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  Dagger Boards for a Catamaran

 How to build dagger boards for a high performance cruising catamaran. Strong and light.

  The shape of the foil in these boards (see right) is pretty much the same as my rudders, that is what I would call "intermediate". I'm hoping for good speed and stall resistance. The trailing edge is blunt but the image is not to scale in that regard.  
   Getting perfect clear hoop pine is no easy thing. This timber set for the spines of the boards came from Boat Craft Pacific. Beautiful timber but hired hands can be a little careless in storage so it came a bit tweaked.
   The central member was a lamination of three pieces to get the right dimension so I arranged the bent pieces to oppose each other and glued them together and clamped both sets together to further straighten and let them sit for a couple days.
   I am using 10mm foam for the skins so glued together the sections here...
   I borrowed a band saw to cut the end caps and pointy bits. I screwed them in place and used a angle to run the length and keep straight. When satisfied I glued them in... checking every inch of the way.. then checking again.. and again...
   until I was sure.. these are set..
   Back to the foam. Laminated the inside with 450 uni 0 degrees. This is actually a BA with most of the glass running north and south and about 20% east and west. The centre of the glass will be my halfway mark. That will determine the board width.
   A little filling to fix up my less than perfect woodworking. You can see here and the following photo, that I put a little piece of tape on the bases of each upright piece. Made me feel more confident and easy to do. Stiffened them right up.
   For the leading edge, the timber I got was a bit to wobbly to use. I had some very heavy foam left over from my rudders and already cut into strip so I put a laminate on the back side and mounted that in place of the timber.
   After the leading edge was set I ground it to shape. Notice I also put in small supports between the other fixtures. Again to stiffen. Added nothing to weight and it did do it's job.
   The foam has been split and trimmed. The trailing edges should be as true as you can make them. The leading edge doesn't matter. Laying on a glue mix.
   Then a timber lined up to stabilise the contour of the foam and ...
   fasten to the ribs..

 Now the bottom side is glued and screwed to the timber underneath toward but not at the trailing edge. Notice have left the foam about 1/4 inch from end of ribs.

the angle is set up so the foam sheets meet at the most stable position possible. The meeting line was close but would have preferred better.

   I wetted a 750 tri ax tape and prised up the ribs from the foam and tucked it under to run the full length. Now applying glue.
   Have spread the glue and leaving more glue mix over the centre of the tape
   And folded the foam over with greatest care so as not to disturb the tape and start clamping the edge and screwing in a mating timber just back from the trailing edge.
   And then start working upwards. I used 4 mm bracing ply scraps to screw into the ribs and spine and timbers toward the top to the leading edge.
   A man can not have too many clamps....
  Glue set and supports removed. 
 On the trailing edge I incorporated a rolled up length of uni to stiffen... overkill really... in with the normal filler mix.
   Here was a missed opportunity! Along with fairing the foam as there were some minor irregularities in the surface, I could have reduced the thickness of the foam by the leading and trailing edges but I got timid. Too worried about loosing stiffness when later I found that concern to be misplaced.
   Next step was to laminate the boards over the foam and the epoxy filler trailing edges. Used one piece off the roll of 450 DB and peel ply
   Then the other side... this time used separate DB sections.. don't remember why but used one piece of peel ply to cover both.

 Time To cap the leading edge using a strip of 12mm foam. That section of aluminium tube is something I have been carrying around for years and have used it to screed concrete and a million other tasks. It is straight as and indispensable. As you can see below, I got a nice true edge because of it.


   Failed to get photos of the process of laminating on the uni on the sides but you can see it here and above. To leave the centre free to place a groove for a hoisting line later, I used two pieces of 850 gr by 150mm wide and folded them over to make two 75 mm strips leaving about an inch in centre. A mistake I made when laminating the uni was in not insuring the boards were perfectly flat whilst the uni was curing. One of the boards set with a little twist as a result. It was about 4mm out on the length of it. Avoidable and inexcusable. I knew better but got careless.
   Shaping the leading edge was done with 7" sander and torture boards to true.
   One edge laminated while the other is gluing.

 There... with some fairing and paint they are done....

BUT NNNOOOOOO!!! These things are too important and I couldn't rest till I was satisfied. The leading edge came off too fat, the trailing edge to thick and I still had that twist in one board.


Jewellers shouldn't build boats.


  I ground off the tape on the leading edge and this is the result. I got a bit finer entry and that is what I wanted.  
   I have a good slab of timber on the foot but I also decored the foam and filled with bog to with stand the occasional oophsie.

 To increase the stiffness of the boards I put a layer of carbon fibre alongside the glass uni already in place. Because carbon is even more resistant to elongation than glass, this is an engineering stuff up. I could have ground off the glass as it would only come into play after a failure of the carbon. But at this point it was a part of the profile too so left it. The notch in the cutaway is to allow a block (cheekplate) to be fastened onto the vertical bit. I embedded a piece of aluminium into the foam to take the screws. This negates the need for a groove in the board or case for lifting lines. The board cavity itself in the case is where the lines will travel. This is a Bob Burgess idea.. thanks Bob!


I used scrap piece of foam for the repair. Already glassed back side and then overlapping tape will laminate the outside. A heavy glue mix was used to fasten pre-taping.

One more thing... notice the thin strip on the trailing edge? That was an idea from Mal Salisbury. I embedded a strip of plastic there to assist in keeping the edge fair and true. Good idea. I cut a line in the bog I had there and set up the strip by eye with glue mix.

   I cut a profile shape out of a piece of ply and used it to trowel on bog lengthwise and then....

 Just as the first coat is going firm, trowel on a layer going the other direction with a mortar trowel to achieve a more even surface and ready to sand the next morning, being careful to use the profile board whilst sanding to keep the proper profile. I figure the profile board to be a permanent piece of the ships equipment and should be safeguarded.


Once sanded, primer and paint.


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