S2 6.7 Junk Rig Conversion

  • 26 Jul 2019 09:10
    Reply # 7797761 on 6872873

    The only time I would ever dream of mixing up a batch of epoxy that big is when I'm coating or laying down fibreglass.  Why would you?  It takes about 15 seconds to pump the epoxy and one minute to mix it.  I cannot see the point of 'saving time' when it means that you are putting yourself under pressure to get the mix out of the container and on to the wood.  If you run out of glue half way through the job, it's vaguely irritating to have to go and mix up some more; if the epoxy starts going off it's a recipe for disaster. As you pointed out, the first thing that goes through your mind is the cost of wasting it, so you are inclined to use it anyway, even though the pot is getting hot and the mix thickening while you use it.  The cost of this pot of epoxy (which is actually, a lot cheaper in real terms than it was 40 years ago) is nothing compared with your peace of mind after the event (did you achieve a decent glue joint?) and, in the worst case, the piece of wood that has to be removed as there are big gaps in the joint because the epoxy had gone hard before it squeezed out.

    I am not renowned for throwing money around, but even I will ditch half a pot of epoxy rather than risk a sub-standard joint.  However, the secret of success is, when in doubt, mix less.  Frankly, I think it's easier to mix another batch than to faff around with iced water, which I would undoubtedly tip all over the wood I'm about to glue!

  • 25 Jul 2019 23:44
    Reply # 7797374 on 6872873

    With the temperatures you are experiencing are you using the slow hardener for your mixes of epoxy?

    Another trick is to have a bucket of ice and keep the mixed epoxy container seated in that. It will not go off until it warms up.

  • 25 Jul 2019 17:48
    Reply # 7796935 on 7796586
    Anonymous wrote:mistakes with epoxy. With the temperature back to a more reasonable dry 68 Deg F yesterday I thought it would be OK to mix up four 'pumps' of West System epoxy at once. I was wrong! About half of it 'went off' in the cup and turned rock hard while I was spreading epoxy. I am trying not to think about how many dollars worth of epoxy I used to make this cup-shaped yellow disk. I will make smaller batches and spread them thin as soon as I can in the future.
    An icecream bucket of ice to put your mix bucket inside may help. Also, using a more bowl shaped mix container helps (to keep the epoxy spread out thin to stay cool). I have used a take out salad dish for a lot of stuff like this. I have also seen people put the epoxy into paint trays (there are cheap paint tray liners that can be tossed when there is too much hard epoxy in them to be used any more) to keep things cooler.
  • 25 Jul 2019 13:43
    Reply # 7796586 on 6872873

    I am still trying to make progress every day. Some days this is just cleaning up the mess from yesterday to get ready for tomorrow. On other days I spent hours in the boat using a hand plane/rasp with temperatures around 90 Deg F and a dew point around 74 Deg F. The result is 7 layers of 1/2" mast step that fit nice and snug against the hull and the layer below. With this done I am sure I could do it again much more quickly and with much less error and rework. Maybe I will test that idea in the future.

    I am moving forward from making mistakes with plywood to making mistakes with epoxy. With the temperature back to a more reasonable dry 68 Deg F yesterday I thought it would be OK to mix up four 'pumps' of West System epoxy at once. I was wrong! About half of it 'went off' in the cup and turned rock hard while I was spreading epoxy. I am trying not to think about how many dollars worth of epoxy I used to make this cup-shaped yellow disk. I will make smaller batches and spread them thin as soon as I can in the future.

    Three layers are coated.

    Progress, I think?

    3 files
    Last modified: 25 Jul 2019 13:47 | Anonymous member
  • 13 Jul 2019 01:19
    Reply # 7777636 on 7777553
    David W wrote:

    [...] The additional area of bond created by the fiberglass is also an addition to your safety factor.

    All the best with the project, David.


    Thank you for taking the time to respond. Thanks also to David T. for the details regarding Weaverbird.

    I will think about fiberglass between layers, running up the hull, and large fillets at the top.

  • 13 Jul 2019 00:24
    Reply # 7777553 on 6872873

    Hi Scott,

    the best way to ensure minimum gaps is to bond each layer separately to the hull and the layer below. If you do this then you can add a layer of fiberglass over each layer of the step and take this several inches further up the hull. The next layer can then be screwed down to the one already in place with lots of epoxy between and filled epoxy along the edges. You may not need the fiberglass between every layer of the step but the peace of mind is worth the extra effort and expense. The additional area of bond created by the fiberglass is also an addition to your safety factor.

    All the best with the project, David.

  • 12 Jul 2019 23:59
    Reply # 7777542 on 6872873

    I am trying to convince myself, one way or the other, that my reworked six-layer mast step is 'good enough' or 'not good enough'.

    There is more than 12" x 12" of potential bonding area and it is a much better fit than my first attempt. But I still think there is more than 4mm gap between the beveled edges and the hull in several places -- and some gaps between layers where I made mistakes.

    I am considering adding two more beveled layers. Each layer would need to be made in two pieces if I want to avoid cutting up the hull liner. I want to avoid cutting up the hull liner.

    Keep thinking or keep working? I never know which one to pick.

    3 files
    Last modified: 13 Jul 2019 01:04 | Anonymous member
  • 30 Jun 2019 19:21
    Reply # 7709263 on 6872873

    Scott, just for comparison, the bonding area of Weaverbird's mast step measures 13" x 13", and this has proven to be entirely satisfactory over 4000 miles. I made it with a 13" x 9.5" hardwood block shaped to match the hull, to start, and then added layers of plywood the same size, finishing with large fillets of chopped glass strands and resin on either side.

    Last modified: 30 Jun 2019 19:22 | Anonymous member
  • 30 Jun 2019 18:54
    Reply # 7709024 on 6872873

    Thank you David W.

    I worked through the math following your advice. I assumed a 22 sqm sail with the COE at 15.0 feet LAP and the top of the mast step at 2.5 feet below the partners.  The result was 1420.848 lbs for the force and 142 square inches minimum contact area. This is roughly equivalent to a 12" x 12" area. I expect I will have at least this much on just one side of the hull. I should end up with double the needed area. I think this may be enough for the '[...] safety factor and peace of mind'.

    I have more significant problems with the way I was trying to fit the plywood layers as a mast step. I am reworking the pieces and hopefully making properly beveled plywood layers this time after some very gracious help from Arne.

    Scott.

    >>> area_m2 = 22.0

    >>> area_ft2 = area_m2 * 10.764

    >>> force_lbs = area_ft2 * 15.0 / 2.5

    >>> force_lbs

    1420.848

    >>> min_in2 = force_lbs / 10

    >>> min_in2

    142.0848

    >>> min_square_side_inches = pow(min_in2, (1.0/2.0))

    >>> min_square_side_inches

    11.9199328857171

  • 27 Jun 2019 01:01
    Reply # 7673655 on 6872873

    Hi Scott,

    fir plywood has a shear strength of about 400 pounds per square inch in the weak direction across the grain. When calculating strength of the bond to the hull, this is probably the weakest link. When gluing to the hull there are a lot of uncertainties such as "is the whole area properly bonded, no gaps", "Is the hull properly prepared so that the bond is as strong as the glue", so I would use a value of 100 pounds per square inch of the contact area to the hull.

    To approximate the lateral force at the step multiply the sail area in feet by the distance from the deck to the center of area in feet; then divide by the distance from the deck to the mast step in feet. The wind pressure on the sail at 20 miles per hour (approximately 16 knots) is one pound per square foot, since you will generally be reefing the sail above this wind speed, this is the maximum force on the sail.

    So if the lateral force at the step is 1,000 pounds then the step area in contact with the hull should be 100 square inches minimum. Anything more adds to the safety factor and peace of mind!!

    All the best with the project.

    David.

    Last modified: 27 Jun 2019 01:03 | Anonymous member
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