Wide Beam Barrelback

Discussion in 'Boat Design' started by TMG, Jul 13, 2010.

  1. TMG
    Joined: Jul 2010
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    Location: New Hampshire

    TMG New Member

    Hi All,

    I'm new to this forum so bear with me if you can. I would like to build an inboard ski / runabout that is classically styled similar to the Chris Craft and Riva barrelback boats. I would like a boat in length between 20 and 22 ft. While I like the designs in Glen-L, I would prefer my boat to have a maximum beam of 8.5', I don't prefer the narrow beam mahogany boats presented on Glen-L. Most of their plans specifically say not to increase beam. I was thinking I may be better to find a design with the proportions I'm looking for and scale it up to the size I'm looking for. For example, Glen-L has a 15' Ski King design with a length of 15' and a beam of 6'. If I were to scale this up 40% I would end up with a boat 21' in length and 8.4' beam. Obviously I'd have to add additional structure / frames to account for the larger size, but I would be maintaining the hull design and, hopefully, the intended performance. I'm also hoping that the added size would allow for a small cuddy forward. Am I on the right track here, or am I asking for trouble?! Would I be better off figuring out how to increase the beam on some of the larger boats? I'm concerned that by doing so, I will change the hull to water shape and it will negatively affect performance. Thanks for your help!
     
  2. Wayne Grabow
    Joined: Aug 2003
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    Wayne Grabow Senior Member

    Scaling of boats from one size to another is best limited to about 10%. Do not attempt to scale a boat by 40%. Displacement does not scale in the same proportion as the dimensions, and you will end up with a boat that does not float on its lines and will look out of proportion. Because the 8.5' beam is important to you and you desire the looks of a traditional runabout, I would suggest that you find a traditional design with the desired beam and then space the frames closer together to bring the length down to about 22'. That will not guarantee that the design will work, but it is a much safer approach to altering a hull design.
     
  3. jg451
    Joined: Jul 2010
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    Location: portland,oregon,usa

    jg451 Junior Member

    Hi TMG,
    I'm a newby here myself. That having been said, I would pose a few questions. If memory serves,l the C.C. barrelbacks were round chine boats. Is the draw of the barrelbacks the lovely jointery deck work? The sleek hull form? The maneuvering caracteristics? How much is aesthetics and how much performance? A wooden coffin will go fast with enough horsepower. Golng fast, maneuvering well, and pulling it off with elegance, aye, there's the rub. A wider, flater stern underbody would give you beam and maneuvering but it wouldn't give you that sexy convex near oval transom the C.C. displacement hull has. Some rationalizing of priorities will bring you closer to your target.
    Regards, Jon
     
  4. tunnels

    tunnels Previous Member

    Dead right rescaling boats dosent work ! Simply draw you own !!
    By all means make it a look alike of the smaller version but do you own drawing and take all the good points from some of the older boats but modernise it a little . Lot of the older boats are quite a shallow dead rise meant for calmer waters plus most have quite rounded sections that lends it self to thumping as the speed increases . The biggest thing to think about is speed , older boats were never designer to do the speeds we want these days , 25 to 30 mph was flying now unless you can hit 50 to 60 you are left in everyones wake .
    Motors have improved hugely ,lighter weight and a lot more horse power .Are you looking at inboard (being a barrel back ) or a outboard ?
    :p
     
  5. TMG
    Joined: Jul 2010
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    Location: New Hampshire

    TMG New Member

    I would like to design my own, I just need a starting point, as I have no clue how to design a proper hull. That's why I was thinking of starting with a design for which plans are available, scale it to the size I want and then figure out the structural requirements, i.e. add more frames, thicker hull, etc. One other poster mentioned that the displacement will not scale properly and that the boat may not float to it's lines. I hadn't thought of that, and that's definitely concerning. In a boat such as a runabout, would one expect the boat to float higher than it's designed waterline, or lower? If it were to float higher, than I can just put in a bigger moter to weigh it down :D
     
  6. apex1

    apex1 Guest


    You are dead right being in doubt about your abilities to design a boat. Redesigning is the same task btw.

    Just leave it.
    The recommendation to draw your own boat was not given by a expert, and is at least dumb and irresponsible. (but it is not his time and money you would waste)
    Drawing a fancy shape has nothing in common with boatdesign. And modern software does not help to achieve a proper result at all.
    The deep understanding of naval architecture would be what you need, but that cannot be found on such platform like this one. There is no instant "howto" in this case.

    Wayne hit the nail.

    Find a proven plan of the right size and change just the styling to fit your aesthetic requirements. A tumblehome shape, like on a Riva, might probably be within the range of possible alterations.

    These (minor) changes could well be discussed with the experts here. Then again you would have to filter out the drivel.

    Regards
    Richard
     
  7. Tad
    Joined: Mar 2002
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    Tad Boat Designer

  8. Eric Sponberg
    Joined: Dec 2001
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    Eric Sponberg Senior Member

    TMG,

    Another option to consider is a kit boat, specifically, the Saetta Classic Runabout: http://www.saettaboats.com/index.php

    The kit is supplied in two parts, the first being all the wood which is pre-NC cut in 3D. All joints are glued. The second part is the hardware, engine, and drive train. You can have conventional V-drive with underslung prop, or I/O drive.

    I designed the bottom for the Saetta, which is similar to the design I did for the Cherubini Classic 20: http://www.cherubiniyachts.com/20.html. Both boats are equipped for waterskiing.

    Both boats are 19-20' long, beams are about 6' to 6.5', and they are deep-V designs rather than the classic concave section fwd/flat-bottom aft type designs of the early 20th century. To see the difference, you can see the redesign that I did on a Chris Craft Cobra a few years ago: http://www.sponbergyachtdesign.com/ChrisCraftCobra.htm. The narrower you go, the deeper the V can be for a given weight.

    I caution you about wide beam. At the size you are considering, an 8.5' beam gives a low L/B ratio, about 2.5. At that ratio, the bottom would have to be relatively flat to float properly at a decent weight. Wide, flat bottom boats are hard to handle, particularly in turns. They also pound more. One has to be very careful how the bottom is shaped, particularly forward, so that the boat remains stable at speed and in turns, and does not porpoise. Of course, the location of the center of gravity, both fore/aft and vertically, is vitally important to good performance.

    My V-designs are all about 20-deg deadrise which I find at this size of boat works really well. The Cobra redesign had a deadrise of 15-deg, as insisted on by the owner, and it did end up with some handling problems, which fortunately he was able to cure. But both the Saetta and Cherubini boats perform exceedingly well--they are stable in turns and plenty fast.

    I hope that helps.

    Eric
     
  9. u4ea32
    Joined: Nov 2005
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    u4ea32 Senior Member

    As always, Tad, you know the most beautiful boats!
     

  10. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    Why do you want a boat with an ungainly length to beam ratio???? Eight point five feet on a 22 footer makes it a tub not a Riva.

    Pay close attention to Eric. He is one of the definitive authorites on this forum. He is also a straight shooter.
     
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