I have a concept - and a lot of questions

Discussion in 'Boat Design' started by ssjothun, Sep 28, 2016.

  1. ssjothun
    Joined: Sep 2016
    Posts: 12
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Norway

    ssjothun Junior Member

    Hi everybody!

    I've been dreaming of building a proper boat since I was a kid, and I think I'm getting closer to realizing that dream.

    I've been incredibly fascinated by the classic runabouts from the 40ies, with their elegant lines and gorgeous craftsmanship.

    The challenge is that, yes, you can buy drawings online and do a Chris Craft or Gar Wood replica, but they're all flat in the bottom, and handling can't possibly all that great. So after some pondering, my idea is to build a boat with the classic lines above water, and a modern hull below, capable of speeds up to 55-70 knots, and be able to cruise in a relaxed manner at 30-40 knots.

    I know almost exactly what I want it to look like (see attachment for sketch, 1:10, 7 meters), and I know the basics of hull design & construction, but I'm not a naval architect, and I don't know how to tweak the different factors just so, to make the hull perform exactly as I want it to.

    That means I either have to spend a year (or more?) reading up on hull design, learning CAD, building and testing models etc - or hire someone to assist me as a consultant. Or something in between?

    Where would be a good place to start?

    -------------
    SPEC:
    I'm still in the very early planning phase, and right now I'm just trying to gather as much information as possible so I can make (semi) qualified decisions. My first task is to specify exactly what kind of qualities I/we want in the boat, then based on that, try to get an idea of what kind of time and cost budget. I've also started doing some first drafts. See attachment for latest update. As you can see, it's still very early on, but I guess you get the general idea.

    General characteristics:
    • Only use in the Rogaland fjord systems. Not very likely to be used in real bad weather, but must handle a decent amount of seas.
    • Primary use is transport to our cabin, with the possibility of day trips around the fjords ( you may have heard of Lysefjorden where the Pullpit Rock is - our cabin is in that area). There will be relatively little need for large cargo capacity beyond needed luggage for a weekend getaway.

    Hull and engine / speed:
    • Must cruise in a comfortable and relaxing manner at 30-40 knots
    • Should be able to handle 50-65 knots (then with far more focus from driver)
    • Extreme cornering is not required beyond avoidance maneuver.
    • Should have smallest possible outboard engine, both in terms of stability/weight distribution and fuel economy/environment. (reluctantly open to discuss inboard)
    • Built light and strong, but not so light that it becomes "nervous", or simply turn its nose wherever the wind wants to point it.

    Material:
    I can make nearly anything in wood, and unless you have any arguments against it, it should be the foundation of the structure. I'm definitely open for discussing specifics.

    Hull:
    2 layers plywood, fiberglass and epoxy. Exterior finish with racing green kevlar/carbon composite. Alternatively mahogany strips.

    Deck:
    2 layers plywood, epoxy, fiber glass, topped with mahogany strips

    Interior / cockpit:
    • In general, we like it clean, simple and functional. Think organic minimalism.
    • Should seat 5, but normally 2-3 people aboard. 2 chairs in front, sofa in the back.
    • Ability to create large sun in the cockpit? Have the sofa slide forward, and flip back rest backwards, and similar with chairs? (My wife loves the concept of bow riders with sunbed in the bow, but we both hate the looks of it with no deck) Just an idea...

    Drawings:
    There's a wood shop nearby that has a CNC machine, so I would like files they could use. Haven't checked, but it's obviously some kind of CAD. For my personal use, a 2D drawing would be plenty, as I would build a 1:10 or 1:5 plywood model first.
    -----------------

    Thanks,

    Sverre
     

    Attached Files:

  2. Mr Efficiency
    Joined: Oct 2010
    Posts: 7,765
    Likes: 273, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Welcome, I think there have been other threads here with this theme, there is obviously interest in retro-styled motor boats. I suppose the difference here is the outboard, which is a little unfaithful to the genre ! When you mention it will be used in a Norwegian fjord, it gets me thinking that the water temperature would be more bracing an than after-shave lotion ! And better not to land on the passengers as spray. The bows of those old-style speedboats don't seem the best if you are running in chop for suppressing spray, but with the vee-hull design you envisage, the bow will be running higher, and drier, I'd guess.
     
  3. ssjothun
    Joined: Sep 2016
    Posts: 12
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Norway

    ssjothun Junior Member

    Hah! It's not that cold, though. right now the sea is about 13C, but drops to 2 to -2C in January/February. My boating season would be April/May till September/October. At least I don't get a spray of Box jellyfish ;)

    Regarding engine, it would either have to be outboard, stern drive, or surface drive if I'm to use a stepped hull (and I'm inclined to) - all which are rather unfaithful to the genre ;)

    It would obviously need some work in the bow (which is basically common logic design), but I'm mostly concerned about what's under water. Though I have some ideas what makes things work, I have no experience with this kind of design work...
     
  4. Mr Efficiency
    Joined: Oct 2010
    Posts: 7,765
    Likes: 273, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Seeing the bottom of the boat is the part that meets the water, and needs to be right for safety and performance at the speeds you envisage, you might be better to adopt the below chine lines of a well-performing existing design, and concentrate on creating what you find most pleasing to the eye, above that. It certainly does not have to be like the boats of old, underneath. At the dock, it will only be visible to the fish. What the eye doesn't see, the heart doesn't grieve ! You don't want to guess about the running surfaces, chances are it won't be as good as a proven hull.
     
  5. gonzo
    Joined: Aug 2002
    Posts: 13,613
    Likes: 383, Points: 93, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Stepped hulls use inboards without problems. Check out the work of Thornycroft.
     
  6. ssjothun
    Joined: Sep 2016
    Posts: 12
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Norway

    ssjothun Junior Member

    That is exactly what I'm after. Probably more a neo-classic approach on top, and definitely a modern racer underneath.

    I have no clue where to look for proven hull designs, though. Any tips?
     
  7. tom28571
    Joined: Dec 2001
    Posts: 2,470
    Likes: 113, Points: 63, Legacy Rep: 1728
    Location: Oriental, NC

    tom28571 Senior Member

    Would be interesting to see how this dream develops. Cruising comfortably and relaxed at 40 knots don't normally belong in the same sentence in a small runabout where you are in such close relationship to the water.
     
  8. ssjothun
    Joined: Sep 2016
    Posts: 12
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Norway

    ssjothun Junior Member

    It's usually not a lot of waves in these fjord systems, so I cruise comfortably at 35-40 knots with one of my current boats, a Fletcher 15' w. 90hp ;)
     
  9. Stumble
    Joined: Oct 2008
    Posts: 1,896
    Likes: 71, Points: 48, Legacy Rep: 739
    Location: New Orleans

    Stumble Senior Member

    I don't know that there is a design like this already on the market that you can buy the plans for. But there are companies that are building this type of boat either as serial builds or semi customs. Take a look at Hacker-Craft and http://www.cherubiniyachts.com/24.html .
     
  10. ssjothun
    Joined: Sep 2016
    Posts: 12
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Norway

    ssjothun Junior Member

    @stumble
    Thanks for the tips, but I'm after something much more agressive than that...I doubt those hulls would handle much over 30 knots?
     
  11. tom28571
    Joined: Dec 2001
    Posts: 2,470
    Likes: 113, Points: 63, Legacy Rep: 1728
    Location: Oriental, NC

    tom28571 Senior Member

    First of this series was the Classic 20 designed by Eric Sponberg. These are not wooden boat though they are engineered and built to look like wood. All are inboard and the 20 lists 40 as top speed.
     
  12. ssjothun
    Joined: Sep 2016
    Posts: 12
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Norway

    ssjothun Junior Member

    Thanks, but I'm after something much faster than that. Like I said in my first post:
    "Should be able to handle 50-65 knots"

    I'm after something light, strong, and with the smallest possible engine to take me to 50-65 knots. The Cherubini boats are good looking, but too slow and heavy.
     
  13. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 471, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Cherubini builds high end, very luxurious boats, so understandably are a bit slower than the lightweight gentleman's racer style of double cockpit, that you're envisioning.

    There are plenty of hulls you can work from, making the topsides an aesthetic you desire. If going for the dramatic deadrise you're showing, you'll need lots of power. If looking for less power, yet similar performance potential, you'll need less deadrise, but will have to pay for this in ride quality at high speed. There are ways to "cross the line" so to speak, having good ride quality and +50 knot potential. The problem generally boils down to what you really need, because +50 knot speeds requires lots of power, because the scantlings need to be hefty, though if you can live with high 40's and cruising in the 30's, the power requirements drop significantly. It's also easier to make the boat lighter and more comfortable in this speed range.

    As far as "reading up for a year" and hoping you'll design a boat with this capability, well this is simply wishful thinking, as it takes a great deal of skill to design stable, comfortable, high speed craft, that can tolerate the loading a 50+ knot runabout will exert on the structure.
     
  14. ssjothun
    Joined: Sep 2016
    Posts: 12
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Norway

    ssjothun Junior Member

    Thanks PAR,
    Just the kind of answer I'm after :)

    I was hoping 150-200hp (not 3-400 which is typical for the more elaborate boats of this size) would be sufficient for what I'm after. I'm thinking in the neighbourhood of 20 degreed deadrise.
     

  15. messabout
    Joined: Jan 2006
    Posts: 2,985
    Likes: 191, Points: 63, Legacy Rep: 1279
    Location: Lakeland Fl USA

    messabout Senior Member

    Here in Florida we have 6 meter bass boats that can run 70 MPH or more. They typically have 250-300 HP outboards and props that cost $1000. The cost of such a fishing boat is considerable to say the least.

    Bass boats have less frontal area than a "gentlemans runabout" which will factor into the speed equation. The runabout may have a better aero coefficient of drag however. Not to worry, you can go as fast as you like if you install enough power.........Big question is: Why do you want to go so fast? Lots of speed does cost lots of money.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.