S2 6.7 Junk Rig Conversion

  • 25 Sep 2019 20:12
    Reply # 7900142 on 6872873

    Next on my 'bottom up' build plan is a tabernacle. I am a little stuck on that until I can decide on the diameter of the mast. I would very much like to use a tapered pole and avoid having a two-part mast that will need assembly. I looked at the 8" pole available at a great price nearby -- to me it is clearly bigger than what I want.

    I am trying to find something that meets all the calculations and rules of thumb I have read. So far nothing meets the criteria.

    I will need to fall short of one, or more, of the goals I typed up:

    1. Mb_yield greater than 1753 kpm

    2. Wall thickness between 2.5% and 5% of the outer diameter

    3. Mass somewhat under 30kg

    If anyone is willing to look at the summary I attached and provide feedback I would appreciate it. At the moment I think going with 'too heavy' might be OK. I am not sure how literally I should take the 'between 2.5% and 5% of OD' rule.

    1 file
    Last modified: 25 Sep 2019 20:18 | Anonymous member
  • 23 Sep 2019 00:24
    Reply # 7893598 on 6872873

    Thanks to everyone who took the time to give advice on the mast.

    I now have 6 layers of 1/2" fir plywood glued into the hull. I also have 4 more layers laminated together for the top of the mast step.

    I am sure I would do it better on the second try ... But I guess I have to learn somehow. I am not terribly proud of the crooked base and I only really got the top two layers glued in the way I would like them all to be. I should have used much more epoxy, I think.

    On the other hand if needed to remove it I have no idea how I could get it out without first chipping all the wood into little pieces and then grinding the epoxy off the hull. Thinking about how difficult it would be to remove intentionally gives me some confidence that it is strong enough.

    I am pretty happy with how the top section turned out.

    It seems that considering epoxy a recurring expense like boat storage makes more sense than thinking of it as a one time expense like buying a boat.

    4 files
    Last modified: 25 Sep 2019 13:31 | Anonymous member
  • 12 Sep 2019 16:12
    Reply # 7877707 on 6872873

    My 7in mast is 36kg bare tube weight, around 40kg after completion and with rigging added. I can just about lift it, but transporting it single handed needs some wheels strapped to one end. Imagining it to be in a tabernacle, I think I could live with raising and lowering it once a season, but doing that every weekend would be a deal breaker for me. It would be sad to have to pass on a bargain, but in this case, it does seem more sensible to hold out for a 6in or 7in mast, somewhat under 30kg.

  • 12 Sep 2019 14:41
    Reply # 7877514 on 6872873

    Its not easy to weigh a mast so I have only an estimate of the  weight of mine - 25 kg at least, and maybe getting towards 30kg. It ended up much heavier than I wanted, and much much heavier than the mast plus rigging it replaced. The boat is only 1700 lb and has no ballasted keel, only a centre board. (There may be a little bit of ballast in the stub.) Anyway, for what that is worth, surprisingly to me, the mast does not seem too heavy, she stands up quite well in a breeze - though it is about the limit for me these days, for stepping. [edit: I can do it though - I have a good tabernacle. For a younger person 30kg would be "up there", but OK I think provided it is being raised in a tabernacle.]

    If your 28' lamp post weighs 35 kg then by simple proportion cutting it down to 24' will bring it to something like (24/28)*35 = 30 kg. (Maybe a little under that, since you will be cutting off the heavy end.)

    James G is carrying a 37kg mast on his 19' (1200 lb) River Rat and says it is too heavy, though he is sailing with it.

    I agree with Arne, but would go further: don't cut the mast at all until you have made the sail and hoisted it high enough to be out of the lifts and lazy jacks. Then you can take the mast down and cut off what you don't need, from the bottom end allowing a little extra, as Arne suggests. Don't cut it too short, is the lesson I learned.

    Last modified: 12 Sep 2019 21:41 | Anonymous member
  • 12 Sep 2019 14:12
    Reply # 7877467 on 6872873
    Anonymous member (Administrator)

    Scott,

    if the price is good and if the boat were mine, I might still jump at it. There are three reasons for it:

    • The boat is not to be crossing oceans or win races.
    • The boat is so light that body weight will have a significant effect on keeping it upright.
    • This mast will still be much lighter related to the boat than the wooden masts of my Malena (her about 85kg mast was 6.1% and her hollow 70kg mast was 5%)

    The solid mast really was too heavy, for sure. The 70kg mast (1400kg boat) was still not ideal, but I could live with it in lack of anything better.

    I would therefore not call a 35kg mast a show-stopper. However, you are to raise and lower that mast before and after each sail, right? Then I guess 35kg would feel on the heavy side.

    Arne

    PS: When cutting down the lamp post to fit your rig, I think you can safely cut from the thick end. Leave a mast about 1-2 feet longer than you think you need, and then trim it down after having gone for a few sails. It's easier to subtract length from a mast than adding more to it...

    Last modified: 13 Sep 2019 14:11 | Anonymous member (Administrator)
  • 12 Sep 2019 13:56
    Reply # 7877444 on 6872873

    Hi David,

    I have an article about Ming Ming II that describes an 8" lamp post being used. I have not been able to find details on the wall thickness of this mast.

    Thank you for the quick helpful response. I found a comment from Arne about mast weight as a percent of displacement here.

    Arne said, "My experience is that I will keep the weight of the mast below 3% (better 2.5%) of the boat’s displacement - and then pile on maximum sail area on that stick."

    So for my 2200lbs boat I need to keep the mast at 55 lbs ideally and 66lbs as an absolute maximum. A quick look with the Junk Rig Calculation Tools tells me that this lamp post will weigh 35.7kg or 78.7 lbs.

    Seems too heavy, I think?

    I do not have an immediate idea how to estimate the weight if I cut it down to something like 24ft, which I expect may be enough.

    1 file
    Last modified: 12 Sep 2019 13:59 | Anonymous member
  • 12 Sep 2019 08:37
    Reply # 7877161 on 6872873

    If I remember correctly, Roger Taylor put a 8in mast into Mingming II, and was a bit nervous that it would 'look' too big, but when the boat was launched and sailing, he no longer thought that.

    I'm happy that my 7in mast can take anything that I can throw at it, so there's no need for the extra diameter in your case. That said, if you were to make an all-wood mast, it would be about 8in at the partners, and probably heavier than the tube that's available to you. If that tube complies with Arne's "% of the displacement" rule, it should be OK. My only other thought is that the tabernacle will have to be large to accommodate it, and take up more room inside the cabin.

  • 12 Sep 2019 01:45
    Reply # 7876901 on 6872873

    Does anyone have an opinion on using the following for a mast stepped in a tablernacle:

    Aluminum alloy tube (specific alloy unknown)

    28 feet in length

    8 inch diameter at the bottom

    4.5 inch diameter at the top

    1/8 inch thick wall

    I was considering a 6 or 7 inch diameter tube. This 8 inch tube has become available locally at a price I like.

    Scott.

    Last modified: 12 Sep 2019 02:06 | Anonymous member
  • 29 Aug 2019 02:47
    Reply # 7854356 on 6872873

    I think I have something more like proletarian paralysis. I ran out of epoxy and I have been working long hours recently. The few places that sell west system do not open early enough for me to go before work and they do not stay open late enough for me to go in the evening. It is a lot easier to find time to read these forums and guess about problems that may or may not exist.

    Today I was able to pick up more epoxy. I now have the top section of the mast step screwed and glued together in the garage. I am almost ready to glue the bottom section, layer by layer, in the boat!

    I do appreciate the points raised by Graeme and Arne. Failing and failing fast is often the most effective way to get something done successfully. Thank you for the advice.

    Last modified: 29 Aug 2019 13:33 | Anonymous member
  • 27 Aug 2019 14:27
    Reply # 7851336 on 6872873
    Anonymous member (Administrator)

    Analysis paralysis,

    what a great word! I don’t dare to think of all the projects which has been slowed down or stopped by AP. When I get attacked by AP, I try to be stubborn and keep on. I have found that I learn at least as much when I get things wrong as when I get it right, first time.

    I recently came to this conclusion:

    It is surprising how much we can achieve if we ignore how little we understand.

    Arne

     


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

                                                              Site contents © the Junk Rig Association and/or individual authors

Powered by Wild Apricot Membership Software