Punt (Cambridge) hull design

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

  1. ashley dalton
    Joined: Feb 2020
    Posts: 10
    Likes: 0, Points: 1
    Location: Uk

    ashley dalton Junior Member

    Hi All thanks for your replies
    Initially, I said 3mph, but this was a miscalculation. the 1.8mph is better.

    The speed is fairly constant. Manoeuvrability is not important, but there can be a lot of stopping and starting to wait for pesky self drive.

    The design must have a plywood base made in one continuous piece. The same goes for the wooden sides.

    I have attached a suggested design and a traditional design for your assessment. just fag-packet sketches of course. Note that the traditional design is 100% symetrical bow to stern.

    The new design features changes in angle of the sides and a more pointy bow. Its gonna look rubbish, so I want to be sure its actually going to improve efficiency.

    checkout my timelapse punt build and punt build video on youtube. i think they're through canterbury punting company
     

    Attached Files:

  2. ashley dalton
    Joined: Feb 2020
    Posts: 10
    Likes: 0, Points: 1
    Location: Uk

    ashley dalton Junior Member

    I dont think that this would be the right look to be honest
     
  3. ashley dalton
    Joined: Feb 2020
    Posts: 10
    Likes: 0, Points: 1
    Location: Uk

    ashley dalton Junior Member

    yes, accept these particular punts are notoriously slow due to the heavy construction which lacks rigidity. The middle one, in particular, is a throwback to the early 2000's when more than 12 pax were permited.
     
  4. ashley dalton
    Joined: Feb 2020
    Posts: 10
    Likes: 0, Points: 1
    Location: Uk

    ashley dalton Junior Member

    Whilst I would love to build a gondola there are a few issues;

    1. im not sure i could build one robust enough (note clincker dingies get smashed on this river very quickly)

    2. I'm pretty sure that grandfather rights to 12 pax on a tiny boat wouldn't apply

    I have toyed with the idea of making the otherwise flat base slightly bulbular. This would however, make re-bottoming difficult as you would need a crazy complex router jig. Does anyone think that a slight curve of say 1" over the 1.4m beam would be advantageous?. Also consider that the wide width tends to make the plywood base curve slightly inwards (opposite direction). Making the base bulbus would decrease surface area to volume slightly.
     
  5. ashley dalton
    Joined: Feb 2020
    Posts: 10
    Likes: 0, Points: 1
    Location: Uk

    ashley dalton Junior Member

    thanks for the calculation. My origional 3mph was a miscalculation, please go with the 1.8mph
     
  6. BlueBell
    Joined: May 2017
    Posts: 1,136
    Likes: 193, Points: 63
    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    At 1.8 mph it doesn't matter what shape your hull is.
    If you want to shave time off the tour, stop throttling back.
    Otherwise, the tour length is the tour length.
    Enjoy it.
     
    Tiny Turnip likes this.
  7. ashley dalton
    Joined: Feb 2020
    Posts: 10
    Likes: 0, Points: 1
    Location: Uk

    ashley dalton Junior Member

    Unfortunately these boats are manually propelled, so there is no throttle.

    I have noticed that some boats are faster than others over the years, so I’m sure there must be an optimum shape.

    I recall watching a documentary about barge hulls, and how changing them slightly using modern science massively improved efficiency. I was hoping to do the same with a punt
     
  8. bajansailor
    Joined: Oct 2007
    Posts: 952
    Likes: 246, Points: 43, Legacy Rep: 37
    Location: Barbados

    bajansailor Marine Surveyor

    I think that you want to try to work with an optimised prismatic coefficient to reduce form and residuary resistance as much as possible.
    You can't do much about wetted surface area, other than trying to keep it as clean and as smooth as possible.

    Re your two sketches for traditional and suggested designs, I think that with the broader transom of the suggested design you might well have more resistance, never mind that it has a more pointy bow.

    Everybody likes to try to 're-invent the wheel' occasionally - hence why shouldn't a punt have a catamaran hull form with two very slender hulls optimised for Prismatic? If you look at it from the side it will look like any other punt - you will only notice it is different when you look at it from bow or stern. And most folk will be looking at punts from the side while (eg) sitting on the river bank.
     
  9. DCockey
    Joined: Oct 2009
    Posts: 4,615
    Likes: 279, Points: 83, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    Ashley, is this your company's video:

    Are these the videos:

    The design lane chart in the Principal of Naval Architecture, Volume II, 1988, figure 62 page 69 shows a "design lane" for prismatic coefficient of 0.86 to 0.89 at Froude number of 0.12 (lowest value on the chart). My guess is the current design is within or close to those recommendations. However at the speed of the punts under discussion (a Froude number around 0.1) wave/form/residual resistance will be so low compared to viscous (skin friction) resistance that changes in prismatic coefficient would have negligable effect/

    That would be an interesting documentary. At higher speeds hull shape will affect drag. But not at the speeds your punts operate at.
     
    bajansailor likes this.
  10. BlueBell
    Joined: May 2017
    Posts: 1,136
    Likes: 193, Points: 63
    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    What a load of rubbish.
    Nuclear fusion and isotope water molecules should ought to do the trick.
     
    bajansailor likes this.

  11. tlouth7
    Joined: Jun 2013
    Posts: 118
    Likes: 31, Points: 28, Legacy Rep: 10
    Location: Cambridge, UK

    tlouth7 Senior Member

    There is (or was) a punt at John's made entirely from 6mm ply. It is vastly lighter than a traditional ladder-built punt and correspondingly faster when punted solo (though required ballast to retain trim). It flexed noticeably when being handled so no doubt would self-destruct pretty rapidly if used in the commercial market. It was also less directionally stable which made it hard to punt efficiently. I suspect that the weight of passengers represents most of the displacement of a double punt, so would not focus too much energy on reducing empty weight (though areas to consider would be the solid sides and the generally over-engineered till).

    I would say that your proposed asymmetrical design would be much worse than the current profile. What laymen rarely realise is that the water flowing off the stern is just as important as the water arriving at the bow. Hence symmetrical plan view and swim profiles make sense. There is (or was) a punt moored near quayside with extremely extended swims (bow and stern overhangs). This could be an area of improvement that would be well suited to analysis using prismatic coefficient.

    The biggest area for improvement is likely to be in the ergonomics of the propulsion. Pushing with a pole on the riverbed is very efficient but the overall stroke efficiency is poor. A study involving an ergonomist would pay dividends in this regard. Also because the forward force is provided at the side there is a tendency for the punt to travel crabwise through the water. This can be minimised by the punter applying torque through their body but that is hard work on the knees and core. Perhaps there would be an improvement by increasing resistance to leeway for example with the addition of bilge keels or lee-boards (these would preclude use of the rollers).

    For those on the forum who are unfamiliar with punts: they are traditionally made from a pair of solid mahogany boards forming the sides, and spaced by frames (the so-called ladder frame). Thin ply forms the base and this is intended to be replaced several times in the life of the punt.
     
    bajansailor likes this.
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.