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

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

  1. SamSam
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    SamSam Senior Member

    The video won't open for me so I haven't seen it, but while motoring at 14 knots are you in rough water or do you make any sharp emergency "Don't run over my baby!" type turns? A straight cruise on smooth waters doesn't tell a whole lot.
     
  2. kerosene
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    kerosene Senior Member

    its not 14.9 knots but mph, so ~13knots.
    Boat has bow quite high and is creating pretty big wake and decent amount of spray. No major turns on the video but it doesn't look like it would be an issue to my untrained eye. No chop much to speak of but I don't think off shore performance at this speed was on the wish list. For sure not ideal hull shape for the speed BUT quite a bit more performance than what the "nay sayers" were predicting.

    So yeah it might not be perfect boat but the OP is right in gloating.
     
  3. rwatson
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    rwatson Senior Member

    I note one "naysayer' got it spot on

    "With a 15 HP outboard you'll max out the hull form around 14 - 15 MPH, but she'll have a nasty bow up trim at that point. This assumes you have enough "punch" to get her over the hump" PAR

    Combined with comparative speed comments by Tom, the performance is much as was expected, except for a couple of posts who were confused by the displacement figures.

    It looks to me that you got exactly what was predicted, - not much nay saying going on at all.
     
  4. kerosene
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    kerosene Senior Member

    Par Ftw
     
  5. fredrosse
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    fredrosse USACE Steam

    rwatson, "It looks to me that you got exactly what was predicted, - not much nay saying going on at all."

    I do not understand, I did not get "exactly what was predicted", I got a test run that worked, was reasonable in turns, went OK, although not ideal. I will post another video of turns soon.

    The issue is, at least in my business, if an cognizant engineer makes a statement as if it is fact, it is taken as fact, and if an expert thinks something might happen, then the person should say "I think it may turn out this way", then it is properly taken as an opinion. Reviewing the original post, the only "Opinion" was PAR's statement that it would make about 15 MPH, which was correct.

    As far as other statements in the original post, none of these things happened at all, yet you say I got "exactly what was predicted". Which of these comments do you think turned out "exactly what was predicted"?


    " first time you put the boat in the water it is going to sink."

    "I would be absolutely shocked if you can get up on a plane"

    "say about 7 - 8 MPH. If you try to push her much faster then this, she'll be come progressively more unstable, sticking her nose higher in the air, the faster you go"

    "She'll start rolling around and eventually get so unstable that it'll flip, swap ends or do some other undesirable thing underway,"

    "When she's standing on her tail with 3/4's of the boat flying out of the water, how much do you think those runners (bilge keels) are going to be engaged?"
     
  6. Saqa
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    Saqa Senior Member

    lol... goodonya mate, I'll have to find your original thread and see who those experts are for a bit of a laugh :)
     
  7. moTthediesel
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    moTthediesel Junior Member

  8. fredrosse
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    fredrosse USACE Steam

    Not crowing, not looking for revenge, just asking how things can be so far off between statements of several in the established boating community and the actual reality of the boat's running. That question still remains unanswered.
     
  9. parkland
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    parkland Senior Member

    I have posted a lot of ideas on here, and while I might agree there has been some negativity at some times, I feel lucky to have the interaction that I get from experienced boaters.

    If a flaw if pointed out in an idea, even if it is'nt 100% accurate or true, I have always enjoyed the fact that someone more experienced than myself has taken the time to look at my idea, and give feedback . Even if the statement is perhaps overly stated, it still gives a guy something to think about, and that to me, is what this is all about.

    Just because an idea works that was thought not to work, doesn't mean all of the issues aren't present, just perhaps to a lesser degree.

    Congrats ! Looks like you made it happen !
     
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  10. rwatson
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    rwatson Senior Member

    I did say one statement was exactly what was predicted, and you recognised it too

    in the same paragraph "only "Opinion" was PAR's statement that it would make about 15 MPH, which was correct."

    I said that PAR got it right - and he did !

    Oh, you forgot the other major part of the statement

    "but she'll have a nasty bow up trim at that point. "

    which it does.



    Regarding the other statements
    " first time you put the boat in the water it is going to sink."

    was before you clarified the displacement figures



    "I would be absolutely shocked if you can get up on a plane"

    You are not fully 'up on plane' in the video - if you were, the spray would be vastly reduced - you cant fully plane with rounded bilges



    "say about 7 - 8 MPH. If you try to push her much faster then this, she'll be come progressively more unstable, sticking her nose higher in the air, the faster you go"

    Pessimistic in speed, but not a bad guess. I don't see you doing any fast turns yet, and this will determine if the 'unstable' bit works



    "When she's standing on her tail with 3/4's of the boat flying out of the water, how much do you think those runners (bilge keels) are going to be engaged?"

    Not much wrong with that. You have it standing on its tail, and if there are any runners up front, they aren't doing much.




    Don't kid yourself - you haven't done much better than the previous comments suggested, and you got exactly what the most experienced commentator promised.
     
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  11. frank smith
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    frank smith Senior Member

    When I was a kid, we would put the biggest motor we had on the smallest boat we had.
    This usually entailed a 5hp on a 8' pram, or an old 18hp on a row boat. Great fun, and learned a lot. I'll bet its real touchy when it comes to steering input .I cant see the video, maybe you can load it on youtube.
    F
     
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  12. moTthediesel
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    moTthediesel Junior Member

    The powered Javelin's bow-up attitude is no worse than most "professionally designed" powerboats that I see on the water every day. At least it seems that the helmsman can see just fine without resorting to the now popular "saggy sheer" design band aid that has become the rage.

    Not fully on plane? Please explain how a 14' hull with only 12hp can be going 15 mph if it's not on plane? And this-- you can't fully plane with rounded bilges. What?? Have you ever sailed a Flying Dutchman in 20knts of wind? Or a Finn? 505? Lazer? Powerboats -- have you ever water skied behind a 15' Lyman with a 25hp OB? I have, all the more remarkable now knowing that round bilged boats can't "fully plane".

    Granted, a hard bilge line may give cleaner separation and less drag, but to say a boat can't plane with rounded bilges is just silly.
     
  13. PAR
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    PAR Yacht Designer/Builder

    She is up on plane and scooting along at 3.3 S/L ratio, which is easily in full plane mode. Of course she's also displaying what was predicted, she's on the verge of longitudinal instability, her bow is in the air, she's tossing spray off her bluff forward sections like a fire hose and I'll bet maneuverability will be very sketchy at that speed (she'll probably flop over), plus other issues, one of which is the following sea that'll climb aboard if you close the throttle very quickly.

    Congratulations, you've taken a Javelin to it's limit and you're burning easily twice as much fuel to do it.

    For example, a simple formula that is somewhat accurate (depends on constant used) is: The Square Root of (Total Shaft Horsepower/Weight ) x Constant = Speed. To get 14 MPH from a boat with your displacement (600 pounds assumed) and HP (10% slip assumed), the constant is about 95, which in real world terms is about half of the typical figures used for a constant on a flat bottomed boat (which is what you have and why fuel use is going to be way up). What this means it your boat has a huge drag quotient related to it.

    The pictures show her planing on her midship sections (something I tried to explained in the previous post I think), which is typical for a sailboat, but really lousy for a powerboat and why she has half the constant of a typical powerboat set of shapes. Lastly you may have a low enough CG to let this boat turn, but I wouldn't do so at speed. It's unlikely she'll trip, assuming she has a skeg and shaft hanging below her, but she may very well test the skippers ability to remain in the boat without a seat belt. It would appear at the very least she need some spray knockers too.
     
  14. tom28571
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    tom28571 Senior Member

    As a reference ot these numbers, the little runabout I referred to in an earlier post made 29mph with 10hp and a weight of about 500# total. The constant for that rig at that speed was an empirically derived 200 compared to your 92 (by my calculation). That constant is used in Crouch's formula that still works very well for most boats.

    So yes, your boat works, but not very well or efficiently. Some "nay sayers" may have been too pessimistic but many of their assessments were closer to the truth than those applauding the results.
     

  15. messabout
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    messabout Senior Member

    At the risk of hijacking part of the thread, here is another planeing sailboat scenario.

    I have a 15'-6" flat bottomed sailing skiff. A flattie. All up weight including the skipper (me) is 310 pounds. She has 80 square feet of sail. It will plane happily at 11 to 12 MPH in a 14 knot breeze. It does not hold its bow high, spray is moderate, the rudder throws a small rooster tail.

    When I drew the boat I used conventional notions that had the deepest part of the bottom forward of mid ships so that the run could be near straight and at a minimum angle to the static waterline. Widest part of the chine is aft of mid ship to try to accomodate the area curve. (Fox, Marchaj and others told me to do it that way)


    If I fiddle with the equation I get a very large constant, or alternatively, an improbable amount of driving force from the sail. Can we assign reasonable credibility to that equation when applied to small sailboats?

    Comments?
     
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