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  1. #11
    Join Date
    Jan 2009
    Location
    Lochaber
    Posts
    6,959

    Default Re: Hull speed Vs GPS sailing app

    My understanding, in simple terms, is a displacement boat produces a wave with wavelength proportional to square root of waterline length. As you increase speed you "catch up" on your wave and your stern is in the trough meaning you need to push uphill to go faster. The height of the wave basically depends on width to length, narrow giving smaller wave. This is why a catamaran is quicker and more fuel efficient than a monohull. Designing lift into the hull and shaping the bow to go through rather than up the wave helps.

  2. #12
    Join Date
    May 2001
    Posts
    14,334

    Default Re: Hull speed Vs GPS sailing app

    Perhaps I am lazy (after all I am past 72) but I tend to rely more on what the GPS tells me regarding SOG, COG, distance to the next waypoint and the ETA at that point. I am not really interested in my speed through the water, which doesn't really matter because this is affected by currents and leeway.
    On a long leg the info that the GPS gives is accurate enough for practical purposes because the values are continually being averaged.
    Upon arrival I am usually amused when I call up the 'MAX speed'. I have a shot of the instrument that shows "MAX 11.34kts"; this on a 32-ft boat that weighs 5 tonnes and with a 24' LWL. Yes, the Centurion32 is certainly not a sluggish boat but hardly "Hull Speed" by any stretch of the imagination; all it means is that in that particular second the GPS aerial was probably going over a largish wave, hence the spurious reading.

    Should we paint what is on a face, what is inside it, or what is behind it?

  3. #13
    Join Date
    Feb 2007
    Location
    Essex
    Posts
    22,028

    Default Re: Hull speed Vs GPS sailing app

    Quote Originally Posted by PuffTheMagicDragon View Post
    Perhaps I am lazy (after all I am past 72) but I tend to rely more on what the GPS tells me regarding SOG, COG, distance to the next waypoint and the ETA at that point. I am not really interested in my speed through the water, which doesn't really matter because this is affected by currents and leeway.
    On a long leg the info that the GPS gives is accurate enough for practical purposes because the values are continually being averaged.
    Upon arrival I am usually amused when I call up the 'MAX speed'. I have a shot of the instrument that shows "MAX 11.34kts"; this on a 32-ft boat that weighs 5 tonnes and with a 24' LWL. Yes, the Centurion32 is certainly not a sluggish boat but hardly "Hull Speed" by any stretch of the imagination; all it means is that in that particular second the GPS aerial was probably going over a largish wave, hence the spurious reading.
    No, young man, I don't think that you have to view the figure as spurious. On a (very few) occasions I have seen my boat speed go up to 11.5kn in semi-surfing conditions. On each occasion the speed has built up steadily over the course of several seconds and the peak speed was maintained for a short while. We certainly got fluke speeds before selective availability was turned off but modern systems seem to be well damped.
    Far away is near at hand in images of elsewhere

  4. #14
    Join Date
    May 2001
    Posts
    14,334

    Default Re: Hull speed Vs GPS sailing app

    Quote Originally Posted by johnalison View Post
    No, young man, I don't think that you have to view the figure as spurious. On a (very few) occasions I have seen my boat speed go up to 11.5kn in semi-surfing conditions. On each occasion the speed has built up steadily over the course of several seconds and the peak speed was maintained for a short while. We certainly got fluke speeds before selective availability was turned off but modern systems seem to be well damped.
    You could well be right!

    However, you might wish to reconsider your use of "young man" when referring to me! I certainly don't feel young at the moment. I am still managing to shuffle slowly with the aid of a walking frame. Last year I fell off the deck onto the concrete when the boat was at the yard fracturing my pelvis on both sides. This year, practically on the anniversary, I managed to lose my balance while trying to hold a line when the boat lurched in a sudden gust. This time I broke my right Femur, necessitating the insertion of a DHS, hence the need for the walking frame!
    Still, I have long learned to take life one day at a time. Maybe this year I'll still be able to fit in some sailing... all in due course.
    Should we paint what is on a face, what is inside it, or what is behind it?

  5. #15
    Join Date
    Nov 2001
    Location
    Eastern Med ish
    Posts
    3,268

    Default Re: Hull speed Vs GPS sailing app

    This week I broke the bracket on the gooseneck .uncontrolled jibe!!!! Don't ask !!! a 1500 Euro boombrake that was not connected.
    So we sailed back to my home base on Genoea alone and was able to get high 8s close hauled and 9.5kts on a reach . not for a few seconds,but, over a couple of hours. Wind speed in the middle to high teens .
    Last edited by rtboss1; 21-06-19 at 15:54.
    never confuse education with intelligence
    Sailing the Aegean

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