designing a fast rowboat

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

  1. NoEyeDeer
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    NoEyeDeer Senior Member

    Ok, you get 0.57/0.59 for Virginia's prismatic? That implies you're using different lines to what is on Nick Schade's site. He claims his boat is a Virginia, except for a modified stem profile above the waterline. His boat has a prismatic of 0.61. That's the lines I was using for my estimate. These comparisons are a bit pointless if we aren't comparing the same thing.


    No, it's not transitional. Guideboats had ridiculously narrow beam at the sheer. It's all to do with saving weight for portage. They reduced the beam right down to the absolute minimum they could get away with. The rowing style had to be adapted to suit.


    The thing that bugs me about KAPER is that it gives resistance curves which have no lumps in them. That's not realistic, due to the way wave drag varies with speed. It goes up and down like a yo-yo, even for skinny hulls.

    ETA: The attached picture is what Michlet predicts for Schade's Virginia at an immersed volume of 0.129 m3 (salt water displacement of 132 kg/292 lbs), with level trim vs one degree stern down trim.

    With the stern down trim, the prismatic decreases from 0.61 to 0.57.

    There's a 10% increase in predicted total resistance, all of which is due to increased wave drag. Given the massive change in distribution of volume between the two trims, I would expect to see some effect.

    By the way, this effect is why I drew my boat so that the keel line effectively duplicates stern down trim (rocker for'd, none aft) for directional stability, but with a curve of areas that is optimised for that trim (ie: not massively shunted aft). Best of both worlds that way. Good wave drag, and good directional stability.
     

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  2. NoEyeDeer
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    NoEyeDeer Senior Member

    Re this Delftship file compatability, I checked on the Delftship site and apparently the auto upgrade doesn't work for the 64 bit free version, which of course is what I'm using. I have to manually upgrade. That's no problem in itself, but the only downloads available for manual upgrade are 5.24.223. So, it looks like I'm screwed.

    Any chance of a scan of those tables of offsets? If I have the actual offsets to the outside of the planking, I can model the hull myself in a format I can use.
     
  3. DCockey
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    DCockey Senior Member

    I'm also using the 64 bit free version. I downloaded 5.24.223 yesterday and installed it. When I opened it, it asked if I wanted to upgrade to 6.13.242 which I did.
     

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  4. NoEyeDeer
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    NoEyeDeer Senior Member

    Oh cool. Ok, I'll manually upgrade my installation to 5.24.223 tonight, and see if it all works then. That would be handy. :)
     
  5. NoEyeDeer
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    NoEyeDeer Senior Member

    Ok, works now. Opens and saves the file. Needs some work to sort the stem and bottom board, but the basics are there. Also needs to have some main particulars set, and a couple of other tweaks, before Delftship will do hydrostatics for it. Got that bit done already.

    This boat is quite a bit finer in the ends than the Grant boat, but there's not much difference in metacentric height or wetted surface.
     
  6. ancient kayaker
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    ancient kayaker aka Terry Haines

    Dick, did you do any more work on the "vertical chine" concept you proposed some (considerable) time ago? The more I look at the GB lines you guys are discussing, the more I suspect the concept may have merit for constructing these boats and also canoes. I was looking at a sailboat design at one time but lost track of it.
     
  7. Clinton B Chase
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    Clinton B Chase Senior Member

    It'd be easy to push that Rushton beam out to 44" so it is legal but also can use oars rather than paddles. A beam of 44", assuming it is at the oarlocks would allow 8'+ oars.

    Using 7:18 inboard to outboard ratio: inboard is 22" + 5" for freeboard and hand overlap = 27". Outboard would be 69 1/2" so total oar of 96.5" or 8'. That sounds good to me.

    I really like the idea of keeping beam manageable for the purpose of having a boat that I could get onto the roof rack and head inland to the lakes. My goodness, I am sounding more like a "guide boater" every post.

    Shall we see what we can do with this Saranac at 18' with 44" beam max. Oh, that wouldn't be legal...17' is max in GB races?

    Flo-Mo mentioned Ben Fuller earlier who I know and have talked to a lot. He has experience in A LOT of trad. boats. He feels that the best waterline length for a normal, pleasure rowboat is a skinny 16'. I have always taken that and figured for me 17' lwl would be good.

    I'd love to see a 17' Saranac....with the max beam allowable in GB races. The hull could always be stretched to 18' for those not needing to compete.
     
  8. NoEyeDeer
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    NoEyeDeer Senior Member

    I'm gonna just use that Saranac Laker file as a guide for marker curves, and re-do it completely in Delftship with a new control grid. The existing grid is so complex that it's effectively impossible to work with in Delftship. You end up chasing your tail for ages if you need to adjust one point. If starting from scratch, a much simpler grid could be used, which would make fairing easy. :)

    ETA: Screenshot of a control curve run down the keel in Delftship. It's way out of fair.
     

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  9. Clinton B Chase
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    Clinton B Chase Senior Member

    Sounds good. Are you doing it as is? Or beamier?

    David, how do the guideboats you mentioned compare with the Laker?
     
  10. DCockey
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    DCockey Senior Member

    I'm working on them. I'm currently modeling Ghost which has a much more detailed and complete set of plans to work from than the Rushton Saranac Laker. By the way, Ghost has an outside beam of around 38" and 8' oars are shown on the plans.

    Adirondack Guideboats who build Kevlar and wood-strip guideboats say their design is based on Rushton and Grant designs.
     
  11. DCockey
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    DCockey Senior Member

    Clint what are your "requirements"?
    1) Absolute musts; won't even look at the design if it doesn't meet these requirements.
    2) Essentials; some wiggle room if not 100% meant
    3) Really want to have
    4) Would be nice​

    Where does being a "double ender" fall?

    Any interest in shapes which are not "traditional"?
     
  12. KJL38
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    KJL38 Senior Member

    With my guideboat built to Nick Schades plans I use 8 ft oars with total inboard length of 22.5" and it seems to work well. The increased gearing isn't a problem as the hull drag is low.

    The narrow beam does make it easier to handle on land as I can stand beside it with a hand on each gunwale and lift it above my head which would not be possible if it was much wider or heavier.
     
  13. NoEyeDeer
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    NoEyeDeer Senior Member

    Well, as near to "as is" as possible, since the idea is to just get another reference model for "normal guideboat".
     
  14. DCockey
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    DCockey Senior Member

    Rules for guideboats in the Adirondack Classic http://www.macscanoe.com/class-specs.html
    GUIDEBOAT:

    Length: 11 - 23 feet (15 to 17 standard)
    Beam: 32 - 44 inches (Outside measurement: 34 to 38 inches is standard)
    Bottom Board Outside Width: 4 - 10 inches.
    Stem Height: 20 - 26 inches.
    Depth Amidships: 11 - 13 inches.
    Construction: Traditional, Strip or mold construction; ribs may be laminated, hull may be wood, fiberglass, kevlar, or composite material, but must
    conform to traditional guideboat shape and character
    in the opinion of the boat judge.
    Oars: Solid wood oars, maximum 8 foot length, 5 1/2 inches wide.
    Hardware: Brass, bronze, or steel pins with 1/2 inch diameter hole to accept pin; may be bushed if out of round.
    Use of wheels: is permitted in the Guideboat Class.
    Footbraces: may be used.
    Seats: Must be fixed, no rolling seats are allowed.
    Oars and Paddles: may be laminated.
    Hardware: Brass, bronze, or aluminum pins.
    Footbraces: must be secured to the boat.
    UNACCEPTABLE for the guideboat class:
    · Roller or sliding seats.
    · Roller, ball or needle bearings in hardware.
    · Outriggers.
    · Boats of design and or construction with pulled- in beams / ends.
    · Paddles and oars with cupped or spooned blades.
    During an event, the guideboat must be propelled by rowing. i.e. the rower can not exchange a paddle for the oars to negotiate Brown's Tract.​
    Note the clause: but must
    conform to traditional guideboat shape and character
    in the opinion of the boat judge.
     

  15. DCockey
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    DCockey Senior Member

    How much do you expect that degree of unfairness would affect your performance predictions?
     
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