can you decrease plate thickness or use FC for a tug?

Discussion in 'Boat Design' started by tugboat, Apr 17, 2010.

  1. BayouDude
    Joined: Feb 2005
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    Location: Louisiana

    BayouDude Junior Member


    There is nothing wrong with a round bilge design on a tug and in fact I am currently working on getting a 120' tug built in the 50's (molded hull 1-1/8" bottom plate) ABS classed. They make better sea keeping boats as you don't get that snappy righting action from the hard chine and get a few more knots of free running speed. The problem at least in our area that you would be hard pressed to find competent ship fitters to build you a molded steel hull tug. Seems to be a lost art down here and it adds to the labor costs of construction.
    In regards to your cooling arrangement, if you have an foil shaped skeg you can use that for cooling (We do this all the time for winch engines) or you can wrap a couple lengths of channel to the bottom of the hull. I have a little 54' tug I can send you an arrangement on if you would like. The house doesn't have quarters as it was used to do day time shift work but you could easily redo the house arrangement. My dad has a little 25'x12' hard chine tug that he uses to move stuff around at my family's shipyard. Has a pair of NA inline 6cyl cummins making about 110 hp a piece. Only has a little control station and no quarters. It pushed the hell out of our house boat before we sold it and now it just moves around work pontoons and helps get tugs off and on of dry dock.
  2. tugboat

    tugboat Previous Member

    Bayoudude, check this out---i believe this is 3/16th plate--the woman building it designed and built this hull- it is round bilge-- she did a great job--perhaps she is a fitter/welder?..anyway after i saw that she just used lots of good clamps to pull in the plate -i reasoned it shouldnt be hard to do it on the 45 ft ST design since those frames are fairly
    gentle curves.
    i have a set of the ST plans coming --would you like to see a copy of the lines plans when i get them?..i could photo copy the lines drawings and PM to you?
    I dont think I would be violating any copyrights...(?)

    see below for the plating details to this round bilge design...amazing how fair it was. I do think the frames were nested and cnc'd...
  3. BayouDude
    Joined: Feb 2005
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    Location: Louisiana

    BayouDude Junior Member

    That is pretty impressive what she accomplished with the limited tools she had on hand. i am not sure why she clamped the steel like she would be building a wooden hull. I would have welded some temporary pad eyes and used come alongs to pull the plate and hold it for welding.
  4. tugboat

    tugboat Previous Member

    bayoudude, I think thats the method taught in Thomas Colvins book(i own a copy). it actually seems like a good idea- but Colvin also does talk about using come alongs...and thats what i would prefer...perhaps she gets a more fair plate by using the clamps?...but it is impressive...i would have tacked
    the plate first then clamped the plate into the hull or used come alongs P&S.
  5. tugboat

    tugboat Previous Member

    Hi Tad -its been a while...sorry..i should have mentioned this is not going to be for commercial use- its purely a character tug for pleasure. but i may use it as a "crossover" vessel. i have seen the "ville class" tugs,
    where plate thickness was varied - some built with 1/4 inch and others 3/16. didnt affect them apparently..
  6. tugboat

    tugboat Previous Member

    update- i finally started the project. I have the keel being ordered. its 1" thick x 4.5 wide" and the stem is going to be bent with commercial equipment. The tug is the navy ST class 45 x 12.5" RADIUSED(not a true round bilge) John G Alden design. The plans are from 1952. or dated such. The engine: traded my d4600 for a d318 cat with 3:1 trans and a 36 inch x 34 prop. I will use bare minimum scantlings. the reducition in size allows the 3/16th plate to work quite well. The tug is for pleasure and maybe some "crossover" work. but i wont register her for commercial use- not going to build it to LLoyds or ABS. just want to look like the st and keep "most" of the strength.

    so my question now us- with the reduced LOA from 45 to 40.5 (10% reduction) shouldnt it be the right thickness at 3/16 instead of 1/4 inch plating (which cannot be worked by an amateur like myself)- and will the reduction cause the boat to look much different?
    im very curious if it will change the profile???
  7. naval ark
    Joined: Mar 2006
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    Location: U.K.

    naval ark Member

    Length Overall has no direct bearing on plating thickness. Of more interest would be stiffener scantling and arrangement, and loading conditions derived from area of operation, in terms of usage at-sea and in-port (especially when considering what you're likely to go bump against).

    I can't imagine that the 1/16" difference itself would be visually discernible, although you may choose to be more careful about your heat levels during construction if you wish to avoid plating distortion.

  8. tugboat

    tugboat Previous Member

    Naval Ark...i truly appreciate your kind support.

    one idea i had was to use longitudinals. as this design has none (no longitudinals- only transverse frames) and that might stiffen up the hull more?

    do you think the profile will change much with a 10% reducion in LOA? the vessel is 45 ft- i could build her 40 for cost reduciton?..or is it not really worth it?

    and will i need to reduce the wheelhouse dimensions by the same 10%?...your help is much appreciated. ill be sequencing my welds. i.e. what i do to port ill do to strbd. do small welds mostly staggered. boats ive looked at seem to be fair doing it this way...

    feel free to PM me ...if you are an NA, i may ask to buy some of your services...
  9. naval ark
    Joined: Mar 2006
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    Location: U.K.

    naval ark Member

    I'm not sure I could provide meaningful answers to any of the questions here.

    I don't really know the details of your situation or even what technical information you have at hand - the reduction of LOA you describe would require a modification to the forebody lines, revision of the weight estimate and longitudinal centers, possibly respacing of the frames, bulkheads, superstructure connections, etc.

    In general, I doubt a significant cost reduction would be realised.

    Changing the hull structural arrangement to a longitudinally framed system suitable for use with 3/16" plate could certainly be done. Whomever you select to carry out this work might even tell you that such a modification would not be necessary, that the structure as currently shown has sufficient margin to carry the thinner plating.

    1 person likes this.
  10. tugboat

    tugboat Previous Member

    Thanks again...
    do you know if its possible to space some frames further apart and just add longitudinal stiffeners to get the same strength? presently the scantlings call for 33 transverse frames. 3/8 inch x 2.5 inch flatbar. if i used 1/4 inch frames and 1/4 x 2.5 inch stiffeners spaced every 16 inches O.C.?
    i guess i also wondered if building the vessel upright or inverted would be better?
    seems like everyone has varying opinions as to which is best...?
  11. tugboat

    tugboat Previous Member

    ["Whomever you select to carry out this work might even tell you that such a modification would not be necessary...."]

    hey again NA

    as mentioned in the book "boatbuilding in steel by Gilbert Klingel" he suggests using the same framing as the thickness of the plate and using longitudinals spaced 16 inches for 3/16th frames. seems reasonable to me at least...
  12. sorenfdk
    Joined: Feb 2002
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    Location: Denmark

    sorenfdk Yacht Designer

    Did Gilbert Klingel have any detailed knowledge about your design (rescaled or not) when he wrote his book?
    It's practically impossible for anyone to answer your questions without any detailed knowledge of the design - we can only come up with guesstimates!
    Are you willing to bet your life (and maybe the lives of your near and dear ones) on guesstimates?
    And what about insurance? I don't know about US insurance companies, but the european companies wouldn't insure anything with scantlings based on guesses made by someone you met on the internet, but don't really know.
  13. apex1

    apex1 Guest

    There is another issue to be addressed here!

    The original design was for a commercial Tug, which required substantial weight to work properly.
    Now you try to save weight (mainly for cost saving I guess), but forget, that this is not possible with the original design! That NEEDS the weight built in.

  14. tugboat

    tugboat Previous Member

    Richard, what do you think the Atkins tug 50 ft design would weigh? just curious since would it mean that a steel tug boat(50 ft) and the wooden one would need to be the same??.if so i could just adjust my scantlings accordingly???...just a thought

  15. Tad
    Joined: Mar 2002
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    Tad Boat Designer

    I can't really get any clear meaning from the above...but...

    There is no way to assess the weight of a vessel by looking at pictures....

    If you build one hull form at two different weights the two boats will float differently....built it lighter and it will float it heavier and it will float deeper......guaranteed.....trim, stability, performance???? all unknown..

    I think Richard tried to tell you this is what will happen if you change the scantlings of a design....the boat then floats different is unknown and pulling figures out of thin air is pointless......if you want a serious answer post construction drawings, hydrostatics, and weight studies for each vessel.....
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