Help me decide how to attach stringers in 23' aluminum boat

Discussion in 'Metal Boat Building' started by Northeaster, Oct 1, 2013.

  1. Northeaster
    Joined: Sep 2011
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    Location: Eastern Canada

    Northeaster Senior Member

    For reference, I am building the Glen L Double Eagle and plans describe (method 1) using tee which is ( i believe) cut in small pieces and placed intercostally (new word got me) between the frames. Or, it says (method 2) you can slot long flat bar but then weld small pieces of flatbar at 90 degrees to the long ones, to basically make tees in place. I think this second method seems better than the first, for getting a fair hull, but seems like overkill.

    Then, a member on the Glen L site here showed me (method 3) how his plans for the snakeshooter show to cut away a long swath of 1 1/2" on the outside edges of the side and bottom frames, leaving the end points, chine points, vee points full width. The 1 1/2"x1" tees are ran and welded in these swaths which allow the hull plate to sit evenly on the stringers and chine points for example(which are not cut away, so are at the same depth as the new stringers. I like the look of this method, and it seems like it would be easier than cutting individual slots at each frame for all stringers. But I am wondering if I would need to add width to my frames - it calls for minimum width of 4" and would be fine for method 1 or 2. But in method 3, if I cut away long swaths of 1 1/2" from a 4" wide frame - obviously this is only leaving 2 1/2" where the tee stringers are welded on. I am thinking I would have to increase my frame width to maybe 5" and cut away the 1 1/2" swaths to be left with 3 1/2". This would take more aluminum sheet and have more waste, as well as reducing interior space marginally.
    As well, I haven't decided on whether to foam the hull or not, but wonder if I want top make any sections watertight for buoyancy - the larger swaths would be impossible whereas the flatbar welded in slots could be made watertight( although the transverse frames do not have to be fully welded to the hull, and there are limber holes in frames as well.)
    I am leaning towards using flatbar only in slots or going with method 3.

    I would appreciate input on the pros and cons or what experienced builders would recommend!
     
  2. Barry
    Joined: Mar 2002
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    Barry Senior Member

    If you cut the swathes for the T, weld the frame to the floor of the hull on each side of the swath and the frame will not lose significant strength.
    If you go with the method of putting in a vertical flat then weld another horizontal flat you could get significant heat distortion in the vertical flat bar. And it will almost be impossible to weld the T to the flat due to space limitations
    When welding in the T or vertical flat to the hull, ensure that the you weld both sides of the flat/T. Ie at the same location I imagine that they gave you the stitch length and skip measurements
    Re foaming
    If you are on the ocean or lake, as compared to running rivers, you could spray foam the interior only if you do not think that you will have to weld on the hull again as a weld can cause the foam to start on fire
    For noise the foam helps a lot
    Not sure on your power config, but find a way to have the engine bulkhead totally isolated from the rest of the boat if it is an inboard. There are many decoupled sheet foams available for noise. The engine manufacturer will have provided an intake spec for air intake.
    You would not want to weld in the frames to the side of the boat in full welds as you will see the imprint through the hull from weld shrinkage and you run the risk of oil canning the sheet between the stringers and frames
     
  3. Northeaster
    Joined: Sep 2011
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    Northeaster Senior Member

    Barry -thanks for the advice.
    I will be using only flatbar now, placed perpendicular to the hull, and do not plan on welding the frames to the hull.

    Power will be a Cummins 4BT 150hp, with a 1:1 velvet drive - inboard, shaft drive.

    I will have a good look at how I can isolate the engine from the hull.
    Not sure about foam yet, as I am concerned about future welding / repair.
     
  4. Barry
    Joined: Mar 2002
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    Barry Senior Member

    We always have a loud sound issue in aluminum boats and foaming the hull gets rid of condensation of water against the aluminum.
    I had contacted a Sound Control engineer several years ago and he said the most important thing to do is to stop the vibrating air, sound, from entering into the area where people will be.

    To prove it, he said to turn on a stereo in an adjacent room, loud, and then begin to close the door, the sound will be loud until the door is just about closed but the final 1/2 inch of closure will significantly reduce the noise. To get rid of even more noise, drop a towel or something along the bottom and the noise will drop again.

    Then the remaining noise will be caused by the door or walls, vibrating with the sound that is then transmitted through the walls
    The step after this is then a sound absorbing material that will absorb the residual energy

    Since that time, we would isolate the engine compartment to stop the air transmitted sound energy. Running a bulkhead along the bottom and sides of the hull, then using a silicone bead to fill the joint. That way we did not have to weld the frame, bulkhead to the side. All control lines and wires were packed with a non flammable packing as it entered the engine compartment.
    We would then line the inside of the compartment with a 1 1/2 inch thick foam sheet that was say 1 inch foam, then a 1/8 inch heavy massed decoupler material, then another 3/16 of foam and glue it to the engine compartment inside. It also had a mylar coating that made it easy to clean.

    The issue then is exhaust and intake air. We built in dedicated air inlets with enough cross sectional area to meet the engine requirement. We would add in another 30 percent. We would use a flexible duct and stuff about a foot of this with stainless steel mesh that we would pick up from a house cleaning store, ie the stainless pot scrubber type, this did not affect the intake but reduced the noise that could come up the pipe to the out side venting

    We also built engine compartment mufflers, large size and put some of the stainless mesh in as well.

    All in all, we would end up with quite a quiet boat.
    It appears that you might have a dog house style engine compartment so it will be more difficult to get the vent inlet to it

    You can also just add in the decoupled foam sheet to all other areas of the hull to help quiet things down
    A trick. When you are shaping the foam, a simple electric jig saw with a fine blade will cut this foam like butter, making the job much easier than using a knife

    If you are just using flat bar, you will have to increase the height if you are not putting on a top flange "T" as the top flat bar T adds a lot to the moment of inertia of the rib.

    While thickness is important in your flatbar, the height is more important as the moment of inertia of the profile increases to the cube of the height. I= 1/12 base times (height cubed)

    Volvo Penta has a manual that I think that might be able to be downloaded from the net. While Volvo specific, it has a ton of valuable information, to help you build your boat. They cover fuel tanks, air inlet sizes and much more. It is a big file but worth the read if you can download it

    Good luck with your project
     

  5. Northeaster
    Joined: Sep 2011
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    Location: Eastern Canada

    Northeaster Senior Member

    Barry - just wanted to thank you for the in-depth post, and all of the advice.

    I am late starting the aluminum work, as I got sidetracked with the engine. I bought a used Cummins 4BT Marine engine. Had to find another transmission, as the one that came with it was a 2.91:1 and I needed closer to 1:1, to run a much smaller prop (16" is max recommed ned on design). Anyway, changed oil, filters, etc and engine runs well. Degreased it and will take off alot of components before final cleaning, and repainting later this winter. Panel works and all gauges are in proper operating ranges.

    Finally got around to cutting out the 1st 3 frames last weekend, and this weekend will cut out the rest. The frames are in two pieces, so will have to be welded together, and an overlapping gusset welded in where they join.
    Then have to weld a 2" flatbar flange on the top of each bottom frame section. Still have to buy some scrap steel for the buiding form / jig, and hopefully will be able to get it set up and starting mounting the frames on it, upside down, within a month or so.
    Then will go about aligning my flatbar stringers, and see how they look.

    Lots to do. Thanks again for the help. Will put the soundproofing advice to use, later in the project -- yes it will be a doghouse, and I will have to se heta my options are for bringing air into the engine space.
     
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