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Thread: Munch

  1. #1
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    Default Munch

    What is the relationship between LOW to hull speed? I have seen 1.35 x square root of length. Does this apply to the new cat boats?

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    Default Re: Munch

    No. If the hull has a length to beam ratio of 12-1 or so, figure 2 x root of LWL. But that's not the upper limit.
    -Dave

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    Default Re: Munch

    I thought it applied to all displacement hulls. I think the theoretical number is 1.42 but 1.25 was considered a reasonable goal. A lot of displacement hulls become semi displacement or true planing hulls when driven above hull speed.

    I have read the theory that some of the clipper ships may have sunk because they exceeded the max hull speed and literally drove into their own bow wave.

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    Default Re: Munch

    Quote Originally Posted by Hooligan navy View Post
    What is the relationship between LOW to hull speed? I have seen 1.35 x square root of length. Does this apply to the new cat boats?
    Which new cat boats?

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    Default Re: Munch

    Its not a hard fixed value, speed vs resistance is a curve. But there is a big big increase in resistance above about 1.34 * (LWL)^.5

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    Default Re: Munch

    The formula is quite old, and was valid for the boats of the day. There is no law of physics that puts a speed wall at 1.34 x root of LWL. It's really about the wave a boat creates as it accelerates, and wider, deeper hulls naturally make bigger waves. A very light boat with long skinny hulls makes very small waves and can consequently go faster for it's length while still at displacement speed. Get the boat up on plane or add a lifting foil and the formula goes out the window.
    -Dave

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    Default Re: Munch

    There's a good explanation of this at
    http://erezbi2000.wixsite.com/proski...-True-Or-False
    The phenomenon falls apart with a boat whose displacement is light enough that it doesn't make much of a bow wave.

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    Default Re: Munch

    Quote Originally Posted by Hooligan navy View Post
    What is the relationship between LOW to hull speed? I have seen 1.35 x square root of length. Does this apply to the new cat boats?
    It applies to catboats, but not to catamarans.



    If this is the kind of boat you're talking about, it applies.

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    Default Re: Munch

    Quote Originally Posted by Woxbox View Post
    The formula is quite old, and was valid for the boats of the day. There is no law of physics that puts a speed wall at 1.34 x root of LWL. It's really about the wave a boat creates as it accelerates, and wider, deeper hulls naturally make bigger waves. A very light boat with long skinny hulls makes very small waves and can consequently go faster for it's length while still at displacement speed. Get the boat up on plane or add a lifting foil and the formula goes out the window.
    That is not correct. It is about the relationship between the crest created at the bow and the crest created at the stern and whether they cancel each other out or add together. A displacement hull at hull speed has the second crest of the bow wave train coinciding with the stern wave, causing a big hump in the displacement curve.
    Now a barge arsed catboat is closer to a semi displacement form so if there is enough power available and the bow is full enough (high prismatic coefficient) it can be driven past that hump in the drag curve, given enough power.
    Not so much for the finer ended counter stern catboats, which I think would be more likely to lock in.
    It really is quite difficult to build an ugly wooden boat.

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    Default Re: Munch

    My brother-in-law law would "surf" his wooden 55' sloop. What a thrill ride. There is something about old wooden boats.

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    Default Re: Munch

    Quote Originally Posted by Woxbox View Post
    The formula is quite old, and was valid for the boats of the day. There is no law of physics that puts a speed wall at 1.34 x root of LWL. It's really about the wave a boat creates as it accelerates, and wider, deeper hulls naturally make bigger waves. A very light boat with long skinny hulls makes very small waves and can consequently go faster for it's length while still at displacement speed. Get the boat up on plane or add a lifting foil and the formula goes out the window.

    Quote Originally Posted by Peerie Maa View Post
    That is not correct. It is about the relationship between the crest created at the bow and the crest created at the stern and whether they cancel each other out or add together. A displacement hull at hull speed has the second crest of the bow wave train coinciding with the stern wave, causing a big hump in the displacement curve.
    Now a barge arsed catboat is closer to a semi displacement form so if there is enough power available and the bow is full enough (high prismatic coefficient) it can be driven past that hump in the drag curve, given enough power.
    Not so much for the finer ended counter stern catboats, which I think would be more likely to lock in.
    You are being a little picky Peerie Maa. Based on the question and apparent minimal knowledge of the OP, Woxbox was feeding some really simple stuff that is enough to get the OP on the right track. This stuff can be and is argued over and over again by supposed experts without either consensus of conclusion. While I'm pretty sure I know what is going on with the water when a boat accelerates, i'm not certain I follow the meaning in your second sentence.
    Tom L

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    Default Re: Munch

    I guess that poses this question. How do you choose a hull shape? One that's narrow, lite, and fast, or one that is wider, heavier, and "spreads the water", and thus slower. They both serve a purpose, I think Alden and Hershoff and others found that balance.

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    Default Re: Munch

    What balance? It's going to change enormously according to the sailor, the area, and the technology, so there is no "balance" but many balances.

    It's also not a case of being narrow and fast or beamy and slow - beamy boats can be very fast if they are light. Look at an Open 60, which is incredibly fast reaching in a breeze. Actually, even something like a standard Beneteau cruiser/racer is much faster than a skinny Herreshoff or Alden of the same LOA. Beam gives stability without increasing weight as much, which is a recipe for good speeds from many angles.

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    Default Re: Munch

    Quote Originally Posted by Tom Lathrop View Post
    Quote Originally Posted by Woxbox View Post
    The formula is quite old, and was valid for the boats of the day. There is no law of physics that puts a speed wall at 1.34 x root of LWL. It's really about the wave a boat creates as it accelerates, and wider, deeper hulls naturally make bigger waves. A very light boat with long skinny hulls makes very small waves and can consequently go faster for it's length while still at displacement speed. Get the boat up on plane or add a lifting foil and the formula goes out the window.



    You are being a little picky Peerie Maa. Based on the question and apparent minimal knowledge of the OP, Woxbox was feeding some really simple stuff that is enough to get the OP on the right track. This stuff can be and is argued over and over again by supposed experts without either consensus of conclusion. While I'm pretty sure I know what is going on with the water when a boat accelerates, i'm not certain I follow the meaning in your second sentence.
    This supposed expert degree qualified Naval Architect does know how wave-making resistance works.
    This second sentence?
    A displacement hull at hull speed has the second crest of the bow wave train coinciding with the stern wave, causing a big hump in the displacement curve.
    A wave train is created by the pressure of the bow pushing the water. The stern crates another wave train due to the pressure created by the water flow leaving the hull.
    As the boat speed changes the wave length of the wave train changes, so there will be speeds at which the stern wave is cancelled out by the trough of the bow wave train, and speeds at which they add together creating a hump in the resistance curve. The last hump occurs when the second crest of the bow wave train coincides with the crest of the stern wave, increasing the size of the wave hence increasing the power that the wave absorbs. Past that point at Fn 0.3, if you have the power to drive the hull you go into semi displacement mode. If you can get past the maximum resistance at Fn 0.5 you start to plane.




    The visualisation of the two wave trains :
    It really is quite difficult to build an ugly wooden boat.

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    Default Re: Munch

    ^I think this is the first time i actually understood the whole bow wave + stern wave-resistance thing
    Ragnar B.

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    Default Re: Munch

    Quote Originally Posted by Peerie Maa View Post
    Now a barge arsed catboat is closer to a semi displacement form so if there is enough power available and the bow is full enough (high prismatic coefficient) it can be driven past that hump in the drag curve, given enough power.
    Not so much for the finer ended counter stern catboats, which I think would be more likely to lock in.
    Bolger claimed, it was the mid sections of catboats which gave them their turn of speed, not the bow or assed ends. He didn't really explain it other than to say their midsections performed really well. . .
    Simpler is better, except when complicated looks really cool.

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    Default Re: Munch

    Wave physics can get complex but there is a useful approxomation formula that shows the relationship between wave length and period for deep ocean wind driven waves. Converting that from metric to feet and knots it turns out to be about

    Speed in knots = (1.34)(square root of length in feet)

    Before you get your boat to "hull speed" the boat will, as some illustrations above show, make waves behind the bow wave. The distance between these waves is a measure of speed. Laden with approxomations, I made the following table for small slow displacement hulls - kayaks, canoes, row boats, small sail boats, et cetera.

    Knots - distance between waves in feet

    1 - 0.5
    2 - 2
    3 - 5
    4 - 9
    5 - 14
    6 - 20
    7 - 27.3
    8 - 35.6

    If you establish sight lines from your usual position over the rail that will register distance between waves with these values, you have a handy speed estimator. It's an especially useful tool if you're sailing in a drifter and want to see the effect of any subtle change in trim. It helps to be profoundly bored.

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    Default Re: Munch

    Quote Originally Posted by Ian McColgin View Post
    . It helps to be profoundly bored.
    AS LOL moment.
    It really is quite difficult to build an ugly wooden boat.

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    Default Re: Munch

    Quote Originally Posted by Paul Pless View Post
    Bolger claimed, it was the mid sections of catboats which gave them their turn of speed, not the bow or assed ends. He didn't really explain it other than to say their midsections performed really well. . .
    The only way that would work in isolation from Prismatic Coefficient and the rest of the hull is if Bolger were talking about their power to stand up to their rig, and hence generate propulsive drive from the sail.
    It really is quite difficult to build an ugly wooden boat.

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    Default Re: Munch

    "How do you choose a hull shape? " - Hooligan Navy

    So there is the question that has bedevilled designers and potential owners for a couple hundred years. The damnable answer is that there is no one 'right' answer - it falls into the 'different horses for different courses' realm. Boat design is a series of compromises made to accommodate the needs (wants) of the boat owner, local conditions in which the boat will sail, stability requirements, local prejudices and that terribly demanding mistress, aesthetics. Alden and Herreshoff certainly were able to consistently find the 'right' compromises for the wants, needs, desires, and aesthetics for mid-century East-coast Americans. Had they been practicing their craft fifty years earlier or later in, say, Germany or Shanghai it is unlikely that they would have come up with the same design ethos. For the same reason of local conditions demanding local answers, consider the bow shape of the Grand Banks schooners such as the Bluenose and Columbia; a fine-entry clipper bow such as those on Chesapeake Bay boats just wouldn't have been as seaworthy as the 'spoon' bow of the Grand Banks fleet. Also, consider the hull form of non-rule cruising boats of the 1980-s through to today - they are not high-sided and beamy for purely stability and sail-carrying-ability, but to provide maximum internal volume to make a comfortable living space for the sailors aboard, and to be relatively impervious to the hull loading as the owners bring ever more 'stuff' aboard.

    One must also not forget that material dictates hull form, too. Probably the most glaring example of this IMHO is what has happened to boat forms since the advent of fiberglass. When each boat was hand-crafted by highly-skilled craftsmen, boat forms varied widely as each boat (or les frequently, each series of boats) could be custom-tailored for the local waters and the prevailing style of the region. With the advent of dozens or hundreds of FRP hulls coming from a common mold, the hulls had to be compromised to meet the demands of a wider range of sailing conditions and owner tastes, so that regional distinctions began to fade away. So now, much like McDonald's hamburgers are bland and characterless so as to not offend the tastes of a world-wide consumer base, boat designs prepared for mass production are stripped of local character and optimised to have the best sailing performance over a wide range of conditions, often to the detriment of individuality and optimal performance in one set of conditions.

    So what is the best hull form for sailing? That depends on where you plan to sail, what you think 'performance' is, and what you expect a pretty sailboat to look like. To get it exactly right, talk to a designer and, like a hamburger from Harvey's, have it made your way.
    Hope for the best, but plan for the worst.

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    Default Re: Munch

    Well, please pardon me, folks, for oversimplifying. But you've got to start somewhere to get a grip on these concepts. In very general terms, I think it's easier to understand if one looks at the energy budget. How much motive force is being used to make waves, and how much is left over to actually move the boat? Some hull shapes waste a lot of energy making waves, while others make a cleaner and "less wasteful" wake. Toss in the fact that there's a squared function relative to speed here, and the math and what you see and feel while on board start to come together.

    Another layman's explanation is dirt simple: as the boat accelerates, the bow is lifted by the bow wave, and the stern drops into the trough behind that same wave. Now you're trying to sail or motor uphill. Right?

    Or we can get very technical, thank you Nick for sharing your expertise.
    -Dave

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    Default Re: Munch

    With respect, I wish it was so simple, Dave. If you only optimise hull shape for aero-hydrodynamic efficiency, you do so at the expense of interior volume. That is fine if you are designing a racing hull wherein everybody on-board is on deck pulling strings, but if you are designing a hull to take a family on a live-aboard cruise in the South Seas, then interior volume - with its attendant wave-making inefficiency - becomes a significant factor. The designer's challenge is to figure out how much compromise in propulsive efficiency is justified in order to make creature comforts available. In other words, how much wave-making resistance are you willing to accept in order to make the boat livable?

    Everything in boat design is a compromise.

    Everything.
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    Default Re: Munch

    MMD, exactly so. Which point could spin off another bit of thread drift about the merits of multihulls. Reducing volumes of information into a few paragraphs will never satisfy, but hopefully at least lead one to pick up one or two of those books.
    -Dave

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    Default Re: Munch

    I am a big fan of multihulls; don't get me started... <grin>
    Hope for the best, but plan for the worst.

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    Default Re: Munch

    I think a pic like this is easier to visualise the waves formed at hull speed than the CFD renderings for amateurs. To simplify, going faster would try to put the stern wave farther aft, dropping the stern into a hole and making it harder and harder to climb out of until planing finally begins.


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    Default Re: Munch

    The thing about hull speed is that most folks really do not understand....they think it is a speed wall or perhaps just a rough guide or has something to do with ferries and unicorns....I'll make an attempt to smooth the waters. I'll consider it for normal displacement hulls....
    Peerie Ma is correct but I don't think it helps. Dave is not entirely wrong.
    When a boat travels through the water it develops drag. There are two main components of the drag....
    1) simple skin friction...the water rubbing against the hull, and more or less directly related to the surface area. As the boat increases its speed, the drag increases in a more or less linear fashion. It does get complicated by skin roughness, protuberances and an assortment of other considerations.
    2) wave making drag...as the hull goes through the water, waves are generated. As the speed increases the waves move further apart according to the formula....it is physics and quantifiable. As the waves get bigger more energy is being consumed making the waves. A big fat high volume boat will generate big fat high waves. A long slender boat will not generate the same big fat waves, but the wave generated will still be the same distance apart at a given speed as the big fat boats wave system. As the second bow wave of the system reaches the stern and merges with the stern wave, the boat sort of drops into the hole between the first and second wave, pretty much as Peerie Ma described.
    The energy lost (drag) in making waves, and skin friction collectively rises until it equals the power that is being applied. The drag curve rises abruptly when the second bow wave merges with the stern wave. That abrupt rise in drag can be modest in a slim boat because the waves generated are small, while the big waves generated by the fat boat really eat up the energy. You can add more power and the slender boat can continue to increase speed, but the fat boat will just dig a deeper hole in the water and if you are powering you might consume twice or more the fuel to increase the speed by a fraction of a knot.
    Cat boats are generally wide, but are relatively shallow body hulls. They do not generate wave the way....say.... a modern fishing boat, designed to carry a big load of fish and are deep belly fat boats.
    I have seen a 19 foot kayak really narrow and slender sprint to 9 knots (measured)under human power. I have also seen a ~30 foot fishing boat digging such a hole ( between the bow and stern waves) in the water the decks appeared awash also doing about 9 knots. Probably the top 0.5 knot costs as much as the first 8.5. The kayak will not carry a couple of tons of fish.
    Naval Architects have found ways of adjusting/fiddling with the drag generating features ....bustles, bulbous bows, counter sterns, and more recently some of these odd appendages on the really go fast sleds.

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    Default Re: Munch

    Academics and analysis aside, it never ceases to be rather thrilling to push one's displacement sailboat past the theoretical hull speed. My tiny sailboat has a WL of only 12.5 feet so it doesn't take much wind to get me excited.

    Jeff

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    Default Re: Munch

    Your point about FRP hulls being "compromised" as opposed to "crafted by highly skilled craftsmen/woman, is a good one, but not much thought about in this new age. The wooden boat builders are becoming few and far between, but the heritage of the grand banks fleet is so strong, that I think "boatbuilding" stays a strong influence in the N E. An apprenticeship at a boat building school is, in my mind, is one great way to start a young adult life. Restoring older wooden boats, although expensive, is also a good way to show how the old timers found the right compromise for the wants of the region. I'm old school, in that, "taking all your stuff aboard" does not play into design. Compromise is key, my favorite is 49' LWL 14' beam 7' draw. dbl gaff rigg . "Pretty wooden boat". TY

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    Default Re: Munch

    Quote Originally Posted by Paul Pless View Post
    Bolger claimed, it was the mid sections of catboats which gave them their turn of speed, not the bow or assed ends. He didn't really explain it other than to say their midsections performed really well. . .
    Do catboats have a particularly good "turn of speed"?? All of the ones around now seem to be fairly sedate performers (NTTAWWT).

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    Default Re: Munch

    Just saw an interesting concept "there are only two straight lines on a sailboat" one the LWL, and two, the mast. Both have to do with speed. Longer straight lines means faster. (Unless your sailing a match box, or one of those "odd" apendeges.)

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    Default Re: Munch

    Quote Originally Posted by Chris249 View Post
    Do catboats have a particularly good "turn of speed"?? All of the ones around now seem to be fairly sedate performers (NTTAWWT).
    Keep in mind that modern catboats do not much resemble racing catboats.



    In addition, modern racing boats are considerably faster than most 19th century racing boats. And if you look at the history, boats like the catboat Una, imported from New York into the UK in 1852, evolved into boats like the Laser.

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    Default Re: Munch

    Quote Originally Posted by Hooligan navy View Post
    Just saw an interesting concept "there are only two straight lines on a sailboat" one the LWL, and two, the mast. Both have to do with speed. Longer straight lines means faster. (Unless your sailing a match box, or one of those "odd" apendeges.)
    Bendy masts have been in use since at least the 1870s.

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    Default Re: Munch

    Quote Originally Posted by Hooligan navy View Post
    Just saw an interesting concept "there are only two straight lines on a sailboat" one the LWL, and two, the mast. Both have to do with speed. Longer straight lines means faster. (Unless your sailing a match box, or one of those "odd" apendeges.)

    Half right...


    Lots of sailboats use mast bend, whatever this is/was and the mighty Star come to mind
    Steve

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    Default Re: Munch

    The old school mast may start out straight, then bend when leverage is applied. I guess the designers used some trial and error to figure out the relationship of keel weight, mast height, boat length/depth. Even today the old wooden boats can hold their own in class racing.

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    Default Re: Munch

    Quote Originally Posted by johnw View Post
    Keep in mind that modern catboats do not much resemble racing catboats.



    In addition, modern racing boats are considerably faster than most 19th century racing boats. And if you look at the history, boats like the catboat Una, imported from New York into the UK in 1852, evolved into boats like the Laser.
    Yep, the difference between old racing catboats (which were very quick for their day) and the modern ones is why I noted that "the ones around now" seem to be pretty sedate. I was wondering whether Bolger was referring to a 19th century sandbagger type or something like a Cape Cod Cat, which isn't a very fast boat.

    As I think you know, I HAVE looked into the history!

    https://sailcraftblog.wordpress.com/.../19/sailcraft/

    https://sailcraftblog.wordpress.com/...ailcraft-pt-3/

    https://sailcraftblog.wordpress.com/2016/05/23/519/

    Personally I'd say that while Una etc were enormously influential, if there was any way to trace the bloodline of Lasers etc it would be that of a canoe, Rater and "sail and oar" dinghy as much or more than catboat. Obviously it's all so mixed the lineage can't really be traced, but the history of the major dinghy class in the place the Laser's creators came from, for example, seems to derive more from canoes and "sail and oar" tender-type dinghies than anything else;

    https://sailcraftblog.wordpress.com/...s-small-boats/


    https://sailcraftblog.wordpress.com/...-200000-ships/

    Of course, the influence of the US catboats in spreading the centreboard concept was very significant.
    Last edited by Chris249; 11-30-2018 at 05:05 PM.

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