A Question or 3

Discussion in 'Boat Design' started by Fred Etheridge, Jun 19, 2022.

  1. Fred Etheridge
    Joined: May 2022
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    Fred Etheridge Junior Member

    I am a bit more interested in larger ship designs .. like a panamax size tanker/bulker-self unloader 20-25 knots hull or LNG/hydrogen . Also a pair of steel sailing ships 1 39x195 wl and 2 65x195 wl but what rigging i am favoring a staysail kind of thing with roller furling.
    what kind of power and hull shape for the first ones?? thoughts?
     
  2. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Have you ever found any Panamax tanker or bulk carrier that can do 20 knots, never mind 25 knots?

    Re what kind of power and hull shape, think of a fast reefer type of vessel, that might be able to do 20 knots at full chat.
    But it won't be able to carry very much, and the fuel costs will be horrendous, compared to a tanker or bulker going at half that speed.

    Are these dimensions in feet for the two ships, being beam x waterline length?
    What do you want these sailing ships to do?
    Carry cargo, or passengers, or........?
    Do you have any sketches that you can post please?
     
  3. jehardiman
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    jehardiman Senior Member

    As Bajansailor says, there is no reason, ever, for a commercial tanker or bulk carrier to go that fast; most are in the 12 to 16 knot range with a 20% DHP sea reserve. Bulk cargo economics do not require speed, they require transport efficiency. As for the hull shape of any bulker constrained to a lock, they are 95% the length, width, and depth of the smallest lock chamber with a cylindrical/elliptical bow, a long PMB with midships coefficient of 95%, and a tucked stern with a block coefficient approaching 0.9.
     
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  4. Fred Etheridge
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    Fred Etheridge Junior Member

    there is in fact a requirement for merchant marine fleet oilers to do at least 20 knots with 2 screws. The sailboats were for cargo with perhaps some deck cabins thrown in . The narrow one a fast clipper like ship..the wide one with a deeper hull for more haulage , with whatever rig seems best , though i thought a lot of a staysail rig.
     
  5. jehardiman
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    jehardiman Senior Member

    As I specifically stated, commercial tankers; fleet auxiliaries and MSC ships are a totally different case.
     
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  6. bajansailor
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    bajansailor Marine Surveyor

    Look no further than the Grain de Sail webpage - they have a very successful 'prototype' vessel (that can carry about 50 tonnes), as featured on the website, and are now building a much bigger vessel.
    Our cargo sailboat Grain de Sail https://graindesail.com/en/content/14-our-cargo-sailboat-grain-de-sail

    The bigger vessel is probably like a slightly smaller version of what you have in mind re your fast clipper like ship.
    GRAIN DE SAIL SIGNS WITH PIRIOU FOR THE CONSTRUCTION OF ITS NEXT CARGO SAILBOAT "GRAIN DE SAIL 2" - Piriou https://www.piriou.com/en/grain-de-sail-signs-with-piriou-for-the-construction-of-its-next-cargo-sailboat-grain-de-sail-2/
     
  7. Fred Etheridge
    Joined: May 2022
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    Fred Etheridge Junior Member

    Thanks for your replies,if i did not undertand all of your comment right away sorry i missed that bit.
    So after looking about and going back over ship vids i was watching the vid on the nelson class hull...23 knots on a 2 shaft hull 35 thousand tons on 45k shaft hp.....so what way would that best be stretched out from 710 feet to 950... 3 80 foot sections 1 middle and fore and aft to full out the lines or just 240' in the middle for something like 50k tons total or so and build as a general fast hull to fit out however.
    the hull already does 23 knots at 710 feet ish some more total W L should in fact up her cruising speed or am i missing something?
     
  8. jehardiman
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    jehardiman Senior Member

    Yes, you are missing something. Just extending the LwL does not guarantee less resistance. A vessels drag is comprised of two parts, the wetted surface friction, and the form drag. Furthermore, the form drag is proportional to two factors, the body shape and the "wave" resistance.
    Realistically, the "wave" resistance is the interaction between the shape of the forebody and the shape of the afterbody (google Wigley hull); this leads "humps" of higher resistance and "hollows" of lower resistance as speed and length interact for most realistically achievable hull forms. See the attached figure.
    Skin friction, on the other hand, is effectively constant for most real ship lengths and speeds (google ITTC Line) and therefor only dependent on surface area. However there is a catch; extending the length of a vessel by some percentage disproportionally increases the wetted surface to some design factor, either wetted surface to volume or wetted surface to length. This has to do with where the extra wetted surface is placed, in the ends or in the PMB.
    So in the final analysis, there is a trade off between volume at speed or length at speed. For a given length/speed "hollow", it is better to increase volume than it is to increase length for transport capacity. And bulkers are all about transport capacity.
     
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  9. Fred Etheridge
    Joined: May 2022
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    Fred Etheridge Junior Member

    so perhaps i should cut some models together and see .
     
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