Busted Daggerboard....

Discussion in 'Multihulls' started by paxfish, Nov 5, 2015.

  1. paxfish
    Joined: Dec 2014
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    paxfish Junior Member

    Ah, The fun continues! I pulled the boards for the first time last night and discovered that one of them took a hit in the past from the side. I am fortunate I didn't lose the board during this, my first season.

    Most of the season I ran only the starboard dagger, because of a persistent leak on this side. Now I know why!

    There is also damage in the dagger well exit (oddly, on the same side as the crack) that I will be repairing as well.

    The board was constructed in a mold with a central glass ibeam, and later filled with high density 2 part foam.

    So how do I attack the board? The dagger well is very tight with no clearance for additional cloth, so I cannot lay cloth along the length of the board with out peeling the whole side.

    I am inclined to open it around the ibeam to ensure that it is intact at the break, and then scarf out maybe 8 inches to either side of the crack. Then lay in several layers of glass.

    I would appreciate the board's thoughts on how to proceed, including cloth weight etc.

    The boat is a 28 foot Malcolm Tennant design called Wildfire: http://www.tennantdesign.co.nz/index.php?page=wildfire

    Sincerely, paxfish

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  2. redreuben
    Joined: Jan 2009
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    redreuben redreuben

    Well you have two choices really, scarf out the damage as you said or build a new one.
    I would repair I think, 90% of the material will need to be uni with some DB to tie it all together. If the Ibeam is delaminated you might have to build a new one.
    Excuse my negativity but like your rudders those leading edges look a bit sharp. Anyone else have an opinion ?
     
  3. paxfish
    Joined: Dec 2014
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    paxfish Junior Member

    No need to apologize! I appreciate your candor. I intend to improve these leading edges as well. It appears there is enough material there to work it a bit.

    After sanding, the cross section reveals a bit more about the construction. Seems like 4 to 5 layers of 17 ounce on the leading half, tapering to maybe 2 or 3 layers in the aft section.

    I was thinking of using this e glass from us composites:

    DB170 - 17oz Biaxial - NO MAT
    +/- 45 degree
    Contains double bias(17oz) stitched at 45degrees.
    Does not have a mat backing.
    Thickness: 0.030"

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    Would I be playing with fire to shuffle in a couple of layers of CF cloth as well?

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  4. groper
    Joined: Jun 2011
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    groper Senior Member

    Fair out the damage, cover it in mold release and take a 2 peice mold off it. Layup a new board in your new mold and it will come out exact same size as the old one so it fits perfect.

    Dont waste your money with CF cloth. If your going to use CF, then you will need to use UD carbon (not woven cloths) for structural work, and 100% carbon, no glass. If using glass for lower cost, forget the carbon and just use 100% glass. The reason is massive difference in modulus, the carbon will take all the strain whilst the glass is still stretching - you cant use them both to handle loads in the same direction. The only alternative for a mixed lamainte is when the fibers handle loads in different axis, so the obvious solution in this case is to use UD carbon in the span direction and dbl bias glass +-45degrees as the lower cost mixed laminate option which will result in a stiffer board than the pure glass board. The glass will do the torsional loads, and the carbon will do the main bending load.

    Rruben gave an indication as to lamainte fiber orientation percentages, id guess around 75% in the span direction and the rest as +-45degrees. Measure the laminate thickess of the old board and use 1mm thickness as 1000gsm (30oz i think?) fiber tex weight. So if the laminate is 10mm thick for ease of demonstration, use 7500gsm in the span direction and 2500gsm -+ 45 degrees. Dont use polyester, use a good structural vinylester if using gelcoat or you could use epoxy if your going to paint them - use a resin with very good mechanical properties.

    The profile could probably be improved, but thats only feasible if your prepared for alot more work and make new trunks for them and install them in the boat...
     
  5. paxfish
    Joined: Dec 2014
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    paxfish Junior Member

    That is helpful info. I better understand the mixing of fibers now. And yes, repair or rebuild, I'll be using epoxy resin.

    From your figures, my .25 inch laminate (6.5mm) would require roughly 4700 gsm uni and 1300 gsm 45 deg. biax. That translates to 8 layers of 17 oz (500g) uni and 2 layers of 17oz DB, which seems like an awful lot! I'll check my math....

    I can improve the leading edge radius a bit without having to change the trunk.
     
  6. philSweet
    Joined: May 2008
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    philSweet Senior Member

    FWIW, I have a suspicion that you are looking at a compression failure, not a tensile one, which would explain why the housing was damaged on the same side. So you might want to examine the compression strength of the foam in the critical area when the board lands on the hull in the fully-to-mostly down position (which is where the crack is, yes?). I would be surprised if the damage hasn't migrated into the I-beam as well.

    Do you have any pics of the housing damage? This could have been instigated with an impact such as a grounding, and then grown. Or it could just be a slow crush/buckle failure. I believe the current recommended approach is to engineer a crush plug into an oversized housing which is a bit more compliant than the board. Need some clearance room to do this though, doesn't help you much. Without a crush zone, You must ensure the board does in fact fail before the hull or housing. Some boards and rudders are also engineered to snap off cleanly. Usually this is just near the tip, so there is still something left in the event of a grounding.
     
  7. Marmoset
    Joined: Aug 2014
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    Marmoset Senior Member

    if you were going mold method you could always cut it back to where a solid round dowl could be attached, and faired, to get a better l.e. not perfect foil but better. this would also ensure your new part is smaller than case allowing case to be filled for fit rather than remade.


    Barry
     
  8. paxfish
    Joined: Dec 2014
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    paxfish Junior Member

    Thanks Phil. As info, 2 part foam was poured in after the initial build (by the home builder) to try to mitigate ongoing water ingress issues. The dagger was initially hollow as per Tennant's design, save for the ibeam.

    I don't have a picture of the housing crack, but your description of a crush buckle failure at the dagger housing exit sounds apt. I have access to the site from both inside and out.

    My intention there is to reinforce from the inside first and then tackle from the outside. Can you provide a little more detail on your proposed "crush plug?"

    Finally, the builder had a 6" foam end on the boards, both of which have long since vanished. I may build new ones for that.
     
  9. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    Always interesting Pax

    It's nice of you to give us something good to get our teeth into.

    I think I can count the layers of glass in the laminate. It seems like about 5 layers for me. What laminate? Can you ask the previous owner/builder?

    Like Phil Sweet it looks like compression failure to me. I think these boards would be more susceptible to compression failure than cedar core boards as the laminate takes all the load. The laminate should be sized for compression rather than tension. Thin laminates will fail by local buckling and it looks like it has done this.

    I am concerned that the beam must have failed or broken a glue line to the laminate to fail. In this case you will need to reattach the laminate to the I beam which will be tricky. If the board was empty you could see inside but the foam makes this impossible. Without checking to see if the I beam is still intact any fix may be temporary at best. Can the foam be removed or is it better to make a new board.

    I had thought of making boards this way for my cat a couple of years ago. You can use the steel mould principle and then do a test fit with an I beam made out of hard wood which is removable. Or just leave the hardwood inside and glue it in.

    The extensions do not have to be easily damaged foam. If you build a new board save the bottoms of this one - about 300mm. Then glue them back onto the (one new) boards you have. I have had this on Kankama for 15 years and haven't hit hard enough to knock them off yet. This board could be used for lots of spare ends. Don't glass them ends on - just glue. Plenty strong enough for sailing and gentle bottom touching.

    cheers

    Phil
     
  10. nzclipper
    Joined: Mar 2011
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    nzclipper Junior Member

    Hi Chris, bugger about the board, but I feel your pain, snapped my GBE board, a few months ago, but could not recover the bottom half. Was beating upwind in big waves and 25 + knots, boat launched in air, landed sideways and snapped off. Have managed to temporarily borrow a board, but tried boards from 5 different GBEs, and none fitted. My point is, be bloody careful about building new board to fit old case. I would be grinding out crack and glassing or carboning back up. my old board had been extended by fariing on a new bottom few feet (yes, prob the reason it broke) so it is definately possible to repair what you have.

    To hopefully make you feel better, all the GBEs down here go through a season or 2 of pain when they start racing and sailing hard, the old boats cant handle it, and in my 5 years of thrashing mine, I have broken 3 beams and replaced all 4 now, broken a board, replaced all the rigging, new mast, new boom, new gooseneck,
     
  11. paxfish
    Joined: Dec 2014
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    paxfish Junior Member

    Thanks Guys.

    Phil - here are a couple more shots. I have good access behind the crack as well.

    There is about 2 inches of solid glass around the perimeter of the exit. However, you can see a lateral crack forward and aft of the hole that likely says the core might be compromised there. I'll be checking that closely as well.

    Thanks for the comiseration NZ. Actually, I'm not discouraged at all. That won't happen until I have to fix the same issue twice! I'm still fired up about the utility and performance of this boat!


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  12. groper
    Joined: Jun 2011
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    groper Senior Member

    wow, looks like youve got some major repairs to do on the case aswell... what is the core made of? if its anything but closed cell foam, then there is going to be water in your core and it will be spreading well past those cracks.

    From an engineering perspective, when the board is down and under load, the highest loaded area on the board and case is where they exit the bottom of the hull. In this area you could include a web in the board so that it cannot be crushed and buckle - which as the others have said look to have happened. The web could be somthing like ply wood or a synthetic substitute like solid glass or coosa board etc. You should also include the same type of web at the board ends aswell. You will find the same thing inside a carbon wing mast where the boom attaches as there is a high compression load there also which adds to the mast buckling etc. The same should happen in the hull. The area around the case exit should be decored of foam or balsa etc and replaced with something that can resist alot more compression - such as very high density foam like coosa board etc, or a high density resin and fiberous filler mixture along with all the extra glass around the area.

    If you value the performance of your cat, then you should strongly consider starting building new boards and cases from scratch. The efficiency of the boards and their lift drag ratio has a rather large effect on your upwind sailing performance. If you use a better profile and do it with good accuracy, you will improve your boats performace at the same time as fixing your damage.
     
  13. paxfish
    Joined: Dec 2014
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    paxfish Junior Member

    Thanks groper. I appreciate your comments about the boards, but don't intend to change their shape at this time. I would have concerns about changes in CLR and balance at a minimum, not to mention the major surgery of building new cases. Oh yeah, and earning the wrath of the wife!

    With regard to the core in that area, I intend to remove any wet cedar and replace with carbon core (coosa equivalent.). There is solid glass 2 inches around the perimeter of the exit that I will take down and replace as necessary. On the inside, I have good access for beefing up that crushed area as well.

    The best laid plans anyway. I started to scarf out the dagger last night and found a hairline crack in the filler near the ibeam, but on close inspection, I-beam appears intact and well bonded to the laminate. I ground through .25 inches (6mm)of uni layers to get there, so it must have been a hell of a hit.
     
  14. paxfish
    Joined: Dec 2014
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    paxfish Junior Member

    Thanks for bearing with me, Guys. as interest, here's a cross-sectional shot of the plans in this area. I circled the area where the crushing has occurred.

    I intend to grind the sole to good glass and lay in several layers of uni, a piece of good foam core about 3/4" thick and then lay on 5 layers of uni and one layer of tri on top. Yes - it'll be a tripping hazard, but I think I can figure that out. No way I'm cutting out the entire length of the sole.,,,

    Then I will fill behind the crushed area (it is apparent from the design why it crushed at that location!) and then rebuild the outer surface from the outside. I think this horizontal column will further distribute the load to the other hull.

    Sound good?

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  15. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    Pax, you don't have to put core around the case. A reasonably good method is to cut around the case and taper the core back at about 45 degrees from the case. This ties the laminate together like a HD insert does. Squeezing a tool to do this will be tricky. I would not be trying to get an HD insert in there - it would be a pain of a job with that sloping side. So I would taper the core a bit and/or make an insert out of silica and epoxy in place. When it gets rubbery add the glass laminate on the inside. Don't add heavy laminate onto a wet cove - wait till it can handle some poking around. BUT don't let it go off without some smoothing either.

    To get around the problem of no glass on the inside of the case to the outer skin you can use a trick with light sheathing cloth. If I need to add some very light cloth in a tricky spot I get some nice fine cloth and cut it at 45 degrees to the roll in say 6cm wide strips. This then acts like DB fabric and is very pliable. It would be good if you wanted to roll a tape inside the case to the hull skin.

    If you are going to work on the case be VERY careful when you relaminate the case in. Unless your cases are very stiff you must put an insert inside the case so that you don't squeeze the case and make it tighter on the board.

    I would also wrap unis from 500mm in front of the case, around the back of the case and back forward, fanning as you go forward. I haven't seen it on any plans but always like to do this to help with sudden case impacts at speeds.
     
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