S2 6.7 Junk Rig Conversion

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  • 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.

    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
  • 26 Jun 2019 22:20
    Reply # 7672105 on 6872873

    I had a nice sheet of 1/2" marine plywood. Now I have a stack of five things. I thought I had a clear idea how to get the mast step done, so I just went for it. I stopped to take a look at the bigger picture after 5 layers. Something does not look right.

    Back at the computer I see that my stack of plywood does not look like Arne's mast step. The thing I have is a lot wider and still not as thick. It is so wide that I can't fit the biggest pieces through bow access in my boat. It is also so wide that I do not have enough plywood left. With the original plan I still need to make 3 more layers to finish the mast sole and 4 more layers for mast step (the collar section to hold the mast).

    I would like to determine how much surface area I need contacting the hull on the beveled edges of the mast step. I hope that I already have more than enough. If that is the case then I will cut off the aft end of each layer such that I have the required amount of surface area. Then with a little more positive thinking this will actually fit in the boat.

    Does anyone have a rule of thumb for how much hull contact is needed in this type of mast step?

    Scott.

    Update:

    After some searching I found the comment I remembered from Arne. It is on the caption of a photo here. He says, 'With a contact area of at least 1500cm2, that thing will stay put'.

    I think I will do some measuring and see how close I am to that number.

    Last modified: 26 Jun 2019 23:38 | Anonymous member
  • 25 Jun 2019 18:16
    Reply # 7655525 on 6872873
    Anonymous member (Administrator)

    Scott,

    I don’t know how good that is, but it looks very much like the way I did it on Ingeborg (and on Johanna and Malena as well...). None of these mast steps has come loose. When thinking of how big the contact area between the mast sole and the hull is, I have no fear.

    Good luck!

    Arne


  • 24 Jun 2019 21:47
    Reply # 7642300 on 6872873

    I made a lot of dust. Much more than I expected. I hope this indicates some progress.

    I might have considered attaching a vacuum hose to the angle grinder some how if I knew how much dust I would be cleaning up later.

    I did not grind the entire mast step area down to translucent glass. I tried to take at least a small layer off everywhere. Any opinions after reviewing the photos would be welcome. I plan on wiping the area down again with towels and acetone just before I epoxy the yet-to-be-made plywood layers.

    The weather is reminding me why I like the inside 'control point' from PJR so much. It is raining and raining. I hope someday I will have a boat that is able to sail in the rain while I am inside dry and comfortable.

    Now I am waiting for dry weather so I can cut and shape plywood. Too messy to do in the garage.

    Scott

    Last modified: 24 Jun 2019 22:06 | Anonymous member
  • 21 Jun 2019 22:26
    Reply # 7592849 on 6872873
    Anonymous member (Administrator)

    Scott,

    Ingeborg’s maststep actually wasn’t so heavy, after all, but if you want to shrinkyours to half the weight of it, it is enough to crimp all dimensions with 20%. The plywood I used was either 15 or16mm thick  -  I don’t remember now.

    Just make sure the mast will never jump out of the step, or else...

    Arne


    Last modified: 14 Jul 2019 21:26 | Anonymous member (Administrator)
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