Fixed keel mod mirror dinghy

Discussion in 'Boat Design' started by Andy Turner, Aug 19, 2014.

  1. Andy Turner
    Joined: Aug 2014
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    Andy Turner Mr

    I love mirror dinghies.
    I have had a few and i know its odd but i just have a thing for them.
    I have come across a sound hull on 'tinternet and had an idea.

    I am a rubbish dinghy sailor and i dont like capsizing and want a small, safe, self-righting, single handed, easy launch, slow dinghy which i can use anywhere with reasonable confidence that despite my slovenly and innept abilities it wont capsize and will self right if it does get knocked over.

    The slot in a mirror is pretty strong and could easily be strengthened.
    I propose making a fixed, longer or deeper keel with some balast, which i could experiment with until happy.
    I know it ruins the whole "one design" thing but the question is...

    Will it work?
    Would a deeper or longer keel be a better option? Considerations are draght and trailering as well as steering.....
    Will it stop the boat going over?
    Will it just break the boat?
    Is it a stupid idea?
     
  2. Eric Sponberg
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    Eric Sponberg Senior Member

    Hi Andy, Yes, it will likely work. You have to make sure that the side seats are truly watertight so that they provide sufficient buoyancy that the dinghy will not flood when over 90+°. (I am not experienced with the Mirror dinghy, but I presume it is designed and built with flotation in mind.)

    A deeper keel is the better option--you want weight down low so that the bulb at the bottom end provides the most righting moment, both sailing and when capsized. It will not prevent capsize--there is always a bigger wave that will capsize any boat. But the deeper the keel and ballast weight, the harder it will be to capsize the boat, and the faster it will re-right. Also, by going deeper rather than longer, you will keep the sailing balance more or less as before. If you go really deep, you will notice different behavior in the boat--you'll feel the lifting effects of the keel more, you'll be sailing the keel instead of sailing the hull. There will be a point of diminishing returns--you can make it too deep where the boat will be difficult to handle, as well as haul around on a trailer.

    If you build it strong enough, it should not break. If it does break, build it stronger.

    No, it is not a stupid idea. You'll probably learn a lot from the exercise.

    I hope that helps.

    Eric
     
  3. philSweet
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    philSweet Senior Member

    You can certainly change the feel of the boat, but actually making it capsize proof is nearly impossible in such a small craft. It just isn't practical to try counter the bulk of the crew with ballast on such a small boat. The ability to self rescue should not be traded off in order to make the boat feel stiffer, and I think this would be the inevitable result. I owned a similar but larger (14') boat which was converted to centerboard and kickup rudder, and I cruised it all over the place. When I outgrew it, I build a 16' version. By the sound of things, you basically need a bigger boat. A 15 -16 foot boat with a 60" beam that weighs about 375 pounds and has a mast height 22-23' off the water that is proportioned like a mirror is a very seaworthy craft.

    Have you looked for a miracle, by any chance? http://www.miracledinghy.org/photo_gallery.html It's a bit more boat.

    <edit> I cross posted with Eric. I got distracted by a couple phone calls and it took a while to hit the post button. These boats are quite weight sensitive and they have very little freeboard considering their basic open design and the bay chop they sail in. Specifically, the pram bow needs to be kept at its design waterline because it is quite blunt spoon shape up front. That's why it is tough to add weight to a board. A centerboard could work, because the weight would be aft going down wind. You would need to keep the rig quite low if you added weight forward, perhaps the original gunter rig would be the best option if you decided to go forward with the mods. If you do add weight, add some additional keel area as well. Basically, these things are pushing their weight limit most of their time already. If they bog their bows, loose speed and loose dynamic lift, then their freeboard goes away and you can't develop any righting moment to power the boat with. The normal sailing mode is with just a little water slipping in over the side and out a bailer. There is already a fair bit of immersed transom, adding weight increases this and decreases the power available to push it. These boats are already about as big feeling as you can get for their length. If you want a bigger feel, try a bigger boat.
     
  4. Skyak
    Joined: Jul 2012
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    Skyak Senior Member

    Andy.
    the idea is far from stupid. It could result in an interesting useful craft. The question is actually more about what needs to be done and what will the result be. The trailering issue could be resolved with a lifting keel and bulb. Then comes the question of how much lead? All that you add takes away from the carrying capacity and handling of this small boat. Being a dingy, it is designed to sail upright, so there is not much useful performance righting to be gained from the bulb. The righting in a capsize is the weight times the depth (presuming sufficient seat flotation)-you definitely want deeper. Along this same line of thinking I would suggest you consider mast flotation. The bulb righting maxes out at 90 degrees so you want to make sure you don't go far past. With these two changes you could add say 40lb X 3ft + 5lb X 14ft = 190 lb ft -should keep you from turtling and the 120ft lb of the bulb should bring you back upright.
     
  5. Clarkey
    Joined: Aug 2010
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    Clarkey Senior Member

    I am sure it would be an interesting experiment, the Mirror is already a great seaboat. From my memories of owning and repairing one I would suggest looking into some serious floor reinforcement - they can flex a fair bit there, especially to the rear of the daggerboard case.

    Otherwise, go for it - the boat has plenty of sealed bouyancy
     
  6. Andy Turner
    Joined: Aug 2014
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    Andy Turner Mr

    Yeah i think i will have a go at this.

    I will strengthn the floor and box.
    I will make a Centre board out of 19mm steel box section with a balast at the bottom and plywood filling.

    I will experiment with different lengths until i find what offers the best feel and stuff.

    I will also play around with the rig options, a little balast and sailing on the main alone with the mast in the forward step should generate a lot less heeling motion and although it will of course be a lot slower that doesnt bother me.

    I know theres the 'get a bigger boat' option but i already have a big yacht.

    What i'm after here is very small boat which i can sail without having to hike out and which is slow and stable and doesnt tip me in the water like my national solo hasa tendency to..... ;)


    Something i can sail sat in the bottom of the boat and get from here to there.


    First thing to do then is buy the hull, fix any little problems with it, make it lovely and buyoant and get the rig set up and do a few trials with the standard centreboard.

    Will keep ya'll posted.....
     
  7. Andy Turner
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    Andy Turner Mr


    Mast flotation! Yes great idea.
    Forgive my being a bit thick but i dont quite understand the....

    40lb X 3ft + 5lb X 14ft = 190 lb ft

    Is that 40lb of weight at the end of a 3ft long board?
    Or
    5lb of weight at the end of a 14ft board.
    I dont like the idea of 14foot draught on a 10ft dinght lmao, so i must be missunderstanding this......
    :confused:
     
  8. Richard Woods
    Joined: Jun 2006
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    Richard Woods Woods Designs

    Have you seen this??

    https://www.youtube.com/watch?v=o6T_l8aKP9M&list=UUhKTQtbKN5BaXFTg2BjcbqA

    more here

    http://sailingcatamarans.com/index.php/designs/27-trimarans-under-25/428-tryst-trimaran

    I think the 40 x 3 refers to the gain in stability from a 40lb bulb on a 3ft keel, and the 5 x 14 the buoyancy from a masthead float

    Personally, having also owned/sailed a Mirror I would worry about the floor strength round the case. And the weight and awkwardness of a heavy board/keel when beaching and launching (presumably singlehanded)

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  9. SukiSolo
    Joined: Dec 2012
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    SukiSolo Senior Member

    Be a bit careful with this project, but it is doable with a bit of care. The Mirror has a 5mm 4 ply (gaboon) floor, at least on the older ones. I assume yours is not new? in which case it will be 5 ply. The floor batten(s) need attention, if you only have one, then add the second as per the newer plans. I recently added 7/8th length ones to stiffen an old one which was holed partly because of this. BTW the original Mirror kits from Bell have very good quality ply, the condition of the hulls is more down to how they were originally put together and how well cared for after. I know of a perfectly good 5 thousand number boat from the 60s' still being used regularly.

    The box is very weak (no hog or keel) so substantial extra bracing, both fore and aft and sideways will be needed. As in small bedlogs and probably a forward knee or brace across the top of the fore part of the box to the side tanks? Probably best to go to the fore tank/cubby bulkhead with reinforcing as this is stiff vertically and athwartships.

    Biggest issue is widening the box (45-50mm?) and replacing it to get enough lead to the bottom. Somewhere between 30 and 45Kg lead low down should do the trick for the reefed sail version. It will depend on whether you want to move sides or sit centrally like an Access (Hansa) type craft. The dagger board will need to be about 100mm longer as in depth, to get enough leverage too.

    As for the mast, I'd leave it alone. It is already floating being solid spruce, hardly needs further buoyancy.

    The hull with its four integral buoyancy tanks will easily float that lead and two people, so nothing to worry about there.

    TBH, the better solution is a sit in keel boat like a 2.4 Meter. After all even a Mirror can plane....;) though whether it could be a mini Sportboat, I doubt.


    Only if you don't sail them right....
     
  10. Petros
    Joined: Oct 2007
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    Petros Senior Member

    welcome to the foum Andy,

    sounds like an interesting idea. You might start out by using a wood dagger board, foil shaped if you want to make the effort, and attach a couple of lead bars at the tip, bolt through. You feed the dagger board up through the bottom and than attach a block, lanyard or pin through the top end so you do not loose it. Than go out and try it.

    If it works out it would not take much to shape the lead bars into two half "torpedo" shapes. even putting 20 pounds down at the end of a 3 or 4' long dagger board will make the Mirror feel very different. when landing it is light enough to lift the board up so the weight is right against the bottom of the boat. this will still allow a beach landing, and make trailoring, or handling much easier.

    making a wood dagger board would be much eaiser than trying to weld up an all metal one. It could be done for only a few dollars worth of materials and a few hours of cutting and shaping.

    Than you also have not altered the dingy so much that is can be used in a club or class, you just go back to the standard daggar board.

    The only real hazard here is that you make a pretty fool proof means to prevent the weighted dagger board from falling out of the slot, a good heavy lanyard or even a large pin or a screw-in-place handle or some such means so it does not accidentally come off and you loose your dagger board on the bottom of the lake. But that can be solved by cleaver design, just keep it simple, but strong and reliable.

    Good luck. Post pictures.
     
  11. Andy Turner
    Joined: Aug 2014
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    Andy Turner Mr

    Yes, thats exactly what i have in mind.

    Strengthen the box substantially.
    Make a replacement daggeboard which goes in from the bottom.
    I was thinking of using some steel box section as a frame with some lead at the bottom and filled out with ply but maybe thats a bit of overkill.

    Loads and load of great ideas from you guys! Thanks so much. Will take pics as i go along yes :)
     
  12. Andy Turner
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    Andy Turner Mr


    Love the look of the Hansa/Access dinghies, never seen them before, thats the kind of thing yes. I dont mind moving side to side and using the tilker but i dont want to feel like i have to hike out on the toe straps to stay upright.

    As for not sailing the National Solo right....... Dead right! I am a crap dinghy sailer!
     
  13. philSweet
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    philSweet Senior Member

    I'm still not really a fan of doing this, but if I did, I'd do it as follows -

    1. Jury rig a weighted daggerboard to fit the existing case. Reinforce the case with a thwart as described by others above.

    2. Retain the jib. I would add a 10" prod and set the jib stay forward on it. Convert to a simple jib boom and increase jib area somewhat. Retain existing halyard arrangement. This gets a bit more bow lift and makes the rig self tacking.

    3. Increase the mast rake if possible.

    4. Set up a roller furling boom with end sheeting and run the boom on a 2' track on the mast. The first reef is to raise the boom on the mast as you furl. This has a tremendous effect on the feel of the boat wrt gust response, and would probably do more good by itself than the board mod.

    5. Once you have the balance sorted out, build a centerboard. I wouldn't go any deeper than the existing daggerboard, but I would add about 15% more area. I'd be tempted to add area to the rudder as well. These mods would really tame the feel of the boat, without slowing it down much in fine conditions. It just makes the boat less twitchy and easier to handle in fluky conditions.
     
  14. Richard Woods
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    Richard Woods Woods Designs

    Don't forget that a ballasted keel is only effective at high heel angles. Below that (ie normal dinghy sailing angles) you rely on form stability - ie hull shape

    And a heavy keel is not going to help much in fast knockdown situations, as the boats momentum takes over. I'm thinking of sloppy gybes for example, often the time most people capsize. Even J boats with ballast keels will capsize then

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     

  15. Andy Turner
    Joined: Aug 2014
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    Andy Turner Mr

    Just a thought......
    Small
    Cheap
    Easy launch and recover
    Self righting
    Safe
    Slow
    Trailerable
    Dinghy with balasted keel/daggerboard

    Doesnt a Lysander more or less fit the bill?
     
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