50' LDL design project.

Discussion in 'Boat Design' started by Robjl, Jun 29, 2014.

  1. Tad
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    Tad Boat Designer

    LCBcurves.jpg

    17 years ago I published the above idealized area curves based on speed-length ratio. The basis of these curves was the best of a series of semi-displacement hulls built in Germany between the wars. I tempered that data with comments from William Garden and Tom Fexas, and my own experience. They have worked well for me.

    More on the subject
    http://www.tadroberts.ca/about/pdf/power-boat-design-form-and-function.pdf
     
  2. Robjl
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    Robjl Senior Member

    Tansl, I generated the hull shape in Prosurf, all NURBS curves, the program saves files as *.srf files. These are peculiar to Prosurf (I think). I converted my file to a DXF as I thought this was openable in most other CAD and Hull design programs, unfortunately the conversion changes NURBS to polylines or splines and creates lots of them to emulate the NURBS. so we get some awkward spots, the blue highlight is an example.
    The top red ellipse shows the knuckle fwd (upper line) and aft(lower line) as it transitions to the transom. I assure you the surfaces are developable from flat panels.
    Any suggestions on a less complicated approach, or opinions on how the hull will perform is why I am here.
    Cheers
     
    Last edited: Jul 1, 2014
  3. Robjl
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    Robjl Senior Member

    Cp ?

    Tad, now I'm confused. Your article has been a vertebrae in the backbone of my reference materials for many years, but in it you state for a "SLR of 1 to 1.2 a Cp of .54 to .59 is correct, and SLR above 1.2 requires a Cp of .60 to .72"
    I know the graph shows suggests lower Cps' and it is odds with what you wrote, in my design I settled on Cp at .58 as being the "optimum compromise"
    Cheers
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Have you got more money and time than you know what to do with, Robjl ? :)
    You outlay a fortune and spend years building a design that is basically unproven, and will be very hard to sell ( at a price that would give reasonable recompense) when the time comes. A huge gamble, and the odds well against you.
     
  5. nzboy
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    nzboy Senior Member

    I think it depends a little on hull shapes Tads graph is very appropriate for most power boat designs where greatest beam is at 50% of length .But your boat more follows delft .So a cp of .58-.60 is ideal for displacement speed. As for LCB in your design word has it (from confidential source )5-10% The greater if your heading into the wild blue yonder in a following sea and surfing down waves .As an aside, recently a12m boat was designed with a 1.3million tag plans were 125k which includes overseeing build. If further plans are sold costing 25k the original owner will credited 25k up to 5 plans where after the designer has ownership of plans. This makes Tads plans fairly cost effective for example
     
  6. nzboy
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    nzboy Senior Member

    I just had a look on TADs website and he has updated his 56 now a 5602 with the galley up . Very attractive boat
     
  7. Tad
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    Tad Boat Designer

    NZ....you are correct, there's a good argument for having a forward prismatic number and an aft one. My view is that water hasn't changed.....:D

    Rob....All these papers and graphs are only guide tools, and "without deviation from the norm, progress is not possible." (F. Zappa) Do look at the available tools, then choose your own path. Some of us must remain more conservative in our design work, as we're spending the client's money. It would be interesting to run a series of these forms through Michelet.

    View attachment npl.pdf

    L-D ratio-2.jpg

    Prismatics.jpg
     
  8. Robjl
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    Robjl Senior Member

    Tad, thanks for the info and the two pearls of wisdom. Frank Zappa has a quote for every occasion. What about "One of my favorite philosophical tenets is that people will agree with you only if they already agree with you. You do not change people's minds."
    In this case I can't agree with Frank.
    When Steve Dashew started his FPB series I thought he was off in the paddock on his own. But we followed his progress and saw his boats in action, some will say clever marketing, but he changed a lot of minds, nobody expected him to sell 64'ers as fast as Circa can build them.
    I have cruised sailboats for 45 years, the sail handling is getting harder (for me) than it used to be, and honestly I motor and motor sail so much that I may as well have a power only vessel. It would definitely let my wife and I cruise for many years longer.
    Others have said this and I am happy to tell all that I agree, over a ten year period the costs of running a cruising sailboat and a similar size power cruiser (displacement hull) are very similar.

    Tad I know you do a very different shape hull to mine, but then Steve Dashew's hulls are different to yours, yet both types apparently perform well. Mine is designed to operate at SLR 1.15...maybe up to 1.4 in emergency or surfing. I have kept the Cp a bit high on the belief that the resistance penalty of a high Cp at low speed is minimal... but a Cp of .53 will be difficult even dangerous to steer as speed gets up. For the same reason I have kept the LCB aft. It is currently at 53.5% and I would dearly like to put it at 55% but am too careful. I have done a weights spreadsheet and LCG =LCB....it all works. Fuel tanks (p&s) are at CG so their use won't change trim.
    I have tried to have a flatter hull under the final run aft to further resist squatting when she surfs down a wave.
    The bump (for the gearbox-engine to sit in) doesn't look the best I agree, it looks less obvious when the skeg is in place, but i left the skeg surfaces off so any-one with the software,skill,generosity could asses the canoe body on its own.
    I am concerned about the aft run and waterflow around the bump and I am happy to hear opinions.
    I have to date built 3 models (.075 scale, LCG and displacement scaled) radio controlled and observed their comparative behavior. The model of the current DXF file is on the way, it has a slightly narrower stern in the water, deck width unchanged.
    I have tried to run the models at wavespeed, SLR 1.34 where the trough stretches from bow to stern. Its a difficult task as the trough is so small. The hull runs sweetly, but of course its not tank testing.
    I made a model and tested it like this on my previous design and I have to say the full size yacht characteristics can be seen in the model.
    Tansl, I have made 3 models by printing the hull panels on paper and tracing them onto ply,solid timber, grp, all flat panels, the only difficult area is the twist in the bottom panel at the bow. definitely all developable.
    Mr Efficiency, what can I say, thankyou for your encouragement, but as far as I know, I only need approval from one person to build another boat... and she said its OK.
    I'll finish with a quote from another well known Frank who said:
    "I'll do it my way"
     
  9. TANSL
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    TANSL Senior Member

  10. Mr Efficiency
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    Mr Efficiency Senior Member

    I am interested to hear how the build will proceed in solid GRP, which would be a very big undertaking.
     
  11. Robjl
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    Robjl Senior Member

    Mr Efficiency, the construction is from flat panels, yes they are large. I plan on starting in summer, and time permitting will post photos if others are interested. I am still keen hear about others opinions on the hull shape.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    Seeing you are going to potter around at 7 or 8 knots, I don't know why you need such fine forward sections. From my experiences of being aboard slowish motor boats in this size range, you actually get a better ride with full ends, less pitch, less roll, and a generally more stable boat. Not to mention more interior space.
     
  13. Robjl
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    Robjl Senior Member

    I reckon we all respond to the weather(seas) we have been caught in. My experience on recent cruises from Adelaide up as far as Port Douglas is that the vessel I have designed will be better than what you suggest. I disagree with you about the full ends, I think the full ends exacerbate pitching in a seaway... and rolling, I don't think it will make much difference. The narrow hull will yaw less and will have better directional stability, ie, easier to steer. I agree about the accommodation but then the narrower hull will be more efficient.
    It's always going to be a compromise.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Some would say it was brave and bold, but it looks like a ridiculous risk to me, to proceed. Even with plans in hand, of a proven design, you have a monster job. What is the budget ?
     

  15. TANSL
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    TANSL Senior Member

    I quite agree with you, but we both need to recognize that, keep in mind that browsing experience with a boat (just one boat) must be taken into account.
     
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