Simple wash rule?

Discussion in 'Boat Design' started by alan craig, Aug 15, 2013.

  1. alan craig
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    alan craig Senior Member

    First the anecdote, then the question:

    Years ago I was on holiday with friends on the Norfolk Broads, UK. We were chugging along at what we considered a reasonable speed - wash not breaking on the bank. We watched in amazement as a river police boat took about 5 minutes to catch us going about 0.01kt faster, wash breaking on both banks, to give us a bo11ocking (a good telling off) for going too fast!

    Now the question; is there a simple rule which could allow low wash boats to travel faster than high wash boats? I realise that in reality there are too many factors to consider, including things not caused by the boat such as the width of the waterway. Are there places which have a rule which is not just a blanket speed limit?
     
  2. michael pierzga
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    michael pierzga Senior Member

    Well...the RULE is that a vessel is responsible for it own wake.

    Speed limits are a way to control wakes. We all know that speed is a poor way to predict wake.

    The problem is.... What other logical system would you use ?
     
  3. rwatson
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    rwatson Senior Member

    Also, wash is only part of the problem. Maximum speed is to avoid collisions as well, not just wake.
     
  4. messabout
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    messabout Senior Member

    Some boats can move at X knots with barely a wake (wash) Other types throw up a wicked wake at the same speed. In my area we have "no wake zones". If you do not create enough wake to disturb another boat or the shoreline you can go at whatever speed. One need not be a genius in order to determine whether a boat is throwing an objectionable wake. All one needs do is look at the water surface adjacent to the boat.

    We also have speed limited areas that are reasonable because of boat congestion, people in the water, or the presence of Manatees (sea cows). Common sense and courtesy should be the criteria. Unfortuneately some of the harbor police types fancy themselves arbiters of such things. Some are very agreeable folk while some are *********. The OP seems to have encountered the latter. There also more than plenty of ******* boaters who make life difficult for the more courteous ones.
     
  5. philSweet
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    philSweet Senior Member

    It's not exactly simple, no, but boats can be, and some are in practice, designed to produce minimal river bank wash. One of our members here has written a lot of computer code to analyze the problem. In addition to his commercial programs, he has released a free version you can play with that will map the wake of a boat. The program is Michlet (and it's optimizing variant, Godzilla) and it takes a bit of getting used to. The Math that goes into it all gets very ugly very fast.

    Within the bounds that are spelled out in the manual, Michlet will let you get a very good estimate of a boat's wash.
     
  6. tom28571
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    tom28571 Senior Member

    Unfortunately many people, even boaters consider speed to be the enemy as for as wake is concerned. Some of my boats are designed for efficiency which translates to low wake in this case but I have been admonished for gong too fast. I also have a little runabout that makes a lot of wake at 4 to 6kts but very little on full plane. One guy on his dock yelled at me for going fast in a very obnoxious manner. I circled back and went by his dock at about 4 to 5 and rocked his boat in the slip pretty well. I know that maybe I should not have done that but his tirade made me angry.

    On July 4th we were going slowly down the creek to the fireworks display when a Wildlife boat came by and admonished me for not having bow lights on. I told him that my running lights were high on the cabin sides but he just repeated that I had no bow lights. I guess someone in the boat finally told him about legal running light locations because he eventually turned away.

    Some of these patrol people don't have even a basic understanding of the rules they are inforcing. Especially true of those temporarily recruited for special events
     
  7. rwatson
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    rwatson Senior Member

    The OP didnt say how fast he was actually going - and the prevailing speed limit.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    Alan

    Firstly you do need to be aware of the Froude depth influence. The max speed of a wave, its celerity (correctly) is sqrt of (g.D). So if the canal/river is 2m deep, the max wave velocity is 8.6 knots and at 5m deep it is 13.6 knots and so on. If your speed is some 10-20% of this speed, you vessel shall experience added resistance and thus increase in wash too.

    Secondly, wash is not or should not be wave height related. It is more about the energy of the wave system. Since other less observable waves often have far more energy than those you see from the side/stern of the vessel. This is where it does get complicated and beyond a simple one liner! However...

    As a simple rule of thumb if you work out your length-displacement ratio that is the length divdied by the displacement the the power of 1/3, you’ll get a value somewhere in the region 3-10. Ata value of 4 the %’age of wash height to Lwl will be roughly 2.5, and at 8 it shall be roughly 5.5. You can roughly interpolate between this for other values.

    So, if your hull has an L/D ratio of 4, and your hull Lwl is say 10, then 2.5% of 10.0m = 250mm wash. Thus you shall expect to produce, roughly, 250mm of wash when under way in non-Froude depth region.
     
  9. Petros
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    Petros Senior Member

    rules have to be simple and applied to everyone, or else they could not be enforced in a uniform way.

    I can drive down a highway very fast in safety in a nice Porsche, and could be a hazard in a truck at the speed limit. the guy in the Porsche gets the speeding ticket even though he has a much higher margin of safety than the truck.

    If you zip down the water way in a foiler making no wake, I am sure they not be keen on allowing it either.

    Enforcement always has to done with a limit or a line you do not cross, even if the stated purpose for the law was not breeched. Also, it seems many jurisdictions see citations as means to enhance their revenues, more than a legitimate concern for safety.

    In recent years they have started using photo ticketing in certain areas, presumably to improve safety. I read a study done a year after it was installed, they have considered the photo ticketing a "Success" since the average speed people drove in the area had gone down, despite the fact that there was no mention nor measure if there was actually any improvement in safety. So many people challenged the ticketing, and filed complaints of poor service, they were losing money on the operation, and canceled their contract with the company they hired to monitor the cameras and mail out the tickets. So, I guess their claim of improving safety was not really what they were after.
     
  10. Leo Lazauskas
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    Leo Lazauskas Senior Member

  11. jehardiman
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    jehardiman Senior Member

    Hummm, I need some time to look closely at that thesis. What I find interesting is that he cites the Washington State Ferries Rich Pass study, as well as his own & Cox's papers on erosion, but never mentions the effect of the Keulegan–Carpenter number that proved so critical to those studies.

    Anyway, for the OP, Ad Hoc said it....
    The issue for near and far field wake wash is too dependent on the "beach" parameters to have a one size fits all. If you don't want a wake, don't sail...but that's not what ships are for.
     
    Last edited: Aug 16, 2013
  12. Richard Woods
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    Richard Woods Woods Designs

    The Norfolk Broads has a 4mph speed limit. Although I believe it only applies to motor boats

    Richard Woods
     
  13. Leo Lazauskas
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    Leo Lazauskas Senior Member

    IIRC, I think he dismissed the WSF study as being too specific to the site to
    be of general use. The WSF limit that was derived from the original study is
    in wide use, but it is probably too strict for many sites and too lax for others.

    Keulegen-Carpenter might also be a bit too rough for scouring studies. It is
    probably better to use other methods that take into account the size of the
    sediment particles, fall velocities, and a host of other site specific
    quantities. But, as you said, there is no hope of coming up with a one size
    fits all technique.

    However, one thing is certain - hull shape is relatively unimportant and
    L/D^{1/3} is far more important as AdHoc noted.

    Another technique I have found useful is to look at various combinations of
    the wave height, H, and the wave period T, e.g. H*T, H*T^2, H^2*T,
    H^2*T^2, etc to emphasise the importance of one parameter over the other.

    In some of my studies I have started looking at the actual velocities and
    pressures on the sea-bed (as in the attached field for a 5900t destroyer
    produced by Michlet). The best fun of all is to invert the pressure field that
    Michlet calculates and to find the shape of the ship that created it :)
    When E.O. Tuck produced a simple version of a plot like that at the David
    Taylor Model Basin in about 1964, he pinned it up on the wall to show his
    colleagues. A passing Rear-Admiral saw it one day and tore it down and
    gave Tuck a bollocking because it could be used to design pressure mines.
     

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  14. FAST FRED
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    FAST FRED Senior Member

    <Well...the RULE is that a vessel is responsible for it own wake.

    Speed limits are a way to control wakes. We all know that speed is a poor way to predict wake.

    The problem is.... What other logical system would you use ?<

    The problem is speed can be measures , a big wake , is a judgement call

    Considering the individuals given a badge and a gun , a speed limit , however poor is a better choice.
     

  15. jehardiman
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    jehardiman Senior Member

    Actually he dismissed the WSF rules being used in other places, but you could apply the the principles using Kc applied to the foreshore. As for Kc being used for scour, that is exactly what it is for, especially if the foreshore is well graded (i.e. has many different sized components). Kc = 2 pi sigma, where sigma is the displacemet parameter for fine sediment transport...but for wash we are looking displacement of all types.

    Another thing that strikes me looking at the paper, is that in his discussion of wave generation and wash effects, he never references Wiegel's Oceanographical Engineering. Which is (was?, 30 years ago) the standard on shore processes and as far as I know, still one of the main data souces (i.e. Wiegel collected the data and provided the theory for the ACE Shore Protection Manual and Costal Engineering Manual which are referenced). A fair amount of chapter 2 & 3, though not wrong, I would have stated differently, he focuses more on how the wave is generated than on the effect of foreshore slope which is covered in Wiegel...<shrug>.
     
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