Aluminium vs Steel

Discussion in 'Boat Design' started by Wynand N, Dec 3, 2004.

  1. D'ARTOIS
    Joined: Nov 2004
    Posts: 1,068
    Likes: 18, Points: 0, Legacy Rep: 321
    Location: The Netherlands

    D'ARTOIS Senior Member

    I think that I shall take the two versions in a visional example: Yago: what do you exactly mean - I quote : the lighter the casco the lower the vcg and therefore the higher the righting moment - unquote

    With what? Do you mean to say that: I take to hulls rigged in same style with dito ballast then the righting moment of the alu hull is higher that the steel one.

    Is that correct?
     
  2. D'ARTOIS
    Joined: Nov 2004
    Posts: 1,068
    Likes: 18, Points: 0, Legacy Rep: 321
    Location: The Netherlands

    D'ARTOIS Senior Member

    Here is an example of an aluminium and a steel version of the same boat; both boats are identical in dimensions:

    Then you will recognise that we have in the alu type a weight difference of
    700 kilo's - NB! in the ballast, as part of the deplacement.

    To say that if you add 700 kilo's in way of extra load does not say that you have compensated for the manco in 700 kilo ballast.
     

    Attached Files:

  3. yago
    Joined: Aug 2004
    Posts: 118
    Likes: 2, Points: 0, Legacy Rep: 17
    Location: Budapest Hungary

    yago __

    D'artois, I am sorry if I misunderstand you, here is what I mean, on your example:

    If you take any steel hull and on this same steel hull you reduce the weight of the hull/deck/superstructure by replacing it with lighter material (thinner steel, or alloy for example) you will get a boat that is overall lighter and the CG has moved down inside the entire distribution of the different masses, simply because the shell's center of gravity before was already above the total CG of the boat. On most boats the VCG is somewhere near the waterline, on modern production yacht usually slightly above, but just by looking at any profile it becomes clear the larger part of the entire shell is clearly above the waterline even if usually the bottom is stronger built. You could very well build a steelhull out of 4mm throughout and 3 mm for the deck, and your shll's CG would most lilkely be be above the VCG of the entire boat. I just did a rough estimate and the CG of the my shell is about 20 cm above the boats CG. (Exceptions would be hulls with extremely heavy bottom plates, that act as ballast in addition to being a structural part of the hull. In this case it would be proper to seperate the added weight from the struturally required weight of the plate and count it as ballast for comparison.)

    If now you would bring this lighter boat back down on its lines by loading it intelligently below the CG, for example by adding tanks in the bilge or stowing beer under the bunks you will have a boat of the same displacement (mass), the same lines and volumes (swims on it's waterline again) and a VCG clearly lower than before, meaning longer arm with identical total mass for a loaded boat.

    That the yard in your example has added some of the gained weight to the ballast will improve the stability even further than if you would just add stores(seven hundred kg will move the CG further down if placed deeper) - and there is still some weight gain left over for beer under the bunks. Result: a more stable boat, stiffer, can carry more sails and thus performs better etc - in my book that makes a superior boat. Carried to the extreme you get uncomfortable boats with jerky movements, but we talk medium to heavy cruising boats here.

    How much you gain and is that worth the price... ? Don't know. ;)
    I choose steel for my own boat, mostly for much the same reasons that you give, it's strong, cheap, easy to maintain & repair, also very pragmatic reasons like building site, tools and simplicity for me... but if I make abstraction of most of that, If I would just fork out the cash and buy it, my Yago would in many ways be a "better" boat in alloy, at least for my personal taste & criteria.
     
  4. D'ARTOIS
    Joined: Nov 2004
    Posts: 1,068
    Likes: 18, Points: 0, Legacy Rep: 321
    Location: The Netherlands

    D'ARTOIS Senior Member

    I am completely with you. But for a world cruiser you better stay with steel and an alloy boat is nice for the freak that can do the job himself. Don't forget that every material has it's own secrets, only known by the craftsmen whose work it is to converse from flat sheets a beautiful hull.
     
  5. Wynand N
    Joined: Oct 2004
    Posts: 1,258
    Likes: 144, Points: 73, Legacy Rep: 1806
    Location: South Africa

    Wynand N Retired Steelboatbuilder

     
  6. Skippy
    Joined: Nov 2004
    Posts: 568
    Likes: 1, Points: 0, Legacy Rep: 12
    Location: cornfields

    Skippy Senior Member

    Yago seems to have a point on the CG issue. The heavier hull adds stability only if its CG is lower than that of the keel, bulb, rigging, crew, and cargo, all taken together but without the hull. And even that is not quite enough, because the lighter boat can get the same speed with less sail area, which means that the rig is lighter and has a lower CG. So you really have to compare the CG of the heavy hull with the CG of the boat with the lighter, lower rig, minus the hull, since that is the true alternative that would be possible with the lighter hull.
     
  7. yago
    Joined: Aug 2004
    Posts: 118
    Likes: 2, Points: 0, Legacy Rep: 17
    Location: Budapest Hungary

    yago __

    As a German designer I would leave it to the skipper's competence to make sure that all beer gets inside his head, not on top of it, whatever the position of the boat. For the american market I will prepare some appropriate stickers with warnings of course ;)
     
  8. D'ARTOIS
    Joined: Nov 2004
    Posts: 1,068
    Likes: 18, Points: 0, Legacy Rep: 321
    Location: The Netherlands

    D'ARTOIS Senior Member

    Wo haben Sie denn dieses gutes Bier versteckt mein lieber Mann? Ich suche aber kan es gar nicht finden, - wo sagen sie? Im Bierkeller......? :D
     
  9. Skippy
    Joined: Nov 2004
    Posts: 568
    Likes: 1, Points: 0, Legacy Rep: 12
    Location: cornfields

    Skippy Senior Member

    Kielgebunden kaltes Blut bewahren.
     
  10. yago
    Joined: Aug 2004
    Posts: 118
    Likes: 2, Points: 0, Legacy Rep: 17
    Location: Budapest Hungary

    yago __

    "Kielgebunden" is very nice, have to write that down somewhere ;)
     
  11. jdhowland
    Joined: Feb 2005
    Posts: 6
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: North Florida

    jdhowland Junior Member

    Gentlemen, this is a great discussion, glad that I found this forum. I can't help but add my two cents worth:

    If you add two U.S. cents (pennies, almost pure copper) to the bilge, bathe in salt water, add a little stray current now and then, the difference between steel and aluminum will be quite evident!

    John
     
  12. Skippy
    Joined: Nov 2004
    Posts: 568
    Likes: 1, Points: 0, Legacy Rep: 12
    Location: cornfields

    Skippy Senior Member

    How bad is it to put a wood or glass deck on a steel hull?
     
  13. JimCooper
    Joined: Feb 2005
    Posts: 81
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Varies, Aberdeen

    JimCooper Junior Member


    You don't need any stray current it makes its own, just need the 2 metals and a bit of saltwater. Skipper had to replate some bottom bits of the afforementioned fishing boat after stowing the spare coil of copper pipe in the storage well, ended up burried under some 300m of wet chain.

    Bout a month later we were wondering where the water was coming from, engineer spent ages finding it. Had a Holiday on full pay in a Norwegian port while the agents found a certified aluminium welder to fix her up and replate about 1 sq meter of the bilge! Should have concreted and painted them I suppose.

    Ever here a scotsman swear ?
     
  14. D'ARTOIS
    Joined: Nov 2004
    Posts: 1,068
    Likes: 18, Points: 0, Legacy Rep: 321
    Location: The Netherlands

    D'ARTOIS Senior Member

    I can imagine something.......look we all know of course what can and may happen with aluminium, nevertheless here, on this forum are many devil's advocates. Sometimes we change roles, the ones in the defence are the prosecutors and vice versa.
     
    1 person likes this.

  15. Wynand N
    Joined: Oct 2004
    Posts: 1,258
    Likes: 144, Points: 73, Legacy Rep: 1806
    Location: South Africa

    Wynand N Retired Steelboatbuilder

    In 1990 I commisioned the Dix 38 (then Force38) as a raduis chine
    steel hull with fibreglass deck. She was designed by Dudley Dix. The idea was to speed up production as the deck take longer to build than the hull and better corners, shapes etc etc can be achieved with FRP and to save weight.

    Only the first steel hull (frameless) was and the plug for the FRP deck mould were built when I decided to close down shop for various reasons. It was a good idea at the time and still is.
    The Force38 is now known as the Dix38 with steel deck.

    I will post a picture later of the adverticement of the said boat when I can lay hands on it
     
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.