Review of dinghy hull form

Discussion in 'Boat Design' started by George T, Mar 23, 2020.

  1. George T
    Joined: Nov 2019
    Posts: 13
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    Location: Portsmouth

    George T Junior Member

    Hi all,

    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 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'm thinking I might need to add in a chine for better performance (or at least remove the entirely rounded form of the hull), would this be correct logic??

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

    made via FreeShip and DelftShip
    efd.PNG efdvs.PNG effed.PNG feacs.PNG fefef.PNG wef.PNG wgsd.PNG
     
    Last edited: Mar 23, 2020
  2. alan craig
    Joined: Jul 2012
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    Location: s.e. england

    alan craig Senior Member

    Every modern planing boat has a much wider and flatter stern. So I guess that is one thing you will need.
     
  3. George T
    Joined: Nov 2019
    Posts: 13
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    Location: Portsmouth

    George T Junior Member

    Yeah, I figured, I was hoping that this might conceivably work like the 420 and 470 hull forms planing wise, but again, that was never the initial intention just an added bonus
     
  4. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    George T, what is the purpose of your boat aside from just sailing around. I get the impression that you intend for it to be fast boat....you are going to use a trapeze to deal with what I presume will be a lot of sail area and seek a fair turn of speed.

    Not to rain on your parade but there is this..........the real hot rods of the day do not look much like what you have drawn although you have done a fine job of it. The modern hot rod dinghy has hard chines aft. They have considerable width in the afterplane area. You have placed the lowest part of the keel profile at midships. Current fast boats move the lowest part of the bottom, in profile, to about 35 to 40% of the LWL when measured from the forefoot. The reason for that, as Uffa Fox showed us, is to minimize the angle of the aft quarter beam buttock angle with respect to the horizontal. That is a feature that is most likely to encourage the boat to plane.

    Tell us more about the primary purpose and hopes for your boat might be. ................

    In fairness we can remind ourselves of the International 10 meter canoe that has a pointy after end. They can, and do, go like a bat out of hell and plane readily, in some cases plane to windward. There is a big difference. The 10M is a skinny, super light weight, over canvassed, boat sailed by athletic maniacs. They also have a sliding hiking plank that helps the skipper keep the boat right side up..........well most of the time. I confess that I was once one of those maniacs. The 10M canoe concept is a hundred years old. I love the 10M but would not design it that way today, despite what wizards like Proctor, Whitman and the others did.
     
  5. Steve Clark
    Joined: Jul 2004
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    Location: Narragansett Bay RI

    Steve Clark Charged Particle

    Looks very much like something from the first part of the last century.
    Too full forward, too small aft. That skeg is a bad ides which will sap performance and make the boat slow to turn and tack.
    As a general rule, max waterline beam should be about 60% aft. Rocker is closely related to how the boat trims at speed. More rocker= more bow up trim.
    Need to target a D/L ratio of under 100. The lower the better. Which will drive you toward long hulls and light weight construction. Midship flare, a la 505, makes live ballast effective while maintaining a narrow water plane.
    SHC
     
  6. George T
    Joined: Nov 2019
    Posts: 13
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    Location: Portsmouth

    George T Junior Member

    Hi, Messabout,

    The boat is intended to go fast (you hit the nail on the head) and to do some club racing and other events all in (handicap style) as this is one of the most popular forms of racing where I am. and secondly thank you for the hint of fullness amidships being slow, I had some thoughts on it but left it for the moment, so that is something I will change, as well as that I think I will add a chine along the water line aft to produce a flatter profile for the hope of planing and I will try and move the fullness forward. However, I do not intend on widening the aft section, I prefer the hull shape as it tapers to the stern and I am attempting something of a fat canoe style shape but I will hope to overcome the planing isssue with a flatter shape and chines.

    As for Mr Clark's comments,

    The skeg is an unintended consequence of the rounder hull shape and narrow stern and I will attempt to try and flare the hull a bit (as you said, a la 505)

    Thank you both for your insight and I hope to post an update with a modified hull shape quite soon
     

  7. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    George, moving the lowest point of keel rocker forward is all very well but to smooth the area curve, the drill is to move the maximum wetted area aft. The concept is to ease the transition from .......prying the water apart to the point where the water begins to close in from the sides of the trough. Catamaran people often have at least two sections with equal displaced area so as to ease the transition. Plot the area curves and try to make the transitions in the Y axis a bit more gentle near the point of maximum displaced volume. What we are dealing with is the acceleration of the water molecules where we want to minimize the acceleration of the molecular structures. That is an oversimplification of the Newton equation F=Ma.

    According to Marchaj and other authorities, the pressure gradient on the bottom of a planing boat diminishes as the dynamic forces of the approaching water pressure moves aft. ( not to the extent that Lindsay Lord laid claim to negative pressure in the far afterplane....he was wrong) It would then be a logical to assume that more area aft would help compensate for the diminishing dynamic pressures (when planing) . Your narrow transom area is suspected of being a fashion notion rather than an established concept of science.

    Please forgive me for being such a critical observer. Your original post appeared to invite commentary.
     
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