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This Aluminum Hull: Is it Thick Enough?

Discussion in 'General Yachting Discussion' started by StuartT, Oct 6, 2015.

  1. StuartT

    StuartT New Member

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    Hi Folks,

    Since starting to look for the 'next' big hole in the water, I have been focused on 55' to 65' trawlers. While the steel group attracts me for the obvious reasons that have been discussed extensively on this forum, I really hate the idea of chasing rust for the rest of my days. And I really don't see my wife and I cruising beyond the inside passage of the PNW to Alaska, so the glass boats keep looking more appropriate, especially when considering the resale difficulties of a steel boat.

    Recently this 72' Darling named "North" got my attention. http://northyacht.ca/exterior.html I really like a lot of the features, especially the aluminum hull which is respected in our area due to the many commercial aluminum boats in similar size ranges. I was starting to really get interested until I read that the hull thickness was .250 (see the tab 'Story'). That stopped me dead in my tracks since it is my understanding that in general, if a steel hull is say 3/8" (again, this size range), you look for a 50% increase in aluminum thickness to approximate the strength of steel, i.e. dent, tear, bend resistance. A steel trawler I was looking at was a 60 footer with a 3/8" A36 hull, so this Darling with a 1/4" hull could be considered, what, less than 1/2 as strong as the steel trawler. My arithmetic: .375 + .1875 = .5625 x 44.4% = .250 or stated in fractions, 3/8" + 3/16 = 9/16 x 44.4% = 1/4).

    I don't want a boat that might be just fine in our Pacific NW inland waters, looks like it should be open ocean ready, but fails the litmus test of strength for future prospective long distance cruisers. What serious owner/captain is going to want to go well offshore if the hull of his boat is a compromise. Maybe the secret is in the fuel capacity. At 1500 gallons and twin 450 C series Cummins, maybe this boat was never designed to be anything more than a costal cruiser.

    Am I over thinking this?

    Stuart Thornton
  2. Capt Ralph

    Capt Ralph Senior Member

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    1/4" allow on the bottom?? Pretty F%^&ng good in my opinion.
    I've run, pounded, abused and run aground Strikers while I was young with much thinner allow plating than that.
    Pushed a 70' Roamer with thinner thru 20 years of Bahama abuse.

    The name escapes me at the moment, some world class custom hit a pinnacle on the left coast (your side) years ago.
    Over built, single engine (engine fell out the hole during salvage). He thought he was tough.
    Since, "boat name" II has completed world trips.

    Not to be depressing,, my point,, Are you breaking ice on the north west passage?
    1/4" alloy (fancy alloy numbers involved) is tough.
    I can not answer your math but a proper surveyor can..

    Just don't hit any underwater mountains.

    I question smart range with 5.9 tuned to 450hp. Not sure here but question it.

    ,rc
    Last edited: Oct 6, 2015
  3. RER

    RER Senior Member

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    The 450C is 8.3 litre. I hope they've changed the oil since 4-9-04.
  4. tristanrowe

    tristanrowe Member

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    6.3(ish)mm is fine for hull plating, ask to see the scantlings as you may find it's thicker on the bottom strakes or even the bilge return - either way as long as people haven't dropped steel fasteners etc in salty bilge water you should be fine. Looks like a handsome vessel, my type of boat for sure.
  5. Capt Ralph

    Capt Ralph Senior Member

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    Brain fart. Saw the older style heat exchanger and assumed it was a 5.9.
    Glad it's the 8.3. Without that computer stuff, may even be better.

    I'm seeing more & more (computerized) of the 8.3's in several hp tunes. Positive comments from owners. We service a few and I really like them.
    We also service an American Tug with a single 8.3/500. What a sweet setup. Slow and gets great range, hit the throttle and she moves.
    The owner just bumps the idle up and runs 6 kts all day. The engine does not load up or seem to have any problem running slow all day.
    As long as nobody wants to go water skiing, twin 8.3s could be a good setup in this 72 footer.
    Last edited: Oct 7, 2015
  6. StuartT

    StuartT New Member

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    Yes, they are 8.3L Cummins engines, not 5.9's. Sweet, also yes. Cruise 10 knots, WOT 13.

    That is a good suggestion to look over the scantlings and see if the critical areas are thicker or were reinforced. I have to assume that this was not overlooked during the engineering of this 50 ton vessel. There is a lot of information online regarding comparisons between steel and alloy as hull material and there seems to be general acceptance that marine alloys will be roughly 50% thicker then marine steel for the same strength.

    For example, from: www.kastenmarine.com/alumVSsteel.htm
    "Since an aluminum structure is designed to a deflection criteria, all scantlings are made somewhere around 50% or so larger than they would be for a steel structure. For the sake of an easy example, what would be one inch of plate thickness on a steel vessel would be approximately one and a half inches of plate thickness on the aluminum vessel in order to achieve the same rigidity of structure. "

    This Al trawler seems to be on the light side of the equation compared to steel yachts I have researched, but then again, I could be missing something about hull design.
  7. Scallywag

    Scallywag Member

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    IMO 1/4 inch is more than adequate. Some naval architects prefer lighter hull design and rely on more ribs/framing to create the necessary rigidity. Others prefer to use thicker plate and less structure. You could say my aluminum hull is very overbuilt and it is 1/4 in places, 5/16 in others, and I believe thicker (3/8) around the prop pockets. I'm sure if you look at the blueprints you will find something similar.

    BTW I have Cummins 6CTA 8.3L at 400HP and love them on my 70 footer for our style of boating. Cruise at 9 knots and WOT is 12.
  8. Scallywag

    Scallywag Member

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    I just read "The Full Story" about North. My boat was built by Marine Builders Inc. in Utica, IN in 1992. Looks like North was built by Darling Yachts in Jeffersonville, IN. I'd be willing to bet some of the same craftsmen worked on both boats.
  9. StuartT

    StuartT New Member

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    I'd bet you are right on that. Must be a lot of inbreeding of craftsmen among the various yards when they are in the same general area.

    I think the 'North' website is an older write up on the boat from circa 2005. There are some 0bvious discrepancies between that and the current listing that are notable, not the least of which is the oil filter date. And I am sure a number of the pictures in the current listing were plagiarized from the 'North' website.

    Seeing it in person will be telling and I hope to do so on a trip to Seattle in a couple weeks. Reminds me of a 60 steel trawler I was interested in which was located in San Diego. The pictures were tantalizing, but when I actually saw the boat, I was disappointed.
  10. olderboater

    olderboater Senior Member

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    That's something I really wish sites would require the photos to have been taken recently. On a home, you don't pull out old photos. The broker goes and takes photos.
  11. karo1776

    karo1776 Senior Member

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    Is this hull thick enough, well yes, but it depends. If the framing system is transverse with plate with ribs (vertical framing) usually the hull plating is thicker than a longitudinally framed boat that has only stringers (horizontal framing). Why is the 'hogging' forces. And, hull plating can be thinner that either framing relying soling with ribs or stringers if it has both.

    Thickness as speed goes up on boats must increase due to the pressure load against the hull from water impinging against it to prevent buckling. Or, in the case of hull plating over the props plating vibration from disturbing the enjoyment of the yacht.

    Hull plating varies with type and purpose of yacht. Sailboats have proportionally thicker plating for the same design speed than power boats (fairness of hull is more important). But the scary think is if you are talking composite hulls their outer skin thickness can be quite thin due to the presence of core to reduce weight.

    Usually even now-a-days hull thickness is more than necessary as safety margin against puncture, and often because of classification societies that still rely on rules of thumb scantlings. Which is how all boats were designed before the computer age. As a kid they still produced ships with a few crack stop riveted seams... now they don't as design technique and metallurgy have made the old by experienced guess or golly largely obsolete... reducing safety margins in plating thickness because we know more and can calculate better.

    Hull plating can be frightfully thin as a ship goes up in size proportionally the hull plating gets thinner. Its like the difference between a typical sized yacht and its plating thickness and an cruise ship.

    Think of it as much like the banking industry where the safety margins get thinner in proportion to the banks size or the outstanding debt held... but in that case the boat won't sink if it hits the rocks as the government will plug up the leaks... by magic of inflation and tax.
  12. StuartT

    StuartT New Member

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    I would have no clue on the proper design criteria but can totally understand where the selection of stringer and ribs, their alloys, structural arrangement, etc. play a huge part determining hull thickness. It may very well be that this 72’ trawler is well designed using ¼” plate.

    It’s just that as I noted above, another 60’ trawler I have looked into was constructed with 3/8” A36 steel plate, which makes the 1/4” plate on this aluminum vessel 33% thinner rather than greater. I recognize that two entirely different design criteria are probably at work here, but the thickness disparities in the hulls (as comparted to Michael Kasten quoted below) is too diverse to ignore without questioning whether the aluminum trawler might have been built with a completely different set of objectives, like strictly inland or costal waters for example.

    And before someone brings it up, yes, a knowledgeable, experienced and qualified marine surveyor will probably be able to review the scantlings, take the measurements and properly advise me. But I have to start somewhere, and posing the question here is a lot less expensive to try and flush out something obvious than blindly ordering a haul out, which requires a written offer with deposit, and a commitment to the cost of the surveyor. Why not try asking first?

    Another reference: http://www.aluminum.org/sites/default/files/Aluminum_at_Sea.pdf

    “According to marine design engineer Michael Kasten of Kasten Marine Design, Port Townsend, Wash., an aluminum hull designed for equivalent strength and stiffness to a steel hull would be about 50% thicker but lighter by as much as 50% and would have a 30% greater dent resistance and 13% greater resistance to rupture.”
  13. karo1776

    karo1776 Senior Member

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    StuartT:
    Perhaps this will help... in the 1980's Jongert made two near identical motor yachts, one steel "Impossible Dream" now, and the other aluminum "Grand Cu" both are on the market and both are owned by members of YF... and both owners know the scantlings of their respective little ships as Jongert provided those routinely to customers... and Herbert Dahm the broker that sold most of Jongerts yachts back in those days was and is always good at serving his clients, real gentleman too. So some research might help you. Jongert is back in operation sort of, the owners are around, though I cannot remember who the owners are (forum names you know) but you could call the brokers, or Herbert or ask around here and likely find out.

    Comparing the two apples to your apple and orange situation might help.

    Also, I bet both owners have pictures of the hulls out of the water... so you can compare hull plating fairness. Why can you do that and get and idea of performance of steel to aluminum... IS because Jongert back in those days made some of the fairest hulls in Holland... and little filler was used. Sadly the Arabs own the thing now after a couple landlubbers.... Jongert and Huisman were really good at hull fairness too... the owners yards were born to demand it... they are all gone now. The motor boats never have fairing done to the hull so you should be able to tell something by seeing the two out of the water after many years (near 30 ) of use. If they are still fair and your smaller boat situation seems to me to have plenty of thickness you should get a sense of proportion. Don't read much into this but an aluminum deforms less from use than a steel hull... bounces back... but just take it into mind.
    Last edited: Oct 12, 2015
  14. Scallywag

    Scallywag Member

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    Keep in mind... steel hulls are built with thicker than required plating to allow for "wastage" as the plating rusts away. So, just because a particular hull was built with 3/8 inch steel, it doesn't mean that is what was structurally required. Plate thickness is increased to allow longer service life.
  15. Scallywag

    Scallywag Member

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    20151011_150804.jpg Just to add some visuals to the conversation. Here is a piece of 3/8" from my hull.
    Last edited by a moderator: Oct 12, 2015
  16. karo1776

    karo1776 Senior Member

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    Aluminum one, owner has CD with scantlings
    Its owner name on YF is rmjranch you can contract here.
    Here is a link to the Aluminum Jongert showing hull out of water, now renamed Grand Cu.


    And here is the current sales listing for the Aluminum one above.... wow owner claims to have plans on CD see...
    http://www.hmy.com/used-yachts/_78_...h_2014_Naiad_Zero_Speed_Stabilizers/4850898/2

    I could not find it on Yachtworld, but it is on Yatco as Grand Cu
    http://www.yatco.com/vessel/info/217025/79ft-24m/1987/jongert-for-sale-aventura-united-states

    Steel one with scantlings
    Here is the sales listing for the Steel one...
    http://www.northropandjohnson.com/yachts-for-sale/3375-impossible-dream/
    and at Yachtworld the listing actually has the scantlings see pasted below.
    http://www.yachtworld.com/boats/198...Fort-Lauderdale/FL/United-States#.VhvyWbQ07dc

    CONSTRUCTION Steel Jongert, Impossible Dream
    Built to German Lloyd’s full classification.

    Hull: 8mm welded steel keel plating, 6mm and 8mm welded steel bottom plating and 5mm topsides plating. Hull framing includes 180mm by 80 mm by 8mm bottom longitudinals, 75mm by 8mm topsides longitudinals, and 75mmby 8mm traverse framing on 400mm centers with 8mm stringers welded to the frames. Integral tanks within the vessel’s hull envelope provide additional hull structure.

    Five watertight bulkheads
    Main Deck: 5mm welded steel plate in 120mm by 80mm by 8mm longitudinal framing and 75mm by 8mm transverse framing.
    Superstructure: 5mm and 6mm aluminum plating welded onto variously sized “T” bar,
    angle and plate aluminum framing on approximately 400mm centers.
    Shafts: 80mm S/S within sealed oilbath tubes
    2” high density foam on hull sides and floors throughout (2007)
    New teak decks (2007)
    Complete exterior LP paint job (2007)
  17. StuartT

    StuartT New Member

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    Scallywag, that is disconcerting to think a steel boat hull specs are "padded". Carrying all that extra weight around just to offset rust? That would be enough to put me off steel right there.

    karo1776, I am not sure what you are trying to tell me. You found some specs on Impossible Dream and posted them. So we have two boats by the same builder (BTW that all aluminum one is called Grand 'Cru', not 'Cu") but we do not have specs on Grand Cru (previously Morning Cloud) so no way to compare them. The owner of Grand Cru apparently has the CD with the scantlings, but it would be inappropriate for me to request a copy.

    You picked out these two for a reason but I do not understand the point you are trying to make or how it helps with my question.
  18. karo1776

    karo1776 Senior Member

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    My point is this:

    1. Simply the two boats were made for the same purpose.
    2. That purpose is similar enough to yours.
    3. The boats were made by the same manufacturer and naval architect.
    4. The boats were of similar size.
    5. The boats were made only a year apart... close enough.
    6. The boats were made of the two different materials you are concerned with Steel and Aluminum.
    7. It situation is rare so a ready practical comparison could be made between thickness of plating, frame spacing and stringers between identical boats, identical manufacturer, at the same time by the same naval architech... etc.
    8. Doing that comparison would help give you a practical sense of proportion to make judgment.

    I am quite sure the owner of Grand Cru would share this information with you as a member of the forum... why not message him and ask ... refer him to this thread... his forum name is "rmjranch".
    FYI, I would chose the Aluminum boat because aluminum bounces back better than steel so likely will retain its fair original state as manufactured. And, the bilges are easier maintained.

    A comment on Jongert and Huisman. Back in the 1980s if you meet the operators of the two yards and shook their hands... you would know... they knew how to build boats. Few do now and run the companies. Wolter's hands were very strong and massive in particular... too bad the stress of building Jim Clarks dreams over stressed him at the point in life they came together. And, I doubt you get a home cooked meal when visiting the yard now. To bad that Jongert is no longer what it was not even faint shadow... and I fear the same for Royal Huisman oh well... J'ai peur d'aimer un souvenir "I am afraid to love a memory"...
    Last edited: Oct 12, 2015
  19. StuartT

    StuartT New Member

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    Okay, now I see where you are coming from. Thank you for that, and I will PM rmjranch and see if he can help.

    Sounds like you go back a ways with these builders. It is sad to see what time often does to successes like these. I don't know of them personally, but it is a tale I hear all too often.
  20. StuartT

    StuartT New Member

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    Well, I guess I need another kind of help. I do not see a way to PM (private message) a member on this forum. Am I missing something or is it not a forum option?

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