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Thread: Hull-speed behaviour

  1. #1
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    Default Hull-speed behaviour

    Suppose a canoe, kayak or surfboard were propelled by some means beyond their theoretical hull speed, would they behave erratically? Expanding on that, if a small powerboat (say, 3.5m) of a Panga-type design (i.e. with a delta planing pad on the bottom) at a particular speed such that when the wetted surface area (due to lift) of the hull were about the same as that of a canoe, kayak or surfboard, would such a boat also behave erratically?
    Hope that make sense
    Thanks
    Norm

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    Default Re: Hull-speed behaviour

    The question confuses some issues.

    "Hull speed" is a bit of a fiction that applies to displacement hulls. It works out to a bit faster in knots than 1.3 time the square root of the waterline in feet. That constant gets higher for very narrow hulls, less for cat boats. Basically it represents the distance between the bow wave, peaking a bit abaft the bow, and the stern wave forming a bit abaft the stern. In other words, the boat has her tail in a hole and the bow trying to climb out and over.

    Planing boats, being on a plane, are not at those speeds subject to the issues of "hull speed". There are separate issues about stability and handling for vessels up on a plane.

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    Default Re: Hull-speed behaviour

    I built a long shaft motor from a big brush cutter for my Macgregor canoe with a 5", 3 blade prop in a tunnel fitting. Not a fixed attachment fortunately.
    It was was way too powerful for the canoe, it became unmanageable at the speeds attained and the rev. control available was inadequate. I only ever tried it again a couple of times. I only threw it out a week or two ago.
    But it certainly was quick.

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    Default Re: Hull-speed behaviour

    Yes and yes.
    "The enemies of reason have a certain blind look."
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    Default Re: Hull-speed behaviour

    Yes, if you drive a skinny boat too fast they go goofy and twitchy. You have to be onto it.
    But panga boats or yamaha longboats, which is what they generally are ,have a few more things going on than just a planing strake. Are you getting one?

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    Default Re: Hull-speed behaviour

    This is a long topic

    Daniel Savitsky ...

    https://web.stevens.edu/facultyprofile/?id=265

    ... published in 1964 an important study on the subject

    Hydrodynamic Design of Planing Hull

    http://www.westlawn.edu/ReferenceInf...gHulls1964.pdf

    your question "would such a boat also behave erratically?" I think that depends on the Beam, the Longitudinal position of the Center of Gravity (LCG) and the Center of the water Pressure

    in a planing boat the "Beam" is equivalent to the "WingSpan"

    https://en.wikipedia.org/wiki/Wingspan

  7. #7
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    Default Re: Hull-speed behaviour



    + big Beam
    + big Beam aft
    + Center of Gravity reculé, LCG 60%-64% LWL

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    Default Re: Hull-speed behaviour

    ICF and Olympic class kayaks can go over 10 mph (16.1 kph) for 1,000 meters (and even faster for 200 meters) and that is well over their theoretical hull speed for a 5.2 meter boat (6.37 mph, 10.25 kph) for a solo boat (C1 and K1).

    I have a marathon canoe that is 18 ft and I can get it over hull speed (6.5 mph) for (very) short sprints and it is fine as far as boat behavior.

    The usual hull speed formulae, such as this one, http://www.psychosnail.com/boatspeedcalculator.aspx, do not work well when you apply them to needle shaped boats.

    Brian

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    Default Re: Hull-speed behaviour

    There is a speed for all hull forms - displacement, semi-displacement, planning, broad, skinny, whatever - at which things go horribly wrong. What that speed is, and how the 'horribly wrong' manifests itself, varies from boat to boat. Displacement hulls can lose lateral stability and roll on their side, lose directional stability and make unexpected violent turns, or lose longitudinal stability and literally sail themselves underwater. Planing boats can start to hobby-horse (porpoising) violently, lose lateral stability, lose directional stability, or suffer a blow-over. Pick your poison; any one will ruin your enjoyable day on the water.
    Hope for the best, but plan for the worst.

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    Default Re: Hull-speed behaviour

    Kayaks beyond hull speed, erratic?

    Not really.
    Creationists aren't mad - they're possessed of demons.

  11. #11
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    Default Re: Hull-speed behaviour

    Yes, kayak beyond hull speed, erratic. Firstly, the kayak hull is shaped so it will plane, so it is beyond hull speed to start with. Secondly, it is only the skill of the kayaker that is managing to keep the hull under control - mostly. Fun? Definitely. Erratic? Certainly.
    Hope for the best, but plan for the worst.

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    Default Re: Hull-speed behaviour

    I my experience with freighter canoes, V stern or square back, it is a non-issue. I do use a "whale tail" (a step up from a "dole fin") anti cavitation plate. A switch made after tearing off one of the dolefins surfing in some big water. The only sensitive situation for steering, for me, is in a steep quartering seas (square waves) in the square back canoe; but that issue is common to any transom boat.

  13. #13

    Default Re: Hull-speed behaviour

    canoe, kayak or surfboard beyond their theoretical hull speed, do they behave erratically.
    [erratic = unpredictable]

    Although touchy, a vessel designed for surfing [and in two of the above, designed to be paddle controlled] is anything but 'unpredictable'. The main unpredicability is in the instantaneous state of changeable water rather than the state of the vessel. Some my most serene [but active] moments have been in planing mode. I'm a fairly basic paddler and make lots of mistakes, but there's nothing like it even for me and furthermore have a fair idea of what the mistakes might have been. Just an ability/knowledge failure rather than a predictability failure
    For WW playboats there's purposeful directional instability [designed to allow spins and more] for surfing kayaks similar to the avatar here, usually with fins [but also edges], there's a bit more directional stability in order to facilitate more directional moves. Hit a cross current or tilt the wrong way [predictable] and you're over now, not now, right now!
    So surfing kayaks or surfbds with sharp edges: unbelievable touchy . . . but is that unpredictable?? Or can we predict that because those sharp edges are so low down but now allow a fair amount of lift over a predictable range at speed it might benefit surfing. It might be semantics, but I don't think so. Ordinary canoes and kayaks [say more rounded hull forms] still surf [assuming = 'over theoretical hull speed'] are harder to predict but still 'controllable' by paddle and are also a tremendous amount of fun when moving that fast.
    bcmarinetrails.org - an attempt, by volunteers, to protect and enable 27,000 km of continuous camping and accesses along and around the whole Wild West Coast of British Columbia - for small beachable craft

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    Default Re: Hull-speed behaviour

    Peter, your freighter canoe has just not been driven to a speed at which it becomes unstable. It is there, trust me.

    Mick, same thing - you have not hit the speed at which the hull becomes unstable. The fact that you are able, by the application of relatively large amounts of power somewhat quickly, to overcome the momentary instability you are experiencing due to wave action, does nothing to correct the inherent instability of the hull form at some presently indeterminate hull speed. You are merely operating the hull in the speed range that it was designed for.

    As for panga boats, there was a scholarly paper published in the organ of the Society of Naval Architects and Marine Engineers (SNAME) a decade or two ago that directly discussed at-speed instability of the hull form. IIRC, this instability exhibited itself in the form of either extreme porpoising or directional instability wherein the hull would suddenly veer off-course, both effects in calm water with no outside forces acting on the hull. With a great deal of complex mathematics, the authors proved that the instabilities were caused by excessive speed lifting the hull out of the water far enough that there was insufficient control surfaces in contact with the water surface to ensure directional stability nor dampen porpoising brought on by a force couple between the centre of dynamic lift and the centre of gravity of the hull.

    If you don't believe me that this issue exists, go strap a 50-hp outboard on a twelve-foot canoe and see what happens at WOT. Be sure to wear your life vest...
    Hope for the best, but plan for the worst.

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    Default Re: Hull-speed behaviour

    Thanks guys

    To put it another way, picture if you will, an un-occupied, uncontrolled canoe, kayak or surfboard being towed behind an offshore racer during the race. To me (in my ignorance) three conditions or more are likely: "squirrelly", upside down, and submarine. Am I fairly close?

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    Default Re: Hull-speed behaviour

    Thanks Micheal that expressed my query much more eloquently than I did. Sorry I didn't see your post before my last post

  17. #17
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    Default Re: Hull-speed behaviour

    "squirrely, upside-down, or submarine"

    Yup, that about covers it.

    Such events have happened quite often, when a large ship undertakes to tow a disabled small fishing boat (or sailboat) to port. The big ship maintains steering at twelve knots; the fishing boat's hull speed is nine or ten knots. The fishing boat usually sinks if the towline doesn't part...
    Hope for the best, but plan for the worst.

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    Default Re: Hull-speed behaviour

    Never the slightest problem towing a sea kayak a sail boat speeds up eleven knots or so. I can't imagine trouble with a surfboard with a hole through the bow to attach the tow line, and keep it on a short leash to keep the bow up. Then it won't matter if it flips over. I don't like towing canoes even slowly.

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    Default Re: Hull-speed behaviour

    Ian, you would have been towing the sea kayak at a hull speed of 2.84 x sqrt (WL), assuming a sea kayak length of 15 feet. Due to the exceedingly high L/B ratio of a sea kayak (often 12:1 or greater), hull speed factor is easily 3.0. How did towing an 8-foot dink at that speed go?
    Hope for the best, but plan for the worst.

  20. #20
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    Default Re: Hull-speed behaviour

    My dink planes easily under tow.

    The most difficult tow was my dory Leeward, a Chamberlain gunning dory. That shape make a huge hole in the water when pulled well past normal hull speed.

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    Default Re: Hull-speed behaviour

    This is a towing a tender thread?
    A friend decided he would take a laser away as his tender one season, he might have even gimmicked up some rowlocks iirc.
    Anyway, every time he hit 9 knots that thing went weird, sheered off and turned turtle. Turning turtle also meant paravane and sudden deceleration.....After the third time he found a friend to leave it with ( trip north to the BOI), talked another friend with a launch to bring it back.
    I know my 15 ft cedar stripper tows at any speed without trouble , does a lazy 10 or 13 knots with a 3.5 HP outboard but I pushed the envelope earlier this year by strapping the 9.8 on it. That became scary, very twitchy and hard to control because of its near constant camber type sections.
    That's very interesting about the panga boats, they were designed for efficient speed while loaded and more unloaded, generally with 30 or 40 hp yamaha enduro motors. As happens , people keep strapping on bigger and better motors until original purpose and function is lost.

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    Default Re: Hull-speed behaviour

    Quote Originally Posted by normil View Post
    Suppose a canoe, kayak or surfboard were propelled by some means beyond their theoretical hull speed, would they behave erratically? Expanding on that, if a small powerboat (say, 3.5m) of a Panga-type design (i.e. with a delta planing pad on the bottom) at a particular speed such that when the wetted surface area (due to lift) of the hull were about the same as that of a canoe, kayak or surfboard, would such a boat also behave erratically?
    Hope that make sense
    Thanks
    Norm
    What tends to happen is that as the bow and stern wave build up and the boat develops a hollow between them amidships. This is when the shape of the ends really matters. A boat with a wide, flat stern will tend to be far more stable than, say, an IOR race boat of the pintail era, famous for their broaching behavior. That's why a boat as fast as Hugo Boss can't have a hull the shape of Ganbare.

  23. #23
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    Default Re: Hull-speed behaviour

    michael-max-munk-6078bf89-2cd2-474c-99ce-9a2faa0f0a0-resize-750.jpeg

    https://en.wikipedia.org/wiki/Michael_Max_Munk

    Moment de Munk

    https://fr.wikipedia.org/wiki/Moment_de_Munk

    IMO ... the Munk Moment is underestimated in many or most of the small boats (6-12 m LWL) and is one of the causes of bad behavior

    I see confusion about the function of the rudder

    in a large merchant ship the rudder fulfills a function: to unleash the Munk moment

    in a small boat the rudder has a function: to cut the Munk moment from the root

  24. #24
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    Default Re: Hull-speed behaviour



    The Earth

    The Force of the earth

    The Earth towing a boat

    Newtons = Boat weight x 9.8 x sin Wave angle

    =

    yaw
    broach
    knock down
    pitch pole

    --- (Edit) ---

    Newtons = Boat Mass x 9.8 x sin Wave

    https://en.wikipedia.org/wiki/Mass

    ---

    and the sailboat crossed the Atlantic on its own after ejecting the crew

    the crew was scared, and we must assume that the sailboat laughed when it managed to shake off the crew

    the sailboat with its behavior of wild bull sinks the spirit of the crew
    Last edited by Juan; 05-09-2019 at 04:12 AM.

  25. #25

    Default Re: Hull-speed behaviour

    Quote Originally Posted by Ian McColgin View Post
    My dink planes easily under tow.

    The most difficult tow was my dory Leeward, a Chamberlain gunning dory. That shape make a huge hole in the water when pulled well past normal hull speed.
    I'm not sure towing a tender is a true assessment of a hull's behavior at speed, as you're exerting a powerful, external stabilising force (the tow line). Any deviation from straight and true (both laterally and vertically) gets pulled back to centre. For some hulls at some speeds this can cause cyclic yawing, but for most it is a beneficial stabilising force.

    Maybe try pushing the hull at the same speed!

  26. #26
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    Default Re: Hull-speed behaviour

    Towing tells you some things and not others, just as powering with an outboard on centerline will be different from sail powering a traditional catboat rig on a run with the center of thrust both forward and outboard.

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    Default Re: Hull-speed behaviour

    Quote Originally Posted by Jackett View Post
    I'm not sure towing a tender is a true assessment of a hull's behavior at speed, as you're exerting a powerful, external stabilising force (the tow line). Any deviation from straight and true (both laterally and vertically) gets pulled back to centre. For some hulls at some speeds this can cause cyclic yawing, but for most it is a beneficial stabilising force.

    Maybe try pushing the hull at the same speed!
    And yet that 'stabilising force' doesn't save boats from their fate such as mmd's example of the fishing boat towed by a ship. For the point being discussed I don't see how a tow line steering a boat is any different to an outboard steering a boat.

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    Default Re: Hull-speed behaviour

    And yet that 'stabilising force' doesn't save boats from their fate such as mmd's example of the fishing boat towed by a ship. For the point being discussed I don't see how a tow line steering a boat is any different to an outboard steering a boat.

    You never noticed that one is in the front and one is in the back? One's pushing; the other's pulling? The outboard is in the water and the towline is out of the water? One acts on a very wide part of the hull; the other acts on the narrowest part of the hull?

    The towline may be a stabilizing force right up until the point when it is not ( until a speed specific to the towed craft is reached).

    Also consider that, if the tow line is affixed such as to build-in a bow up attitude to the towed craft, then the towed craft isn't interacting with the water as its design intended.The shape of the wetted surface might be changed and so the designed centers of buoyancy and gravity may be displaced. Without considering a specific boat towed at a specific speed we can't state whether this creates a more stable or less stable form, but we can probably state that it will be form changed from that which was intended by the designer.

    Kevin
    There are two kinds of boaters: those who have run aground, and those who lie about it.

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    Default Re: Hull-speed behaviour

    Quote Originally Posted by Small boats rock View Post
    And yet that 'stabilising force' doesn't save boats from their fate such as mmd's example of the fishing boat towed by a ship. For the point being discussed I don't see how a tow line steering a boat is any different to an outboard steering a boat.

    A tractive or pulling force applied to the bow of a boat will tend to be more directionally stable than a pushing force applied near the stern. Both of these will be different from an off-center pushing force applied to one or more points located along the centerline of the boat. This is important if the loss of stability when attempting to change course happens before the loss of stability when traveling in a straight line. It is also important when operating under power you more or less control.

    When being towed for whatever reason, you can ask the master of the towing vessel to reduce speed. He may do so. If not, the only control you have as master of your vessel is to either ride along or take an ax to the tow line.

    When you are controlling the power, you have more options. You can reef or lower sails or reduce the throttle(s). If obliged to change course, you will need to do this to avoid powering yourself into trouble even if you are only operating at 100 percent of your hull speed if you are making a drastic heading change. But what if you refit your boat with enough power to easily exceed your maximum safe speed. In what axis does your hull depart controlability first? Is the departure gradual or sudden? Will a rapid reduction in power save the day, or will that actually send you careening off in a random and boat-destroying direction. For that matter, a twin-screw and a single-screw boat will behave differently even with the thrust from both screws of the twin-screw being equal and equaling the thrust of the single screw. Likewise, an outboard or i/o drive will behave differently than an otherwise identical rudder-steered boat since with the former the thrust itself maintains the course or causes the course change directly instead of indirectly.

  30. #30
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    Default Re: Hull-speed behaviour

    I would guess that most hulls get a bit squirrelly at some speed; I certainly have felt that at 45 (mph) in a planning hull; it is not an issue at 20 knts or less in a frieghter canoe. Towing a hull and its vagueries, might have as much to do with the tow eye placement as the hull shape, and where the towed hull sits on the stern wave.

  31. #31
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    Default Re: Hull-speed behaviour

    I had a 16ft Thomson canoe that I made a bracket for & hung a vintage 1.5hp Neptune outboard on. It would start to squat and get real squirrely if run to fast.

  32. #32
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    Default Re: Hull-speed behaviour

    Side note on towing the dink. Especially in a following sea, it can be a problem if, as is true most of the time with most cruising sailboats, the overtaking waves are just that - overtaking. The dink will surf forward and slam into the mother ship or will veer off, broach, and maybe capsize.

    My best defensive measure, short of getting the dink on deck where it belongs, is to tow a cylidrical fender astern of the dink. Some experimentation is required to find the right size fender for your boat's power and typical speeds but the right size fender will skim along the top of the water not adding too much resistance at your nominal speed but at higher speeds like the dink surfing up your stern, will build bow and stern waves, basically towing itself under (just as Michael described for a larger boat) and thus build huge resistance fast. Hold the dink back and prevents the broach.

    By the way, there used to be an Australian product, a cylindrical drogue that deployed flaps at set speeds, adding huge resistance when needed.

    G'luck

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    Default Re: Hull-speed behaviour

    Quote Originally Posted by Ian McColgin View Post
    By the way, there used to be an Australian product, a cylindrical drogue that deployed flaps at set speeds, adding huge resistance when needed.

    G'luck
    Sounds interesting. Couldn't find one with Google, but I di find the Seabrake, which appears to do something similar by increasing drag as the speed increases. https://www.oceanchandlery.com/seabrake.html
    Management is the art of counting beans. Leadership is the art of making every being count. --Joe Finch

  34. #34
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    Default Re: Hull-speed behaviour

    That Seabrake looks simpler and more reliable than the earlier Australian product I so dimly remember from the early '80s. Clever those Antipodeans.

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    Default Re: Hull-speed behaviour

    I usually just throw out an old sheet or halyard coiled up and with about 10 ft or so free over the back of the dinghy if it's going to be rough. Some solar showers inside for a bit of ballast, but the best thing you can do apart from setting the painter length to suit the swells , is run two, one to each quarter of your boat.
    Even on a narrow transom that both snubs the yaw and creates some drag. I like one high and one lower on the stem too. In fact the high one over the breast hook and fastened inside the dinghy.j
    I've come close in a serious blow and very heavy rain, but never lost one dinghy in 30 years. That was special, we were surfing at up to15 knots in big coastal seas, what went wrong was the rain filling the dinghy and free surface sloshing up to the bow when it was it's turn to surf the wave. She lost her manners that day but we didn't lose her.

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