Fast Displacment Hull, Plywood, Tunnels and Hull Speed!

Discussion in 'Boat Design' started by Dhutch, Oct 11, 2012.

  1. Dhutch
    Joined: Oct 2008
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    Dhutch Junior Member

    I have a bit of an itch to make a fast boat and would like to make a fast displacement boat, simular to the Thames and Windermere fast launches of yesteryear. But made of plywood in order to allow a suitably quick build process, and powered by a petrol engine.

    I understand displacment boats are typically limited by their hull speed, at which point longer is better. Using the below formular which appears fairly well reguarded, you get the below figures;

    V = 1.34 * L^½.

    16ft 5.3knots
    20ft 5.9knots
    25ft 6.7knots
    30ft 7.3knots
    50ft 9.5knots

    However the umpire launch Consuta (google it if you wish) which I have been on can achive more like 24knots at her 50ft lenght, which gives a hull speed constant of more like 3.39 which is much higher than most appear to allow for.
    But if I could replicate that at 20ft, I would get around 15knotts which is more like my target.

    I am also interested in keeping the wash to a minimum to allow the boat to be used on small inland and canalised rivers hence if nothing else, the desire to use a large propellor partially in a tunnel.


    Daniel
     
  2. Tad
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    Tad Boat Designer

    The reason Consuta is fast is that she is very small (beam, depth, and weight) for her length. Trading the steam gear for a modern lightweight engine will help you but your hull must be tiny to achieve these speeds. To maintain the correct proportions your beam is 3' on 20' length, and draft is a few inches. This results in a tricky (unsafe for some) handling boat. See Rob White's Rescue Minor and Ankle Deep by Uffa Fox.
     
  3. tom28571
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    tom28571 Senior Member

    I think the best boat to look at to see how your goal may be achieved is Weston Farmer's Coyote 11. Designed for maximum speed/minimum drag in the size range you want. You must give up something to get something. You give up carrying capacity, stability and rough water capability.

    Someone will suggest multihulls and that is one option that gives back stability and more carrying capacity.
     
  4. eyschulman
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    eyschulman Senior Member

    How fast do you want to go? There are 36-40ft lobster boats that with plenty of HP run from 25-40knots. At slower speeds these same boats are seaworthy and fair on fuel burn.
     
  5. Dhutch
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    Dhutch Junior Member

    Ok. Thank makes sense, presumably the boat will then effectively push through its bow wave with its light weight and high power thin hull. Your certainly correct about consuta being a very narrow hull for her length but prehaps I hadn't given enough credit to just how narrow that actually is. Im willing to accept compromises to the design to achieve some of the goals but I dont want to make anything thats unmanageable or unsafe in the process. I will look at the Coyote 11 design.

    As I say, currently its a pipe dream, but somewhere around the 15knots was what sprung to mind.

    I dont fancy a multihull, but might look semi--displacement (whatever that means) 'runabout' type designs.

    Another source of my inspiration it the below video. Arlette is 30ft by 5'6" beam and Oberon is simular at 29ft by 5 both built by the same owner with a displacement of around 1ton managing 15mph or so with 15ihp. Further details available on the steam boat register link
    http://www.youtube.com/watch?v=BgzSazdIy5s


    Daniel
     
  6. tom28571
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    tom28571 Senior Member

    In order to make sense of the possibilities, you have to first define the parameters that bound what you will accept. Looking at displacement craft is one way. Changing to either semi-displacement or full planing makes fundamental changes.

    Displacement means minimum combined wavemaking and frictional drag. That leads to the long and lean hull with a fine entry and an exit at or above the waterline. Some will say that the S/L ratio of Coyote places it in the semi-displacement category but I don't consider it so. I think S/L ratios can be misleading unless comparing boats of similar S/L.

    Full planing means you can go to more beam but the exit must be flat, or nearly so.

    Both benefit from light weight but weight is more critical in the full planing hull. Both need high efficiency hull design to minimize power requirements.

    Semi-displacement is somewhere between these and is not generally compatible with a minimum power, high efficiency program.

    Not everyone will agree with these general statements but they suit me. All are subject to some degree of interpretation.
     
  7. tunnels

    tunnels Previous Member

    put the two together !!

    Just make a old style look above the waterline and below a fast planing hull ! marry the two designs . Now stand back and listen to all the negative objections and it cant be done !!and you shouldnt do that its not proper !! :D:p:p:)
     
  8. liki
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    liki Senior Member

    Lovely boats those two steam launches. Any chance of acquiring more information about Oberon or Arlette?

    Maybe you should for a while forget the "displacement hull" from the equation and read about e.g. Nigel Iren's LDL designs on the web and about Whio from Woodenboat issue #190? The issue can be bought as a digital copy for a few USD from their online store.
     
  9. Dhutch
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    Dhutch Junior Member

    Depends what you want to know, although I have met the owner several times, ive never seen the boats and hence most of the infomation I am aware of is lifted from the SBA steamboat register I provided a link to in my above post.
     
  10. Dhutch
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    Dhutch Junior Member

    Clearly before building a boat the objectives need to be clear and the design and expected outcome settled. That is part of the purpose of this thread.

    One of the main issues is the requirement to build a boat that I can make use of. I would really like to be able to tow the boat to a slipway, launch, and cruise up and down our larger rivers at 15knots on a sunny day on a 50ft fast launch, stop in a town, spend an hour, and cruise back to the slipway. Sadly however you cant tow/launch a 50ft boat in that way, all of our rivers in the UK have speed limits on them around the 4-6knots limit, maybe 8knots beyond the tidal limit on a few. Which is where a fast-displacment hull appeals. Something that can do 4-6 very happily at low revs, but can then get up a bit on the strighter deeper sections and make some progress, withot excesively drawing attention to itself.

    I have a freind with a 14-15ft 'salcombe flyer' based sport rib with a 90hp outboard which we take out to sea during a annual weeks summer holiday to the south coast, which is great fun but far from civalised even on a flat day and although the adrenaline of being about get to 40mph in very little time at all is appealing it, very quickly gets tiring but does solve the problem of river speedlimits by going out beyond them. And is where a larger but more powerfull semi-displacment type boat appeals, while prehaps also being usable on rivers.

    I also (as some might know, or have guess) have ties with the steam boats and steam launches which is via my owning a steam canal narrowboat but while I can join in on one of there rallies that takes place on a canalised river all other events require the boat to be trailed and hence rules me out and It would be quite nice to have a suitably 'classic' boat (all be it not a steamboat) to take to a number of these events.

    Other links found:
    http://wolfeboats.com/index.php
    http://sportboats.apolloduck.co.uk/feature.phtml?id=256586
    http://www.duckworksbbs.com/plans/wf/piute/index.htm

    The main reason for the project in my eyes is the build, Ive got inchy fingars to see what I can put together in what time frame, play with for a few years, and then sell on, hopefully covering the build costs.


    I if was going to make a planing hull, which is something I had also considered, it would be in a classic 'runabout' sort of style.
    If not quite a ChrisCraft : http://www.old-outboard.com/images/Boats n stuff/Jeff's 1941 Chris Craft.jpg
    Prehaps it might end up more like : http://www.hartley-boats.com/Sportwin.html
    Or a bit lighter/faster : https://www.glen-l.com/designs/inboard/inboard-images/dsn-ram.jpg

    Im also slightly limited in that while I have no objections to making alterations, I do not in any way intend to get into hull design in the duration of the project, so will be looking to local and make use of suitable existing plans.


    Daniel
     
  11. Crag Cay
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    Crag Cay Senior Member

    Don't get tied up with the technicalities at this stage. Stick with real constraints based on your perceived use. You've made a good start with the river speed limits etc.

    If you want to use it on the canal, you have a beam limit of under 7ft, draft of less than 2ft 6, etc. But if it's rivers and estuaries, then the beam could be up to 8ft 3. Then if you want to tow it behind a car (not 4x4) etc, it probably limits its displacement to 1 ton loaded and including the trailer, etc. Towing something 28 ft long is a probably the absolute limit for length.

    Do you want to overnight on it? make a brew? have a portable toilet? etc.

    What sort of 'style' do you like?

    Fitting a wooden envelope round these sorts of parameters is the easy bit and can come later.
     
  12. liki
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    liki Senior Member

    I also found the information in the register, but would love to have some more pictures of them, especially some showing their underwater shapes.
     
  13. Dhutch
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    Dhutch Junior Member

    Absolutely. I have no real desire to take in on the canal network, although it would be nice to get up on the plane down the BCN mainline that doesnt sound like a very clever idea to me! Nor does locking through a varnished plywood boat.

    Towing wise I understand the limit for trailer size if 2.55 metres and the length limit for a trailer is 7 metres (circa 20ft) , but 2m overhang is allowed with a flag or 3.05m with in illuminated board, presumably ontop of the 7ms. bring the total to 10m or around 30ft, allowing for clearance to the towing vehicle. I passed my test after 1997 and have not taken my B+E test so am currently limited to around a tonne and drive a large hatchback, but could take my test, which would allow more with a suitable tow car. I have experience of towing a 14ft 1100kg box trailer with a kitcar inside it.

    I would like to have the option to 'camp' over night in it under a canvas cover of sometime and may seek to have seating that can let into a bed, but am not expecting to do this often or in great style. No requirement to make hot drinks or cook, no real desire to have a toilet onboard although I do have a portapotti an may allow of a cupboard large enough to carry it.

    Daniel
     
  14. tom28571
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    tom28571 Senior Member

    http://wolfeboats.com/index.php
    http://sportboats.apolloduck.co.uk/f...html?id=256586
    http://www.duckworksbbs.com/plans/wf/piute/index.htm

    The first one is Denny Wolfe's electric version of Coyote and is very nice. I have had the pleasure of riding with Denny in it. Not cheap by any means and limited to what can be achieved with electrics. Go for it if that suits you.

    The last one is Weston Farmer's Piute. Harry Brian's Handy Billy is a new version of this design that is at the upper end of the semi-displacement group. One of the early attempts to get beyond displacement speed in a modest sized (length) boat. Could be nice also.

    Most of the rest are from the classic runabouts popular in the early to mid 20th century. These boats are big wave makers and are not happy between very low and high speed. You will make lots of enemies with these on canals or narrows, especially near other boats at docks.
     

  15. Dhutch
    Joined: Oct 2008
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    Dhutch Junior Member

    Yes, well this is sort of the issue with any planning hull really. Which is why the fast-displacment boat appeals, but might not give the baot (and speed) required within a sensable lenght/beam.

    Been having a quick looking in the 'outboards' section which appear to havea few more 'quick and dirty' type builds. I dont how suitable the larger ones would be putting a inboard engine in, this forinstance seems to say all the right things, if smaller than Im planning.

    https://www.boatdesigns.com/13-Tuffy-runabout/products/367/

    Daniel
     
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