Review of dinghy hull design *update*

Discussion in 'Boat Design' started by George T, Mar 27, 2020.

  1. George T
    Joined: Nov 2019
    Posts: 13
    Likes: 0, Points: 1
    Location: Portsmouth

    George T Junior Member

    Hi all,

    (this will be familiar to those who have read the previous thread and the statement below is the same but I have updated the design itself, once again, all comments are appreciated)

    I have spent some time creating a hull form for a 16ft two person trapezing dinghy (similar concept to an FD or 505), I know that I could have used these existing designs but wanted to practice and have the satisfaction of "doing it myself". The boat will have 3 sails (main, jib, symmetrical spinnaker) and will have one person on the trapeze, it is designed for use on the Solent (UK) so somewhat sheltered but still rough waters occasionally, it is designed for two larger sailors like myself (all up crew weight of around 150-190kg). It will have a metal centreboard (not designed yet), I have no idea if this will plane (opinions will be welcome) but I hope it has the potential to considering it will have large sail area downwind (not finalised but around 27m2 like a 505) designs and dimensions below (incl. linesplan)

    Feel free to address my SOR and hull design in general, don't hold back on criticism as this is what I am looking for to better the idea. I have now added a chine and moved the fullness more forward and flattened the aft to encourage planing, I have left a narrower stern still (using the concept of a fat ******* style IC)

    Length (waterline): 15.5ft
    Length (overall): 16ft
    Beam: 4ft 3in
    Draught: 1ft
    Height of hull: 3ft

    (I am aware that I have some issues with lines coming together on the centre, i forgot to check these and it is not a design problem)

    made via FreeShip and DelftShip
    csazca.PNG egdsfv.PNG faedcs.PNG rgsdvz.PNG wdasc.PNG
     
    Last edited: Mar 27, 2020
  2. tlouth7
    Joined: Jun 2013
    Posts: 115
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    Location: Cambridge, UK

    tlouth7 Senior Member

    I'm intrigued by the tiny transom, is there a specific reason you have brought the beam in so far at the stern?

    I would suspect that the extremely low shear at the stern will make the hiking position for the helm rather strenuous and wet.
     
  3. George T
    Joined: Nov 2019
    Posts: 13
    Likes: 0, Points: 1
    Location: Portsmouth

    George T Junior Member

    Hi Tlouth7,
    I designed this boat for use with me as a helm, personally I prefer putting my weight in my leading leg whilst hiking and find it more comfortable to do so, also as a heavier chap I find I do not need to be sat as far back to achieve the correct trim, this is why I have done this, thank you for your point and as long as this (let me know if it does) doesn't affect the performance of the hull then I am happy

    Many thanks
     
  4. wet feet
    Joined: Nov 2004
    Posts: 482
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    Location: East Anglia,England

    wet feet Senior Member

    I think you may have designed a bit of a submarine.The freeboard looks a bit scant and combined with the vertical topsides forward,the water isn't exactly strongly discouraged from surging over the foredeck and into the cockpit.The beam in the central section isn't what I would expect to see if the shrouds are to do their job well,what shroud angle do you envisage at the hounds?A situation that won't be made any easier if two plus size crew need to be carried.Why the metal centreplate?I can't think of a trapeze dinghy designed since the fifties that has one as they provide almost no righting moment until the boat is almost capsized and the weight inhibits planing and may be unwelcome when you pull the boat out of the water.

    The width of the transom may not be an issue but is the transom tall enough to fit the very popular rudder stocks that have 170mm between the bearing surfaces?For comparison could you find an Osprey and look at the stem height,transom height and beam at shrouds?It might also be instructive to draw a rig and particularly a spinnaker chute.
     
  5. George T
    Joined: Nov 2019
    Posts: 13
    Likes: 0, Points: 1
    Location: Portsmouth

    George T Junior Member

    Hi, wet feet,

    I shall answer your questions in order,

    The free board is slightly smaller than I had anticipated and can be worked on without adversely affecting my characteristics at the water line and below, there is also flare at the water line induced by the Chines although if it looks to be problem I can and will adjust it.

    Secondly, the beam is a problem and I will be moving the full beam further forward to level with where the mast will be (I will post an update with right design soon) this will hopefully induce the width needed, however it is designed to work with an SB20 style shroud system ie: lower and uppers which in my experience sailing those boats at a high level have negated the need for a massive beam due to the shrouds.

    Thirdly, most of my experience has been in boats with a metal centreplate and I find that the extra moment provided is beneficial as well as the weight helping when righting after a capsize which is a benefit as I normally sail with minimal safety cover due to my free time and amount of cover I can get where I sail due to distance, as well as this, with the sail area planned the extra weight should not be a problem and as for recovery we have a beach tractor for our club and the next club along has a tidal slipway and a winch, so that is not a problem.

    Lastly, the popular mounting systems are not a problem as I will be modifying a rudder and mounting I already own from a similarly sized dinghy.

    I hope these answer your queries and let you know that I understand the points you make design wise and will be incorporating them into a revised design

    Many thanks
     

  6. George T
    Joined: Nov 2019
    Posts: 13
    Likes: 0, Points: 1
    Location: Portsmouth

    George T Junior Member

    Hi, all, in response to the questions above which I have already replied to, here are my updated hull designs with rig plans soon to follow. These have the same spec as the previous designs but feature an increased beam abeam of the mast step and more flare to dissuade water flowing over the bow, as well as a slightly milder slope to the transom for increased comfort on my end.
    many thanks. update 1.PNG update 2.PNG update 3.PNG update 4.PNG update 5.PNG
     
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