will likely use mini frames or similar to provide strength and support near bow, and cut long stringers out there.
     
  7. Barry
    Joined: Mar 2002
    Posts: 1,428
    Likes: 229, Points: 63, Legacy Rep: 158

    Barry Senior Member

    Just a few things to look out for moving ahead and particularly if you are going to use 1/8 in the boat.

    If you are going to weld any vertical frame or any other item to the side of the boat, chine to sheer, with 1/8, there will be distortion from the weld bead. If you make full length vertical welds on the side of the boat, your boat will look like a starved cow.

    We always welded in a few side stringers, trays is what we called them, and they would be 1/8 by 5 inches (with a small flange on the keel side of the tray for stiffness) . We would turn down the heat, to where we would get some penetration but not the same as say the weld settings for a fillet structural weld, make a 3 inch pass then skip 6- 8 inches and then another 3 inch pass. There will be an imprint on the outside, a raised area where the weld bead is. And of course after this had cooled we would do another 3 inch pass on top of the tray at exactly the same bead location as on the first pass, etc

    You can sand out the bump of the imprint, if you don't, you will see these bumps when you paint the boat.

    Because you might be using 1/8th, you have to be careful that you do not terminate a stiff structural member in the middle of a 1/8 panel as this termination point will be stiff and can cause cracking as the too thin 1/8th will be flexing around this stiff point We have seen this many times in home builts boats that we have repaired.

    Re grinding,

    One of the biggest first time builders mistakes occurs is when they have done all the welding and want to take a grinder to smooth welds or take hard corners and soften them before painting.

    There are three types of discs in play here
    The hard grinding disc, the flap disc which is ridged but the sand paper inserts have some flexibility, and the sanding disc.
    Each has a specific function

    You need a depressed wheel grinding and flap disc (Normally they have the nut already in the plastic disc) so the nut that holds the disc on is below the face of the disc.

    Assume the plate that you are welding is horizontal in all directions and you are working on a butt weld

    Very carefully you grind the weld to within a 1/16 of the plate face without the disc touching anything but the weld bead. Then you have to ensure that the disc is perfectly parallel to the plate, continue to take the bead down ensuring the disc is flat to the plate
    and when the weld bead is almost gone the entire disc face has to be engaged to the face of the material
    If you try to grind by using only say the front of the disc by raising the back of the grinder you will most certainly undercut the weld and it is almost impossible to fair (certainly with epoxy but this expensive and time consuming)
    I used the butt weld first to make the point of full disc engagement to the plate.

    The flap disc is a little more forgiving but still can undercut easy. It normally takes material off slower so you have time to gauge the progress. If you use the flap disc after the grinding disc, the material should be good enough to paint without further sanding but as it does not take a lot of time to sand, it never hurts to take a sanding disc to the surface. Finer grit of course

    The sanding disc is used to give a slight round edge to a sharp edge. Norton and others I expect, have a 7 inch rubber disc that has say 6 - one inch holes in the disc. The sand paper has the same holes and when the paper is attached to the disc and the disc is turning you can actually see the spot that you are sanding.

    So the a butt weld grind procedure would be to lay the 7 inch grinding disc with full contact on the plate to grind the weld area, follow with the flap disc held flat and then if you want, give it a final sand with the sanding disc

    Now for a 90 degree joint. (or 45 or)
    The trick here is not to use just say the top of the disc to grind off the weld as it is almost impossible to keep the disc parallel to the plate. So say you are at a desk and the bead is on the corner closest to you. You use the bottom of the disc to cut the weld bead material off so that the disc is parallel to the plate. The entire diameter of the disc is over the material, which ensures flatness of the grind.
    ( I used the example of the desk to make the point to keep the entire disc over the material, normally you can work keeping the grinder at 45 degrees to the work and use the 4 oclock position as the grinding spot assuming that you are grinding to the right side of your body)

    Then follow with the soft sanding disc to smooth the sharp edge.

    As an experiment, when you get some plate in, take a 12 inch by 12 piece of 1/4 inch plate, oops, you might not have any 1/4 inch, weld a 12 inch cold weld across the plate,( by cold I mean that you will have a 1/4 inch bead width and maybe 3/16 inch tall) .

    Follow the above method, the most important point is to keep the first two discs flat so you can feel the disc engaging the plate, (after you have remove most of the 3/16th inch height) for this test, do not touch it with the sanding disc.
    Then take a can of black gloss spray paint and spray the plate. If you have done a perfect job, you will not be able to see where the weld bead was.
     
    Last edited: Mar 13, 2014
  8. Ad Hoc
    Joined: Oct 2008
    Posts: 6,973
    Likes: 917, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Perhaps this will help:

    ally boat comments.jpg

    It is not easy to comment on a 2D image of a 3D shape, but....it appears from your images the stringers are not following a natural faired line. However, as you can see on my comments on the picture, it appears, could be wrong, that the 2 frames identified have a change in shape over a short distance that is creating the problem.

    The stringers cannot follow their initial natural lines without being forced. Thus don't force them, let them run naturally. Depending upon the shape of the hull and how you set the stringers these can run out going up to the gunwale or down to the stem. Usually it is up to the gunwale, as I've roughly indicated in the pic.

    You can see evidence of this, apart from your difficulties in attaching them, in that you have 'flat spot's. The FBs should be a smooth natural curve, but you have straight sections, which is where the change from a smooth curve to being forced into a direction which is no longer natural is occurring, thus a straight sectional bit. These areas are highly stressed, which means you're restraining them prior to welding; which you must never do. These shall eventually crack....sometimes, depending upon how much force is holding them, can crack immediately after welding.

    Also the cutouts for your FBs will crack. You have hard cut ends, you need a relief hole at the ends, see below. Also I can't see a mouse hole between the frame and the stringer too...to avoid biaxial stress raiser.

    ally FB cutout.jpg
     
    1 person likes this.
  9. Ad Hoc
    Joined: Oct 2008
    Posts: 6,973
    Likes: 917, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Barry,

    I'm interested in your comment here.

    I generally design in the 20-70m range not smaller boat stuff. But have done some smaller vessels over the past few years. I hear this comment frequently with smaller yards; notably none commercial/Class build yards. Rather than put my take on this and why it occurs, i'd like to hear from your experience why you get a lot of distortion.
     
  10. Northeaster
    Joined: Sep 2011
    Posts: 265
    Likes: 8, Points: 18, Legacy Rep: 45
    Location: Eastern Canada

    Northeaster Senior Member

    Barry - thanks for the detailed advice.

    I don't have them on yet, but you can see the slots are cut on all but the fwd few frames, for the side frame stringers - same material - 3/16" x 1.5" flatbar. These will be welded to the hull. Not sure if I will weld teh frames to the humm on sides or not, for reasons you mentioned. Document on welding, which came with plans, says to weld frames to hull after stringers, but it is a generic writeup, and not specific to this deisgn, so this was on my list of questions to ask when I am a bit further down the road.

    I will practice grinding/sanding as you described - I would have likely only used the front edge, as I often do on steel projects - removing metal in a hurry.

    Adhoc - a few notes and likley further questions.

    - The plans show limber holes in the frames at chines and keel, but I have not seen or read anywhere to leave a "mouse hole" to avoid stress where the flatbar stringer passes through the slot in the frames.
    To be fair, the plans proposed two ways for arranging stringers.
    1. Fitting / welding t-bar stringers intercostally / between frames.
    2. Laying long pieces of flatbar (as i have done) passing through the frames, and then fit pieces of flatbar intercostally at 90 degrees "underneath" the long sections, thus creating a tee in place.
    I asked several questions about the best way to do this on a couple of boatbuilding forums and most thought the tee was overkill and to use flatbar, as I am doing.
    re: the apparant large change between frames near the bow - I am attaching a few pics of boats built in ply and aluminum, so it looks possible, despite my perhaps flawed attempts at running stringers.
    I hadn't looked at these pics in a long time, so now looking again, I see one has wood stringers that do see to trend downwards towards ths gunwales.

    If you observe anything else from these pics, or othewise have further advice, please feel free to give it - I really appreciate all of you experienced guys taking the time to help a novice homebuilder.
     

    Attached Files:

  11. Ad Hoc
    Joined: Oct 2008
    Posts: 6,973
    Likes: 917, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    To be fair, amateur and/or small boat yards wont have heard of such or do not employ this procedure in their builds. The reason is ignorance, nothing else. The knowledge that commercial yards and the standards that they must adhere to to satisfy Class design and surveyor build, generally it seems does not filter down to the smaller yards and amatuer boat builders. Thus there is often a gulf in knowledge and experience between the two.

    In commercial vessels and larger vessels, steel or aluminium, there is a real need to avoid a weld over a weld. Thus when you weld the frame web to the shell, it will "join" that of the stringer weld running long.t'ly. So firstly you have a weld over weld...in steel, not so bad, but in ally, a no-no. Welding over a weld weakens the joint. Secondly, you have a weld running transversely and a weld running long.t'dly. Once the structural member is being loaded, from say a slamming load, these welds shall be under load too. BUT..and here is the whole point. If you nominally take the weld as a unit stress value of 1 in each weld, the resultant stress at the joint can be sqtroot(2), or 41% greater than anticipated. It is simply a higher load. If you draw a straight line of length 1 and another of length 1 at right angles to the other, and at 45 degrees draw another but forms a square (with 2 triangles), that diagonal is 41% longer....and is the resultant load in the weld. Thus it will crack there first. Thirdly welds over welds always produce over size weld beads. Oversized weld beads are also stress raisers since they create localised structural discontinuities which increases the stress even higher. A cold weld over the first pass will also introduce voids if not done with care and hence crack quicker too...so, again, a big no-no. So the up shot is, that the welded cross over joint, of the 2 welds joining together can be as much as 2 times the stress as the welds in the long.t or transverse, or even higher if welded porrly. This is why there must always be mouse holes. No class surveyor would allow without mouse holes.

    This means there shall be 'gaps' under the stringer between it and the shell plate once curvature of the hull sets in. Since the stringer is straight/unbent, unlike the shaped hull.

    Also if stringers are intercostal, unless the same depth as the frame, requires tripping brackets either side, otherwise it is...yup...a stress raiser in the structural joint with the main transverse frame.

    For the 'intercostal' part, see above.

    But this is dependent upon the structural strength of the joint and hence the boat. Unless you know what the displacement and speed and wave hieghts the boat is designed to meet, it is all a guess. A Tee of 30x25x3 is not as stiff as a deeper/thicker FB as a simple example, so which to choose?...well, the design says Tee,....etc etc. But unless you or the designer knows what the slamming loads for the hull are going to be and then design the long.t stringers to satisfy that load, how do you know what section to choose? You don't..!

    Some sections are easier to use than others. Which is why, on small monohulls FBs tend to dominate. Easier to bend.
     
  12. Barry
    Joined: Mar 2002
    Posts: 1,428
    Likes: 229, Points: 63, Legacy Rep: 158

    Barry Senior Member

    To Adhoc
    If you take a 2 foot long 1/4 by 4 inch flat bar and put a 2 foot by 2 foot sheet of 1/8 or even 3/16 over top to make a T and then weld the flat bar to the sheet, the contraction of the weld bead will pull the sheet down and cause distortion.

    There is just not enough rigidity from the moment of inertia of the thin plate to keep the plate fair. Of course you can keep adding in stringers at very close spacing to take
    some of the deformation out.

    If you take this same 1/8 inch thick 2 foot square plate and tack on a 2 inch flat bar all around the perimeter at 90 degrees, then weld the flat bar onto the plate, the center of the plate will move in the same direction as the welded side. ( of course the flat bar as it is welded on one side will form a curve due to shrinkage)


    The very first 21 foot jet that we built in about 1985, we used 3/16 bottom plate and 3/16 stringers, and we had some deformation even in that. When we began using 1/4 the problem went away and we could weld 1/4 inch stringers to the 1/4 plate with not issues.

    Of course we were building 4,000 pound prox boats to take a hit on a rock at 35 mph and would never have been able to get by with 1 1/2 inch stringers.

    The 1/8th vertical welds on the sides will cause similar issues. I have seen several home builds where the rookie welded in a frame completely vertical in the side and had the thin plate deform inwards. The effect is further multiplied if the frame is welded in where the plate is curved at the front of the boat, the heat relieves the stress and the plate flattens out.

    Good catch on the stringers where they did not maintain the arc, that the flat bar would be flat.

    Many boat builders up to about 45 feet do not use frames before the boat shell is welded. They will take the CAD generated cut plates, lay the opposing floor pieces together, put in a couple of temporary frames to maintain the deadrise, and begin tacking the plates from the transom forward. As the opposing pieces make contact the bow curves will form themselves and be a developable surface.
    After the sides, and bottom are tacked, the floor stringers are installed. then the side stringers, then however they are doing the sheer etc. Some bulkheads are installed that make contact with the sides if the material is over 1/8 inch thick.

    Re your comments on the ceramic backing plates for butt welds to enable one sided welding. Certainly an acceptable procedure if you have them available. Where we were absolutely unable to get to both side of the weld when it was in place, we would tack in an aluminum backing plate say 3 inches wide behind where we were going to make the butt joint. We would then lay in the second piece of the material and leave enough gap
    so when we welded we would be able to capture all the four corners/edges of the two pieces and use the tacked in piece behind to act as a heat sink for the weld.

    Often these locations were at the corners of boat at the transom where the narrow ledge at the sheer joined. We never had a crack occur here.

    Our range of boats was 21 to 40, material thickness excluding keel protection would be up to 3/8 of an inch.

    I would never build a boat over 18 feet without it being 1/4 inch. At 1/8 inch, there will have to be a lot of material used to keep the hull straight that it could weigh as much as 1/4 inch without the headaches.

    Northeaster, forgot to mention in the earlier comment that when you are grinding any curved surface you are better to use either the flap disc or the sanding disc to deal with these areas, still keeping the disc as flat as possible to the work.
     
  13. Ad Hoc
    Joined: Oct 2008
    Posts: 6,973
    Likes: 917, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Barry,

    Thanks for coming back, really appreciate it.

    For me, it is all about sequencing. The sequencing is the key and more often over looked owing to the production manager shouting why is the boat behind…stop welding XX first weld YY instead…..it takes some courage to weld a boat in a sequence that often belies the intuitive way.

    The moment of inertia of the plate should not be an issue. Without going into too much detail (and taking over the thread) the hull plating should have been all fully welded, in the correct sequence well before the long.ts stringers and frames are welded to the shell plating. (Of course the weld connection of the frames to the stringers is done before the shell plate is fitted, for ease of access).

    So, the shell plating is all fully welded, thus any inertia you speak off, is now automatically there as the whole shell is very stiff.

    The welding sequence of the stringers and frames relies upon 1) the correct size weld (not too much heat). Since over size welds (because the welder thinks more weld is good) can make matters worse, 2) the sequence starting at midships on the centreline, then slowly progress in equal steps fore and aft and from centre line to outboard both port and stbd again in equal steps. So the weld, and shrinkage/distortion is kept to a minimum and the hull is being balanced by the equal and opposite welds.

    The temptation is the start at one end, and then weld in one direction only. ..this must be avoided.

    I have plenty of examples, though not decent photos to see clearly, where I’ve deigned catamarans almost 30m in length, with 4mm plating and here a 27m monohull with 4mm sides, and a 33m monohull with 5mm sides.

    27m monohull.jpg 33m monohull.jpg

    As you can see, not a hungry horse effect at all, very smooth and fair. Going to thicker plate adds cost and weight. If you can change your sequence to produce the minimal distortion and effects, it saves in the long term.
     
  14. Kevin Morin
    Joined: May 2013
    Posts: 83
    Likes: 10, Points: 0, Legacy Rep: 72
    Location: Kenai, AK

    Kevin Morin Junior Member

    fairing the frame

    North,
    I don't usually build with frames as you show in your posts but I'd surely like to comment on them hoping there may be some help in my remarks.

    First, not about frames. Aluminum sheet 5052, 5086 and 5083 all bend compound naturally. That is; if you take the developed shape of the 'orange peel' of the bow forefoot of the bottom and pull up the keel edges there will be a natural 'bulge' from keel to chine.

    IF you continue to tighten the cylindrical curvature of a sheet it will flatten somewhat. The bow or 'lay' of the hull sheets if visible if you take time to stand a 5'x25' sheet on the long edge. All sheet will show a bow to one direction or the other, I build with the bow, by marking it and using it outward during the layout of the hull panels.

    I've heard this is from rolls stock cooling at the mill from the outer edges and therefore a very tiny contraction over the edges but whatever its cause? I lay the bottom halves and topside sheets facing each when I do layout for the hull panels. I can't often afford NC cut parts.

    This bulge or convex shape of the 'natural' bend is very slight, but it will not happen in plywood because of ply's layers and grain directions. This small compound shape can be removed by pulling the sheet up to the framing or left but it exists; so Plywood as a frame fairing aide is a very mixed blessing.

    The two materials form differently and ply on metal is not a 'true indicator'. It will help aft the masted station but is not as reliable forward where the cylindrical sections give way to conic surfaces.

    Next, on the only framed hulls I've built the lofting or framing was not accurate in the flat plans to layout the longs- they had to be laid out on the frames with extrusions or battens to find the notch locations accurately. So we built the frames, then stood them on the keel and then wrapped angle extrusions as battens to 'line off' the notches, then used small squares to locate the drilled holes, drilled and used a jig saw to cut to lines to the tangents of the drilled back of the notches.

    I don't know of another way to do this work unless the designer had taken time to fair the longs or they were computer generated frames and longs?

    That key step seems to me, to be a major contribution to the lack of fair surfaces in your frame - the notches clearly show the longs are not notched into the frames correctly. The forward few frame notches are clearly not fair to the rest of the longs' curves aft.

    I agree with AdHoc; the frame notches in aluminum should have a drilled inner end of notch. AND I'd add that I would say it was not good sequence to weld the longs to thee frames before the hull was tacked up with its plate/skin/hull surfaces. I also agree that the chines, keels, longs areas should (must) all have a 'mouse hole' 'limber hole' to provide an avoidance of the two welds AND to allow the MIG torch's gas flow and bead to continue past the transverse frames at the keel, chine and any other intersections of the frame. This allows an inner chine weld without involving a weld stop at the transverse frame's intersection with the chine weld seam.

    The reason that smaller framed boats of this class do not have the longs welded fast to the frames is when the hull panels are all tacked on, then welded at chine, keel and transom, there is some small added convex expansion to these panels. This is from the small contraction of these panels around their edges as they're welded out- both in side and outside. Now, the longs are pressed outward, usually a small amount to touch the hull panels' inside surfaces and this is why they are not welded fast to the transverse frames before final hull weld out.

    The transverse frames need not be welded to the hull panels unless they do touch. They will do their job in structural support by supporting the longs even without touching the hull panels. In the case of bulkheads that will be continuously welded to the hull, I personally loft/draw/design these transverse frames/panels/sheets with a small curvature (convex) from keel to chine or from chine to sheer so they hold the panel in a small convex bulge, alloying the future full length welds.

    regarding welding.

    I plan the welds of the entire boat on copies of the hull's lines or even by hand sketching. I bevel every weld of a hull long into the extrusion piece so the weld is not a true Butt T Fillet. Instead I want as much of the root face buried under the flat bar or angle extrusion leg edge as I can get.

    If I have transverse bulkheads or frames, I bevel anywhere this will be welded to the hull's inner surfaces. All beveling is planned off the copy of profile/body plan and plan so I know when and where all the welds will be placed.

    I agree - totally and completely- with Ad Hoc its a matter of sequence. I tack just inside the ends of each weld site, and then dress all tacks with a carbide burr and then weld the race track over those tacks. This method may seem like more work but it results in fair hulls of welded aluminum.

    As to welding to 1/8" hull sheeting with heavier stringers. I would like the even thickness welds as they're easier but if the bevels and wire feed were adjusted well, and your reflexes could work at the travels speeds needed, these welds are possible, but in a keel down position I would consider them unrealistic for a first time builder, low hour welder. I did similar welds in the past but could no longer do them at this age, unless the entire hull were on a rotisserie fixture to all the welds to be done overhead, or down hand.

    [​IMG]

    as an example, here is the bow deck, anchor locker added externally above the finished deck of a recent 25' skiff. The bulkhead is 0.160" and the edges are beveled to accommodate the seal welds.

    [​IMG]

    The bow deck/locker's print through is very minimal and the boat will not be painted so there was not fairing of this surface. But the point here is that by beveling the deck, and the transverse bulkhead's vertical edges after they were fit and before tacking in; the weld is 'buried' into the edges of the two materials and therefore the lever arm of pull of the face contraction is reduced. This reduction leaves the hull fair, there always be print through or the weld expansion of a good penetrating aluminum T fillet type weld, but it can be minimized with weld joint preparation.

    Another visible example of the prepared weld bevels are the spray rail shown here. This 4"x3" angle is tapered about 10' at the bow, the outer edge has been routered to a 1/2" bull nose to remove the sharp edge. The weld are 3" long and beveled fully into the edge of the angle its 1/4" thickness. They are all TIG welds (not hand TIG, cold wire feed) and the extrusion was placed with tacks at each weld end initially. When the weld out came for this hull long, the welds had a narrow face and deep groove to fill so they were put in at 300 A, ( Argon/Helium mix @ 30ft^3/hr. 250Hz, unbalanced AC, pure 1/8" tungsten, Miller Dynasty 300DX, 5386 filler wire 0.035" dia.) at MIG speeds and the side is fair and the rail reasonably well attached.

    [​IMG]

    This image is along that locker's deck, on the port side looking at the inside of the stem, of the skiff above. This is a TIG weld, and the face width is about 5/32" to 3/16" wide the topsides are 0.160"-5/32" and the deck is the same. There is a 'naturally occurring' V at the leaning topside to the deck, but that was widened after this deck plate was fit, in order to more fully bury the root face of the weld to align that fusion with the central axis of the plate.

    This is why I take time to layout the welds on the plans, then mark the longs and bevel where they will be welded. It is why I bevel bulkheads, as shown here as I think the resulting hulls come out cleaner. I'm not saying my first couple dozen skiffs turned out this way, but I'd like to think my next dozen will?

    Your stringers as laid out and notched seem to me a headache you cannot overcome as they ARE NOT on the same surfaces as the frames.

    Cheers,
    Kevin Morin
    Kenai, AK
     

  15. Ad Hoc
    Joined: Oct 2008
    Posts: 6,973
    Likes: 917, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Thanks for wading in Kevin. I’m always interested in other peoples view and take on how they build, especially boats which are much smaller than I work with.

    I think may be you’re referring to the “line” created by the rolling. The rolling that is required even for a basic “O” temper, to flatten and obtain a tight dimensional tolerance. Shown below:

    Ally plate and rolling lines.jpg

    Therein lies the issue. These days everyone just click buttons and hey presto a hull and a set of lines. And then click and double click, hey presto, a set of longitudinal stringer runs. What may seem great, quick and hey this is easy, on a monitor with a software that is doing all the work for you, in reality, on the shop floor can be a real mess. The problems is that when I graduated we had a mould loft and highly skilled staff that could see a wonky line from a thousand yards in the dark. It is a real skill and talent to fair lines and even more so the produce a set of faired runs for stringers. This skill and art has been lost and is “assumed” by button pushers running freeware or if they buy software with a bit more capability still unable to understand the concept of production faired lines. Asked to pay for a set of proper decent production faired lines and stringer runs….and most will baulk at the cost and time involved when faced with software that does “it” in seconds! :eek:

    You’re referring to “floating frames”. You can get away with this on smaller boats, but not on larger boats. In fact you can get away with many things on a smaller boat owing to the law of diminishing returns and scaling. Which is often why small boat builders wonder why those like me (bigger boats) insist on detailing the way I do, which they’ve usually never heard of.

    Not sure how small boat yards do it..but I always produce a welding plan dwg, which clearly sets out the location, throat thickness, size, length and thickness of the welds…along with correct produce for different and dissimilar plate thicknesses.

    Agreed.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.