Punt (Cambridge) hull design

Discussion in 'Boat Design' started by ashley dalton, Feb 22, 2020.

  1. ashley dalton
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    ashley dalton Junior Member

    Dear all

    I wish to optimise the hull of a traditional Cambridge punt.

    I can adjust width, bottom curvature (rocker) and the slope of the sides. These factors can vary down the length of the boat.

    I wish to know how I can optimise hull efficiency

    it’s a displacement hull travelling at around 3mph. Water depth is around 5ft. Negligible flow.
     
  2. DCockey
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    DCockey Senior Member

    What do you define hull efficiency? Resistance at a specified speed? Speed with a specified input power? What are the constraints and requirements?

    Is this a university/academic project?
     
  3. ashley dalton
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    ashley dalton Junior Member

    Thanks for your response

    I’m from a punting company

    the average speed would be around 3km/hour.

    Each tour lasts around 45 minutes. 2.2km. Each chauffeur must do up to 10 tours in a day.

    we would like to shave a few minutes off each tour.
     
  4. ashley dalton
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    ashley dalton Junior Member

     
  5. fishwics
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    fishwics Quiet member

    Not for nothing are racing punts long and narrow - it minimises wetted area. I suspect you are more constrained by payload (weight of customers), space needed (number of bodies and how big), maximum overall length (will it fit in the workshop), etc, n'est-ce pas?
     
  6. ashley dalton
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    ashley dalton Junior Member

    The boats are 1.4m wife by 7m

    they take 12 passengers

    the racing punts are symmetrical end to end because they must go forwards and backwards, we don’t have that constraint. I can change the slope on the sides, the curvature of the bottom and the width down the boat
     
  7. gonzo
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    gonzo Senior Member

    Long and narrow increases wetted area. It reduces wave making resistance; to a point.
     
  8. clmanges
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    clmanges Senior Member

    If it's a displacement hull, can it still be called a punt? All the punts I've seen have pure planing hulls. Still, I suppose that you can call it a punt based on the method of propulsion, as a lot of boats are called kayaks only because they're driven with a double-bladed paddle.

    If you're willing to redraw so much of the boat's shape, you might consider melding it into something closer to a gondola.
     
  9. bajansailor
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    bajansailor Marine Surveyor

    Welcome to the Forum Ashley.
    Clmanges has suggested tending towards something more gondola shaped - how far can you deviate from a traditional Cambridge river punt 'look'?
    Is the beam of 1.4 metres a definite constraint, or would you be able to go a bit wider?
    Could you post a few photos of the punts currently in use please?
     
  10. DCockey
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    DCockey Senior Member

    Lets look at some numbers.

    Average speed is 0.815 m/s = 1.58 knots = 1.82 mph

    3 mph is considerably higher than the average speed of 1.8 mph. I'll assume that the tours do not occur at a constant speed and 3 mph represents the speed while move without obstructions. 3 mph = 1.341 m/s

    Assuming a waterline length of 6 m the Froude number based on length for 0.815 m/s is 0.106 and for 1.341 m/s is 0.175. At these Froude numbers unless shallow water effects are significant viscous/residual resistance will be dominant.

    Check of Froude number based on water depth to see if shallow water effects may be important. 5 ft = 1.524 m. Froude number based on depth for 0.815 m/s is 0.211 and for 1.341 m/s is 0.347. At these Froude numbers based on depth the increase in wave making resistance due to shallow water effects should be negligable.

    A reduction of 2.5 minutes is a 5% reduction. Assuming the controlling factor for speed is the power output of the chauffeur then to achieve the higher speed with the same power the resistance at the higher speed would require the resistance at the higher speed be 95% of what it currently is at the lower speed.

    The obvious way to reduce resistance at the given speeds is to reduce wetted surface area by reducing the beam or length. Reducing beam will reduce initial transverse static stability. A reduction of 10% in beam will reduce initial transverse static stability by approximately 19%. Reducing length will reduce initial transvers static stability but not as rapidly as beam. A reduction of 10% in length will reduce initial transverse static stability by approximately 10%.
     
  11. DCockey
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    DCockey Senior Member

    The punts under discussion are the type used in Cambridge and Oxford, England. The use of the name "punts" for these boats predates the concept of a planing hull.
    https://upload.wikimedia.org/wikipedia/commons/d/dc/CambridgePunt.jpg
    Punt (boat) - Wikipedia https://en.wikipedia.org/wiki/Punt_(boat)
     
  12. DCockey
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    DCockey Senior Member

    Why would you want to go wider given the speed?
     
  13. DCockey
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    DCockey Senior Member

  14. bajansailor
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    bajansailor Marine Surveyor

    Just a probably crazy idea about having a 'catamaran punt' (for want of a better term) with optimised hull shapes if considerable deviation is allowed from the 'traditional' punt look.
    The rowing regattas usually have support and camera boats with cat hull forms - but then they do have to travel at much faster speeds than a punt in order to keep up with an 8 man rowing shell going at full chat.
     

  15. clmanges
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    clmanges Senior Member

    @DCockey, I've studied these quite a bit; even thought about building something like them.

    Perhaps we have differing definitions of 'planing'. I use it in this context as opposed to 'displacement', which displaces water side-to-side. I'm certainly not conflating this hull shape with one that's designed to be gotten up onto a plane on the water.

    One of the things I thought remarkable about these traditional punts is that, at the speed they're operated, they produce no wake at all; the only indication they're disturbing the water is a very slight turbulence trailing them. That's no surprise, though, as they're only moving at walking speed or less.

    Another consideration for @ashley dalton is passenger comfort and safety. The traditional design is all initial stability and zero secondary, which is perfect for the slow-flowing canals they operate on. They're freight barges, after all. If the hulls were reshaped for more efficiency it might make them a bit tippy. Some of your clientele may have never been on any kind of boat before, and the traditional design is very forgiving of dynamic and/or off-center loads.

    Another thing: some of these waterways have a cheap substitute for a lock, where the boat is hauled up a set of rollers. If your tours involve negotiating one of those, you'll want to have enough of a flat bottom to keep it level.

    Finally, cost. The traditional design (as a freight barge) is dirt-cheap, made mostly of straight planks, and IIRC, meant to be disposable. Changing that shape would certainly cost more to make.
     
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