How light is too light?

Discussion in 'Boat Design' started by curtis73, Jul 21, 2015.

  1. curtis73
    Joined: Mar 2002
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    curtis73 Junior Member

    I'm in the very beginning stages of building a boat. When I say beginning, I mean that I don't even live in a place with a big enough garage to do it, so think like a 5-year project kinda thing. I have restored many boats so this seems like a logical next step in the hobby.

    The boat will be a medium-deep vee hull, probably 19-21 feet, bowrider. I haven't decided on drive, but most likely I/O, also considering jet.

    The most likely candidate for construction would be plywood/fiberglass, but I was crunching numbers on raw materials and composite foam/carbon fiber is not outside the realm. Considering a normal retail buyer might spend $30k on a new boat of this caliber, an additional $3k on materials in my build might make for a light exotic boat that drafts a little less for shallower water exploration

    Provided I can maintain the proper CG with a light hull, how light is too light? I don't want to be charging down the lake at 70mph and find myself, oh, I don't know... upside down and dead? I have a million years of boat experience, but most of it is nothing in the performance realm; trawlers, cuddys, offshore fishing, jon boats, etc.

    My latest boat (and my frame of reference) was a 19' baja that weighed in at 2130 dry, so easily 2600 with equipment, gas, and me. Very stable, but sluggish and it did prevent me from some exploration in shallower water.

    So I guess the question is, at what point does light weight and speed go from mash-the-throttle-and-you're-safe, to this-requires-real-skill-to-not-die?
     
  2. gonzo
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    gonzo Senior Member

    You can't consider weight by itself. It is one parameter of the design.
     
  3. curtis73
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    curtis73 Junior Member

    Agreed, I'm more interested in the weight's factor. I'm fine with design and engineering parameters of hull shape (or at least learning what I don't know yet). I'm concerned with the point at which weight becomes an issue.

    I'm not asking for numbers on my hypothetical boat design, I'm opening a discussion on the effects of weight on stability and safety.
     
  4. NavalSArtichoke
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    NavalSArtichoke Senior Member

    Speed kills, on Land, Sea, and Air.

    If you don't know what you are doing, don't think about going fast. It's not worth it.
     
  5. DCockey
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    DCockey Senior Member

  6. messabout
    Joined: Jan 2006
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    messabout Senior Member

    Operation costs for a 70 MPH boat will be substantial. If shallow draft is a factor, then deep vee is less than the best option.

    It will be well for you to make a statement of requirements (SOR), then the members can give you some good commentary.

    DCockey has given you the first best suggestion.
     
  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Weight is a byproduct of the data crunching. Simply put, once you've established your loads and accepted a safety margin you can live with, your previous decisions, working around the spiral, provide you with the weight options.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Ultra light weight and heavily veed hulls doesn't work well either at rest or underway. If the intention of it is to gain speed, into the range mentioned above, it really isn't a suitable arena for guesswork.
     
  9. PAR
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    PAR Yacht Designer/Builder

    What most are trying to say Curtis is, engineering for anticipated loading hasn't asy real short cuts unless you have a great deal of experience and expertise with the particular set of problems you're looking at. Sometimes you can employ a rule of thumb, though this usually doesn't produce light weight, but often does produce stiff and strong enough for an application.

    All this said, you're correct in that lighter is faster, usually. Composite design with an eye on light weight mean a highly engineered structure and there's not short cut to the required engineering. So, the question remains, how much engineering understanding and experience do you have? Another key question would be your understanding and experience with hydrodynamics? I this area you can decrease loading with clever design tricks, relieving some of the structural requirements you might need. Building techniques can also come to play, though this is the engineering and experence side of things too.

    I think what you're looking for (my huge assumption here) is you have a design you'd like use and you're wondering how the laminate schedule might be changed to suit different fabric choices. There are ways to do this and even a few books, that can help with these types of conversions, but they do require some serious understanding of what's going on with the composite end results. Maybe you can be a little more forthcoming in what you're trying to do, what plans you're working with, your experience level with the various subjects necessary . . .
     
  10. TeddyDiver
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    TeddyDiver Gollywobbler

    IMHO assuming you calculate the skins with ample safety factors you can't go too light. Comparing gf on ply, strip planked core and foam core they all have different schedules for the skins, foam cored has the thickest and heaviest skins. Any fast planning boat can flip just depends of the wind, sea state and the idiot on the wheel (no punt intended). Anyway if the design is about the same foam cored gives you most load carrying capability which you should use too assuming it's not a racing boat. Additional fish well, fridge or extra tankage what ever your priorities might be so in the end you have a boat about the same displacement whatever the core is..
    BR Teddy
     
  11. curtis73
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    curtis73 Junior Member

    Thank you, David. I will order that as soon as I get back to the states. (on vacation now)
     
  12. curtis73
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    curtis73 Junior Member

    Engineering experience is healthy and pretty complete. I am not formally trained in engineering, but have plenty of aptitude and a lot of experience. By trade I'm equal parts artist and fabricator... in automotive fields. I build cars, I'm a machinist, weldor, fabricator. I have a lot of experience with laminates, but 90% of it is in a non-structural applications. Put it this way... when it comes to boats, I've done nearly everything except build a hull. I've taken existing hulls and stripped them bare, added new stringers and floors, made custom gunwales and coaming, but I was building on an existing structure. Any structural laminates I have done have been simple; engine rooms, decks, floors, dashboards, etc.

    The hydrodynamics part is coming to me quickly, but aside from a pilot's license and a minor in physics, I have a lot of growth to do. My thought was to do another boat starting from the hull up and capitalize on someone else's labor, but I want the learning experience and satisfaction/pride of taking raw materials and making it all myself. I do anticipate using existing hull plans if I can find one that suits me. I'm not at the point (yet) of engineering a complete hull design from scratch, nor do I think my needs and use would necessitate a clean-slate design.

    I actually have vague design things in mind; LOA, beam, depth, intended use. This post is my first actual informational gathering part.

    ... And I may learn that its beyond my ability and I'll find an existing hull to modify, but that's one of the reasons this is a long term project. I don't mind putting in the time and effort to learn what I don't know.
     
  13. curtis73
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    curtis73 Junior Member

    SOR:

    Purpose: transportation/day trips on the water. Primarily exploration, occasional skiing/wakeboarding possible. Typical use would be to pack a lunch and spend the day exploring on the water. No overnighting anticipated, but if I do I can always take a sleeping bag and sleep on the deck. Ingress and egress are very important. Frequent shore mooring in rocky, wilderness areas.

    Waters: Primarily lake use. Mostly THIS ONE. It is about 13 miles from the bottom to the top. Deepest point is about 90ft, but as you can see there are a zillion little shallow bays and side lakes that have narrow entrances and shallow access. Chop can reach 2' on windy days. I have seen 3' chop, but if its that windy I just don't go out. Occasional use in salt, mostly inland waters around Chincoteague, VA and I anticipate possible use in Lake Travis TX and inland waterway around Miami/FL Keys. Given the intended usage and relatively low-chop waters, deep hulls are not required.

    Number of passengers: Typically 1-4, but physical capacity for 6-8 might be needed from time to time for transport to and from islands. It only needs to comfortably accommodate 4, but capacity for 8 would be helpful as transportation.

    Speed: a max speed of 70-80 mph would be nice, but cruise in the 25-40 mph range.

    Power: Single I/O or possibly jet. Given my fabrication and automotive engineering experience, any or all powerplants are on the table; diesel, gas, forced induction, EFI, whatever. Obvious nods go to Ford/Chrysler/GM V8 for the inexpensive and plentiful marinization parts, but I'm not ruling out some rather oddball engine ideas. Fuel consumption not really an issue. I have ready access to fuel.

    In a nutshell... if I were to be purchasing a boat today, I would be looking for something in the 19-21' range. I would probably lean toward a fish 'n ski style (basically an outboard bass boat with a full console) or something like a Baja SunSport I/O. The reason I haven't gone with that style is because of their drawbacks; the fish 'n ski scores a zero on passenger comfort and a 2 on style and appearance. The I/O drafts a lot and is just heavy and sluggish.

    If I were to have a hull that is somewhere between those two on deadrise and depth, I think it would be fantastic. Topside I can make however I choose.
     
  14. Jamie Kennedy
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    Jamie Kennedy Senior Member

    In general, a boat should be lighter than water, but heavier than air.
    About mid way between is a good start, but I would tend to err on the side of air. ;-)
     

  15. curtis73
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    curtis73 Junior Member

    Maybe a helium-filled pontoon boat?
     
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