finding the arc between two points

Discussion in 'Boat Design' started by Mcgyver, Aug 8, 2007.

  1. Mcgyver
    Joined: Aug 2007
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    Location: Maine

    Mcgyver New Member

    Im starting to build a hydroplane from the 60's (sporthydro) that I found on the net, and I can't figure out how to find out the curve of the BOWPLATE. I have all of the dimensions that I need but cant figure out the curve. I know I can just wing it and get something close but I want it to be a little bit more precise. is there a more precise way of doing this or do I have to guess. I have also clipped a picture from the plans so you can see what I'm talking about
    Please Help

    The New Guy

    Mcgyver:confused:
     

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  2. LP
    Joined: Jul 2005
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    LP Flying Boatman

    Without going into detail, I just drew it up at a 62" radius.
     
  3. Pericles
    Joined: Sep 2006
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    Pericles Senior Member

    Use a batten to create the curve, using the measurements given on the blueprint.

    Pericles
     
  4. Mcgyver
    Joined: Aug 2007
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    Mcgyver New Member

    So string and a pencil is the simplest way to do this? if there is a better way to do this, can you point me in the right direction?

    Mcgyver
     
  5. lazeyjack

    lazeyjack Guest

    take the beam, and divide by 3
    then take the crown height and mutliply by these factors
    at 1/3 beam out from centre, multiply height by .8818, and 2/3 out by .5393
    draw straight line between outside ponts and divide into the thirds, now square up and mark the heights you obtained , I do all my decks this way, if there are 40 beams I do each and everyone separately, makes very fair deck and beam,
    you can then use a batten to draw in the line from points or do it in rhino or something
    is this what you are wanting?
     
  6. lazeyjack

    lazeyjack Guest

    here in rhino
     

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  7. lazeyjack

    lazeyjack Guest

    here it is split up, it is a very accurate and fast way
     

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  8. Michael Chudy
    Joined: Apr 2005
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    Location: Eastport, Maine

    Michael Chudy Yacht Designer

    If you want the math, here it is.

    To find the radius of an arc when you know the height and the length:

    You know the height of the arc = 4.5"

    The length is 48 " minus twice the length of the opposite side of the triangle whose adjacent side is 3" and angle is 15 Degrees.

    Opp = Adj * tan angle. So Opp = 3" * tan 15. Opp = .804"

    Length = 48 - (2* .804) = 46.392

    Using Pythagorean theorem:

    Radius = (4h^2 + L^2) / 8h

    Radius = 62"

    All this information and more is available online at

    http://mathcentral.uregina.ca/

    Have fun,
    Michael
     
  9. Mcgyver
    Joined: Aug 2007
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    Location: Maine

    Mcgyver New Member

    Thank you very much for your help guys. I got the desired arc for the bow plate and it came out perfect.

    Thank you

    Mcgyver
     
  10. BWD
    Joined: Aug 2006
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    Location: Virginia, US

    BWD Senior Member

    Glad the deck arc came out right.
    Wanted to ask if anyone has used the method from Gougeon's book, getting ready to do so myself.
    Very simple, draw circle with radius equal to deck camber, measure arc heights above diameter, transfer to template/stock and spring batten. Seems straightforward, but no reason not to ask before cutting...;)
     

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  11. SkipperSki
    Joined: Sep 2007
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    Location: Olympic Peninsula, WA

    SkipperSki Junior Member

    Mcgyver,

    I assume the picture is from a magazine, the easiest way is first determine the scale of the drawing, using the 6" dimension, on a scale ruler it's say, 3/8" which is a half foot of 3/4" scale.

    Now tape the picture to a sheet of paper, and using only a drawing compass, pick two points along the arc. place the compass on one point, and scribe a 180 degree right arc, perpendicular to the arc, place the compass neddle at the intersection, and scribe a 180 degree left arc, the intersection points of the two scribed arc's, place a straight edge on these points and draw a line, out on the drawing paper, this drawn line is perpendicluar to the design arc, do the same at a wider point of the design arc, this second line drawn to intersect with the first, is the vertex of the designs radius, measure the design radius from the found drawing scale. Or draw it out full size on your material.
     
  12. tom28571
    Joined: Dec 2001
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    tom28571 Senior Member

    Most of the above methods will give an adequate answer.

    The formula is: r = 4(h^2) + (c^2)/ 8h

    r = radius of arc
    h = height of arc = 4 1/2
    c = span of arc = 48

    So the radius of your deck beam is 64.55"

    If the span is far greater than the height, a close enough answer is r = c^2/8h which give 64". close enough and easy enough to remember.:cool:
     
  13. lewisboats
    Joined: Oct 2002
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    lewisboats Obsessed Member

    I take it no one reads the entire post...he already has the darn thing done!
     
  14. SkipperSki
    Joined: Sep 2007
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    SkipperSki Junior Member

    Aye, Aye, Mate !

    But someone may learn from it next year !

    The compass method is fool proof, when doing a boat part copy/restoration.
    by drawing out the arc full size, and find the center of the arc/circle (s)
     

  15. tom28571
    Joined: Dec 2001
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    tom28571 Senior Member

    Aye Lewis, but knowing the simple formula will work at small scale on paper or at any scale next time anyone wants to do this. The true mensuration formula from a math book is much more involved but this one works well enough.

    Of course if anyone just likes to plug numbers into an internet table, just look here:http://www.handymath.com/cgi-bin/rad2.cgi?submit=Entry

    Also kicks out the length of the arc.

    Good knowledge never wasted.
     
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