Some dinghy design questions

Discussion in 'Sailboats' started by sawmaster, May 22, 2012.

  1. CT 249
    Joined: Dec 2004
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    CT 249 Senior Member

    Sawmaster, why not quickly calculate the CLR/CLE/lead etc of some well-known designs of similar proportions, using the rudder? Surely that would give you some idea.

    Modern dinghies have rudders that carry a large proportion of side force, as you can tell by having your rudder pintles break and seeing what happens! I wouldn't be surprised if it was impossible to sail a Laser or Tasar, (two of my boats) in a practical manner without a rudder, although they can be sailed very easily with the rudder down but at a fixed angle. Surely the speed at which these boats round up without a rudder indicates that the rudder is bearing a very large proportion of side load?
  2. sawmaster
    Joined: May 2010
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    sawmaster Senior Member

    To ct 249 -that sounds like a reasonable approach.--I will try to find some dinghys with similar proportions-here are the particulars:loa-4.11m max beam,(less wings)1.29m lwl 3.65m-draft(less appendages)4 in/.1014 m(sorry,I'm new to converting english to metric) hull wt including rig 59 kg- sail area 9.29 sq meters/100 sq ft.Which current designs do you know of that are similar enough that their parameters might be of use?--The current iteration has the center of the mast placed 3.75ft (sorry,I mean1143mm?)aft of the stem sta 0 --It carries a main of 7.43 sq m--jib is only 1.86 sq m.I have also experienced the sudden round up resulting from rudder loss and came to the same conclusion.However,Ive also experienced trying to bear off in a strong breeze and had the boat continue on in a straight line just dragging the rudder.-I actually had to through off quite a bit of mainsheet before the boat would bear off--possibly the result of including too much of the rudder in the clr calculations as mentioned in an earlier post.-As to posting a drawing I would love to do that to get some input and (hopefully CONSTRUCTIVE) criticism, however there are some issues with using this computer as it isnt mine--belongs to the library and I'm not very techno-literate.I may try to photograph some drawings w/my digital camera ,post them to my email and provide a link.Thanks again to everyone who has responded to this post.
  3. gggGuest
    Joined: Feb 2005
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    gggGuest ...

    It sounds as if you stalled out the rudder. That's pretty normal. If that happens the trick is to centreline the rudder again so the flow reattaches, and steer more gradually and progressively. But you should always be easing the mainsail when you bear away anyway - the sails will almost always be able to overpower the rudder. The only time that won't happen is if you are dragging a young barn door behind the boat instead of a rudder blade, which lacks elegance...
  4. HakimKlunker
    Joined: Aug 2009
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    HakimKlunker Andreas der Juengere

    Sailing dinghies usually respond very much on weight trim. And also the position of the centre board (dagger boards are a little less significant)
    When you want to bear off and the boat heels much, you will have the same problem. That goes of course with a perhaps stalling rudder (too hard).
    With your jib significantly smaller then the main, you have the unwanted tendency increased.

    My thought: Before you begin to modify the boat, you may first improve the test conditions. Some of your unwanted effects may come from incorrect handling? Not that I mean to criticise anyone's sailing skills, but here might be one source for errors.

    In your boat specs there are no c'board and rudder areas mentioned, neither the positions. Perhaps some photo may be of help.
    If a boat of 4.20m length is designed like a 'Moth', the data of the 420 will not help you much :)
  5. philSweet
    Joined: May 2008
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    philSweet Senior Member

    Klunker, I don't quite follow this bit.

    Because of the higher CoE of a main?

  6. HakimKlunker
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    HakimKlunker Andreas der Juengere

    It is more about handling: When you bear off, you let out the sails.
    Looking from aft, the CoE's move sideways; they are offset from the centre line. Sail's lift forward and the lever arm (distance c/l to projected CoE) produce a part force to windward.
    When sailing you would now keep the jib in a little longer so that it counteracts. But with a small area only, the effect is less.

    Cheers: Hakim
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