Ocean Crossing In A Small Boat

Discussion in 'Sailboats' started by venomousbird, Jan 28, 2009.

  1. Tcubed
    Joined: Sep 2008
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    Tcubed Boat Designer

    Hoorah! IT IS an ITCHEN FERRY. I'de give you rep but the program won't allow rep twice to same.
  2. Chris Ostlind

    Chris Ostlind Previous Member

    Here's the interesting part... the square root of the LOA (not LWL) is 4.69 and when multiplied by the suggested factor of 1.34 for displacement hulls of this type, you get 6.28 knots of hull speed.

    When the 6.28 value is multiplied by 24 hours, we see a suggested, hull speed distance of 150nm.

    Even more remarkable is the 48 hour distance covered of 385nm, resulting in a speed of 8 knots AVERAGE. This means that the boat regularly saw speeds in the 10-12 knot region for lengthy sections of the 48 hour period in order to get that very high average... and all when the boat was at its most heavily laden condition, at the very beginning of the journey.

    What is not being told is the current speed, the wave action, the wind conditions, the sail area being flown and the ever-present sailor's fudge factor for boat's of their own.

    You want to say, Hmmmmm? What's up with that?... I wouldn't blame you.

    "By the hand of Neptune she was spirited along at a frightening pace..."

    There are lots of very solid, 40' blue water cruisers who would like to see 24 and 48 hour distances such as these being reported. 40 footers, mind you, with modern underbodies, bulbed fin keels, the latest foil shapes and very powerful rigs.

    So, TC... tell us the rest of the story, would you?
  3. Tcubed
    Joined: Sep 2008
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    Tcubed Boat Designer

    If you look at cruisers daily averages and compare it against boat length the expected correlation is much weaker than expected by simply looking at root(lwl) . This is a pretty well established fact. I would say it is no doubt due to small boats being much easier to drive hard and also because small boats don't suffer as much in light winds.

    Those couple of days it was blowing pretty strong at close to forty knots from behind with a big breaking sea. I had a partly reefed main , staysail and the storm spinnaker up so no shortage of power.

    The current in that area is about 0.5 knots to 1.0 knots typically (in my direction) although who knows it might have been even a bit more. But not much more or the sea state would have been calmer.

    So that gives an average speed length ratio of between 1.53 and 1.64.

    There is nothing in fluid dynamics that says that once you reach 1.34 all further speed is impossible. What it does say is that it takes rapidly increasing amounts of power - for these kinds of disp/length ratios. On that day power was not an issue. Also as it was two consecutive days and the day before and after were also high i know that it is not due to navigational error. I know full well the boat was exceeding hull speed by simply observing the position of the stern wave.

    To me this is not so impressive as the fact that the boat was so well behaved that it would self steer (sheet to tiller method) well enough to allow me to drive it so hard, although i must admit there were a couple of instances when i saw a particularly gnarly wave come up from behind and i seriously wondered if it would be able to take it (as in not broach). It did though.

    PS the [...most heavily laden..] is a moot point . I had less than a hundred kg of food onboard and a bit less than 80 liters of water compared to 3200 kg disp makes almost no diference.
    Last edited: Jan 28, 2009
  4. alan white
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    alan white Senior Member

    I forgot. Check out stbd frame 19 for a minor crack just behind that discolored ceiling plank.
    On the serious side, I love itchen ferries.
  5. Tcubed
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    Tcubed Boat Designer

    Actually this one had no ceiling, which i prefer as it allows for visually inspecting the entire hull.
  6. Landlubber
    Joined: Jun 2007
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    Landlubber Senior Member


    Why is it that the boat required does not necessarily have to be comfortable, as you stated in the openimg lines.

    One thing about ocean sailing, you never know what tomorrow may bring, and the best thing i have found is to be ready for tomorrow. That involves plenty of rest, good food and a balanced boat that does not really require you at the helm. Comfort is relative to each and everyone of us I know, but a comfortable boat, both for yourself and for the boat too, makes for a successful voyage, and also the desire to continue from one port to another.

    Personally, I would rate comfort as being a serious requirement.

    Those smaller boats, Vertue etc are certainly good sea boats, but again, personally, I prefer over 30 footers, they are not necessarily more expensive either. You can get to just over 40 feet without getting into "serious" sized winches and sails, rigging etc. The loa is very important for daily averages, no way a smaller boat is going to average the same as a larger boat over time/distance. Speed is of course not the main driving point, but it is nice to have a boat that can sail well and fast.

    Another point worth consideration, design....good design cost no more to build than bad design......yet the value and real worth of a good design is invaluable. Never build a crap design just because it is cheap...the poor man pays twice.
  7. Guest625101138

    Guest625101138 Previous Member

    The 1.34 is not a "suggested" factor it is a defined parameter related to the celerity of the transverse wave group formed by the hull that has the same wavelength as the water line length of the hull. The term hull speed was coined a long time ago when it was determined that heavy displacement hulls found it difficult to exceed the so-called hull speed unless they had very high power.

    In deep water the celerity of a wave train is given by:
    V=sqrt(g x Wavelength / (2 PI)) (g is gravitational constant 9.8m/s/s in mks or 32ft/s/s in British units)

    If you have WL in ft and speed in knots then it is:
    V= sqrt(WL*32/2/pi)*3600/6080 kts
    = 1.34 * sqrt(WL)

    Rick W
    1 person likes this.
  8. Chris Ostlind

    Chris Ostlind Previous Member

    Cube, the ratio does not change, just because you are moving in a favorable current. The VMG might change, but the hull speed coefficient remains the same, as does your ability to move through the water.

    You can get a faster hull that planes, or you can get a faster hull that is a heck of a lot longer and skinnier, but that hull you have there is going to be... that hull. Unless, of course, you have a magic new kind of math that wiggles around the realities that have been accepted for a century now and are applied by naval architects the world over..

    Yes, you are right, you might be able to go faster with that hull... provided that you mash her into the sea with a prodigous sail rig (which is not in evidence, no matter what you may have been flying) and produce an incredible amount of power well beyond anything you indicate that was safely available... as in something like triple the generated horsepower and the boat is trimmed and built, to benefit... and you have the sand (some would say it's actually something different than sand) to go out and do it.

    Take a long look at the start of this season's Vendee Globe, as it left from Sables d'Olonne, France. The fleet is composed of some of the best, solo monohull sailors on the planet, with the absolute best equipment that can be used. Yet, they tried like hell to run with a storm that was pounding the Bay of Biscay with winds and sea states similar to those you describe and they got, collectively, their butts kicked but good. This is the very same region of the Atlantic that you describe in your narrative.

    Many of the VG sailors (Michel Desjoyeux, "The Professor", for one) had to return to shore to repair broken bits and wait for the thing to subside. The ones that did press on were doing all they could to stay alive and safely knock-out the same kinds of boat speeds you say you handled easily with a boat about one third the LOA.

    Like I said, Cube, I love the story and have a great deal of affinity for those who can tell a good yarn. I'm going to leave it at that.
  9. Tcubed
    Joined: Sep 2008
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    Tcubed Boat Designer

    Sound like a lot of armchair talk from someone who has a poor grasp of the fundamental equations .
    I do not have the time to counter attacks from someone openly insinuating i am not reporting the truth of the events as they were.
    You display the primary characteristics of prejudice, pouring scorn over what you do not understand, instead of engaging in rational discussion to see where the truth really lies.
  10. RichR
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    RichR Junior Member

  11. Chris Ostlind

    Chris Ostlind Previous Member

    8 knots easy in a gale with a loooong sprit and a chute flying for max effect. Dude hooked-up and disappeared over the horizon with a white whale on his, errr, transom and pink dolphins leading the way to safety.


    McNally is quite the guy. Certainly one of but a few on this planet, but quite the guy, nevertheless.

    Attached Files:

  12. Guest62110524

    Guest62110524 Previous Member

    chris ostlind
    something I am missing then, my last 48 foot wl was very happy sailing in full gale at 13 knots,, her 48 sq rt 1.34 giving theoretical 9.34 and under power flat tit she would do 10, Guess this is what T cubed was saying Nimbus is not a planing sailing yacht
  13. Chris Ostlind

    Chris Ostlind Previous Member

    Good for you, Whoosh.

    Don't get too excited, though. My 20 ft. trimaran does 30knots all day and night without the screacher up and man does that water spray hurt. ;-)

    Please refer back to the post by Rick Willoughby from this afternoon. Have a good look at the info shown there and get back to us.
  14. Guest62110524

    Guest62110524 Previous Member

    nah I sleep well, surfin down waves with my 8mm formed ally bottom, Ill, pilot your boat, then get off when dark comes:))

  15. Landlubber
    Joined: Jun 2007
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    Landlubber Senior Member


    Don't loose faith mate, I am sure there are many of us on this forum that have exceeded calculated hull speed for many days at a time, you say you did it, you did it, no worries mate.
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