Confined channel wave resistance effects - narrowboats

Discussion in 'Hydrodynamics and Aerodynamics' started by dpaws, Mar 8, 2016.

  1. dpaws
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    dpaws Junior Member

    Gentlemen good morning - help!

    I'm considering design factors for UK canal narrow boats in confined channels.

    I've been privileged to enjoy the personal advice of Dr Barrass regarding the squat considerations, but whilst discussing with members on the canal forum, one member has suggested that we have both overlooked two considerations - I've pasted the relevant comment below:

    "As for the hydrodynamics behind hull resistance and squat in shallow and restricted channels, it seems to me that in everything you have either expounded or referred to so far you have ignored two very important and influential considerations applicable to narrowboats in the very small and very restricted English canals as opposed to barges and ships operating in restricted, but relatively far less restricted, channels elsewhere.

    Firstly, operating speed for a narrowboat on a narrow English canal is proportionally much lower than for a barge or ship in a much 'bigger' canal, and in general the transverse X-sectional area of the immersed hull of a narrowboat in relation to the X-sectional area of the waterway will be much greater than that of a barge or ship in what to them is a restricted channel.

    Secondly, and relevant to the pressure head loss influencing squat at the stern , I don't think you've given any consideration to the effects on pressure head under the stern of the differing wake percentages associated with the 'fineness' or otherwise of the run aft to the propeller, which in a narrow and shallow canal can bring all the the water that's rushing back along both sides of the boat to an abrupt standstill roughly level with the stern."

    I recognised the first factor as the channel block co-efficient, and by using the Barrass short form squat calculation I have been able to illustrate graphically that the block co-efficient can be practically disregarded under these conditions.

    However, the second part of his concern I have struggled to comprehend. I don't doubt his observations, he is an experienced canal boat helmsman, but I can't find a method to quantify the effect that he is describing. I've considered the max hull speed, but even with a favourably adjusted constant the speed remains outside the maximum speed permitted on the canal, 4mph.

    For approximate numbers: LWL = 18.0m, draft 0.80m, beam 2.0m, canal depth 1.1m, centre channel width 4.5m, Cb around 0.68 (fining in progress)

    Now I have every confidence in Barrass, (and why wouldn't I!) - I'd be very surprised if there was a significant factor that I should be considering that he failed to mention, he doesn't strike me as being the forgetful type...

    Would someone be kind enough to point me in the right direction? All comments most welcome....

    Much obliged
     
  2. HJS
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    HJS Member

    A very interesting topic.

    What strikes me is that it focuses solely on the block coefficient, that does not describe anything about the stern design.

    I wonder how a narrowboat really looks at stern. I've been googling, but have not found anything useful.

    Grateful for appropriate links in order to make a better picture of the boat.

    js
     
  3. dpaws
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    dpaws Junior Member

    For sure - thanks for your interest.

    This is the site of a respected builder of traditional working narrowboat replicas, amongst many other seemingly magical skills!

    http://www.brinklowboatservices.com/new-builds/steam-tunnel-tug-hasty/

    On this page there's a good foto, but browse under the "new builds" tab and I'm sure you'll discover more. The dimensions vary between models but the general arrangement and swim / counter stern form is consistent throughout.

    You'll find more here - less replica, more modern equivalent - subtle variations and generally a simpler construction

    http://www.tylerwilsonboats.com/index/

    Hope that helps :)
     
  4. DCockey
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    DCockey Senior Member

    "Hull speed" based on waterline length alone has little relevance with the water depth is a small fraction of the waterline length.
     
  5. dpaws
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    dpaws Junior Member

    (thank you) - OK, which equation would you use in this situation?
     
  6. DCockey
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    DCockey Senior Member

    The dispersion relationship for finite depth water.

    If the water depth is large compared to the wavelength the wave speed asymptotes to being dependent on wave length only. That's were the "hull speed" equation comes from.

    If the water depth is small compared to the wavelength the wave speed asymptotes to being dependent on water depth only.

    Have you studied water wave theory? Is this a school project?
     
  7. dpaws
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    dpaws Junior Member

    Many thanks David - Yes, most definitely a school project in the sense of being a naive but curious student!

    No, I haven't studied the wave theory, but I will if it will help me to understand the scenario better. I hadn't understood its significance in such a confined channel.

    I'd been contemplating the passage of the hull through the confined channel, with particular regard for squat and therefore more inclined to look at the water speed relative to the hull and the consequence of the subsequent pressure variations.

    For me waves are (were) surface effects so I'd assumed that the water beneath the surface would not exhibit such an oscillation.

    Of course, what I'm attempting to do is to quantify such effects on the hull as a component of the total resistance to its forward motion through a body of water. The aim ultimately is to optimise the bow and stern profiles to minimise the total resistance of the hull when underway and thus increase propulsive efficiency.
     
  8. DCockey
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    DCockey Senior Member

    Studying the basics of wave theory would be very beneficial. Water waves are not a surface effect only, particularly in shallow water. The pressure distribution around the hull in shallow, confined water will be directly affected by the waves.
     
  9. dpaws
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    dpaws Junior Member

    Forgive me, no doubt a dumb question but...

    If these waves are present sub-surface and are of significance to my calculations, why do you not experience a pulsing effect against the hull surface between the pressure peaks and the troughs? I'd have expected the sheet sides to be pulsing in flex, in and out like a pair of lungs.
     
  10. DCockey
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    DCockey Senior Member

    The waves system due to the boat moves with the boat, so it is steady from the boat frame of reference (assuming boat moving at constant speed, and constant channel cross section).

    What are you studying?
     
  11. dpaws
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    dpaws Junior Member

    I'm not studying in the formal sense or as part of a structured course - I have the possibility to influence the hull shape of a new build narrowboat.

    I started to ask questions about which factors were most relevant for consideration, but it seems that every question leads to another five!

    The intellectual journey so far has been both enlightening, thanks to patient and knowledgeable people like yourself, and highly frustrating because it's so incredibly complicated, on paper at least.

    The traditional narrowboats were optimised for their cargo capacity and sailed in deeper channels than exist on the UK canal network today which is why I thought it desirable to optimise the hull for my own needs and gain propulsive efficiency from a lower Cb.
     
  12. HJS
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    HJS Member

    I think there needs to be a lot more input than just varying the block coefficient.

    js
     

    Attached Files:

  13. DCockey
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    DCockey Senior Member

    Where did you get that information? Is that true everywhere on the UK canal network?
     
  14. DCockey
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    DCockey Senior Member

    Stern shape should depend on several factors, which will have different importance depending on the particular set of requirements. These would include:

    - Resistance at target cruising speed.

    - Resistance at other speeds.

    - Interior volume including space for propulsion system.

    - Cost of fabrication.

    s
     

  15. dpaws
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    dpaws Junior Member

    I couldn't tell you verbatim as I don't have comparable surveys, but idle chatter amongst the community online would suggest it's a common issue, due to a lack of dredging when the network was in decline in the 70's I suppose.
     
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