Extending a hull

Discussion in 'Boat Design' started by Herr Kaleun, Feb 7, 2014.

  1. Herr Kaleun
    Joined: May 2013
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    Herr Kaleun Junior Member

    Does anyone have any experiences, knowledge or ideas on how practical it would be to extend a hull LOA/LWL forward.

    Vessel: LOA - 70' [21M]
    Beam - 15' [4.54M]
    Draft - 8.5' [2.59m]

    Keel: Long Keel
    Bow: Minimal rake
    Stern: Round/counter
    Rig: Bermudan stays'l schooner on aluminium spars.
    Eng: 6 cyl/120 hp
    Prop: Long-shaft to single 3-blade prop.

    Hull constructed in steel. Suspected of being designed as a naval pinnace (?), or auxiliary boat. Now an established and sturdy sailing vessel.

    This is a design feasibility study; taking into consideration no restriction to cost, other than materials.

    Envisaged 4.5m extension to be 'grafted' onto existing bow, commencing with shallow rake to just below waterline, then angled to steeper rake to stem head. Stations extended from point of maximum beam. Strengthening to existing hull at 'point-of-graft' and of the whole extension, for added safety in arctic conditions. [Nor'west Passage anyone?]

    Inclusion of WTBs/WTCs and hatches throughout hull.

    Essentially, I just would like to know how often these design changes are considered, with obvious stability, drag and efficiency factors accounted for.

    Have downloaded loads of stability and ship-building pdf gumpf from interweb, but there's nothing like the practical and theoretical opinions of nautical folk to bounce off...

    Any pointers anyone??? :cool:
  2. alan white
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    alan white Senior Member

    If you are trying to increase the strength of the hull forward, it's assumed you get this by doubling the plating but in doing so, your hull is very heavy forward. To balance this would be to essentially build a whole new hull, doubling aft plating and adding a lot of weight to say the least.
    Without much analysis, it looks like your safety and survivability is comprimised rather than enhanced. This doesn't get into performance or cost, which are a huge part of the equation.
  3. Herr Kaleun
    Joined: May 2013
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    Herr Kaleun Junior Member

    Hi Alan,
    Well yes, that would be my reservations to such a project. A sensible reply. But....

    I am only musing over the question as to how often these modifications are made. Plus, I would argue that perhaps the extra buoyancy would partially compensate for the extra weight. My concept takes into account that the original bow might be retained, making the 'flying fore-peak' as I call it, a water/airtight compartment. Though I concede that the weight of the redundant bow may be an issue...

    Alan, this is purely a 'what if' question really. I am merely interested in the idea of changing a vessels role, characteristics etc... The RN are renowned for changing the role of their vessels, turning old cruisers into commando carriers or adding an angled flight deck to conventional aircraft carriers, adding hangers to old frigates, etc... Vessels are also retro-fitted with bulbous bows, or sections of the stern cut away or duck-tails added, so clearly it does happen. I can however, see that these examples are mostly on large ships, not small, steel, vessels such as my subject schooner, size matters...

    I am not interested in the answer - buy a bigger boat!

    If one bought a vessel, because it was cheap or available at the time but, it was not quite big enough for ones requirements and that it was perhaps an old design that might benefit from technical advancements, would the notion of extending its hull form be considered?

    A fellow forum member has PMd me saying that he has considerable experience in this field, so it will be interesting to hear his take on this.

    Otherwise, I would in practice, agree with your concerns. Thanks for your input Alan.
  4. messabout
    Joined: Jan 2006
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    messabout Senior Member

    It seems to me that such a lenthening, by the bow, project would require that the plating be faired nicely. That implies that the new skin might extend from the new bow alll the way to near the center of the boat, some 35 feet plus the new 4.5 meters of it. That would be a lot of new steel, and a lot of added weight. The section of maximum area would therefore be sent farther aft which could ultimately affect the pitching/plunging behaviour of the boat.

    As the pointy end of the extended boat, being rather triangular, a disproportionate amount of weight versus displacement might become problematic. Hmmm. This is going to need some serious study.
  5. JSL
    Joined: Nov 2012
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    JSL Senior Member

    it would be nice to see a photo or drawing of the profile. Sometimes adding a new midsection is easier.
    I helped with a 60' to 72' conversion (steel schooner) some years ago and it went very well.

  6. Mr Efficiency
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    Mr Efficiency Senior Member

    So, what is the object of the exercise ?
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