Mast position for small sailing dinghy

Discussion in 'Stability' started by jantheron, Aug 12, 2010.

  1. jantheron
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    jantheron Junior Member

    Is there a general rule of thumb, or a good reference I can use to design the mast location on a small sailing dinghy I am starting to design? I want a planing hull, about 10ft LOA, wooden construction (no hard chines).

    Jan Theron
    www.kanoefabrik.com
     
  2. Petros
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    Petros Senior Member

    The simple and reasonably reliable way it is usually done for a dinghy is to take an accurate drawing of the profile of your boat with sail plan and center board shown. You want the centroid of the area of the centerboard to match the centroid of the area of the sail. That is how you place the mast relative to the centerboard. And than you fine tune it in the water by adjusting the forward or aft rake of the mast so you have minimal side pressure on the tiller when on a reach. Usually the mast angle change is so small that most will not notice it is not dead vertical. On an unusual sail design you might want to allow some kind of adjustable step to allow you to move the mast for and aft a few inches.

    The interaction is very complicated, but this gets you close enough to make it manageable, and it is fairly simple.

    Have fun, and good luck.
     
  3. alan white
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    alan white Senior Member

    Also allow the centerboard, if you have one, to cant forward and that will give you another way to balance the boat. Rudder position and area also can be played around with.
     
  4. jantheron
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    jantheron Junior Member

    Centerboard

    Thank you for your replies! And now the next question is.... how do I position the centerboard?

    ;-)

    kanoefabrik.com
     
  5. alan white
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    alan white Senior Member

    I thought that question was answered. You see, the mast and board positions are interrelated--- they are mutually dependant. If one changes, the other must also change. On the same hull, moving the mast forward requires the centerboard move forward as well. Not the same distance necessarily, but their relative positions must consider the center of effort of the sailplan and the center of lateral resistance of the boat's underbody.
    Making a cardboard cutout of the scaled down underbody is a good way to find the lateral center of the underbody including all appendages. All you do is see where to place a fulcrum under the cutout (a pencil will do) and that is the center of the lateral area. On a plan this vertical line is shown by an arrow pointing up at the waterline. You'll see the arrow on most study drawings.
    The center of the sail plan is also shown by an arrow pointing down at the waterline. The two arrow positions are not always lined up. Usually, the CE (center of the sailplan, or center of effort) is ahead of the CLR (center of lateral resistance).
    The difference can be as great as 15-17% of the waterline length or as little as 0-5%. Each hull and sailplan will be a little different. What matters is that the relationship between the two positions is correct for the factors involved. Actual mast position can be changed dramatically just so long as the relationship between the sails and the underbody remain the same.
     
  6. jantheron
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    jantheron Junior Member

    Thank you for your comprehensive reply. I think you misunderstood earlier how really really new I am at this. Your explanation lights up the evening sky in my head. ;-)

    Thanks again.

    Jan Theron
    www.kanoefabrik.com
     
  7. Wavewacker
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    Wavewacker Senior Member

    Alan, I had no idea! Didn't have a clue as to the relationship, now it makes sence...Thanks!
     
  8. jantheron
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    jantheron Junior Member

    Weatherhelm

    I recently read, after your guidance, that one of the reasons the CE is ahead of the CLR is to ensure a degree of weatherhelm, so that in case you capsize, for example, the boat heads into the wind rather than sail away from you. ;-) Makes a lot of sense in a perverse kind of way.

    Jan
     
  9. PAR
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    PAR Yacht Designer/Builder

    The answers provided will get you a rough location for the CE in relationship to the CLP. A more refined location would be to position the CE directly over the leading edge of the center/dagger/lee board, if it's fairly high aspect and the center of the leading edge if it's a "slice 'o pie" type of board. This relieves the rudder, which is a common problem in small craft with an inattentive skipper.
     
  10. HELLICONIA54
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    HELLICONIA54 HELLICONIA54

    Boat heads into the wind,Then stalls(stops.)You swim to your boat and continue on.lol better than watching it sail away from you...lol I'm pretty new to this as well.good luck.
     
  11. JRD
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    JRD Senior Member

    Don't worry about location of CE or CLR when your boat capsizes. It doesn't tend make all that much difference when its upside down and you are in the water. Its a good time to start thinking about how much bouancy your design has.....

    Jan, I think what you have read is a bit misleading, the further the CE is ahead of the CLR the more lee helm there will be. i.e. the boat wants to bear away rather than luffing up, which is the opposite to what you have quoted.

    Alan has correctly defined the theory for relating the position of CE and CLR. Respectfully, I would like to expand on this a small amount. The % by which the CE is ahead of the CLR is called "Lead". In a simple world the CE would be above or very slightly aft of the CLR, this would give balance with a small amount of weather helm, giving you feel and the safety of letting the boat round up if you let go of the tiller.
    In the real world, a hull moving through the water making waves and heeling over, has a tendancy to steer its self toward the wind due to the pressure build up on the lee side and the underwater shape is no longer symetrical. As Alan quoted this is hugely affected by the hull shape, fullness of the bow and trim. Basically the CLR is dynamic in location. It is close to, but not tied precisely to the geometric centre of lateral area. Think of the lead as a correction factor that is applied to the geometric centres of area of the yacht and sails. The keel yacht experts here (I'm not one of them) get good at predicting this based on observation, experince and an understanding of the movements of hulls through the water.

    I am personally interested in how this theory relates to a dinghy. My own understanding for planing dinghies is that the affect of lead is minimal (or even negative) as you are aiming to sail the boat as flat as possible.

    My own experience with a light planing dinghy is that I modified a crewed boat with jib to a single hander with mainsail only. I read all the theory I could find and duly tried to apply a new CB location based on what is actually keelboat theory (There is very little published about centreboard yacht design, I have found). The initial location had the CE was toward the leading edge of the board and it just never felt right. After a couple of years perserverance and adjusting the mast rake, I folded and cut the case out of the boat and relocated it 200mm (8") forward. This made a big improvement and it now seems about right in all the conditions I have sailed it in.

    Interestingly my original location was where Paul has suggested above the leading edge. It is now directly above the trailing edge of the board. I think this just serves the point that there are different designs built for different styles of sailing. I am interested for any other comments about this for other designs of racing/planing dinghy.

    Cheers
    Jeff

    P.S. Jan, how is your design progressing? I'm not to sure that a 10ft round bilge sailboat with an adult aboard is going to get into planing mode very often, have you progressed your thinking on this?
     
  12. jantheron
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    jantheron Junior Member

    Jeff

    Thank you for your reply. Your comments hit the mark. I have in the meantime converted one of my flatbottom canoes (about 14ft LOA, 30" beam) to a sailing canoe by adding a mast step and making a rudder and some leeboards that will be attached to the gunwales with an adjustable / removable lee bracket. I am in the process of making a hollow mast out of Ash with bird's mouth joints, very much like my hollow canoe paddles. To keep the sail CE fairly low I made the mast only 12ft long and will stretch the boom aft by about 8ft. Once it is going I will be able to play around with the leeboard position to get a better feel for the CLR effect. I think that is the way I will go initially, and once I have some experience I will shape my intended dinghy hull.

    As far as the slack bilges are concerned, you are correct of course that a round bilge will struggle to get onto a plane, but rather than having a very sharp chine, I'm going to go for a hard bilge; something I can strip-plank with narrow planks. The aft sections will be pretty flat (like FIMA, the modern rendition of a Sonder-class yacht modeled after Herreshoff's Bibelot) and the length will also probably stretch well beyond 10 feet. I still want it to be a fast single-hander though, and I'm still in two minds whether to have stays on the mast so I can also add a trapeze.

    Jan
    www.kanoefabrik.com
     
  13. JRD
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    JRD Senior Member

    Hi Jan,

    I have never seen a Sonderclass before and had a look at some webbsites. They are fabulous looking classic yachts, in saying this they have remarkably modern looking lines for the 20's, perhaps with the exception of the long overhanging bow.

    Have you looked at the IC - International Canoe class? These have been around for quite a few decades and still retain some classic appeal. They are damn quick if your balance is good, using a long sliding seat instead of trapeze. Ive never seen one here in New Zealand, but they seem to be actively sailed throughout Europe, USA and Australia. From what I can see, many of the class members are actively designing and building their own boats and rigs within the box rule constraints and there is quite open collaboration about the design and innovations.

    :) For me, give the trapeze a go, its great fun and and can give you a very quick boat when you get it right. On the off chance you havent sailed a trapeze boat before, especially helming from the wire. I suggest you hitch a ride on some one elses boat first, even if its crewing for some one. Steering from the wire is not for everyone, and once you have built a rig with enough sail area for this, you are stuck with too much power for a hiking boat in all but the lightest winds. There are quite a few things to consider in your design if you are going to sail if from the wire. If you are interested in trapeze single handers, have a look at Contenders, Farr 3.7s, MustoSkiff and Swift-Solo.

    Jeff
     
  14. jantheron
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    jantheron Junior Member

    Jeff

    Yes, they are beautiful, aren't they? FIMA has been clocked at 18 knots... definitely in planing mode. I have had a look at the IC10s, but that is a bit extreme and the kind of thing you'd expect the Aussies to embrace (no negative sentiment intended). I am looking for something a bit more traditional, also not like the extreme skiffs that seem to proliferate down under. Something more like an updated, cool-looking Finn with a trapeze option. I like the Farr 3.7s and the Swift Solo, but the Solo's lines are not pleasing to my eye and the Farr's chine is a bit radical for me, although I do like its fuller bow sections. I really like the Fireballs and would rather use more design cues from it than any of the other boats (the squared-off bow is a bit offensive to my eye though). Check out some nice hull photos here:

    http://www.fireball-international.ca/docs/archives/BuilderPhotos/collinh.html

    My main criterion is that is has to be built out of wood (Ashcroft technique or strip-planked rather than stitch-and-glue which I find a bit too inexact - although I am open to be converted), be traditional-looking yet fast with a planing hull - and if the chop gets a bit rough, that it can also handle that with a reefed sail. We can get some rough conditions out here (although I build my canoes in the desert, I am only an hour or so away from the coast) and I'd like the boat to be able to handle rough water.

    I have some plans for the Finn; maybe I'll loft those full-scale and use that as my point of departure.

    Thank you for this discussion and your suggestions; it has really helped me to get more focused on what I want.

    Jan Theron
    www.kanoefabrik.com
     

  15. JRD
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    JRD Senior Member

    Hi Jan,

    I see where your'e coming from. A finn style boat with a trapeze would be very cool, finns are surprisingly quick for their size and weight. Have you seen the Devoti D1? You might not like the hull lines, but it has been developed by an Olympic champion in Finns, and there may just be some rigging ideas of interest if you go down the unstayed mast route.
    In NZ we have a 11' boat called a zephyr, cold moulded and very much a classic small boat, (The designer Des Townson built every one of them himself until recently I beleive, in the interests of maintaining a strict one design). Not a trapeze boat, but it has a stayed rig, and I would suggest the potential for a few design ideas for you.
    http://www.zephyr.org.nz/

    There is an International 14 being built out of ply using the Ashcroft method below the chine. Its been a long term build but he is making an amazing job of it.
    http://forum.international14.org/viewtopic.php?f=2&t=865&start=0

    If you are going to make it a Trapeze boat, think carefully about the sidedeck design. You need to have something to push out with at foot level. Alot of dinghys have nice curved wide side decks that are a long way from the floor - great for hiking out. When wiring in strong winds you just kind of jump out and your there, but in marginal wiring conditions, you are trying to move out gradually as the breeze comes up and you cant find anything to rest your feet on to push from, do a sketch and you'll see what I mean. In this case you might want to consider a false floor that is no more that 8-12" below the side deck and some 2" rails on the floor

    Good luck, perhaps you will show us the results some time soon.
    Jeff
     
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