Is there any cost to surfing a boat's wake (to the boat in front)

Discussion in 'Boat Design' started by jakeeeef, Mar 8, 2023.

  1. jakeeeef
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    jakeeeef Senior Member

    Whenever I get the car ferry over to the Isle of Wight, I look at the big, curling stern wave on the port side about 40 feet behind. There isn't such a big one on the starboard side ( it's got Voith Schneider Propulsors so throws out an unusual wake). It's a double ender so probably not a massively efficient hull either.
    I think about one day building a boat optimised to surf its wake, complete with single line pull system to simultaneously lift and shut off the engine, and wonder whether once surfing it, a free ride across the Solent would be possible. It tends to do between 10.5 and 12 knots on the 50 minute crossing.

    Anyway, it makes me wonder if the ferry operator would be 'losing out' in any way and would have any valid point. They certainly won't like it one bit and will bleat all manner of tenuous safety related excuses. Of course where in the system the energy is being depleted from wouldn't be measureable in this case as the ferry is so huge and something that could surf it's stern wave would be very light and small.

    But in principle, once a boat has left its stern wave behind, in deep water, can another boat using an amount of its energy effect it in any way?

    I suspect it can, as there is bank drag, shallow water bottom drag etc, but the bank/ bottom is a stationary object but someone surfing your wake is going the same speed as you are so it's not quite the same.

    I also wonder the same thing when slipstreaming other people when cycling, when drafting other paddle boarders in paddle board races and when slipstreaming lorries on the motorway.

    In paddle board racing, the 'urban myth' I have occasionally heard is that when two 14 foot boards draft each other very closely, the water, to an extent 'sees' a 28 ft board, so it benefits them both. I don't buy this at all, as when you are the board in front, you don't feel any speed difference when someone comes up close behind.

    The difference when you are the board behind is huge though, they say 20% less effort for the same speed, which I can believe. When you draft someone you have to ease right off to not run right up their stern. I remember one race, in the foot or so behind the board in front there were a couple of leaves just circulating in his wake for about 10 minutes, and the bow of my board was in that water, with me hardly paddling to stay with him. A huge psychological boost, apart from the clear physical advantage. But was this 'costing' him, other than that I was conserving lots of energy ready to overtake him? Was he pulling me up the river in any way?
     
  2. jakeeeef
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    jakeeeef Senior Member

    I also wonder if maritime law has ever looked at this. If one supertanker followed 50m behind another across an ocean the one behind would save a small fortune on fuel!

    But what could the one in front say? It's not like they have tied on to the one in front with any physical connection. Or they could have a deal. One goes in front one trip, then they swap next time...
     
  3. jehardiman
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    jehardiman Senior Member

    If you are talking about "surfing" like a surf/wake board on the transverse wave, then there is no loss to the generating vessel. That horsepower is already out of the barn.

    Otherwise if you are taking advantage of the wake velocity behind the body, then again it is not costing the generating vessel unless you are similar sized and hard on its transom thereby changing the flow aft on the leading vessel. Often when sailing in SF Bay we would try and tuck our small boat up inside the wake of a larger racing vessel. The velocity of the wake can be subtracted from the SoG to get a lower velocity through the water. In this way we found a 22 ft LwL boat could keep up with a 40 ft LwL which normally, sailing separately, it could not.

    Yes, people have looked at a "sea train". There are other effects that have more influence the overall efficiency that make it not attractive to shippers. That said, there is a huge amount of commerce moved by river towboat floats and open ocean towed barge strings.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    You'd have more of a chance following the Redjet, but she does 35knots..and the Wightlink FastCat, at around 22knots.

    Otherwise....what JEH said...:)
     
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  5. jakeeeef
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    jakeeeef Senior Member

    It sounds like my paddle board example with 2 almost identical 14 foot boards with bow within a foot of the stern in front is this, but that the Isle of Wight car ferry example is something else.

    To clarify, on the car ferry I'm not talking about surfing the transverse stern wave. It's not steep enough. I've followed it in a RIB and it requires the throttle to be shut right off to stay on the wave, so it's worth doing, if a little taxing on the concentration and a fair bit slower speed than you often want to be crossing the Solent at. It's a great place to sit if it's choppy- I used to have a 4.5m RIB that on a windy day was a very wet ride in the Solent, but sitting on the ferries transverse stern waves was very dry and very reasonable on fuel. And 10-12 knots was actually all I'd be doing anyway in that boat in metre high waves, behind the ferry it was same speed made good, great fuel savings, completely dry. But the transverse wave is not steep enough to keep a small boat on it with zero throttle and engine lifted.
    I'm talking about the quarter wave, emanating diagonally from the aft quarter which in a 20 foot section on the port side is about 4 ft high, curling over beautifully. I've done a fair bit of surfing and that bit of that wave looks highly surfable.
     
  6. BMcF
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    BMcF Senior Member

    We built and tested two "sea train" models for the Office of Naval Research a number of years ago. Four "cars" coupled using ATB-type coupler and all propulsion in the rearmost unit. The test results supported the CFD analyses...one of the intermediate "cars" (I've forgotten which) was getting a free ride. In other words, the drag of a 3-car train was almost same as drag for a 4-car train. The concept design that we modeled was about 1100' LOA with 4 cars. There was some commercial interest..but not enough to get any metal cut.
     
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  7. jehardiman
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    jehardiman Senior Member

    Yeah, I have found that a lot of concepts pop up every 15 years or so when they have been forgotten/never heard of in the present cadre of admins/engineers. IIRC, the concept generally chosen is the FO/FO LASH ship with LASH/SEEBEE barges of ~100/1000 tons. Used by the Navy and several east coast power companies.

    Lighter aboard ship - Wikipedia https://en.wikipedia.org/wiki/Lighter_aboard_ship

    http://www.shipstructure.org/pdf/75symp06.pdf
     
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  8. BlueBell
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    BlueBell . . . _ _ _ . . . _ _ _

    That's how I got @DogCavalry little commuter over to him.
    Surfed the wake of the 19-knot, BC Ferries SPIRIT OF OAK BAY.
    Good thing, fuel consumption on his little 70HP two stroke was considerably higher than expected.
    I wouldn't have made it without the push.
     
  9. DogCavalry
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    Pulled up to our dock on the last few drops
     
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  10. mitchgrunes
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    mitchgrunes Senior Member

    When a bike drafts another bike, they are so close that an accident could occur, especially if the front bike weaves side to side. In addition, if the rear bike peddles too fast, they again might collide with the front bike.

    It seems like the same would be true for boats, and that the safety concerns are valid. Also that a collision could damage the front boat's propeller(s).

    It also seems that if you front surf a bow wave, that is exerting a force on the water - you might even cause the surface current to flow backwards, or make it break. You often see that in whitewater boating - a whitewater boat front surfs a smooth wave formed by a submerged rock upstream of it, and the sometimes boat causes the wave to break and become turbulent. (I think that is a very cool effect - that a lightweight boat can substantially effect the motion of the water it is surfing.)

    It seems like that has to have some effect on the generating ship, because

    1. you would to some extent be driving the wave front backwards, changing the angle the bow wave makes with the ship.

    2. Yes the water that you are pushing backwards has already left the ship, and is moving away from it. But wouldn't there still be a little increased drag due to the water's viscosity, which would slightly drag the water next to the ship to move a little less fast forward?

    3. If the boats actually bump, the lead boat would be affected too.

    I point out, BTW, that migrating birds frequently do draft off each others' winds. One bird flys in front, and the other birds fly in a spreading V behind it. I assume (but don't know) that the birds occasionally switch off who is in front. But clearly, there is a net efficiency gain for the flock. Something similar occurs to schools of fish - to some extent fish can swim in each other's "drag wakes".

    Sometimes you see fighter jets flying in formation like that at air shows. But it is dangerous.

    BTW, there are people who claim that a drafting cyclist actually benefits the lead cyclist:
    www.myccr.com/phpbbforum/viewtopic.php?f=16&t=26417&start=0

    And that thread, some people argue that for boats that are close enough, both boats are affected, and the boats are dragged towards each other when they are along sides, and create a turning force.

    It seems to me this is a complicated enough question that you really need experiments to settle it definitively. Hydrodynamic fluid mechanic calculations are limited in accuracy by the fact that some eddies sometimes form at close to a molecular scale, so the number of calculations, and the amount of memory needed to simulate hydrodynamic flow with perfect accuracy are prohibitive.
     
  11. Blueknarr
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    Blueknarr Senior Member

    Birds do swap out who is leading.

    Depending on the exact geometry between drafter and draftee there could be benefits to both or a negative effect on the leader.

    Jets NEVER fly in another's wake. It will interfere with airflow to flight control surfaces and engine intake.
     
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  12. jehardiman
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    jehardiman Senior Member

    Mitch and Blue; there are significant differences between hydrodynamics, land bounded aerodynamics, and flight aerodynamics. For a full discussion of everything in your two posts, consult Hoerner's Fluid Dynamic Drag.

    FWIW, two or more land vehicle always benefit from drafting (i.e. all use less power), but the leading vehicle always expends more power compared to the others. Additionally, when flying in a diamond or finger four the aircraft are staggered vertically not for efficiency, but for visual cueing.
     
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  13. mitchgrunes
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    mitchgrunes Senior Member

    Wow! I just skimmed a little of Hoerner's text from ftp.demec.ufpr.br/disciplinas/TM240/Marchi/Bibliografia/Hoerner.pdf. Incredibly complicated.

    He makes a point of saying that he is presenting empirical engineering approximations, not theoretical mathematical ones, but would what one studies about fluid dynamics in standard undergraduate physics courses (Navier-Stokes equation, Poison equation, Reynold's number, along with a brief mention of numeric methods of solving partial differential equations a la jrper.github.io/training/2014/IntroToFEM.pdf) be even remotely close to being enough to even roughly calculate hydrodynamic drag and other interactions for boats and ships?

    E.g., that there would be any practical point in my trying to optimize kayak shape using those simple theoretical equations?

    I was already aware that estimating the stresses and dynamic shapes of a skin-on-frame kayak are way beyond the sophistication of what I saw in a generic undergraduate (theoretical, not engineering) 1970's era physics. But I sort of hoped I could figure out roughly optimal shapes.
     
  14. jakeeeef
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    jakeeeef Senior Member

    Refering to the natural world examples/ biomimicry. Yes, definitely sea creatures as well as birds. Look at where a baby orca cruises alongside and at the rear quarter of its mother, not just for protection, also gets an almost free ride.
     
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  15. The Q
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    The Q Senior Member

    On the Norfolk Broads, back when I sailed things like a laser, I often used the admitted slow wake to assist the boat going forward.. unfortunately there were also a lot of boats going the other way.
     
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