Cadet sail area?

Discussion in 'Sailboats' started by Hobitz, Mar 18, 2015.

  1. Hobitz
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    Hobitz Junior Member

    Hi there!

    Many years ago I with my father built a small plywood dingy based on modified Cadet class. 3.6 meters loa (~12ft). We completed the hull, but did not complete the mast and sails, nor did have chance to put it in water.
    Recently have been thinking of finishing it.

    If I remember correctly, mast height was supposed to be around 5.2 meters. Mainsail 3.9 sq.m. and jib about 1.4 square meters.

    Originally this boat was meant for two teenagers. Would it be reasonable to get a bigger sail area/highter mast, in order for adults to sail it and have some fun?

    We have a chance of mostly sailing it in lake or sea shoreside in light wind.

    What sail area and layout would you suggest? Can I take the original mainsail/gib ratio and scale it up, or would i need more specific layout?

    Possibly I could modify centreboard, if that would make difference?
     
  2. gggGuest
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    gggGuest ...

    Dinghies are really very tolerant of different sailplans, you can just rake the mast back more or less to suit, although ideally the jib shape is adapted to match the final position. This is even more the case if you have a pivoting centreboard.

    The Firefly dinghy is a good example of a moderate 12 ft boat that sails nicely with two adults, I think if you copied its sailplan you wouldn't go too far wrong. The class rules are here: http://www.rya.org.uk/sitecollectio...Documents/Class Rules/Class Rules Firefly.pdf and give you enough dimensions of the sails to work from. Use the 'racing' sail plan. If your mast position and deck layout is very different you'd probably want to juggle a bit with the jib shape, but if you can draw your boat out on top of the drawings in those rules you ought to be able to work out something that's close enough to work reasonably.
     
  3. WhiteDwarf
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    WhiteDwarf White Dwarf

    Is this the Jack Holt design?

    There are a number of designs called Cadet. Are you talking about Jack Holt's design?

    The 1947/8 Yachting Year provides the following...

    Mainsail - 42 sq ft, luff 13 ft 6 ins, foot 6'2"
    Jib - 13.5 sq ft, luff 9', foot 3'
    Spinnaker is shown, but no dimensions.

    The position of the centreboard will be critical to balancing the rig. Generally, it is easier to move the rig, if that can be achieved.

    Hope that is helpful.
     
  4. Hobitz
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    Hobitz Junior Member

    Yes, I believe it is the same as shown here: http://www.cadetclass.org/the-cadet
    Only difference, mine has a pivoting centreboard and pointy nose instead of flattened/cut as original. Because of that it is a bit longer (3.6m, not 3.2m). Hull crossection and centreboard shape looks exactly the same.
     
  5. SukiSolo
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    SukiSolo Senior Member

    Modifying the daggerboard will make a difference, mainly make it longer and not a full round at the tip. The Cadet as standard makes a fair bit of leeway, I'd hazard a guess at about at least 150mm deeper and squarer. To give some idea of really efficient 12' dinghies upwind, the Nat 12 uses a c/board 1200mm deep under the keel (it is limited by the rules, longer has been tried).
    My own experience has led me to believe that around 1 meter is enough with reasonable width of foil - area is the more important. However IF you have the standard Cadet thickness of daggerboard - 13mm, it will not be strong enough if lengthened too much. Note that solid timber laminated and glass sheathed boards are needed even for the youth sailors. The old ply ones just break!.
    It sounds like the boat is more limited by the centreboard case size, so put in the biggest possible. You can always move the pivot around, for balance!. Aim for max forward from transom to leading edge of board wth it vertical or even a couple of degrees forward.

    As gggGuest says a decent sail area for a 12' is around 8 sq m or a little more. Maybe an old RS200 rig (no kite) would do? The Cadet itself is a bit small for two adults, but OK with one adult and a child, where it can be a useful trainer.
     
  6. Hobitz
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    Hobitz Junior Member

    Thanks for suggestions. Currently I am looking in to using Firefly sails as they are readily available to order online. Mainsail is about 8 sq.m. and jib 2.4 sq.m. Fits quite well in existing layout.

    For the taller rotating centreboard will have to modify board box to accommodate it. As I am thinking of making modifications to the deck, it should be no problem to modify box also.

    I am a bit surprised about weaker plywood boards. Was thinking to build 18mm thick ply board covered with layer of epoxy fibreglass. Possibly layering it from three 6mm ply sheets? Solid timber board would be stronger?

    Could I reuse Firefly centreboard design, and move it little bit forward from where it originally is in Cadet? Firefly board is about 1100 mm deep under keel, 300mm wide.
     
  7. SukiSolo
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    SukiSolo Senior Member

    A solid laminated board is much stiffer than ply as all the grain works to give stiffness in the lateral direction. Ply has 50% the wrong way!. You need quarter sawn timber, I'd suggest Spruce, Sitka or clean European Spruce throughout if you intend 18mm thickness and even this will need glass coating. Personally if I had a free thickness choice, closer to 24 or 25mm sheathed and 28mm is unsheathed. These thicknesses are strong enough to right a boat when capsized and yet flex a little upwind correctly. I assume no body standing beyond halfway along the board btw. As for section and assuming it will not be a 'slab' something along the lines of one of the 64 Naca or even the 00 series 12 will give reasonable performance. The Firefly shape is not too bad, but probably would be better if a fraction more elliptical. You can use the rocker of your hull to give a leading edge 'curve' to the front of the board as it will maximise area but tuck in fine.

    I think you'll find total sail area is 8.36 sq meters with a reasonable area jib as you state. This can be handled by a child from age 8 or so upwards, a little depending on wind strength. Certainly a 12 year old can usually cope in a large wind range. I've quite enjoyed Fireflys' for team racing, just the kicker position and the kicking strap made it 'fun' when tacking.....;)
     
  8. gggGuest
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    gggGuest ...

    I should just make the biggest board that will fit in the existing box and see how it goes. The thing about centreboard area is that the worst case is not whilst sailing along, but when getting going again after a tack: once you are moving it will almost certainly be big enough, but if its too small you may experience trouble with it stalling coming out of a tack. You could always make a larger daggerboard as an interim measure if you do find you want a longer one.

    A plywood board will very likely break if you stand on the end when recovering from a capsize, but the solution to that is to be very disciplined about *not* standing on the end and only put your feet at the root of the board and lean back on the jib sheet to right the boat. That's how generations of sailors avoided breaking their plywood centreboards! It won't break sailing along. This forum has a fixation about plywood foils breaking which seems exaggerated to me after seeing and using them for many years. If you do use plywood though you mustn't shape it too much, because all the strength is in the outer layers of wood (which must be lengthwise grain), so you must leave it flat for at least 3/4 of the chord and only shape the edges, and of course that makes for a much less efficient foil. Thickness wise Enterprises, for example, have a maximum width of 23mm, but 18mm is common. If it works for them it will work for you.

    If you are going to go to the trouble of glass coating a board and making a proper hydrofoil shape then it makes sense to do the job thoroughly. This is a pretty good article. http://www.uk-cherub.org/doku.php/tech/foils. A cedar core will do nicely for you. Carbon fibre is probably way over the top for your boat, but if you use 50% more glass than is listed for carbon then it should be OK. As you're worried you may be short of area it will make sense to have the board as nice a shape as possible, because that will reduce the tendency to stall coming out of tacks (see above), so on the whole its probably worth making a full aerofoil board with a cedar core.
     
  9. SukiSolo
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    SukiSolo Senior Member

    If you want a nice relatively modern design with too small a board which exhibits the behaviour you describe (correctly) try the Laser Bahia......;)
    Ideal for training boats, one boat length sideways before it goes forward enough to stop going sideways more, tut tut!

    Actually I've only seen 3 ply boards break so far this year....

    AR Cedar is OK but on an 18mm thickness, my preference would be a composite timber core with spruce in the main load bearing part and cedar further out. Yes, you can (and I have) scarfed one into the other along with other light materials for really light stuff. Agreed if you can find 15 (or so) layer ply at 18mm it will be OK, just a bit more flexy, not to mention hard to find to buy, which is another good reason for the laminated option.
     
  10. Hobitz
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    Hobitz Junior Member

    Only 3 have been broken this year, that is not so bad, isn't it? :D

    As I will rebuild board box it should not be difficult to account for fatter board. So 24mm would be good choice? As for box length, I could plan it for maximum board length of 1200 mm. If larger board will be needed, I could build wider board, with same length. For construction, there does not seem to be much difference in difficulty to build either one, ply or solid wood board. Where I live, European spruce is very common, so I think will go with the solid board. Cedar would be more difficult to acquire, if not impossible. Carbon fibre would be overkill, will probably use common fibreglass cloth.
    How big of a factor is board weight? Suppose spruce board weights less than usually denser plywood. Would not want to make boat much more heavy, as it is quite small.

    Regarding sail plan. What effect on boat is achieved if mast is moved forward? It seems that I will have to move mast forward by about 100 to 200 mm, as Firefly has a longer mainsail foot. Sure I could tilt a mast back little bit?!

    I do not have much experience in sailing, more into building stuff, so this boat seems to be quite good choice to get into it. May be sailing alone or with friend. Does the main limitation would be weight and draught, if two adults is to be using it, or is it mostly small size and usability?
     
  11. SukiSolo
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    SukiSolo Senior Member

    You could do a lot of work searching for optimum foil shape, but to be honest any half decent section will do. You could stay at the 18mm with 2 layers of 270 gsm cloth around a light core and it would be fine. If you go up to 24, only one layer will be needed, the same weight cloth. Rudders can be a shade lighter say 200 gsm if you wish to sheath, not always necessary btw. However I would recommend a minimum dimension of the max chord thickness of 20 to 22mm for the rudder. These sort of dimensions will give many years service with good performance and pretty light weight.

    Pick some nice straight grained, light (400 Kg Cu Meter or lighter) knot free European Spruce that is Quarter sawn or close to it and it will be fine and lighter than most plywood. You can easily find guides on good foil building using the stacked laminate process, also the glue only needs to be a marine type, so it can be urea formadehyde, resourcinol, or other not necessarily epoxy, though that is fine of course just a little more expensive. Epoxy is the choice for sheathing though.

    The mast only needs to be moved to replicate the position of the balance against the board (CLR) of a Firefly. It should be raked aft, this alters the jib slot - a lot. I would guess at 250 to 300+mm of rake on a standard Firefly rig with current sails. Generally moving the rig forward results in lee helm which you don't want at all! The boat tries to bear away from the wind all the time and on all points. Moving the mast aft gives weather helm which makes the boat head up into the wind. You need a very small amount of this for optimum balance and controllability. Depends a bit on what you want the boat to do, but one option is a second mast position further forward which uses no jib , so the balance is maintained. You may well find another dinghy with two mast steps, and this is why.
     
  12. Hobitz
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    Hobitz Junior Member

    Thank you for a valuable advice.

    So it's a bad thing to move mast forward! Generally it is preferred to have CE behind CLR for boat to have a little bit of weather helm, and balance it with a moving centreboard and rudder rake?

    How about PVA D3 type glue (polyvinyl acetate waterproof glue). It has various names/brands around the world, but here it is simply known as PVA D3. Is it strong and reliable enough for wooden mast or centreboard construction?
     
  13. gggGuest
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    gggGuest ...

    People get over excited about weather helm and balance, especially here. In practice on a modern racing boat both the rudder and the centreboard are taking up lateral resistance, and all moving the mast back and forward does is to alter the balance point between them, whether it be 60/40 daggerboard rudder, 70/30 or what makes little difference. The more sophisticated racing classes rake the mast back and forward a phenomenal amount in different conditions without getting excited about balance.

    If there's side load on the rudder then the tiller will be tugging on the helms hand, but this need not be weather helm, even though its often mistaken for it. The key factor is whether, with the boat flat, the helm is on the centreline when sailing a straight course. If the helm is pulled up to windward to maintain a straight course then you have weather helm, and a lot of unnecessary drag. If the tiller is on the centreline then all is well.

    Of course if the boat isn't bolt upright then the typical vaguely wedged shaped hull will generate weather helm, and need a lot of tiller input to keep it in a straight line, but this is very little to do with the rig position too.

    Its really not worth getting terribly excited about centreboard position etc on a dinghy, especially if you have a pivoting board. You have so many options for keeping things within the very wide area that will work satisfactorily that its not a key factor.
     
  14. Hobitz
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    Hobitz Junior Member

    For anyone interested! Finally got hold of some relics. It is original drawings of boat in question. They are from 1964 Russian sailing magazine. As it appears, it is not exactly modified Cadet, it is something different, although some features look similar. Not sure if it is soviet era copy&paste design, or something original. I can't find any info online of this boat being built or sailing, so its probably not very popular.
     

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  15. gggGuest
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    gggGuest ...

    Man, that looks like a low boom! I think I'd want to change that. Must have been designed by a Finn sailor!

    It doesn't immediately resemble any UK class I'm familiar with, so I wouldn't be at all surprised if it were 100% a native design.

    My second thought is that the board looks small and a long way aft by contemporary standards. I certainly would't want to be moving the mast forward.

    The hull shape doesn't look a mile away from the state of the art at the time. The low bow chine is unusual, but not unique and certainly not "wrong". It might be a bit noisy in a short chop but it would probably also be quite effective in flat water and lighter winds.

    I should just get her on the water and go sailing, and a Firefly rig or similar should do just fine. If you want to change stuff do it next winter in the light of experience, not this spring by guesswork.
     
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