Rough rule of thumb for max lengthening of an aluminum hull?

Discussion in 'Boatbuilding' started by John McCrary, Jan 17, 2024.

  1. John McCrary
    Joined: Oct 2021
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    John McCrary Junior Member

    I am working with a medical charity that wants to field a long-ranged, mid-sized (100-150 tons displacement) disaster response boat. They have found an aluminum motor-sailer schooner project boat that is to the stage of having its hull assembled to the tack welded stage. Their thought process is that at that stage of construction, the boat is relatively easily refitted to accomodate their needs. They ideally wanted to add a dedicated cargo hold by stretching the hull amidships.

    I was curious about how much of a hold could be added, but it appears the original architect is no longer in business so I can't just shoot them an RFP for the changes. Is there a rule of thumb for how much an aluminum hull can be stretched? I am asking because I am making the assumption that it would be significatly easier/faster/cheaper for the original architect to design the hold since they have not just the blueprints but also all the various things like CAD files and stability calcs. My thought process is that if there is a rule of thumb, it would allow us to judge if it was worth the extra expense of having a third-party reverse -engineering the design to add the hold. eg We do not want to spend tens of thousands of dollars if it would only net us an additional 100 cubic foot of cargo space (for exmple).
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Hi John

    It is not as simple as that.
    It is all determined by the stability and the routes the vessel shall engage upon.
    You need to engage the services of a NA, who can look at the influence on the increase in length + the cargo on the LCG/VCG and hence how much stability and more importantly, reserve stability the vessel has under varying loading conditions and heeling levers.
    You should not attempt any changes, until this has been investigated by a NA.
     
  3. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    The possibility of lengthening the boat really depends on its hullshape. Sailing yachts often don't have constant midship sections, stretching such a design isn't possible by inserting a piece in the middle. With some yachts its possible to strech the stern and move some accommodation there, freeing up midship space.

    It's worth trying to contact the original NA (or his estate), maybe he is willing to do some consulting or just sell some documentation.
    The simplest thing is to pay a local NA for a few hours of his time so that he can visit the boat in person and give you some answers. It's just another form of survey, a standard expense for a buyer.
     
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  4. John McCrary
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    John McCrary Junior Member

    Thanks,

    Don't worry Ad-Hoc, no home-grown modifications. This boat will essentially be run like a commercial vessel, and I cannot see any insurance company being willing to insure a hull with a major DIY hull modification. Nor could we as medical professional operate a vessel in good conscience unless we know it meets industry standards. We really understand the meaning of "We're professionals, don't try this at home."

    As for the other info you mentioned. The schooner was blue-water expedition design to be used just about anywhere that you didn't need a full ice-breaker hull. The projected weight was right on the line between ultra-light and light, but the hull is all aluminum and thick, and stability-wise, it meets the USCG standards for ocean-going commercial vessels. Estimated cruise speed with either motor or full sails was estimated around 12 knots with a motor-sailing estimate of 17-18 knots. That suggests to me that the design can easily trade speed for payload without major buoyancy issues. My main question would be if the structural integrity is as over-engineered as I think it is.

    Rumars, I have tried emailing the NA's company email address multiple times with no luck. The former company phone number now goes to his ex-wife, and there was no response to the number she gave me.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    In that case, since you have, or at least access to:
    Just supply that data to a NA who can give you a quote for the work that you require.
    As the data already exists, a NA can give you an indicate of what you can and can't get away with and things you must do, based on the original data.
    It is a straight forward task for an NA.
     
  6. bajansailor
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    bajansailor Marine Surveyor

    What is the particular appeal of this particular boat - is it because she can be acquired relatively cheaply?
    If this is the case, remember that the cost of the hull is only a small fraction of the overall cost of the vessel.

    I presume that the tack welded together hull is currently sitting in a boatyard somewhere (in Alaska?) - have you talked to the guys building the boat, to see what their thoughts are re your proposals for lengthening the hull?

    Re how you want
    This is a big project and you are looking at a cost of millions of dollars, hence you want to be absolutely certain about the design before you jump in too deeply and start flinging a lot of money at it.
    Has the Charity established a detailed Statement of Requirements (SOR) for this vessel which clearly outlines things like :
    The budget available,
    What work it is expected to do,
    Whereabouts in the world it will be working (I am guessing Alaska, or should it be able to operate anywhere, apart from the extreme polar regions perhaps),
    How many crew / passengers it has to be able to accommodate,
    The minimum average cruising speed required when underway, along with the range (how far in terms of distance, and / or how long it can operate without being re-supplied with fuel and provisions)

    These are just a few suggestions above for your SOR - the more detailed your SOR is, the easier it is in many ways to design the vessel that you want - try to establish as many constraints as possible.
    When doing so, you might find that some are already in conflict with others......

    Are you at liberty to post any drawings of the boat (I presume that they have been using drawings to get the boat to it's current stage of completion) and photos of her currently (and would you have any photos that show the frames being set up?).

    Who estimated these speeds - the original naval architect behind the design?
    The motor sailing speed of 17 - 18 knots could be a bit optimistic, but then we don't know what power the engine is, or how much sail area she has, or what the shape of the hull looks like....
    If you mention who designed her, a member here might know what happened to the company, and if they might be contactable under a different name?

    Finally, changing tack (but hopefully without too much thread drift) did things progress any further with your previous thread (link below) a few years ago re a long slender multihull (or it might be called a stabilised monohull?)

    Where to find scantling formulas for a long, flat-bottomed hull with an extreme length to beam ratio https://www.boatdesign.net/threads/where-to-find-scantling-formulas-for-a-long-flat-bottomed-hull-with-an-extreme-length-to-beam-ratio.66179/
     
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  7. jehardiman
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    jehardiman Senior Member

    I second this. The real issue with AL is fatigue. The US Coast Guard shut down a "B"..billion USD project because of stress issues cause by lengthening vessels. What happens when you lengthen a vessel is you increase the load/stress on midships. With AL, more than most metals, when you increase loading cycles you shorten life. This needs a good look.
     
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  8. gonzo
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    gonzo Senior Member

    In that case, what Class was is designed and built to?
     
  9. rangebowdrie
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    rangebowdrie Senior Member

    Isn't 100 CuFt in measurement rules = to 1 ton?
    That's about or less than 1% of the vessel.
    Move the interior around and/or eliminate a cabin, now you'll have more then 100CuFt.
    Be creative.
     
  10. gonzo
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    gonzo Senior Member

  11. rangebowdrie
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    rangebowdrie Senior Member

    Yes, I'll admit I used a poor analogy.
    The CG has my boat at 5 tons Net, but it displaces a lot more than that.
     
  12. Milehog
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    Milehog Clever Quip

    In some cases lengthening a recreational vessel by up to 10% can be done without too much drama.
    You are starting with a light recreational vessel and contemplating adding length, commercial duty and cargo. Too complex for a rule of thumb.
    You must hire a NA to see if it is even feasible.
     
  13. John McCrary
    Joined: Oct 2021
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    John McCrary Junior Member

    ugg, just accidently closed my browser and lost the all but done post I had spent over an hour on in order to answer your questions and give more insight into our thought process/reasoning. I will redo it this evening after work.

    In the meantime, here are links to the design's webpage and a short video where the original owner describes his vision.

    https://www.kastenmarine.com/zebulun.htm



    I will say that a major reason we are looking at this design is that it is spec'ed and designed for an almost exact match to our mission requirements and the NA also explored variations on the design up to 120 foot which suggested to use that he had a decent idea of what would be needed for a stretch. This was before we found out that we could not get ahold of him.
     
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  14. Milehog
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    Milehog Clever Quip

    That vessel has been brought up here before IIRC.
    If it is the one I am thinking of she was stitched together up the Columbia River in the Tri-Citys, barged down to ~ Portland, then disappeared.
    It is good to know Mr. Kasten had envisioned larger versions. If that will translate to this vessel is the question.
     

  15. gonzo
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    gonzo Senior Member

    It may not be possible to convert a recreational boat into a commercial vessel. Usually it needs to be designed to comply with a Class and regulations.
     
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