Pocket cruising boats

Discussion in 'Sailboats' started by Guillermo, Apr 30, 2006.

  1. Windvang
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    Windvang Yacht Designer

    You can design a "light" boat to carry a big payload. Lighter boats are much dryer to sail, faster and need less sail area, making crew work much easier.

    I designed a "light" Trintella 52 that only needed a self tacking jib to go much faster than the old "heavy" 150% genoa version. It is 14.2 tonnes and designed to have 3 tonnes of payload.

    The movement against big waves is less than the heavy version, but a bit quicker. This is not so much a result of the weight as it is of the increased speed. It is like the difference between a BMW versus a Chevy, it all depents on what you like. Me, I get seasick in a Chevy.
     
  2. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    OK, I see what you mean :)
    I think you are right.
    But if, for some reason, you want only 26 feet, should the boat weight 1 ton or 3 tons to take a load of 2 tons?
     
  3. sharpii2
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    sharpii2 Senior Member

    True. True.

    But the 'under funded' cruiser is happy enough to be on the water. She/he will hardly notice such trivialities.

    Also, light boats seem to have proportionately bigger rigs and smaller appendages than heavy boats.

    But I do agree with your premise. The over loaded light boat will be slower than the less overloaded heavy boat with the same load.

    Bob
     
  4. Windvang
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    Windvang Yacht Designer

    2 ton load for a 26 is a bit much anyway. I agree with you on the long and slender theory a 3 ton 30' is abouth the same cost as a 3 ton 26. Yet if you save weight on a 3 ton 26 you can trade part of that in for performance and part for extra load.

    My opinion is, if you only want to have 26 you have te be prepared to travel somewhat "light" that means more pitstops for taking on fuel, water and food. That might not be a bad compromise as the 26 will also be slower and less comfortable.

    The attached example is a 40' Kvase I reengineered from steel into foam composite. The hull was build in kevlar in order to get the same strength as the steel. Originally 15 ton, now 13 ton including extra generator, 250 l extra fuel and 120Kg. extra batteries. The only difficulty is getting rid of 3 m3 volume without sacificing to much interior space.
     

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  5. sharpii2
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    sharpii2 Senior Member

    52 ft is an awfully big boat. Especially for this thread. Smaller boats have to carry proportionately way more payload.

    The 20 footer I'm designing is expected to carry at least 1,000 lbs. And she'll be about 2,000 lbs light ship.

    She will be slow, however (3kts anticipated average).

    Bob
     
  6. Windvang
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    Windvang Yacht Designer

    I think 2,000 lbs is not extremely heavy for a 20' 1000 lbl payload is a lot however.

    What if you increase the length of your design by 10% and keep the same weight? You would need the same gear, sailarea and engine (if any). Assuming you build in ply (sharpie) that will still be 3 plates lengtwise for the hull sheeting and so probably the same cost, maybe 1% up on the overall price.

    Arthur
     
  7. Vega
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    Vega Senior Member

    Nice post!
    I agree with most of what you say, but not about this (if we are talking about a 28ft).

    Even if the light boat has no negative stability (or an equal AVS), it would be a boat easier to capsize. The light boat, for having an equal RM it would have a very deep keel with a small bulb and its roll moment would be much, much bigger compared with the heavier boat, making it easier to be capsized by breaking waves, or very disturbed seas.
     
  8. Vega
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    Vega Senior Member

    Nice post!

    I agree with most of what you say, but not about this (if we are talking about a 28ft).

    A light boat, even if it has no negative stability (or with an equal AVS), would be a boat easier to capsize than the heavy boat. For having an equal RM, the light boat would have a very deep keel with a small bulb and its roll moment would be much, much bigger compared with the heavier boat, making it easier to be capsized by breaking waves, or very disturbed seas.

    Yes, a small light weight boat can be relatively seaworthy, but not as much as a good designed heavy boat. For that (relatively seaworthy) it would have to be an expensive and very unpractical boat with a big draft, not to mention about safety problems related with a fragile keel. Not the kind of boat the market offers as the mass production 27ft, or the one sailors want as an oceangoig cruising boat.
     
  9. Windvang
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    Windvang Yacht Designer

    Why not the same depth of keel with a big bulb? The you can lower weight and increase RM.
     
  10. Guillermo
    Joined: Mar 2005
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    Guillermo Ingeniero Naval

    I've found this nice design from Albert Nazarov (http://www.nazarovdesign.net/design.htm):

    'Albatros 800' Transportable cruiser
    LOA=8.00m
    LWL=7.44m
    B=2.25m
    DSPL=1270kg
    SA=43.8m2

    This is in the far end of the spectrum, if we consider it against Golden Hind 31 (By the way, this boat is Design category A!) or Vancouver 28, i.e.
    What do you think about it?

    Cheers
     

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  11. Vega
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    Vega Senior Member

    To have the same RM of a heavy boat, with the same depth of keel (cruising sailboats have a small draft), the bulb would have to weigh so much that it would not be a light boat anymore:D
     
  12. Vega
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    Vega Senior Member

    That is a very incomplete information. What about draft, Ballast and type of keel (bulb?, weight of bulb).
    Show me the Rm curves of both boats and I will say something.;)
     
  13. Guillermo
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    Guillermo Ingeniero Naval

    I'm a little bit confused with your statement :confused:
    For the same size of boat, RM curves are quite different for, let's say, a long keeled narrow 'wine glassed hulled' motorsailer and a wide flat planning hulled one, with bulb at the end of a deep keel, both having the same displacement.
    Let's suppose they have the same freeboard and comparable deck arrangement. Watertightness supposed all around 360º.
    Most probably the flat and wide boat will have an all around better RM curve, with higher initial stability and also higher AVS, so a much bigger area under the RM curve than the motorsailer, thus providing a much better dynamic stability curve, which is really what we have to analize to judge the resistance to capsizing. Please correct me if I'm wrong, or if I misunderstood you.

    Note: I do not consider the possibility of a keel failure or other concepts. We are talking only stability here.
     
  14. Guillermo
    Joined: Mar 2005
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    Guillermo Ingeniero Naval

    Some interesting data for this boat:
    D/L = 332
    SA/D = 16.31
    6*HP/D = 12.04
    CSF = 1.67
    MCR = 31.71
    Cutter rigged.
    Lay-out: 3/4 people.
    Design Category: A

    Here we have a very nice ocean going, safe, heavy, sail-oriented motorsailer. I like very much this one.
     

  15. Vega
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    Vega Senior Member

    Let’s see :

    In the other post we were talking about two sailboats, one light and another heavy, both with no negative stability (AVS 180º).
    Both have the same RM ( with no negative stability) and that means that the force necessary to capsize any of the boats would be equal. This does not mean that the boats have the same curve (they don’t), it means that the area behind the RM curves would be the same (not the curves).
    As you have said, the lighter boat would have a sharper curve, with better initial stability and the heavier one would have a more rounded curve with worst initial stability but better final stability.

    Being the force for capsizing each boat the same, the boat that would be easier to capsize would be the one with bigger roll moment of inertia and that would be the one with the lower CG.

    That’s why a sailboat that has lost his rig is a lot easier to capsize, even if without the rig its CG is a lot lower. That’s because, without the mast, its roll moment is a lot bigger.
     
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