Help on first sailboat design (exercise)

Discussion in 'Boat Design' started by daniGG, Nov 25, 2019.

  1. daniGG
    Joined: Nov 2019
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    daniGG Junior Member

    Hi All!

    I am new to the Forum, so first of all… great to be here !!

    I'm posting because I’d like to undertake the project of designing a sailboat and I know I'm going to get stuck along the way so I think I can find help here and learn a lot!

    The idea of designing the sailboat is more to experience (for pleasure) all the steps of the exercise than to design the boat of my dreams (or any decent sailboat) so expectation on the outcome is aligned with the experience (none ;))

    I did the initial research and read avidly some of the classic books recommended to get into sailboat design and construction, I have the brief and I have the main KPIs fixed (LoA,LwL etc).
    I also have the "lines" of a couple of design I'd like to start from as a base and I thought to share it here to validate anything that I might have gotten wrong to before moving forward.

    Would this be the right place to post this?
     
  2. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    Welcome to the forum.

    Post your designs if you're able, there may be a delay in your privileges to do so however...
     
  3. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    +1 re a warm welcome to the forum.
    Please do post (in this thread) the lines plan(s) of the designs that you particularly like, and want to use as a basis - please remember that pretty much all boat design work is a subtle evolution of another design (sometimes very subtle), so don't feel that you have to be too radical on your first design.
    Which are the classic books that you have read so far?
     
  4. daniGG
    Joined: Nov 2019
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    Location: Paris

    daniGG Junior Member

    Thanks guys for the warm welcome!

    Here are some books I read so far (plus research done online on different topics) : Principles of Yacht Design, Elements of Yacht Design, and another couples (not in english) on designing and building wooden boats. I also have an Introduction to Naval Architecture and some other more technical ones but mainly to check some specific topics along the way. Being new to design aw as well I started a course on Rhino.

    Following the steps from the above books I drafted a brief for my exercise that is the following:
    • A seaworthy sailboat for a family of 2-4 to do extensive cruising including trans-oceanic passages in “safety”.
    • Ability to be sailed shorthanded, actually for the most part by a couple also in bad weather.
    • Sailboat comfort, e.g. no need to have more rooms than a boutique hotel and in general, safety aspects will take priority on fancy stuff, if a tradeoff has to be made.
    • Pure cruiser, no need to win races, but has to be able to move with no engine help in light winds (ok to do so with specific sails).
    Things to try to include
    1. Chart Table, in particular in the direction of the motion
    2. At least one bed close to the cockpit for night passages
    3. Marconi rigged and no running backstay
    4. Long “enough” Traveler
    5. 2m height below deck under the coachroof area
    6. Long keel: assumption here is that it would be easier to build in case a DYI build project should arise and potentially can help create space for the point 5?
    7. Tiller steering (no wheel) for simplicity of construction, maintenance, robustness
    8. Decent galley – cooking will be an important activity J
    9. One double cabin
    10. 2m long beds (I know…we are tall L )
     
  5. daniGG
    Joined: Nov 2019
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    daniGG Junior Member

    For the sake of the exercise I took some reference sailboat that I like (from what I read) and extracted some KPIs to start from:

    Sailboat I selected
    • Cabo Rico 38
    • Hans Cristian 38
    • Tayana 37
    • Island Packet 35
    • Rustler 36
    • TRADEWIND 35
    • Baba 35
    • Pacific Seacraft 34
    • Westsail 32
    • Baba 30
    Final KPIs for the project:
    LoA: 33ft
    LwL:31ft
    Beam: 11,60
    S.A. 950 ft2
    Draft: 5,7ft
    Disp: 21050 lb
    Ballast: 8700 lb

    Ratios:
    SA/Disp: 19,9
    Bal/Disp: 41%
    Displ/Lenght: 320

    What I like in terms of lines are the classic channel cutters:

    [​IMG]
     
  6. Dolfiman
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    Dolfiman Senior Member

  7. daniGG
    Joined: Nov 2019
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    Location: Paris

    daniGG Junior Member

    Thanks Dolfiman for the info! Really nice looking boat and actually allow me to introduce my first doubt.

    I'm trying to draft the initial lines of the sailboat, faaaaaar from being close to anything worth posting (especially aft :( - img below).
    I quickly validated that, with the current "design", a balance can be found to accommodate desired LWL and Displacement (through the displacement of the submerged part of the hull).

    However when I try to draft the potential ballast in the lower part of the keel (assuming a lead ballast - 2nd image) it is clear that there isn't sufficient "volume" to make the 8700lb (400kg+-) that I'd like to have as per my KPIs.

    My questions then are
    1. Is it correct to assume that all the ballast should lay in the lower and thinner part of the keel for those types of "classic/long keel" designs?
    2. Are 10cm (0,32ft) too little as width of the bottom part of the keel? (I checked boats as baba35 and others and they seems to have a "thin" long keel. Shall I just make it generally deeper to allow enough height in the section to make the proper weight?). I thought about make deeper the front part but I guess that then tacking will be impacted (?)
    Apologize for the images and if I'm asking stupid questions! Thanks!!!

    upload_2019-11-27_22-37-35.png

    upload_2019-11-27_22-36-47.png
     
  8. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    I'd first calculate a first approach of total ballast the boat really needs.
     
  9. daniGG
    Joined: Nov 2019
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    daniGG Junior Member

    oh ok thanks!

    Just to check if I got it properly, the idea is then to draft the rest of the boat (interio, deck, mast etc) and identify the weights of each element and their position so that I have an overall view of the total weight of the boat and therefore of how much ballast I should have (and also it's distribution along the keel)?
     
  10. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    Studying the stability of the boat, the maximum heel you want for the boat in normal tight conditions, will give you an idea of the necessary ballast.
     
  11. daniGG
    Joined: Nov 2019
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    daniGG Junior Member

    thanks!!

    Ok so it is correct to proceed like this as per below?
    1. Define normal tight conditions (wind speed I guess) and desired heeling of the boat with a certain amount of canvas (e.g. full main/jib vs reeffed etc)
    2. Calculate the moment that the boat is experiencing from the sails and equal this to the righting moment that needs to be generated to be at equilibrium
    3. Since I know (I'll have to calculate) the center of buoyancy position (as it is only function of the submerged part of the hull and its sections) then I should be able to identify the desired righting arm and from that the position of the Center of Gravity.
    4. If I know the Center of Gravity position and I know the weights distribution of the boar without ballast then I should be able to know how much ballast roughly I'll need?
     
  12. TANSL
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    TANSL Senior Member

    Yes, I think that procedure is correct. There may be some other way to get it but what you describe, imo, is correct. On the other hand, although it is not valid exactly for the type of boat you want, the attached spreadsheet might perhaps help you.
     

    Attached Files:

  13. daniGG
    Joined: Nov 2019
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    daniGG Junior Member

    Thanks Tansl once again, I'll crack on the calculation and revert with the outcome.
     
  14. daniGG
    Joined: Nov 2019
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    Location: Paris

    daniGG Junior Member

    Whilst I'm dealing with the process of defining a HL view of the interior and it's weights, what I did was (pictures below):
    1. Defined a draft position of the mast at 43% of the LWL from the bow (should be in the reasonable sloop cutter config range if I got it correctly)

    2. Drafted a height from WL for the mast that provides me with a 19,36m mast from the deck. It seems to me maybe a bit tall for this types of boats but once fixed the above point 1, this was the height that would allow me to have a main of the size I needed to get to the targeted SA/Displ ratio of 19.9. Any advice here would be great :)
    3. Drafted a mainsail, Jib and a (permanent?) forestay for strong winds. The assumption is not to use the jib and the forestay together (not sure if it is a reasonable assumptions, I thought would limit the sizing of the mast+rigging to avoid running backstays, checkstays etc). So the idea is to use main and jib (without the staysail) in normal cruising (up to reef 1-2?) and then to move to reefed main + forestay with no Jib.

    4. Calculated the Center of Effort of the Main+Jib and Calculated the Center of Lateral Resistance of the hull (without the rudder as I still have to properly design it) and the lead between the two (horizontal distance between the CLR and the CE) is 14% of the LWL that seems to be in the recommended range according to empirical lead rule quoted in "Principles of yacht design - Larsson etc" for this type of yachts.
    So far so good?

    upload_2019-12-3_10-38-10.png

    upload_2019-12-3_10-52-54.png
     

  15. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Just going on 'looks' - the mast does appear to be too tall, the side profile of the transom is very strange, and the aft buttocks in your earlier drawings do look rather steep / tight (not too sure how to best describe it) which could suggest extra resistance.
    Have you seen Chuck Paine's Expannie?
    If I were in your shoes I think I would be using something like this as a reference boat for starting off with.
    36′ Offshore Voyager EXPANNIE – Chuck Paine Yacht Design LLC https://www.chuckpaine.com/boats/36-offshore-voyager-expannie/
    I also really like how he has managed to get some balance in the rudder, despite it being transom hung.

    Edited to add some photos.
     

    Attached Files:

    Last edited: Dec 3, 2019 at 10:58 AM
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