S2 6.7 Junk Rig Conversion

  • 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.


    >>> area_m2 = 22.0

    >>> area_ft2 = area_m2 * 10.764

    >>> force_lbs = area_ft2 * 15.0 / 2.5

    >>> force_lbs


    >>> min_in2 = force_lbs / 10

    >>> min_in2


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

    >>> min_square_side_inches


  • 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.


    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?



    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.

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


    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!


  • 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.


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


    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...


    Last modified: 14 Jul 2019 21:26 | Anonymous member (Administrator)
  • 21 Jun 2019 16:33
    Reply # 7592231 on 7589014
    Arne wrote:


    PS: Check this: http://bit.ly/2sf2tsW

    Hi Arne,

    I have been studying your mast step and partners document for some months now. I have never considered any other way to build a mast step. My only doubt is if I should use the same total thickness on my S2 6.7 (998kg) as you did on your Folkboat (2150kg). I am not concerned about how much labor is involved. I am concerned about adding too much weight to the bow of my boat.

    In addition to the very helpful instructions I also enjoyed these comments:

    "With the sail finished (Jan. 2015), it was time to set to work on the hard bits."

    "to have a mast step come loose [...] would turn the big mast into the mother of all can openers. "

    When I was working on the sail it did not occur to me that the hard part was yet to come!


    Last modified: 21 Jun 2019 16:34 | Anonymous member
  • 20 Jun 2019 09:07
    Reply # 7589014 on 6872873
    Anonymous member (Administrator)


    this plywood maststep may not be the quickest to build, but it surely is strong, and you have the advantage of being able to move it a little on top of the base, to get the mast standing right. My present boat was afloat while I fitted the step, so the boat was only so and so level with me moving around on it.

    Four things may be of interest:

    ·         I rubbed the hull with a little angle grinder to make the epoxy stick as well as possible. (Edit: Wise from experience, I brought the vacuum cleaner on board with the angle grinder...)

    ·         I sealed the plywood thoroughly.

    ·         I made a drain notch in the mast step. Even with sealed plywood, I don’t want the mast to stand in a puddle of water. (so far the step has stayed bone dry, thanks to the mast coat).

    ·         I have later secured the mast from ever being able to jump out of the step.


    PS: Check this: http://bit.ly/2sf2tsW

    Last modified: 24 Jun 2019 22:20 | Anonymous member (Administrator)
  • 19 Jun 2019 22:34
    Reply # 7588409 on 7586679
    Anonymous wrote:

    Phil, if you are following along I would appreciate your opinion on how thick and wide to make the mast step. As you may recall the displacement of your ComPac is close to the displacement of my S2 6.7

    Is 6" tall, total, too much? Is 2" for the collar too much or too little? 

    I think I had six beveled layers of 1/2 inch ply for the base, enough to to have good contact area on its bottom and the beveled edges. The upper section that held the mast is probably 2 inches thick.  Brenda B’s hull is flatter than some so the bevels may be flatter with slightly more surface and contact area for the epoxy, first well coated (plywood edges soak it up pretty well) then glued with thickend epoxy. I also glasssed top edge of the base to the hull.  If I get out to the boat tomorrow I’ll double check.

       " ...there is nothing - absolutely nothing - half so much worth doing as simply messing about in junk-rigged boats" 
                                                               - the Chinese Water Rat

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