Power Cruiser for Florida/Bahamas

Discussion in 'Boat Design' started by Starhop, Nov 23, 2017.

  1. Starhop
    Joined: Nov 2017
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    Location: United States of America

    Starhop Junior Member

    Hey Y'all!

    Been lurking here for a while. Anyway, I will be attending college down in Florida next year, and I am looking to build a overnight cruiser capable of safely making the gulf stream crossing between Florida and The Bahamas between now and then. I envision something in the 25-30 foot range with a proper head and galley area. Ideally, the boat would be a full displacement trawler capable of cruising at at least 6 knots, with a small tender in tow (or perhaps I'll skip the tender and lash a couple of 2 man sea kayaks to the rails). I am unsure if I will use steel, aluminum, or wood/fiberglass construction. I would love to have inboard power, but suspect I will end up using an outboard system due to simplicity. Luckily, I am blessed to have access to facilities and tools that could support such a project. I realize that this sounds quite ambitious for a first time builder, especially one 18 years of age, but I have grown up around boats and the ocean for my entire life and have a decent understanding of life at sea, spending many of my summers cruising. I am hoping to get some advice as to where to start with the design process. I would like to be safety oriented in my approach, over engineering rather than skimping on anything. Does this sound at all sane? Are my expectations ridiculous? Does such a project seem possible, and if so, how should I best start my journey? These are all questions I'd love to get advice on. Sorry for a long rambling post, lol.

    Kind regards,

    Starhop
     
  2. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    Welcome to the forum and I hope that, in it, you will find what you need.
    To design a boat it is very convenient, even necessary, that you have knowledge, experience, in navigation. But you must also know something, some think not much, about naval architecture and marine engineering. What is your level in these subjects? Do you want to design the boat for yourself or have the collaboration of a professional?
     
  3. Starhop
    Joined: Nov 2017
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    Starhop Junior Member

    Thank you for the warm welcome, sir! I'd have to say basic, in both. I plan to take a comprehensive course in marine navigation before I go much further with this project. My current experience in navigation is limited to that of coastal cruising, which I managed successfully. As far as marine engineering is concerned, the best I've done, I'm afraid, is restoring and working on boats <30 feet. I am open to consulting with professionals, but I'd like the challenge and learning experience of doing most of the design work myself. At the very least, I would want a professional to review my work before I start the actual build.
     
  4. TANSL
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    TANSL Senior Member

    Do you know at least how to draw, create 3D models, can you use some naval CAD program ?.
     
  5. Starhop
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    Starhop Junior Member

    Yes, that I can do. I have the free version of DELFTship right now, but I am also open to other software.
     
  6. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    6 or 7 knots is horribly slow, it gives you little or no capability to out-run local weather events like storms, and strong currents can drastically alter your progress. It also means time spent where you want to be, is sharply reduced, especially for week-ends etc. Sure it might be a lot cheaper to run, to settle for slow, but if the budget allows, going at least twice that speed appeals much more.
     
  7. Starhop
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    Starhop Junior Member

    I fear that going faster than about 7.5 knots for a 30 footer would require me to consider a planing hull, if my mental mental math is correct?
     
  8. TANSL
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    TANSL Senior Member

    Not at all and for a comfortable passenger ship, you should go to a displacement hull.
     
  9. Starhop
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    Starhop Junior Member

    I thought that assuming a displacement hull, the max hull speed for a 30 foot boat would be 1.34 x √30 = 7.34 knots?
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Obviously it depends on what time frames are involved, if you are restricted to a week-end, it will be time to come home soon after arriving, well, almost ! Yes, a pure displacement hull won't have much speed potential beyond that 7 knots, but will be more comfortable, by and large, than boats designed for greater speeds. And your running costs if using a diesel inboard will be somewhat lower. But you will still be bobbing around for a long time in what is a small boat on a big sea. This is potentially "big water" and seaworthiness has to be job #1. A boat that jars and pounds, will soon cure you of the desire to repeat the experience very often, though if you are very selective in picking your days (week-ends always seem to be rough) :rolleyes: planing boats can do the job safely and with tolerable comfort.
     
  11. TANSL
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    TANSL Senior Member

    For me, who have designed some passenger ships, planing is not a desirable condition at all for passenger ships.
    According to the ISO standard, attached, a boat of 30 feet (9.1 m) at 15 knots at full load displacement, is considered as displacement.
    Snap41.jpg
    V, knots
    Lwl, meters
    We always have semi-displacement mode.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    15 knots for a 25- 30 foot boat, of "normal " proportions, not a catamaran or shaped like a rowing eight, will be either semi-displacement, or planing. The latter would need to be relatively light, and/or of modest deadrise, to be running at that cruise speed, effectively. Maybe he should consider a displacement cat, that way the higher cruise speed is achieved, and with a couple of smallish outboards, and the security of two engines.
     
  13. TANSL
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    TANSL Senior Member

    From what is said in the first post, speed does not seem to be a condition for the boat. As in any boat, not pleasure, the OP will have to calculate the most economical speed.
    We can talk about many things but without having an SOR it is not possible to say what type of hull is the best.
     
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  14. Starhop
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    Starhop Junior Member

    I can post my full SOR, if that would be helpful :)
     
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  15. Mr Efficiency
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    Mr Efficiency Senior Member

    Certainly !
     
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