designing a fast rowboat

Discussion in 'Boat Design' started by nordvindcrew, Oct 13, 2006.

  1. Clinton B Chase
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    Clinton B Chase Senior Member

  2. DCockey
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    DCockey Senior Member

    Previous post was 250 lbs total displacement.

    For 310 lbs total displacement:
    Volume Displacement = 4.83657
    Wetted Surface Area = 27.8692
    Waterline Length = 15.888
    Maximum Waterline Beam = 2.74325
    Water Plane Area = 24.9775​
     
  3. NoEyeDeer
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    NoEyeDeer Senior Member

    Yes, but if you look at the rest of the figures he's using a displacement of 405 lbs, which presumably is meant to be boat + 2 people.

    This is why I said make sure you are comparing at the same displacement. An AGB with one person on board (even a monster like you) is only going to be displacing around 280-290 with everything, so the wetted surface is going to be lower.
     
  4. DCockey
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    DCockey Senior Member

    Annie (without outer stem and outer keel) at 405 lbs in seawater:

    Volume Displacement = 6.31991
    Wetted Surface Area = 30.6863
    Waterline Length = 16.0334
    Maximum Waterline Beam = 2.84803
    Center of Floatation = 8.81074, -9.28784e-16,0.469
     
  5. NoEyeDeer
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    NoEyeDeer Senior Member

    How much does the outer stem and keel add?

    ETA: Assuming 17 feet of length (including around forefoot etc) and a 1" square keel, that's 17/6 square feet, or almost 3. So that puts you about 13% behind the guideboat at the same displacement. If you build it without an outer keel, you'd only be a little bit behind (around 3-5% maybe).
     
  6. El_Guero

    El_Guero Previous Member

    Have you looked at SOF?

    Even for two men, you are talking some significant weight in these boats for racing. Don't you think?

    And I would think some narrower beam would lessen the water you have to move. Don't you think?
     
  7. NoEyeDeer
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    NoEyeDeer Senior Member

    I've looked at the weights for SOF boats of various sorts, and honestly they don't seem to come in much lighter than lightweight wooden construction. They're cheaper and easier to make, which is a good thing for some people, but the weight advantage isn't much.
     
  8. flo-mo
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    flo-mo Junior Member

    Here is something I want to share with you. It's work in progress and will arouse more questions than it brings answers but I thought it's a good time to make it public.

    A spreadsheet comparing speed for the same resistance and same displacement for a number of small craft (mainly rowboats and canoes). I made 3D models of the hulls and did the hydrostatic calculation with ProSurf 3 and choose Kaper for resistance prediction.

    To make it simple I choose the same displacement for all designs (single: 135kg/double 225kg).

    The numbers I choose for resistance should reflect:

    -cruising speed
    -pushing hard for a longer distance
    -sprint

    I also added the righting moment for 15° heel for comparison of stability.

    Although I tried to do my best while creating the 3D models the accuracy of the resulting numbers is not absolutely reliable. Nevertheless the spreadsheet should more or less reflect the characteristics of the various designs at different ranges of speed and displacement.
     

    Attached Files:

  9. flo-mo
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    flo-mo Junior Member

  10. DickT
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    DickT Junior Member

    Herreshoff

     
  11. NoEyeDeer
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    NoEyeDeer Senior Member

    Ok :D I noticed that the Herreshoff 17 is listed as having a speed of 5.11 knots (I assume it's knots) in your 6 kg "sprint" chart. I can tell you for a fact that those boats can sustain 5.6 knots average speed over 1.5 nautical miles, without needing an Olympic athlete at the oars.

    In short sprint (200 metres or so) they can get to around 6 knots. :)

    At your 5.11 knots, Michlet predicts a resistance of around 4 kg.

    ETA: That 4kg is at a displacement of 113 kg, which is what you can get if you build them lightly.
     
  12. flo-mo
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    flo-mo Junior Member

    The reference for my assumptions are the results form the Sulkava race (single and double).

    At the Sulkava Race the fastest men single average 6 knots for 58km, the fastest men double average 6.5 knots.

    So I was wrong to say sprint – I should have said: Top athletes in a long distance race.
     
  13. DCockey
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    DCockey Senior Member

    I added the outer keel and stems per the original design of Annie, and adjusted the waterlines to maintain the same displacements. The model I used previously had the stems and keel flush with the rabbet. The outer keel protrudes about 1/4". Molded width of the outer stems vary but is about 2". Increase in wetted surface area with the outer stems and keels added is under 3%.

    250 lbs:
    Volume Displacement = 3.88677
    Wetted Surface Area = 26.5067
    Waterline Length = 16.3099
    Maximum Waterline Beam = 2.64616
    Water Plane Area = 23.234

    310 lbs:
    Volume Displacement = 4.82515
    Wetted Surface Area = 28.6447
    Waterline Length = 16.4094
    Maximum Waterline Beam = 2.73496
    Water Plane Area = 24.8557​


    410 lbs:
    Volume Displacement = 6.32912
    Wetted Surface Area = 31.5765
    Waterline Length = 16.5428
    Maximum Waterline Beam = 2.84308
    Water Plane Area = 26.9482​
     
  14. DickT
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    DickT Junior Member

    Herreshoff below WL at speed

    FWIW here's my boat at an indicated 6mph on my GPS.
     

    Attached Files:


  15. NoEyeDeer
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    NoEyeDeer Senior Member

    Ok, so by those figures it looks like Annie has about 5% more wetted surface than the GB at the same displacement.


    Ah. That makes more sense. :)

    I'm not sure about KAPER. It doesn't use the actual lines, just a collection of coefficients. I did notice that when I tried to apply KAPER in Delftship it gave some very odd results at times.

    Then again, Michlet isn't 100% accurate either. From some of the full size testing Leo has done, it seems that for this sort of boat Michlet probably underestimates by around 10%. It should still be pretty accurate for comparative purposes though. It has the advantage that it does use the actual lines of the hull.
     
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