Aluminum Transom Thickness

Discussion in 'Metal Boat Building' started by Dakotaly, Jun 27, 2024.

  1. Dakotaly
    Joined: May 2023
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    Dakotaly Junior Member

    I am trying to decide what thickness of aluminum tube to use on my transom. It will have a 150HP on it. my transom design is very similar to this one. My tubing will extend all the way to the sides of the hull. My hull plate is .100" and originally I was thinking .125" on the 2x4 tubing. Should it be beefier? upload_2024-6-27_13-55-39.jpeg
     

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  2. BlueBell
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    BlueBell . . . _ _ _ . . . _ _ _

    Probably.
     
  3. C. Dog
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    C. Dog Senior Member

    I would have thought that ¼" wall would give adequate margin.
     
  4. Dakotaly
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    Dakotaly Junior Member

    I just reached out to get a quote on 3/16" & 1/4" my only worry is it has to be custom orderd so price jumps significantly!
     
  5. BlueBell
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    BlueBell . . . _ _ _ . . . _ _ _

    Dakotaly,

    What does your transom look like?
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    Be grateful it is not this one.
    This is a very poor structural design and even worse fabrication/welding....shocking.

    Depends whether it will be a structural member, or merely a member for rounded off edges?
     
    baeckmo and bajansailor like this.
  7. Dakotaly
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    Dakotaly Junior Member

    @Ad Hoc Funny enough this is actually a transom from a major aluminum boat manufacture. What makes it structurally unsound and what about the welding looks wrong so I don't make the same mistakes? My current boat (99' Princecraft Ventura 191 150HP) has a very similar design just instead of aluminum 2x4 it has a standard plywood transom with 4 perpendicular tie-ins to the hull that are bolted on. I don't have a picture with me currently of the mock up I have in mind but it was originally based very similarly off of the original photo. I can get one later this evening.
     
  8. Dakotaly
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    Dakotaly Junior Member

    Below is what I currently have. The lower deck and upper deck will be welded in providing additional support along with a "secondary" transom that will tie in to the knee braces adding even more support.

    upload_2024-6-28_17-10-25.png
    upload_2024-6-28_17-11-6.png
    upload_2024-6-28_17-12-6.png
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    The channel section, creating a box, is an issue on several fronts. Once welded, how do you know if there is an corrosion inside, you don’t. Way too many hard spots created by those 90 deg angle joints, thus at the corners there will be stress concentrations…

    It all looks like it has not been designed at all, but just either drawn up, or built on the job thinking – yeah this is nice and stiff, it wont budge, by a fabricator.

    The welding here
    upload_2024-6-29_9-45-55.png
    And here
    upload_2024-6-29_9-46-48.png

    Is shocking…sooty indicative of poor prep and poor fusion. It is tack welded & fully welded all over the place, the sequencing is terrible. Welding into those tight corners of the box created by the channel section, next to impossible, so they will over weld and blow away the ‘meat’ and then it will also be another stress concentration coupled to the fact it is also a tri-axial stress raiser.

    I could go on….but it is shockingly bad.

    Is it an outboard or inboard...and what power size/weight?
     
  10. Dakotaly
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    Dakotaly Junior Member

    This will all be in fresh watter I'm not to worried about corrosion we have had alumnium boats here and never had any worries about it. As for the other question it is a 150 O/B.
     
  11. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    It is difficult to state exactly with dimensions and how many o/bs etc.
    But all you need to do is take the weight, thrust and bending moment on the transom and take those loads into the surrounding structure.
    In a simple schematic way like so:

    upload_2024-6-29_10-41-48.png

    So large knees/bkts with riders in the long.t direction that are inline with the supports mounts of the o/b.
    Then Tees or L's made from web and rider bar flange, running across the transom that become part of the knees. This stiffens up the transom and helps to transfer the load long.t and transversely.
    Then support for those transverse members on the transom, long.t - to transfer the load.

    Try not to over think it.
     
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  12. Dakotaly
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    Dakotaly Junior Member

    Glad to hear we are on the same page mine is very similar to that the only difference is the horizontal cross structure extends all the way across. Thanks for the help!
     
  13. Barry
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    Barry Senior Member

    Another issue with the rectangular tubing is that it appears that the mounting bolts are going through the tubing. When tightened the opposing flats of the tubing will bend inward and the tubing will flex without providing enough tension in the bolt to rigidly hold the OB

    If this builder insists on rectangular tubing as a transom stiffener, the best concept ( I am assuming 1/2 inch bolts) is to drill through both sides of the tubing and transom plating with a 1/4 inch drill. Perfectly
    perpindicular to the tubing/plate. Drill the transom and stern side flat with a 1/2 inch drill and the bow flat with a drill as close to the OD of the pipe which is .840 inches.. (or something that will suit)
    You will need then 4 only 1/2 inch sched 80 pieces of pipe that will go through the larger bow side hole and weld it on both ends of each pipe. This will keep the flats from collapsing
    when you tighten the bolt. Also never use a lock washer against aluminum as over time they can wear the contact area at the split an not hold the nut from loosening.
    Flats and a nylock will do the trick.
     
  14. fallguy
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    fallguy Senior Member

    My brothers aluminum boat transom is corroding badly and only used in f/w. It is only surface corrosion. The source is road salts from the boat being hauled in winter it seems, or perhaps that one trip to Florida before he bought it.
     

  15. Dakotaly
    Joined: May 2023
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    Dakotaly Junior Member

    I would think something like this would negate the issue of the tubing collapsing. Or even a larger one that would span the width of the tubing. This is my current plan.
     
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