So much for the "Nay Sayers" (Powered O'Day Javelin 14.9 mph)

Discussion in 'Boat Design' started by fredrosse, Oct 31, 2013.

  1. daiquiri
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    daiquiri Engineering and Design

    You guys who use mph for measuring sailboat speed always leave me wondering why... Why mph? :)
    My first thought when I see a mph figure is: does he really means (statute) mph or did he mean to say knots (which is nautical mph)? 1 kt = 1.15 mph, making a sensible difference.

    Messabout, can you tell us how did you measure those numbers (sailboat and wind speed) - to let us comprehend better how big is the ballpark.

    Cheers
     
  2. PAR
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    PAR Yacht Designer/Builder

    You can equate wind speed with HP to a degree, so assuming fresh winds (for a day boat) you're making in the range of 1.6 to 1.8 HP on that flattie. Of course, wind speeds vary, so will available HP.
     
  3. rwatson
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    rwatson Senior Member

    I don't know what sort of boats you are used to, but if I owned one that threw that much spray, I would never exceed a few knots out of sheer embarrassment.


    You need to brush up on hull hydrodynamics.
    http://www.staatsgeheim.com/wp-content/uploads/Savitskyreport.pdf

    "Negative Pressures on Convex Surfaces.
    One other major hydrodynamic phenomenon associated with high speed itself is the generation of negative pressures along convex surfaces of the hull i.e. convex longitudinal buttock lines or convex curvatures in the transverse plane—particularly in the bilge area. The influence of this effect on hull form selection is also discussed subsequently. "


    It is impossible to 'fully' plane if you have convex curves of any sort in the hull, because of the negative pressure generated.

    How about an accurate quote = "Not fully on plane"

    You know what fully means ? maximum planing efficiency is fully on plane.

    And "Less drag..." ? Lets be honest and admit we are talking 4 - 6 times less drag.
    Talk about damning with faint praise ! You are talking major planing degradation with round bilges.

    Those sailing boats you quote suffer from the same planing inefficiencies as this one, not matter how spectacular the sight of sailing craft creating huge amounts of wash may be.

    Tom2851's figures are the best empirical evidence that this design exercise is just stubborn attempt to defy the laws of physics, and failing.
     
  4. daiquiri
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    daiquiri Engineering and Design

    True, although sailboats can do without the optimum planing efficiency because they have the propulsive power for free.
     
  5. rwatson
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    rwatson Senior Member

    There's nothing 'free' about sailing boats - which is why the vast majority of commercial boats are not sailing.

    You can outperform just about any sailing boat with a 6hp outboard with fuel, for 1/3rd of the price of mast, rigging, sail wardrobe, sheets, winches, sail covers and assorted fittings.

    I could also add on the complexity of hull building, cost of ballast for many types of sailing boats.

    Dont give me 'free' :mad: - I have owned a bunch of 'em !!!
     
  6. daiquiri
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    daiquiri Engineering and Design

    That is correct. I meant to say - petrol-free while underway. Certainly not cost-free in a general sense. :)

    Cheers
     
  7. PAR
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    PAR Yacht Designer/Builder

    You can be fully up on plane with convex surfaces. Many a round bilge boat does it all the time, including the one I own, but yes, the shapes used in these sections can be limiting, particularly at higher S/L ratios than shown in the video.

    The problem with this boat is she's in full plane mode, but on the sections (hull area) typical for a sailboat, which is different then a powerboat. This boat was designed before more modern approaches to planing dinghies, so it has a fairly bluff bow and quickly flattening section forward of midship (hence the spray), so she has bearing area and buoyancy to get up and scoot. A powerboat typically has a higher power to weight ratio, so approaches it's plane efficiency different, with much less volume in the entry, it's CG located further aft and considerably (comparatively) more bearing area (planing patch) aft. If this boat wasn't what it is and was a more modern dinghy, she'd have more efficiency in her after sections and would likely plane better. In fact some dinghies can plane at 3.3 S/L with their sailing rig, pretty easily.

    The real question is if she's practical. At 14 MPH, I'd say no, but reduce speed just a bit, say 10 - 12 MPH, which will save the engine and fuel consumption considerable and she's fine, if a bit wet.

    I'll also add that yes the rig does cost more initially, but after not all that many rides on mother natures breath, you've surpassed the cost differences and yep, the rest is free.
     
  8. tom28571
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    tom28571 Senior Member

    Finally, something to really laugh at in this thread. I am currently working out the cost of owning a "free" sailboat.

    A feature of the North Carolina offshore fishermen that go up to 80 feet LOA is that the bottom DOES usually have a bit of convex shape near the transom. With megapower engines, these can go fast enough to satisfy anyone's idea of planing. They also work in some of the roughest water to be found. There are reasons for that but this thread is already complicated enough.
     

  9. messabout
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    messabout Senior Member

    Daiquiri; You are right about the silliness of mixing units like MPH and Knots. One day we Americans will come to our senses and adopt the metric system so that we are all on the same page.

    I measured boat speed with a GPS. In typical US fashion, it displays miles per hour. The handheld windspeed indicator displays knots. My numbers would not withstand requirements for rigorous proofs. Pretty good ball park estimates nonetheless.

    Is the weight quantity in the equation to be taken in pounds, kilograms, long tons, or the now defunct Brit units of stones?
     
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