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Thread: Double ended dinghy symmetry

  1. #1
    Join Date
    Dec 2011
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    North East England
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    Default Double ended dinghy symmetry

    I designed and built a 12’ x 30” canoe a few years back, it has symmetry both side to side and fore and aft so the four quarters of the canoe are made from identical panels - stitch and glue. Aesthectically and functionally this works fine.

    I like the idea of a double ended dinghy, I am going to say peapod but may be technically wrong. When I have looked at plan views of double enders some have more fullness towards the bow, some look almost symmetrical fore and aft and others are fuller at the stern. I can appreciate the full stern allows the crew to sit in the back and have a finer entry, more powerful(?) but other than that can’t understand what the advantages of each type might be.

    So the question why the variation of fullness position?
    For a boat for sailing on sheltered water what are the downsides of craft that has fore and aft symmetry?

    Personally I prefer the full stern like SEI but that is just based on aesthetics.

    If this works all of these examples should all have their bows to the right





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  2. #2
    Join Date
    Dec 2015
    Location
    Dorset, UK
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    777

    Default Re: Double ended dinghy symmetry

    Tink,

    At higher froude speeds, it's found that the most efficient position of the LCB is slightly aft of the mid point.

    At lower froude speeds it's found that the most efficient position of the LCB is slightly forward of the mid point. It can be calculated exactly for the speed you want to optimise, based on tank testing results.

    It's also a matter for length to displacment to beam. Effectively as a boat becomes innately more hydrodynamic by making it long and thin for it's displacement, the optimum position of the LCB has less influence/ matters less. So it's about optimisation of 'fat' Western boats, due to harbour LOA taxation.

    It also relates to wavemaking resistance, as and aft lcb gives more length forward to achieve a lower vertical and horizontal entry angle, and alonger run of attached flow before it detaches at higher water flow speeds. The down side is that it gives a higher aft exit angle and detached flow there...

    For sailing boats that are more concerned with higher froude speed efficiency sailing to windward, we see aft positioned LCB's, which probably helps where people naturally sit too, though you could achieve a similar effect by sitting a bit aft in a peapod. Pea Pods are at 50% LWL. You can row it in either direction and not be far off!

    Some boats like older Dories and 30ft Whaleboats have a forward LCB as they travelled at low froude speeds if you look at the plans. Its not wrong, just a different optimisation.

    The aft biased waterlines of moderns sail and oar boats, sitting under mizzen, are actually now optimised for travelling backwards and making leeway. It has crossed my mind that it would be optimum to row most boats backwards at lower speeds...just the rollocks and aft keel fin will be in the wrong place. You can in effect put the rowing thwart a bit forward to bring the LCB more forward in rowing mode, if the LCB is a bit aft as on most sailing boats.

    As you look at oar and sail, sail and oar to sail you will see the LCB move aft accordingly. Is it all a wash? Well at low speeds, the biggest drag is wetted area, so the advanatage of a forward LCB isn't much. At higher speeds, a lot of drag is wavemaking resistance, where the LCB position has more of an effect comparing them both, so most plump for a slightly aft LCB whatever.

    Aesthetically there is an effect. A boat with an aft LCB and topsides placement of maximum beam, will be bluffer to the aft stem. It will need a tad more sheer immediately towards the stern to avoid any powderhorn effect that the wider angle optically creates when viewed from an aft quarter.
    Last edited by Edward Pearson; 06-13-2018 at 05:42 AM.

  3. #3
    Join Date
    Jan 2010
    Location
    Northern Europe
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    8,835

    Default Re: Double ended dinghy symmetry

    Ed summed it up nice. Many of the old trad style Swedish local sail and oar boats are more mackeral head, showing they were not intended for higher speeds. Some of the Finnish boats even more so, but much of that was also related to conditions and having bouyancy where needed. I have just sold my Vattern snipa to a guy who has a collection of sail and oar boats from different areas of Sweden, they all differ depending on use, but the mackeral head is prelevent in most.

  4. #4
    Join Date
    Dec 2011
    Location
    North East England
    Posts
    1,126

    Default Re: Double ended dinghy symmetry

    Thanks for the replies, especially the detail you went into Edward. I will play about on DelftShip and decide which compromise I will prioritise now understanding a little more about where to place the volume

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