Building a scaled down International 505 dinghy

Discussion in 'Hydrodynamics and Aerodynamics' started by Senrab, Aug 2, 2013.

  1. Senrab
    Joined: Jul 2013
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    Location: Suzhou China

    Senrab Junior Member

    Hi, this is my first post. I am at the planning stage of building a wooden dingy based on the International 505. I will be building this using strip plank + veneer or all veneer with wooden spars. Its not my first build and I know there is lots of work but I have a good workshop and tools.

    I've considered an Argie 15 and an Iain Oughtred Gannet but in the end I've chosen the 505 as a base because I like its shape, performance and righting-after-capsize ability (built in flotation at the rail - reduced bailing, no rescue craft here!) and, also, I have the plans for it.

    I have two main reasons for scaling down. The first is because my wife and I want to be able to handle the craft both in and out of the water and 5.05 metres may be a bit of a handful for us. This is especially true because we are based in Suzhou in China and will have a degree of man-handling, though we will use a trailer of sorts. (this is because sailing in China is virtually unknown except in one or two coastal centres, so there are no slip-ways in our local lakes.) The second reason for scaling down is to slightly de-power the craft because, although I am fairly experienced, I want to sail with my wife and some friends who are (so far) not experienced.

    I've done a fair bit of reading on the internet and also studied all the relevant bits in Dave Gerr's "The Nature of Boats". Following this, I've scaled down the plans using Autocad and "Freeship" for calculation of some of the hydrostatics.

    The scaling I've used is not uniform. I understand and have read all I can find on scaling similitude For instance, I've scaled the length by 0.83 and the beam and profile by 0.91. I've attached a set of metrics.

    There are two things I'm a bit worried about:
    The first is the Lead. This is about 290mm in the 505 and is about 350mm in my scaled version. This is the only thing that's gone up in scaling and is because despite reducing mast height, I've raised (relatively) the head of the jib which increases its size and therefore moves the CP forward. Two reasons for this: firstly because I was worried about it being underpowered and secondly because I think that the increase in curvature of the hull (resulting from the non-uniform scaling) will increase the lee helm so I felt the lead should be increased a bit.

    The second concern is the power. Have I powered it down too much, or not-enough? We still want to be able to get up on a plane without sailing in harsh winds. As far as I can see from the theory, it will plane at about 8Kts I've done some power and heeling calculations but its all a bit theoretical and back-of-a-fag-packet.

    My table of scaling factors is shown below. I'd be really grateful for any constructive input, not only on my concerns but also on anything you feel I should change or do more research on.

    Sorry the table doesn't format well in the post. There a three columns: Original 505, New and Scale Factor.

    Dimensions in mm -------------- Original 505 New Scale Factor
    Length OA --------------------------- 5050 4200 0.83
    Length WL --------------------------- 4500 4000 0.89
    Beam -------------------------------- 1860 1700 0.91
    Draft --------------------------------- 147 134 0.91
    Disp with 2 crew (Kg)--------------- 257 250 0.97
    Mast height -------------------------- 7260 6000 0.83
    Boom length ------------------------ 3120 2590 0.83
    Mainsail area sq m ------------------ 11.74 7.87 0.67
    Jib area sq m ------------------------ 4.68 3.9 0.83
    Total sail area sq m ----------------- 16.42 11.77 0.72
    SA/D^2/3 ---------------------------- 37 27 0.73
    Height of CP from w/l --------------- 2494 2263 0.91
    Lead (CP fwd of CLR) --------------- 290 350 1.21
    Horsepower at F3-4 ---------------- 3.35 2.40 0.72
  2. gonzo
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    gonzo Senior Member

    Have you considered an existing design on that size, like the 420?
  3. Senrab
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    Senrab Junior Member

    Hi Gonzo
    Yes, I've considered an International 420 and searched the internet but it seems not possible to get plans for a self build.
  4. El_Guero

    El_Guero Previous Member

  5. DCockey
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    DCockey Senior Member

    Have a look at Dudley Dix's Paper Jet. It's a "performance" dinghy the size you are interested which is designed for "amateur" construction. Plans include three rigs of varying size, complexity and performance.
  6. Senrab
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    Senrab Junior Member

    Thanks for your suggestion. I've looked at it but to be honest I'm looking for a reply to my questions rather than just suggestions to build something else :)
  7. dskira

    dskira Previous Member

    I use to sail the 505. The waterline beam was narrow, and the boat not design in anyway for leisurely sailing. Little initial stability, trapeze as the wind piping up. But fast and very exhilarating.
    Bad choice in any cir constance for sailing with wife. It was and is a racing dinghy.
    When capsized, it was not easy to right it up. needed some strength, experience, and very good personal shape.
    In my opinion, choose an other design.
  8. Mikko Brummer
    Joined: May 2006
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    Mikko Brummer Senior Member

    I find Senrab's project interesting, even if I do concur to some extents with Dskira about comments on the original 505. The original 505 has little initial stability, compared to the FD or 470, for instance. Your non uniform scaling should help, though. BWL is missing from your table, important for stability.

    Where did you get the DSPL of 257 kg. As I recall, the 505 weighs 130 kg, plus a crew of 80+90 kg, or a total of 300 kg? Plus the sail on top of that? How much do you and your wife weigh? I assume you plan to keep the trapeze for the crew?

    I would not worry much about the lead, some 505s have aspinnaker tunnel with the jib stay 30 cm more aft than some that don't have a tunnel, with no effect on balance. But I dont know if heightening the foretriangle is a good idea, the rig depowering depends on the length of the mast above hounds.

    Could you post a sailplan and some renderings of th hull?
  9. Senrab
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    Senrab Junior Member

    Hi Mikko
    Thanks for your interest and comments. I attach a set of renders and a table of hydrostatics. I think you may be right about the displacement. I'm 80 Kg and my wife is 50Kg so perhaps it may max out at about 300Kg. As you can see from the table of hydrostatics, the BWL at 300Kg is about 1.2 metres.

    The lines are not very well faired, especially round the stern because I've had difficulty with adjusting the curves generated by Freeship after import of the stations' co-ordinates. However I dont think this has a material influence on the hydrostatics since the distortions are above the waterline.

    I'll build in the ability for a trapeze but probably not use it to begin with. Initially we wouldn't use a spinnaker and would sail in light winds. The righting issue raised by Dskira worries me a little because, as I mentioned there wont be any rescue craft so if/when we capsize we will need to right the craft ourselves. I plan for wooden spars though so unlikely to invert. However I dont think this would be worse than righting a Hobie 16 with an aluminium mast - I had one of these some years ago and it wasnt too bad righting that, though I must say I had a heavier crew to help.

    Attached Files:

  10. Munter
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    Munter Amateur

    Maybe try to find some info on the old 3o3 class which was a kind of scaled five oh. Not much info on the web about them though.
  11. Petros
    Joined: Oct 2007
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    Petros Senior Member

    I like your idea, I would go ahead with such a plan. looks well thought out and it would be a fun experiment, might get something going in china if you can use all locally sourced low cost materials.

    There are several things I would change however, One thing that would mitigate the issues with a low time crew, and for casual sailing comfort, is to make the hull more flat in the center. A flatter bottom would not be as tippy and uncomfortable for inexperienced crew and guests, and it will come up on plane in lighter wind too. easier to car top or trailer as well.

    I would also make it a bit larger at perhaps 4.3 m, it will give you a bit more room and is more typical for size for a sailing dingy in the USA. I would not change anything else.

    You can balance the rig by adding or removing rake in the mast. Or perhaps build the hull so you have several mast steps to choose from when you assemble the boat.

    good luck.
  12. CT 249
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    CT 249 Senior Member

    As Petros says, a slight flattening along the keel line will improve performance, all else being equal. One of the advances that the brilliant 505 design made was to go flatter along the keel line, which is a trend followed by most modern boats (apart from the Bethwaite designs where it's less important due to their dimensions). The 505 appears to have been slightly flatter than the "prototype" 505, the Coronet, as far as I can see from some small plans of the two. Your design appears to be more Veed than the 505, although that's just an impression from looking at plans on screen.

    The 303 that Munter mentioned was a two-teenager baby 505 developed from the "fofMoth", which was a late '50s/early '60s design by Westell for the Moth class, using the 505 concept. I have a small photocopy of the lines somewhere.

  13. Senrab
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    Senrab Junior Member

    Hi Petros and CT249
    Thanks for your insightful comments. I reduced the length to 420 because 505 seemed pretty long for us to man-handle while launching, but I must say, with some reluctance. I think I'll follow your advice and make it a bit longer.

    I also think the boat looks more V'd than the 505. I think this is because of the scaling ratios that I've used (0.83 for length and 0.91 for both beam and height).

    I imagine the Coronet and the 505 were designed with open sea racing in mind so a more deeply V'd hull would probably be more appropriate. We will be sailing on lakes and I'd like it not to be too tippy so I think the idea of flattening the hull slightly is an excellent idea.
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