Mast/centreboar/transom equation ?

Discussion in 'Boat Design' started by Burkie, May 29, 2010.

  1. Burkie
    Joined: May 2010
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    Burkie Junior Member

    Hi All,

    I'm trying to tweak a 16ft sailing dinghy,she has a lot of weather helm and I was told that the mast was slightly too far aftand too near the centreboard ,but obviously the person couldn't tell me where exactly it should be placed.
    I would appreciate any help in its proper placement.

    Regards

    Eugene
     
  2. Grant Nelson
    Joined: Feb 2005
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    Grant Nelson Senior Member

    What is basically happening is that the REAL center of the force created by your sails, is behind REAL center of lateral resistance created by your boat in the water. You need to either move the center of force of the sails forward or the center of resistance back.

    To figure out how much, you can take two approaches:
    1. experiment with the sail rigging and board and rudder positions, or
    2. calcuate where the centers should be and make a design decision what to move where.

    I advise you to try approach 1, get an idea if you can solve the problem without changing the fundamental construction of the boat, or failing in fixing it, get a practical idea of how bad the problem is, which will help with approach 2.

    Point 1. has the advantage of being practical (up to a degree), fun if you like experimeting, and does not require you to have an accurate profile design of your boat.

    Point 2. in the case of dinghies, has few advantages as a first move. I did a parametric study of several popular designs (470, 505, lightening, flying scot, Laser Stratos, and a few more I do not remember) and found that the actual measure of the distance between the center of sail effort and of the center of lateral resistance ranged all over the place, so its not likely that, based on the design drawing and calculating the centers, starting with calcuations will help up front. You can still do this yourself, at the same time as approach 1, to see if its way off the typical range, and start considering your options, or have some fun making a drawing for each of your changes in step 1, to keep a record of where the theoretical centers where.

    For the forum to help further with either, a profile picture, with sails, sailing and on a trailer if possible, and a profile design, showing accurately the sails, hull, centerboard (and how its location chages when you start raising it) and rudder is needed. If you can also show your deck layout, we might be able to offer some ideas on changing that. Based on this we can suggest ways to modify the rig, etc. in simple ways, so you can see if you can correct the problem by, for example, leaning the mast further forward or sailing with the board (if its a pivot type) a bit up, or trimming your sails differently.

    To start tackling approach 2, you can take an accurate drawing of a profile of the boat, and meaure the area of the jib (only the part before the mast), the main, and the part of the boat (hull, rudder and centerboard) below the waterline, and then find the center of area of all three. You then need find the combined center of area of the two sails. What designers then use is the amount the center of the area of the sails is forward of the center of the area of the hull under the waterline, and compare that to the values of existing boats.

    (this last aspect is confusing, because these areas are sometimes called the Center of Lateral Resistance (CLR)and the Center of Effort (CE) but in reality they are not. This whole method is based on using the same approximating method through out the industry so that the resulting measures can be compared to HELP make a decision on design. It works well for larger sailing boats, but as I said, for smaller boats is is less predictable)

    A published range that the sail area ahead of the submerged hull area should be is 13- 17%. In my quick study, the amounts where significanlty less (-1 to 6%), but I also had to depend on profiles I found on the internet as images, and these are often distorted. Still it suggests being cautious with making significant changes based only on using approach 2.

    Check out this book if you can, pages around 297 will tell you a lot of what I just did. Its well worth buying if you interested in understanding how boats work. The Nature of Boats, Sails
     
  3. messabout
    Joined: Jan 2006
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    messabout Senior Member

    Grant has covered the technical part very well. The practical part goes something like this.......The sails themselves can cause excess weather helm. If they are old and a bit bagged out you will probably have increasingly annoying weather helm. Be advised that some degree of WH is much better than any degree of lee helm. Another factor is boom length. If you have something like a Cape Cod Cat boat then you have a long boom. When running free or broad reaching, the sails center of effort is way forward and waaay outboard. The longer the boom the worse it gets. Nothing much you can do about that kind of design quirk.

    If you are sloop rigged, there can be some jib sheet angle problems. Or if you are not a veteran sailor, you may need to modify the way you sheet the jib. As for the main, you may not be vanging it sufficiently. If the clew end of the boom is waving up and down you are probably going to feel the result in the tiller. The leech of the main should not be but very slightly curvy. If it makes much of a curve to leeward, then you'll feel that too.

    There is another recent thread here that discussed the same issue at some length. Use the search function to find it.
     
  4. wet feet
    Joined: Nov 2004
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    wet feet Senior Member

    Its usually much easier to adjust the rake of the mast than the position and this would be my first line of experimentation.Try moving the masthead forward and see if the problem goes away.The other choice is to find a good dinghy sailor and persuade him to take the boat for a sail and act on his recommendations,some beer might be involved in the process.
     
  5. Burkie
    Joined: May 2010
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    Location: Ireland

    Burkie Junior Member

    Thanks guys ,VERY comprehensive replies,I'll try the rake first ,if that doesn't work I can alter the mast position forward a few inches, I also have a friend who used sail professionally in 29ft Dragons who would be more than willing to help.
    Grant ,I'm going to print off your post as it's so detailed and informative I'd need a little time to take it all in properly.

    Thanks all

    Regards

    Eugene
     
  6. Grant Nelson
    Joined: Feb 2005
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    Grant Nelson Senior Member

    Glad to help. I edited my first post to take out most of the spelling and grammer errors that probably would have left you a bit confused... :D
     

  7. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    The crew placement affects the helm on a dinghy too. Place yourself further aft to help reduce it. That is if it doesn't make the stern drag.
     
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