First Attempt: Design to Build a Log

Discussion in 'Boat Design' started by Bayden, Feb 3, 2020.

  1. Bayden
    Joined: Feb 2020
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    Location: Philadelphia

    Bayden Junior Member

    Good Afternoon All,

    While I have built several boats I have never designed one before. So I am starting this iterative log to follow the multi year journey from design through to launch. There is a good chance I will not be successful but I will enjoy the process none the less. I will be learning EVERYTHING from scratch, including the design software (although I am CAD/Blender proficient. So I absolutely appreciate any design feedback, suggestions or information that might be useful.

    This may end up being close to the OB-1 in concept.

    High Level Design goals:

    Area of operation: Great loop, down the east coast, and in to the Bahamas. Weather depending of course. I am not interested in true deep offshore work.

    Trailerable: Not in the sense that I plan on taking it too and from home every trip but it should be able to be taken home at the end of the season for iterative updates and re launched in the spring.

    Main Function: Family Cruising. Husband/Wife/Son for week long trips or greater. Little fishing while cruising would be nice too.

    Hull Type: Semi Displacement with an aim to avoid slamming as much as reasonable.

    LOA: 28 to 34 ft. I would love to go bigger but that is about the max myself, my friend, my family can handle and anything beyond 30 feet will be a significant uplift.

    Beam: 8' 6"

    Performance: comfortable at displacement speeds but with the ability to cruise at 14-16kt with a maximum speed of 18- 20kt.


    Range: This is a bit of a TBD. As far as possible. But I think I will need to get a bit further in the process before I am confident of what is feasible. At a minimum 250 miles with a 10% reserve at cruise.

    Power: I am still a bit up in the air on this. I would prefer an inline diesel/shaft. But I am also considering a stern drive and even a V drive. I would prefer to not run a OB if possible. The biggest + to a stern drive to me is the ability reduce draft as I figure most operations will be in the Bahamas (Exumas specifically) and may want to beach the bow. Generally speaking the water where I plan to spend the most time will be shallow.

    Accommodations: 1 bunk (wife and I) and the ability to convert the main deck seating into a bunk for my son (will be 4-5 by the time this is complete). Toilet/Sink/Shower. Small as possible galley. Small generator and AC.

    Construction Method: Plywood cored fiberglass construction for the hull. Foam cored fiberglass for cabin. I might go full foam as I get further into the design so TBD. Any thoughts would be appreciated.


    Anyway I think that is enough for now. Wish me luck this might be a spectacular failure.

    Regards,
    Bayden
     
  2. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    What do you need, specific help in certain calculations or the complete project of the ship following the SOR supplied by you?
     
  3. Bayden
    Joined: Feb 2020
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    Bayden Junior Member

    Afternoon Tansl. I do not actually need anything specific really. I would ask that if anyone sees an egregious errors in anything I post or if you have a suggestion please point it out. If you know of existing plans that are similar in nature I would appreciate that as well (Some framework to base it off of is always easier for an amateur).

    This is a significant undertaking for me (probably somewhat of an understatement) and I do not know what I don't know yet. I figured if I keep a public log of what I am doing not only can I solicit some help but it might be useful for others down the road too.

    Currently I am in the early process of building a 17 ft shallow V hull for fishing and general running about. Until I am finished with that I am just teaching myself more about marine architecture in general and working through the theoretical implications of this larger vessel. I doubt any physical work will begin on the larger vessel for 12 months.

    Regards,
    Bayden.
     
  4. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    I see. I'd like to help you so I am available for inquiries about specific questions.
    Start with a statement of requirements, as complete as possible, and a general layout plan where you see that everything you want is, with the correct dimensions.
    Use personal messages, if you prefer.
     
  5. Ad Hoc
    Joined: Oct 2008
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    Location: Japan

    Ad Hoc Naval Architect

    Welcome to the forum Bayden

    Well your route needs to establish two main objectives to see if it is indeed viable.
    1) The design
    2) The cost

    The two are dependent upon each other. So your first task, is to draw up either on scaled/squared line paper, a sketch of what you would like. If you can use/draw in CAD, even better, not not 100% essential.
    Once you have drawn up your layout, does it fit inside the dims you wish to use?...for example, no point showing space for a head when the width is only 400mm when measured, for example. Similarly no point showing an ER, but no room to actually fit an engine.
    Thus draw up the basics of the whole design layout you wish at a true 1 to 1 scale. And just draw any hull shape under it, for now. A simple mono or cat, which ever is your preference.

    Then from that sketch layout, you need to confirm what the weight of the whole thing is, since without this, you're always guessing and most likely it will be wrong if you just build it without this data.
    So for the hull...you will have drawn a hull shape that "looks" about right..in terms of length depth and beam. Just establish the girth of that hull, by simple sketch. Then use this to multiply for the whole length to give an area.

    Then if you know roughly what kind of weight per sq/metre your lay up is, then you have a weight x area x 2. The 2 for errors and unknowns, for now.

    So, you'll then have a worked up weight of your design. Everything from structure to engines to galley to whatever, then add a margin +10% to this weight.

    Then see if this weight suits roughly the hull...in other words, does it float, does it float up right and does it float on the draft you hope?
    If the answer to any of these is no...repeat the previous steps until it does.

    When all seems ok...then you need to establish the cost of this design and layout.

    Does it sit inside your budget?
    Again, if not, refine and repeat until it does.

    Until you have done the above procedures, you will not know what will or will not work and not how much it will cost.
    No point doing anything beyond these first simple steps... if you want the boat to be successful.

    The detail design comes after all this, not before, as far too many do .... and end up making errors which become costly to recover from.
     
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  6. Bayden
    Joined: Feb 2020
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    Location: Philadelphia

    Bayden Junior Member

    That is great information thank you. One of the reasons I started this process so early was figuring it was going to be very iterative. To say I have ALOT to learn would be a mild understatement. I have heard boat design defined as both art and science.
     
  7. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Bayden,

    There is always the overwhelming urge of amateur/home builders to - start building something. To them it defines progress and progression towards the goal....it is being built, yippiiee..finally started.
    But the euphoria soon wanes as the costs spiral, the design doesn't fit or work etc etc..all because the most important steps, noted above, were ignored, not done, or simply dismissed as not necessary, as it impeded the self imposed immediacy for progress!
     
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  8. Bayden
    Joined: Feb 2020
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    Bayden Junior Member

    If I am being honest if I had tried to do this 10 years ago... you would be describing me to a T... Thankfully I am a little older and a little wiser, at least I like to think so. These days my free time is scheduled so to speak. I am much more interested in the philosophy of measure twice cut once. Or measure 20 times and cut once.
     
  9. Bayden
    Joined: Feb 2020
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    Bayden Junior Member

    I was playing around with some designs and it is incredible how much more proportional a a 10ft beam feels vs an 8'6" with a 32'LOA (not counting the swim platform)


    8_6_vs_10_beam.png side.jpg
     
  10. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Re your initial SOR, my first thought was something like a classic lobster boat - there are lovely designs by Paul Gartside and Tad Roberts (and many others).
    But then you would ideally like to have shallow draft for cruising the Exumas - and for this a power cat would be hard to beat.
    Richard Woods' Skootas are very nice - but they have O/B motors, which you are not so keen on.
     
  11. romeomikehotel
    Joined: Feb 2019
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    romeomikehotel Junior Member

    Go for 10 on the beam. I know you’re worried about trailerable width but unless you’re trailering past highway patrol regularly, it won’t be an issue. City cops don’t care and/or don’t have any reference for determining a boat is over width.

    My next boat coming up has an 11 ft beam and I’ll be trailering it weekly from the house to the ramp and I’d bet the title of the boat I’ll never get hassled.
     
  12. Bayden
    Joined: Feb 2020
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    Location: Philadelphia

    Bayden Junior Member


    Good Morning bajansailor. I will look at both of those today. Thank you for the reference.

    I did actually review the Woods design, and like them alot. They would absolutely be a dream in the Bahamas. but will not work for me for two reasons. 1) The Beam is too wide for me. 2) Half the fun of this process is going to be designing it myself and I think I will be struggling with a monohull I think a multi hull will be beyond me.

    If I ever win the lottery though a Lagoon 630MY is my first purchase!!!.
     
  13. Bayden
    Joined: Feb 2020
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    Location: Philadelphia

    Bayden Junior Member

    I think your right. I can get a wide load permit here... It is just a longish trip from my house to the water figure an hour 20 before I am somewhere I can launch it. Longer to where I WANT to launch it. Ah Well C'est la vie
     
  14. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Re 'designing it yourself', it is worth noting that probably 99% of new boat designs are based on existing designs that have been slightly tweaked or modified - very few start from a completely blank sheet of paper (or blank computer screen.
    And there is no shame in using other vessels for reference - the more that you can find that you like, the better.
    And your design can then be an evolution of all the nice aspects that you like in these designs.

    You mention using outdrive legs re the shallow draft criteria for the Bahamas - I think I would prefer to have O/B motors any day, but you are not keen on them. Is this because of the higher cost of petrol / gasoline, and / or not so much availability in the Bahamas, or ?

    Another boat that you could perhaps add to your reference list could be this MJM 35 -
    MJM Yachts 35z https://mjmyachts.com/35z/
    She has a very nice hull form (you could do a multi-chine version for plywood if desired) - but she has 11' beam (rather than a max of 10'), she is 35' (a bit bigger than your 34' limit) - and she has outboards.

    But then no boat is ever 'perfect' - they all have compromises made along the way.
     

  15. Bayden
    Joined: Feb 2020
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    Location: Philadelphia

    Bayden Junior Member

    That is a good point about most designs heavily leaning on earlier ones. It is less of a willingness to modify other designs and more of an issue with my confidence in modifying a muti-hull. From my amateur perspective (and I may be wrong) there are less questions with a monohull and I am taking as much of a KISS philosophy as I can with this.

    I do really like that MJM35 though.

    My issues with OB has more to do with past experience then anything else. It is very possible modern engines do not have the same limitations I have worked on in the past. I am envisioning this running at displacement speeds greater then 50% of the time. With an OB I have found they are not as efficient at low speeds and they seem to last the same amount of time no matter how hard you run them. 1500 hours or so.

    From a logistics perspective getting an OB serviced in the Bahamas is likely easier then an inboard. I have just enjoyed working with an inboard diesels more personally. At the end of the day I may not have a choice. My design may not allow for enough below deck space. At the moment I am trying to get a feel for how much the vessel will weigh less the power plant.
     
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