Single Dagger Board vs Double Boards

Discussion in 'Multihulls' started by CatBuilder, Apr 26, 2011.

  1. oldsailor7
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    oldsailor7 Senior Member

    Quote ["To me the top out configuration just looks like an accident waiting to happen, if a board jams or it needs to be pulled up in rough weather."] Quote

    Thats why I don't like outboard daggers in Trimarans---- from personal experience.
     
  2. rayaldridge
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    rayaldridge Senior Member

    Phil has correctly interpreted my vague theorizing, Red. He makes a good point that in a big boat, it may make handling easier to divide the board area into two. I guess that was often the rationale for ketches, as well.

    There's an interesting piece by Shuttleworth on the design of a 70 foot cruising cat. He had the budget to do a lot of tank testing, and concluded that on efficiency alone, the single daggerboard was the winner. But for pragmatic reasons, he went with stub keels and a single board-- the stub keels were to protect the rudders and props.

    I worried over the inboard vs. outboard placement of the board when I was drawing Slider, but I couldn't see where I'd put my feet to haul up a stuck outboard dagger. Looked like a back injury waiting to happen.
     
  3. hoytedow
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    hoytedow Carbon Based Life Form

    If you have 1 daggerboard, it takes 100% of the strain. 2 daggerboards share the load, halving the strain and the chance for catastrophic failure. Redreuben said it best what you would be if that single board fails.
     
  4. rayaldridge
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    rayaldridge Senior Member

    I'm not sure breaking a board could really be counted as a catastrophe. A lot of boats will still make some ground to windward, even without a board-- my little cat does. I lose a few degrees, but sometimes beating across a flat will save me a lot of time, so I just haul up the dagger and go. Even in ocean passages, prudent sailors plan their passages to take advantage of prevailing winds, so even then a broken board wouldn't do too much harm.

    A real catastrophe would be to lose the rudders. Off and on I've considered a single rudder and board set up on a central spine, for my next boat, a folding cat. The redundancy of double rudders does seem to be a safety factor. On the other hand, Derek Kelsall was putting board, outboard and rudder down the centerline 40 years ago, and still likes the setup.
     
  5. bus_labi
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    bus_labi New Member

    Bending stifness is proportional to cube of material thickness, so it's not a problem to design a board with somewhat larger chord/thickness to have double the bending strenth of smaller one.

    One could even carry spare board under the bridgedeck like spare tire (which of course does not help if damaged board is stuck).
     
  6. keysdisease
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    keysdisease Senior Member

    Your memory is not failing you old sailor, the early MacGregor 36 cats had just one board.

    I owned one and there was a slight but noticable difference in leeway reduction on one tack vs the other. Tacking was quicker when pivoting on the hull with the board too (and I do seem to remember it being the port hull)

    For a larger heavier boat like Catbuilder is building he will be the only person that might notice any difference, and then only when the wind is forward of the beam.

    The boat will most probably be slightly faster with only one board off the wind because of the reduced weight and drag from the missing board and trunk.

    Boat = compromise

    Steve



     
  7. keysdisease
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    keysdisease Senior Member

    For the use Catbuilders boat is going to get breaking a board will be a PITA but not a big deal. He's going to do captained term charters and he'll sail off the wind or fire up the diesels if "the" board breaks. I don't believe his cruising grounds will be taking him very far offshore.

    Steve

     
  8. CatBuilder

    CatBuilder Previous Member

    Couple of added facts about my personal situation to add to the thread:

    It was asked somewhere what my beam is. Overall beam is 25ft.

    Also, we will be doing charters, but do plan a lot of offshore sailing, including ocean crossings.

    I'm all turned around about this now. I had thought the advantages would be great, having one board, but then didn't realize it would have to be a huge board that might be difficult to handle.

    It would cut down a lot of work building one board and one trunk, as well is make my galley a lot more useable. (board is in the galley down)

    It looks like there is no absolute consensus here. I'm just plain stuck.

    Thanks for all the input, everyone. Gives a lot to think about.
     
  9. Alan.M
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    Alan.M Junior Member

    I have my board canted in (out at the top) but only 5 degrees. It's mounted outboard in the hull. There is a downhaul line so I shouldn't need to be out there pushing the board down, but even if I did need to, it can be done without leaning outside of the boat.

    Again, even if I lost the board (or the use of it) I can still sail to windward, just not as efficiently. Seriously, without the board you are NOT "screwed"! You won't win races, but you'll get where you're going! I'm forming the opinion that the people saying you would be either have'nt sailed a daggerboard boat, or tried sailing one to windward without the board down. You have more leeway, but it's not a disaster.

    I've got a similar sized boat - 44 feet - the board isn't difficult to handle, although it is quite big. It's around 20% longer and wider than each board would have been if I had two. I can carry it myself, although it's much easier with 2 people. Not that we have needed to carry it since the boat was launched. Weighs around 50kg.

    Lifting or lowering is easy - uphaul and downhaul are led to a winch. Mostly When I'm using it I just let it down to where it floats - around half down - unless I am sailing to windward in pretty light stuff - less than 10 knots - when I'll haul it down more.
     
  10. CatBuilder

    CatBuilder Previous Member

    Thanks, Alan. Fancy seeing you here. :)

    I am definitely over the idea of being "screwed" if I lost a board. It's not the end of the world and if I was really worried, I'd make a second board and stow it in case I wanted to have it for insurance.

    Or... just pull up the damaged board and have epoxy/glass around to fix it up if in the middle of nowhere.

    Not an issue, IMO.

    So I guess the uphaul and downhaul lines make it simple to handle? Somehow, I was thinking it would be difficult, but if everything is just done off the winch, I suppose it doesn't matter what size your board is.

    Did you have data from Bob O saying that your single board was 20% wider and 20% longer than the double boards would have been? Is that right from the designer?

    I ask because I have no option for a single board on my plans, but would really like to fit one, I think.
     
  11. oldsailor7
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    oldsailor7 Senior Member

    All the Wharram classic cats had no boards.
    Admittedly they were not exceptionally close on the wind----but they never seemed to have too much trouble when ocean cruising. :cool:
     
  12. Alan.M
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    Alan.M Junior Member

    Yes, Bob Oram did include specs for a single board if desired. I went a bit longer again, because I had increased my sheer height for more headroom. I don't have the plans with me just now, they are back in storage, so can't give you specific details though.
     
  13. CatBuilder

    CatBuilder Previous Member

    That's still a pretty good start for me to make a decision. Thanks.

    I'd say 20% wider and longer (even including your sheer height mods) is not terribly huge compared to the regular boards.

    I am very strongly leaning toward this.

    I asked my designer (even paid!) if he could work up a single board configuration for me, but no luck. He must be MIA this week. Sucks because I'm currently joining together the port hull, which is the hull where the single, large board will go and have to cut the bulkheads accordingly. :(
     
  14. Petros
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    Petros Senior Member

    The redundancy issue is moot, all cruising monohulls have only one and it never occured to monohull sailors that they needed another. Redunancy is NOT a reason to have two daggerboards. The asymmetric drag can have a significant effect over long distances.

    Many years ago a friend and I entered the largest Hobie cat regatta on the west coast with the then new Hobie 18. We sailed two days over many different conditions in and around the Newport Beach CA harbor. We were newbies so we got to sail in B fleet with the other newbies and less skilled Hobie 18 owners. Other than reading some books, and me owning a old 14' sloop, we had little experience in the Hobie 18, and none in racing. My friend Don had just bought the 18 and only been on it twice before, and ask me to crew for him since I was the only guy he knew with any sailing experience.

    We kept a careful eye on the guys in A fleet to see how they were setting their sails, course they took, etc. We were jockeying back and forth for first potion with one other B fleet team over the two days, the only ones with any sailing skill at all in the B fleet. On one of the final legs we had a long run straight down wind into the jetty, and through the harbor back to the starting point.

    Air was very light and all of us were hardly moving at all, we just crept along with barely enough rudder power to keep us straight, all of the fleet were bunched together just feet apart. I was noticing most other crews were pulling one dagger board up when going down wind, presumably to reduce drag, so we did the same. With both up rudder control went to zero, so we needed something down to assist steering. I noticed that the tiller was just off center in exactly the direction to trim out asymmetric drag caused by the single dagger board. It was Don's boat so he was on the helm most of the race, I told him I was going to try something to help. I rolled across the trampoline deck and pulled the dagger board up so the just the lower 10" or so were in the water, and set the other one the same way so they would cause the same amount of drag. Sure enough the tiller came back to center, and we slowly started creeping ahead of B fleet and eventually won it because of that piece of insight I learn in engineering school (I was a student at the time).

    What was funny was as we pulled ahead of only other skilled crew in B fleet, they were frantically looking over our hull and rigging and theirs (we were only about 10 feet apart) trying to see what we were doing to suddenly gain some speed on them. We were all in identical boats. We sat there grinning at them like Cheshire cats, they were convinced we were cheating. The skipper of the other boat calls out to us before we were out of ear shot "what are you doing?" I just said "it is a secret" as we slowly increased the distance between us.

    So the asymmetric drag IS noticeable and does make a difference. It has to be overcome by trimming the rudder, and this creates slightly increased induced drag on the rudders. It also means you will have more rudder authority in one direction than the other. Over long distances this extra drag will mean it takes longer to get to your destination. There is a reason to have symmetrical hulls.

    If you want only one dagger board, I would put it in the center of the deck. The surface Perice foil will be less efficient too, but as least you will not have asymmetric drag.
     

  15. oldsailor7
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    oldsailor7 Senior Member

    Whether you have one board or two, the immersed area of the board(s) should be 2% of the projected sail area. (main and fore-triangle).
    Basic rule of thumb. ;)
     
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