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Thread: Hull-speed behaviour

  1. #36
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    Default Re: Hull-speed behaviour

    Quote Originally Posted by Ian McColgin View Post
    That Seabrake looks simpler and more reliable than the earlier Australian product I so dimly remember from the early '80s. Clever those Antipodeans.
    Maybe you think better upside down?

    The Seabrake might have been invented by the same guy (John Abernathy) as the earlier one that you saw.
    https://www.sail-world.com/Australia...?source=google
    So John went home and started to refine the idea, By 1982, he had developed a full blown two stage moulded plastic drogue.. He worked with the Australian Maritime College to develop his first model, which was used in the BOC Round the World Race. ‘It’s still a fantastic product today,’ he says, ‘a lifesaver, but it was a bit bulky.’

    So John went back to the drawing board, using traditional canvas material with a stainless steel wire frame and finally producing today’s internationally successful ‘Seabrake’
    Management is the art of counting beans. Leadership is the art of making every being count. --Joe Finch

  2. #37
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    Default Re: Hull-speed behaviour

    I carry one as our drogue and emergency steering option, never had to use it as that so far. But it does double duty as our flopper stopper, a job it does nearly as well as the big diving triangle I made a few years ago but not taking up the room. Useful thing.

    That's s great idea with towing the fender too Ian, I'm going give that a go sometime for sure.

  3. #38
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    Default Re: Hull-speed behaviour

    I think NM Dave is right. Gonna get one. Or two.

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    Default Re: Hull-speed behaviour

    Yes we think better upside down since all the blood flows to our brains.

  5. #40
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    Default Re: Hull-speed behaviour

    Crappy day here meant no varnishing or other boat work, so it's back to the drogue set up, new bridle and thimbles for the line.
    20190512_165333.jpg
    I plan not to use it.

  6. #41

    Default Re: Hull-speed behaviour

    Was given a tow for a couple miles once by a motorboat, sitting on my paddleboard with their waterski rope, well above the board's hull speed.
    Was impressed by how well the board tracked straight; may be the fin on back of the board made all the difference.

  7. #42
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    Default

    Quote Originally Posted by Waterwytch View Post
    Was given a tow for a couple miles once by a motorboat, sitting on my paddleboard with their waterski rope, well above the board's hull speed.
    Was impressed by how well the board tracked straight; may be the fin on back of the board made all the difference.


    All due respect, how do you know what the hull speed of your board is?

    It may be that its speed, when paddled, is limited by so much, " human power." Its max speed paddling may be a function of power input not design or shape.

    Kevin


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  8. #43
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    Default Re: Hull-speed behaviour

    "Hull speed" is about surface wave formation, which is why issues of "hull speed" are fairly silly for light boat boats that if provided adequate power (usually by tow since they lack the buoyancy to hold a big power plant) just go straight to a plane.

    Just to illustrate how how the surface wave separation matters, the reputedly fastest submarine, the old Soviet submarine K-162, could about double "hull speed" when submerged.

    Another side note: We see the surface waves and how they affect "hull speed". These forces are three dimensional. There's in essence a bow wave that flows down to the depths. I used to sail a boat that, I think due to her canoe sections, had an especially strong bow wave downwards. At the right depth, that wave would reflect of the bottom (if hard enough like sand, less so if soft mud) and foul the water flow around the bottom of the fin keel. I could feel it and use the effect as a sort of depth sounder when playing the shoals.

  9. #44
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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
    It's not your imagination, I remember them too. My thoughts at the time, was that iit seemed to be a very finicky system to have to trust in extremis.

  10. #45
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    Default Re: Hull-speed behaviour

    Quote Originally Posted by John B View Post
    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.
    Nice idea; thanks.

  11. #46
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    Default Re: Hull-speed behaviour

    Quote Originally Posted by Juan View Post


    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
    Where is the "wild bull" behaviour? If the boat crossed the Atlantic alone it sounds as if the boat's behaviour was very good.

  12. #47
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    Default Re: Hull-speed behaviour

    Quote Originally Posted by John B View Post
    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.
    People seem to get Lasers to tow happily up to claimed speeds of 15-20 knots, either manned or unmanned. Maybe they are doing it in flatter water?

  13. #48
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    Default Re: Hull-speed behaviour

    I don't know the details ,Chris. I'll ask him sometime. It was behind Iorangi, the 50 ft classic so a low towing angle, but we never had issues with that. I'm wondering what he did about the centre board casing.

  14. #49
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    Default Re: Hull-speed behaviour

    Quote Originally Posted by Chris249 View Post
    Where is the "wild bull" behaviour? If the boat crossed the Atlantic alone it sounds as if the boat's behaviour was very good.
    I don't understand your point

    The fact is that a helicopter of the Portuguese Air Force at the very limit of his autonomy rescued a scared crew because the sailboat behaviour was a succession of

    yaw
    broach
    knock down,
    and wishes to pass the stern over the bow (pitch pole)

    On the return trip across the Atlantic - the sailboat appeared in the Bahamas - there was no one to enjoy the behavior of the sailboat

    ---

    My interpratation is

    (1) The Earth towing the boat with a Force (Newtons) = Mass x 9.8 x sine Wave angle

    that is: twentieth century sailboats were artifacts designed to be moved by the Force of the wind, and encounter problems when they are towed by the powerful Earth Force

    +
    (2) The Keel Hydrodynamic Center ahead of the Center or Gravity

    +
    (3) The Munk Moment is underestimate or completely forgotten

    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

    the fact that you can not find "Munk's Moment" in the Wikipedia in english speaks volumes about the laziness and laziness in the design of sailboats
    Last edited by Juan; 05-13-2019 at 03:51 AM.

  15. #50
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    Default Re: Hull-speed behaviour

    The fact that the crew got scared could be utterly irrelevant. They may not have been experienced sailors, or known how to handle the boat or the conditions. If the boat made it across the ocean without sinking, even without being cared for by the crew, it is likely to have been fairly seaworthy.It looks like the boat is a S&S Swan 411, a very, very reliable and well tested breed.

    Why blame the boat because one crew got scared? Colin Archer designs have pitchpoled and drowned crewmen - does that mean we should condemn them all? Any car can hit a wall if it is driven carelessly - does that mean we condemn all cars?

    I seem to recall lots of discussion of Munk's ideas in thin airfoil theory and the application to sailboats. It is silly to claim that sailboat designers are lazy. The VPPs and other design tools they use are very complex. The ones I know work hard. What gives you the right to insult them?

    All the designers I know of are very aware of the forces created by waves and the issues associated with "surfing" down swells under the force of gravity. It's something they discuss in person and in print.

  16. #51
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    Default Re: Hull-speed behaviour

    A couple of references to start

    Nomoto K., Tatano H., Balance of Helm of Sailing Yachts, A Ship Hydrodynamics Approach on the Problem, 6th HISWA Symposium, Amsterdam, 1979

    (it sounds like a joke, but Kensaku Nomoto had to come from Osaka to remember that the sailboats have bows)

    Keuning J.A., Vermeulen K.J., The Yaw Balance of Sailing Yachts Upright and Heeled, SNAME 16th CSYS, Annapolis, 2003

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    Default Re: Hull-speed behaviour

    Came across this video of a 5HP outboard on a fishing type kayak, driver says that GPS shows a speed of between 14-16mph. He gets underway at about the 13:50 mark. Seems pretty stable at speed. Looks like fun, LOL! Pete.

    Ignorance of ignorance can beget complacency and unjustified arrogance.

  18. #53
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    Default Re: Hull-speed behaviour

    <sigh>

    As I stated in my first post (post No. 9), "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."

    There is a speed at which - for any hull - the dynamic forces become so precariously balanced that, should any opposing force be met, be it wind, wave, lillypad, whatever, the balance of the boat will be upset and the boat will go out of control. For instance, in the video above the young fellow has installed enough power to get the kayak to plane. He is smart enough to realize that he has to move his weight forward to balance the C of G of the boat, motor, & himself over the centroid of lift of the hull. So far, so good, If he puts more power in the boat and goes faster, more of the boat's bow will be carried above the water surface and less is actually in contact with the water. This trend will increase with increasing speed until, at some point, there is so little boat in the water that there is insufficient hull in the water to maintain directional stability and the boat is liable to lose directional stability and something will trigger a catastrophic fall off of plane. This could manifest itself by chine-walking violently, or suddenly veering off to one side (possibly resulting in a barrel-roll), or simply dropping off plane and stuffing the bow. The speed at which this happens is unknown - it could be at 30 mph, or it could be at 18 mph. I am betting that it is not much faster than what has been shown in the video. But it WILL happen.

    Guys, just because you have had a dinghy that tows well behind your cruising boat, or a surfboard that momentarily stays on plane, or whatever, does not mean that you have successfully disputed the laws of physics. It merely means that you have found a 'sweet spot' in which your boat attains a fast speed without problems in a specific sea state and (if towing) with an external stabilizer aiding the hull's stability. Think of it this way: If all hull forms planed well if only they have enough horsepower, then we would never have developed hard-chine planning hullforms, only more powerful engines, and we would still be racing Hackers and Ditchburns from the dawn of the past century.

    But if you are certain that you have found a way to make non-planning hulls stay on plane and perform well, I would suggest patenting the concept, 'cause if you don't and you are right, somebody else is gonna get rich.
    Hope for the best, but plan for the worst.

  19. #54
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    Default Re: Hull-speed behaviour



    Professor emeritus Daniel Savitsky and little girl Paula surfing the Marosa Wave

    The funny thing is that an Optimist is a 200 D/L carene

    On the other hand this Wave - "la Marosa" is called - on our scale of older gentlemen would be perhaps equivalent to a Wave of about 9-13 meters high

    Daniel Savitsky, Hydrodynamic Design of Planing Hulls, Davidson Laboratory, Stevens Institute of Technology, 1964

  20. #55
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    Default Re: Hull-speed behaviour



    Transquadra 2018

    Look at the winner (the pink boat): Bepox 999, plywood (okume)

    Wow

  21. #56
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    Default Re: Hull-speed behaviour

    Quote Originally Posted by Juan View Post
    A couple of references to start

    Nomoto K., Tatano H., Balance of Helm of Sailing Yachts, A Ship Hydrodynamics Approach on the Problem, 6th HISWA Symposium, Amsterdam, 1979

    (it sounds like a joke, but Kensaku Nomoto had to come from Osaka to remember that the sailboats have bows)

    Keuning J.A., Vermeulen K.J., The Yaw Balance of Sailing Yachts Upright and Heeled, SNAME 16th CSYS, Annapolis, 2003
    What is the relevance of those papers to the claim that yacht designers are lazy? I can't see anything that indicates they are not the sort of papers that designers I know, and many of those I have read about, would have read. The second uses the Delft series, which as I understand it was initially funded by H Irving Pratt because of his interest in yacht rating rules. I know that top designers know it very well; some have used it to develop their own VPPs which they believe to be superior. For example, about 20 years ago there was a big dispute about the IMS' treatment of hull volume about 3/4 of the way aft. Rolf Vrolijk and the ORC supported the Delft series drag calculations, while Farr etc said that their own VPPs were superior and proved that the "normal" values obtained from the Delft series were incorrect. So given that the designers know the Delft series that well, and that some of them have got enough sponsorship dollars to run their own tank tests and design their own codes, and that some of them are qualified NAs who are fully aware of Munk etc, where is there evidence for your insults?

    Apart from everything else, it doesn't seem logical. There are big bags of cash available at the top end of yacht racing, and when that sort of money is available there are people who will work hard for it rather than just being lazy.
    Last edited by Chris249; 05-13-2019 at 05:15 PM.

  22. #57
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    Default Re: Hull-speed behaviour

    Quote Originally Posted by mmd View Post
    <sigh>

    As I stated in my first post (post No. 9), "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."

    There is a speed at which - for any hull - the dynamic forces become so precariously balanced that, should any opposing force be met, be it wind, wave, lillypad, whatever, the balance of the boat will be upset and the boat will go out of control. For instance, in the video above the young fellow has installed enough power to get the kayak to plane. He is smart enough to realize that he has to move his weight forward to balance the C of G of the boat, motor, & himself over the centroid of lift of the hull. So far, so good, If he puts more power in the boat and goes faster, more of the boat's bow will be carried above the water surface and less is actually in contact with the water. This trend will increase with increasing speed until, at some point, there is so little boat in the water that there is insufficient hull in the water to maintain directional stability and the boat is liable to lose directional stability and something will trigger a catastrophic fall off of plane. This could manifest itself by chine-walking violently, or suddenly veering off to one side (possibly resulting in a barrel-roll), or simply dropping off plane and stuffing the bow. The speed at which this happens is unknown - it could be at 30 mph, or it could be at 18 mph. I am betting that it is not much faster than what has been shown in the video. But it WILL happen.

    Guys, just because you have had a dinghy that tows well behind your cruising boat, or a surfboard that momentarily stays on plane, or whatever, does not mean that you have successfully disputed the laws of physics. It merely means that you have found a 'sweet spot' in which your boat attains a fast speed without problems in a specific sea state and (if towing) with an external stabilizer aiding the hull's stability. Think of it this way: If all hull forms planed well if only they have enough horsepower, then we would never have developed hard-chine planning hullforms, only more powerful engines, and we would still be racing Hackers and Ditchburns from the dawn of the past century.

    But if you are certain that you have found a way to make non-planning hulls stay on plane and perform well, I would suggest patenting the concept, 'cause if you don't and you are right, somebody else is gonna get rich.
    Not surprisingly, what you say does seem to be shown in the performance of real life hulls. At top speed even fast hulls like those of 18 Foot Skiffs or windsurfers start chine walking, nosediving etc.

  23. #58
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    Default Re: Hull-speed behaviour

    mmd's posts seem to be the most easily understood way of understanding hull instability; but I still have a bit of trouble understanding the ease that pangas (or frieghter canoes in the north) seem to handle 70 or 90 hp outboards, without apparent difficulty at speed.

  24. #59
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    Default Re: Hull-speed behaviour

    Pangas -or freighter canoes - have a long waterline length and relatively narrow beam, so even when driven at speed by, say, a 90-hp motor they still have a generous length of hull in the water, which adds to directional stability. Put a 200-hp motor on one, max out the throttle, and I suspect that it will carry enough of the bow clear to make handling a bit squirrely. Once the hull's CG gets too far out past the centre of dynamic lift on the hull, the boat will steer erratically, porpoise, chine-walk, or chine-steer. Or maybe several of those at once.
    Hope for the best, but plan for the worst.

  25. #60
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    Default Re: Hull-speed behaviour

    Here is a link to an article I wrote about this very subject. It may be of interest because of the particular boats involved, and because of the photos of actual behaviour.

    http://rosslillistonewoodenboat.blog...hulls-for.html

  26. #61

    Default Re: Hull-speed behaviour

    [QUOTE=Breakaway;5891009]All due respect, how do you know what the hull speed of your board is?

    It may be that its speed, when paddled, is limited by so much, " human power." Its max speed paddling may be a function of power input not design or shape.

    Kevin

    Hull speed of a 12 ft waterline is about 4.7 knots according to calculations. Has nothing to do with means of propulsion. We were moving right along at more than 5 knots was my point.

  27. #62
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    Default Re: Hull-speed behaviour

    I'm pretty sure I don't understand the original question, but I'm not sure why people are talking about hull speed for surfboards and paddleboards.

    With very few exceptions, these are always planning watercraft.

    I have gps tracks on my 4'6" kiteboard at well over 25 knots for moderately long distances. From experience, it never operates at displacement speeds.

    windsurfers too... Planing watercraft. If it starts behaving badly at high speed you probably need a smaller fin (or take out the centerboard if you are riding some antique monstrosity).

  28. #63
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    Default Re: Hull-speed behaviour

    A surfboard being paddled out through whitewater isn't planing. Nor is a SUP that is not on a wave, or a typical short windsurfer in very light winds. In fact a typical day at somewhere like Lake Garda sees hundreds of shortboard windsurfers schlogging around at displacement speeds.

    The typical "antique monstrosity" windsurfer is faster than most modern boats in the light winds that are typical of most of the world most of the time, because they work fairly well at displacement speed.

  29. #64
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    Default Re: Hull-speed behaviour

    Funny hull-speed story: some years ago a builder at Lowell's Boat Shop tried a very custom dory-build for a customer who wanted an outboard well in a very short dory. If I remember the details aright, this builder was not one of the regular builders, but some kind of temporary back when Lowells Boat Shop was reforming itself into a nonprofit museum shop. Anyway, the customer came back in a rage -- under power something bizarre would happen, and I believe giant stern waves would come in and deluge him. I saw the dory: it did look very weird, with a huge rocker curve situated in a way meant to account for the customer's design specs for the outboard well. Lowell's took it back to avoid problems. Apropos of discussions made above, there was talk of diving, climbing, hobby horsing, and what-not. I got this story when the regular builder was back and steaming about the crisis that shouldn't have happened. I left him pondering whether the boat could still be ethically used as a planter after grinding off the Lowell insignia, or whether if would be best to just saw up the hull (and grind off the Lowell insignia). -- Wade

  30. #65
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    Default

    Quote Originally Posted by Chris249 View Post
    A surfboard being paddled out through whitewater isn't planing. Nor is a SUP that is not on a wave, or a typical short windsurfer in very light winds. In fact a typical day at somewhere like Lake Garda sees hundreds of shortboard windsurfers schlogging around at displacement speeds.

    The typical "antique monstrosity" windsurfer is faster than most modern boats in the light winds that are typical of most of the world most of the time, because they work fairly well at displacement speed.



    Yes, but being operated at displacement speed is not the same thing as being a displacement hull.

    Examples:

    A fishing boat planes out to the grounds before trolling for fish.

    A rescue craft races to a scene before slowly criss-crossing through the wreckage.

    A ski boat slows to transit a, " no-wake zone, " enroute home.


    Planing craft are often operated at displacement speeds.


    Kevin


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    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

    Yes, but the point was the poster I was referring to appeared to be saying that planing craft never operate at displacement speeds or that they do not have "hull speeds". We agree that even planing hulls can operate at displacement speeds, and we know that planing craft generally do have "hull speeds" but that they can exceed them easily and dramatically.

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    Default Hull-speed behaviour

    Sent from my iPhone using Tapatalk Pro
    Last edited by Breakaway; 05-15-2019 at 09:14 PM.
    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 Breakaway View Post
    Yes, but being operated at displacement speed is not the same thing as being a displacement hull.

    Examples:

    A fishing boat planes out to the grounds before trolling for fish.

    A rescue craft races to a scene before slowly criss-crossing through the wreckage.

    A ski boat slows to transit a, " no-wake zone, " enroute home.


    Planing craft are often operated at displacement speeds.


    Kevin


    Sent from my iPhone using Tapatalk Pro
    Wiping the tears of laughter away after your ski boat example.

  34. #69
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    Default Re: Hull-speed behaviour

    Hahaha, yeah, like that ever happens.

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    Default Re: Hull-speed behaviour

    Quote Originally Posted by Small boats rock View Post
    Wiping the tears of laughter away after your ski boat example.
    Oh, I've seen ski boats with a hull speed of less than six knots slow to a seven-knot speed limit. This maximizes their wake.

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