Stern taper on semi-displacement hulls

Discussion in 'Hydrodynamics and Aerodynamics' started by Will Fraser, May 15, 2018.

  1. Will Fraser
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    Will Fraser Senior Member

    I have read a couple of articles gleaned from other threads on motor/mega yacht design. Both articles (one by Van Oossanen et al, the other by Blount and McGrath) make reference to the NPL series hull form.
    Blount and McGrath shows the benefits of the round-bilged NPL over an equivalent semidisplacement hard chine design (62 series) at speeds under Fn = 1. Van Oossanen again shows the superiority of some "Fast Displacement Hull Form" - FDHF - over the NPL hull at speeds below Fn = 3.5 - 5.
    The FDHF seems to have hardly any taper in the waterplane lines aft of midships which, to my non-NA eye, seems counter intuitive for good performance at high displacement speeds. Both the NPL and FDHF forms appear to have similarly straight aft buttocks.
    Have I missed something or is it therefore enough to just raise the transom with a little bow-down trim in order to reduce the ratio between immersed transom and max section area?
     
  2. DCockey
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    DCockey Senior Member

    The wide stern with very shallow transom immersion appears to be popular for power boats designed for fast "semi-displacement" speeds, Froude numbers from around 0.35 to 1,0. The buttocks are long and straight or close to straight.

    My understanding is on advantage of the wide, shallow stern is it reduces stern sinkage at higher speeds. It also usually increases initial stability and provides more deck area than a stern which is tapered in planview.
     
  3. HJS
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    HJS Member

    There are much more relative data that must match than mentioned above. An example of a semi planing or semi displacement boat is presented in this link. The boat is indeed much smaller than a mega yacht, but the principle is exactly the same.

    http://sassdesign.net/Keyhaven Skiff, a semiplaning lightweight skiff.pdf
     
  4. Will Fraser
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    Will Fraser Senior Member

    Those are exactly the properties I am after for a low powered fishing skiff. No megayacht, but it looks like the ratios scale perfectly for a 13-14ft, 5hp craft that will perform well with one or two on board.
    If built with low weight in mind, the weight of a motor on the transom would require an ungainly long tiller extension to reach a helming position forward enough to provide optimum trim.
    A number of SUP-like skiffs have resorted to a slotted transom to move the motor forward, but this cut-out simultaneously robs the stern of volume. Is there a way to efficiently close off the well aft of the motor? The Bartender 19 seems to work fine but I cannot find drawings or photos showing details of the aft part of the motor-well.

    Another question relating to such motor-wells or slots: one of the article mentioned above discusses the minimum speed for a transom of a certain immersion to run dry. Would the displaced transom (or the side walls of the slot for that matter) still run dry at a similar speed?
     
  5. Will Fraser
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    Will Fraser Senior Member

    Hi Jurgen,
    It was precisely that article that had me wondering about why the wide stern on the FDHF seemed to deviate from the norm.
    Would a wide, slotted stern be equivalent to a narrower stern of same immersed draft and area?
    Any other guidelines I should consider to optimise the slot - interceptors, side wall angle etc?
     
  6. HJS
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    HJS Member

    There is no norm, only calculations where all factors must co-operate to an optimal result. The result must match the client's wishes and requirements. The requirements must contain at least the maximum and minimum payload in addition to the maximum and minimum speed. Then it is the designer's task to present an acceptable alternative.
     
  7. PAR
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    PAR Yacht Designer/Builder

    Pinching in the stern a bit on displacement and semi displacement hull forms, also helps them maneuver at lower speeds with less fuss. They tend to "tuck in" as they toss the helm over and ride the midship lee wave train through the maneuver, which is generally a little more comfortable.
     
  8. jehardiman
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    jehardiman Senior Member

    There is too much that depends on LWL/B to comment. See the series 62 data when plotted against LWL/B for insight. Or look at German S-Boots vice British MTBs for an appreciation of what goes on.
     
  9. HJS
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    HJS Member

    Calculate, no guessing
    As I wrote earlier, there is much more that must match. The total weight position must be close to or aft of the center of the waterline area. Otherwise there is a danger that the boat will dive or broach. And the total weight must be placed in relation to the waveforming length and the selected speed.
     
  10. Will Fraser
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    Will Fraser Senior Member

    Thank you for all the replies so far. I have obtained a paper with various numerical solutions for predicting resistance, notably one by Holtrop and Mennen which takes into account various form parameters.

    A few checks of metacentric height for candidate hull shapes indicate that the minimum beam required for stable stand-up casting would drive the L/B ratio down to around 4 and the B/T ratio up to at least 9. I note that some systematic tests do include data in these shape ranges, but that there is also a drag penalty compared to skinnier shapes with more draft and less wetted area.
    One shape in particular for which I have never seen published data is the inverted-V/Sea Sled. It is naturally much more stable for a given beam, but does anyone know how they perform in semi-displacement mode?
    I assume most of the Sea Sled models were designed specifically for the planing regime, but I did see a magazine article by Hickman himself (Motorboating, July 1935) in which he refers to "slow speed, strictly displacement Sea Sled hull". So were there any "strictly semi-displacement" models as well?
     
  11. Will Fraser
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    Will Fraser Senior Member

    I have created a spreadsheet of Mercier and Savitsky's numerical method which incorporates the transom/max section area ratio. (Section 4.4 of the attached article).
    Results are gives as total resistance (RT/W) based on friction for a 100000lb displacement. I used a cad model of the NPL series to get realistic values for L, B, T, iE, areas etc. At first glance the results look like they fall in the right ballpark.
    Scaling to different displacements however results in an increase in RT/W that seem way out of proportion. To illustrate, I have scaled the reference hull (100000lb) down by only 1%. Uncorrected Rt/W values are almost indistinguishable. Corrected values increase by as much as 60%.
    Scaling (eq. 19) : RT/W-corrected = Rt/W + (Cf-ref - Cf +CA)^(1/2)*(S/D^2/3)*(FnV^2)
     
  12. HJS
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    HJS Member

    Try the Delft Systematic Deadrise Series. As I perceive it, it is much more reliable than Mercier - Savitsky.
     
  13. Will Fraser
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    Will Fraser Senior Member

    Thank you for the suggestion.
    I used the comparison of your Victoria hull vs C600 in one of your articles (PB Design Challenge 2009) to test the formulas. I used both the FAST and HSMV formulas, and in both cases the C600 shows lower drag than the V600 from Fn = 0.9 and faster. Have you also found this to be the case?
    Another question is about the suitability to extrapolate beyond original test parameters. The HSMV formula accepts deadrise as an input but the lowest deadrise used in the model tests was 12.5deg. Entering values lower than that (e.g. 8deg for the V600) shows much increased drag beyond Fn = 0.7. Is this to be expected or just an inappropriate extrapolation?

    The hulls used in the Delft Deadrise series appear to be exclusively planing hulls, evaluated in the semi-displacement speed range. Are these formulas just as applicable to designs with much finer angles of entry and round bilges?
     

  14. HJS
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    HJS Member

    WF

    I have not gone back and compared the calculation models in that presentation (PB Design Challenge 2009). The data reported are based on measured values. It is always important to check which surveys are the basis of the calculation model. Usually you should not extrapolate outside the underlying survey. I myself have found that de Groot and DSDS match well up to planing speed.

    JS
     
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