Fire Fly

Discussion in 'Boat Design' started by WBC, Jun 9, 2010.

  1. WBC
    Joined: Aug 2008
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    WBC Junior Member

    I was on Svensons site for free boat plans and I have been looking at this 14ft step hull of Fire Fly. Has any one here have an opinion here on this boat. Thinking about a 25 maybe a 40 e-tec motor.
    What do you think she will draft with 2 men aboard?
    Do you think I could change the camber (less camber)on the deck beams and achieve the same strength?
    Think she will slide in the turns?
    Thanks for your input
     
  2. tom28571
    Joined: Dec 2001
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    tom28571 Senior Member

    Another of the very prolific W D Jackson plans that got more home builders on the water than any other designer in mid 20th century. Its a dated design but still worthy of building if that is your interest. Why are you concerned with the hull draft? The draft of any small outboard boat is determined by the motor and is usually about 23 inches, give or take a couple.

    Reducing the deck camber will definitely reduce strength but it is much more than I have used with good success so I would not hesitate to cut it back.

    As shown, it will skid all over the place in turns but I expect Jackson intends that you add a skid fin to stop that errant behavior. With the motor raised, the draft will be determined by the skid fin, which you cannot do without.

    An Etec 40 would be a great motor for this boat. I would expect about 50mph with that setup, properly tuned and lightly loaded.

    Using more modern construction and materials, you might get the weight down a bit but not a whole lot.
     
  3. tinhorn
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    tinhorn Senior Member

  4. WBC
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    WBC Junior Member

    My apologies gentleman for wasting your time. I have had a tuff time uploading so it it is. I was thinking of deck beams having say 1/2"
    to the foot camber fwd deck to sta 2 aft deck to sta 5 to transom open in the middle but maybe a seat fwd with fuel tank underneath seat where middle deck is fishing skiff is what I'm looking to achieve. Tiller steering.
    thanks for your comments
     

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  5. WBC
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    WBC Junior Member

    Shallow draft to fish in shallow waters push pole around the shallows.
    It draws out good and it is a straight lint line from step to transom. . Slide in turn, a small keel say 3/4x 1-1/2, no?
     
  6. messabout
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    messabout Senior Member

    Small keel piece, No...A skid fin on such a boat consists of an aluminum fin of perhaps 12" long x 8" deep. It has a flange at the top for attaching to the boat bottom. Finding just the right place to put it is the puzzle part. Someplace on the afterplane no doubt. These things are for sale at various suppliers.

    A 40 on that boat with tiller steering is going to be pretty scary. Tiller steering at 40 to 50 MPH is very sensitive to the slightest input.
     
  7. tom28571
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    tom28571 Senior Member

    The boat is going to rotate about the skid fin (mostly). The fin should be placed near the forward part of the hull bottom that is in contact with the water so that is is fully immersed at all times. You don't want the fin too near the stern because that would counteract the turning moment of the propeller in a turn. Some lateral resistance is offered by any hull bottom so the best location varies. The center of this lateral resistance varies depending on how much of the hull is in contact with the water at a given time. Its never going to be perfect more than in small increments of time.
     
  8. WBC
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    WBC Junior Member

    My thinking is a 25 on the back of her. Shoot can't go over 30mph anyway.
    Won't the step help in getting you on plane faster without the bow going skyward on the initial thrust. Forcing the water to break away on that initial thrust? Using the term jumping out of the water.
    Would it be better to eliminate the step, I have just enough design experience to be dangerous. I have attended boat school in Me, and have worked for some great builders over the years, and have been hard headed at times guess that kind of thing rubs off on you from these builders. Those folks are now gone and really had no interest in small planing craft. There is not much information on small planing craft out there so I'm turning to you guys for help in answering some questions.
    I understand the skid fin but that would not be practical in my application. The boat at rest being poled though the water would defeat the shallow draft to get into the shallow bays when hunting fish, yes I said hunting fish.
    Hell's Bay has a boat out that has 5in draft, two men , 40hp motor, Hull weight dry 350lbs , and this article claims 300 hull weight every thing but the motor. I don't know call me what you want but I'm racist against ester sisters,
    you know poly and vinyl. Still a firm believer that wood is best.
    Well we will stop there and I just want to say thanks again for all the input
     
  9. tom28571
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    tom28571 Senior Member

    I allowed myself to stick my nose into this thread so have no one else to blame. Forget about this boat. I had no idea that you wanted a fishing boat in the beginning. It is not nor will it ever be a good fishing boat. Don't even think about removing the step. You will wind up with either a bastardized boat or have to completely redesign the boat. Can you so that?

    If fishing is the main goal, look for one of hundreds, maybe thousands, of other possible choices out there. With low to moderate power, a jon boat would be fine and there are a lot of aluminum versions to choose from that are pretty cheap. Or, a shallow V with a small full length keel will do the trick. Add lots of power and you need something to control that power in maneuvers. A 40hp Etec or 4 stroke is going to cost a LOT more than the boat no matter how it is built in that size.
     
  10. WBC
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    WBC Junior Member

    Well I have drawn it out in CAD and the step would not be that hard to take out, and really your only redrawing two of the stations. I have designed a couple boats in the past but it was all guess work only because the older guy I worked for was not really giving up any information. If I asked a question he would walk off cursing at me and saying " I'm not a designer , I'm a mathematician" Well he held 7 world records runs in a sport fish from Miami to New York. So he knew some thing. Just was not telling, and he went to his grave with that info, sad in a way but that was how it was in his early life. Every body was out to steal his ideas, even though he admitted that there wasn't any new lines left.
    Here is a pic of a creation I designed and built but.... I built her too heavy. She draws about 10in fully loaded and the hull weighed in at 1100lbs. Too heavy, she rides nice and she is so beamy 6ft6in on the chines at the transom and at 15ft6in loa the hole shot stinks. Besides with the price of gas these days I'm looking to down size the motor to a 25. There are so many designs from the 40's-60's that there's got to be a design out there to fit the application. Wood and epoxy is still the way to go in my mind and I guess that's where I'm bullheaded. Thanks for the input.
     

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  11. tunnels

    tunnels Previous Member

    draw it full size !!

    If you want to redesign the step out of the hull simply draw the boat out full size on sheets of ply !!
    By drawing the complete hull /deck etc full size can save a load of time and you can see what you are actually going to be making .
    Even cad drawings for boats 10 ft to 20 ft long if you dont have access to a CNC to cut frames for you simply draw them out .
    If you have a wall some where its best to fix the sheets up and draw on a wall ,That way you can stand back and see if it looks like you want it to be . :D
     
  12. WBC
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    WBC Junior Member

    Yea I was working for a guy here in the cape and watched him draw on a wall and yea he had no clue how to loft. Nor did he have an eye for any kind of line be it straight or curved. I guess hat was why when I built the boat off his lofting the boat was 3/4" longer on one side than the other, but could tell him anything he already knew it all, like no way you can loft bearding, rabbet, or apex lines on the stem. I was humored and watched him struggle, never offered another suggestion.
    And gentleman I am in the process of drawing this thing out without the step. When I'm finished I will post here.
    Thanks for your comments
     
  13. WBC
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    WBC Junior Member

    Ok here it is with just a few minor adjustments. Will run what little calc I have.
    If you look the roiginal transom angle was 22 degrees, I changed to 12.
    If I take the rise out of the transom and make it dead flat there will that make it float shallower with a higher ppi?
    I'm not too fimiliar with the foam but would that say give her a higher negitive number in flotation resulting in a greater ppi?
     

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  14. WBC
    Joined: Aug 2008
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    WBC Junior Member

    I came up with a displacement of 1058 lbs and a ppi of 265 lbs, Sound about right at 6in.
    Thanks for your input
     
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