hello, playing with a 28fter

Discussion in 'Boat Design' started by mj_lover, Feb 21, 2008.

  1. mj_lover
    Joined: Jul 2006
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    mj_lover Junior Member

    hi there everyone. I've been lurking around on and off for a while, and figured I should say hi, and become a little more active.
    As of late the desire for something a little bigger has been playing in my mind (I sail a laser 2, and have a 14' speedboat from the 70's). While browsing the school library I stumbled upon this little number that looks really nice. She is 28' long, and 6' wide designed for a 8hp diesel. 6' seems incredibly narrow for a cruising boat that I'm looking for, so I'm thinking of a 30% beam increase, as well as a little more free board, plus the addition of a small cabin.

    thought this would be the right place for some feedback. Great forum by the way!
     

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  2. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    MJ, You really shouldnt be smoking that stuff while you are making decisions about changing a boats design.
    Read more in this forum and learn how to use the search feature, and you will find some really great boats that fit your requirements.
    BTW, I have been eyeballing the design you selected for several years, with an economy cruiser in mind. I envisioned it with a canvas dodger stretched on battens to midship for a cabin, and maybe a bimini over a center console/engine box. She's a sweet hull, and only 8 horses needed to power!! Sure its narrow, but thats where the beautiful numbers are derived from. Adding 30% to the beam changes everything, most notably fuel and hp requirements, as well as redesigning almost every aspect of the drawings.
     
  3. deepsix
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    deepsix Senior Member

  4. mj_lover
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    mj_lover Junior Member

    there are some nice designs in there deepsix, all the ones I found while cruising thru the web look not as nice. Thanks for pointing them out.

    KapnD, in my defense, I wasn't smoking anything heavy, at the time :p The book I got the design from has a discussion of how to go about a beam change and the affects on performance. so i was just playing with the idea. I am however going to try to play with this search feature, see if I can work it good to find stuff.

    thanks for the replys!!
     
  5. chandler
    Joined: Mar 2004
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    chandler Senior Member

    MJ lover,
    If you have any elementary math skills you can rework and redesign anything.
    Get some good books...chapelle, skenes. Make it work the way you want, don't be dictated by what the original designer thought would be best for you.
    For all the you need a "naval architect" to do that; stretch out on a limb and and do something for yourself.
    For all the naval architects and self proclaimed designers on this web site, I would guesstimate 3 of 10 actually have a degree from a university that offers a degree in naval architecture.
     
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    The design for the 28' powerboat above is intended to be an efficient displacement speed craft. Her chine and slightly submersed transom suggest she could handle more power, though she'll likely max out around 15 knots and you'll be looking at the sky and chewing up lots of fuel.

    With an 8 HP engine she'll do 8 MPH and that's what she'll do all day long, sipping fuel like a bird does water.

    Increase her beam and you'll need a whole lot more power then 8 HP. You'll need a V8 mounted midship. Add some freeboard (read weight) and you'll need more power.

    Frankly, there are lots of 28' power cruiser plans out there . . .

    [​IMG]

    (including my own) . . .

    and doctoring up what seems a well thought out shoal draft, displacement/semi displacement craft into something it will have difficulty becoming, just doesn't make a lot of sense.

    No offence intended, but you're best advised to find a design more closely suited to your needs.
     
  7. mj_lover
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    mj_lover Junior Member

    no offense taken taken, I just really like the design of the boat. I am aware I would need more power, was thinking a 60 hp diesel, as i only need 9-11knts.
    the boat you showed is nice, not my style, but point taken. I will keep looking around, just very picky in design i guess.

    I'm also going to invest in some good books see if there is anything i can play with that is closer to my requirements then the 1st boat.
     
  8. alan white
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    alan white Senior Member

    Best not to widen that design. It's narrow for a reason having to do with fuel consumption and while few 28 ft boats are so narrow nowadays, neither are many people willing to go so slow---- but long and narrow is key to efficient displacement travelling. The hull shown could easily cruise with a five-eight horsepower inboard. Nowadays, a 4-stroke outboard would be a good choice. A small single cylinder diesel would vibrate a bit and cost a lot more.

    Alan
     
  9. erik818
    Joined: Feb 2007
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    erik818 Senior Member

    MJ lover,
    The speed you desire, 9 - 11 knots, is not easy to achieve with good economy. Displacement speed for a 28' hull would be something like 7 knots. Typical planing speed would be 20 knots or so, for a boat designed with a planing hull. I definitely sympathise with your sentiment that 20 knots isn't really needed and 7 knots is too slow. The speed just above "hull speed" is usually the worst one for fuel economy.

    There are still many traditional 7 - 10 m long displacement boats where I live, and I have one myself. They are usually not as slender as the boat in your drawing. From what I've seen of such boats, my guess is that 20 Hp or so would be suitable four your boat if you increase beam with 30%. The speed would still be 7 - 8 knots. Going faster than that requires a different hull shape, but maybe it is possible to gain a few knots with a ridiculous amount of horsepowers. My opinion is that you violate the beauty of the design by pushing it a few knots faster using brute force.

    If you are not interested in a properly planing boat, but are not satisfied with hull speed, I suggest that you have a look at the box keel threads on this forum. I'm tempted by that type of design myself, as I find my displacement boat too slow many times but still have no need of proper planing speeds.

    Erik
     
  10. mj_lover
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    mj_lover Junior Member

    http://www.glen-l.com/designs/workboat/noyo.html

    hmm...I think I found what I was looking for yay!

    Reaches the 9 knts I was looking for, and has good deck space. Not sure what box keel to look for, searched the forum, but still very fuzzy...
     
    Last edited: Feb 24, 2008
  11. kapnD
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    kapnD Senior Member

    MJ, My apologies. (cough, cough) Went back to look at the 28 x 6 you originally mentioned, and lo and behold there are instructions included on how to stretch the beam by 20%! No extended horsepower/speed calculations, but I'm sure a 10-15hp outboard would do the job.
     

  12. mj_lover
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    mj_lover Junior Member

    haha, no worries mate
     
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