Another mast problem

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  • 18 Sep 2019 08:30
    Reply # 7886826 on 7851186

    Many thanks to all who have taken time to reply to my postings.

    Martin, I will consider all the points you have mentioned as there is plenty to think about.

    Arne, I am delighted that you have given me your thoughts on the design for the rig. I will now go away and see if my old brain can digest the information. I will reply in a few days time.

    Again thanks to all.

    Steve

  • 17 Sep 2019 12:01
    Reply # 7884693 on 7851186
    Anonymous member (Administrator)

    Stephen

    Now I had a little go at trying to design a usable junkrig foro your mast.

    The drawn mast is only 9.4m, with a little of it below the wl, as shown. I guess if you have two tubes, 5m long, this would let you join them with 60cm overlap.

    On the shown rig I have reduced the battens from 4.9m (Ingeborg) to 4.6m. By picking a master sail with AR= 2.00 and scaling it to B=4.6m, the sail area ended up at 33sqm, giving an SA/Disp. = 18. This sail makes use of the 9.4m mast without pinching it.

    Fore-aft position.
    This is always tricky; in particular with long keel and integral rudder
    I have put the mast as far forward as I dared. The actual position of the mast and sail depends on, among other things, how the boat balances with the original rig. If the tiller balance is next to neutral, then the JR may well be shifted aft about 20cm. If the BR gives some weather helm, then I suggest you fit the JR in the shown position.

    Something like that...

    Arne



    Last modified: 17 Sep 2019 18:20 | Anonymous member (Administrator)
  • 17 Sep 2019 09:53
    Reply # 7884561 on 7851186

    Hi Stephen,

    Some random thoughts:

    • Can you redesign the sail plan to make the mast shorter? A shorter mast will be easier to make, easier to raise and have less effect on the stability of the boat. You only need to chop a bit off the mast to make 2x5m tubes viable.
    • One key consideration with a mast is to taper the strength of the mast to reduce stress concentrations. This means adding a sleeve inside the lower mast to reinforce it is a good idea. The sleeve can also extend the lower mast tube down to the foot, although given the stresses at the foot and the consequences of failure you probably don't want to do this. Sleeve could be wood or aluminium. You can then reduce the weight of the main tube.
    • Aluminium tube has a tolerance - typically +-1mm in both internal and external diameters. The diameter you get depends on how worn the extrusion dies were when they made your tube. I was very lucky in that my topmast/sleeve was a sliding fit in the main tube. On my dinghy the tubes were cheap enough that I could afford to buy the topmast tube and take the risk it wouldn't fit.
    • A wooden topmast is easier in that you can make it to fit, plus you can taper it externally to reduce windage. However it is more work and probably more expensive than an aluminium topmast. You will also need somewhere to build it.
    • If a wooden topmast would strong enough with a solid section that sounds like a good approach. Calculate the weight you would save with a hollow topmast to check it is significant enough to justify the extra work.

    Mast design does seem to involve going round and round in circles running endless calculations. You'll get there in the end!

    Cheers

    Martin

  • 17 Sep 2019 07:55
    Reply # 7884496 on 7851186

    Hi David,

    sorry it has taken so long to reply but I have been unavailable. The total length of the mast is 9.7m. With only a 5m aluminium tube available I would not get the bury between the two tubes. Unfortunately this project has to be a real budget one as the funds are just not available; I will be doing all the work myself and openly copying Arne's design. I will by necessity be using readily available materials.

    Steve

  • 04 Sep 2019 22:27
    Reply # 7864175 on 7851186

    Hi Steven,

    have you thought about reinforcing the wood top section with a layer of carbon fiber. This should strengthen it sufficiently for the stresses you have indicated. In New Zealand you can also purchase carbon fiber maxi yacht spinnaker pole sections that would suit the design you are looking at and reduce the weight of the mast. This might be a possible avenue to look at in the UK. Just the electrolysis issue with CF and aluminium to be careful with. By the way what is the total mast length you are looking at?

    All the best with the project, David.

    Last modified: 04 Sep 2019 22:30 | Anonymous member
  • 04 Sep 2019 20:07
    Reply # 7863882 on 7851186

    Hi Scott, I am planning on using 6082 T6 tube as that is readily available. I have calculated that I would have a bending moment at the join of 1334kpm. The nearest diameter of tube available to get near that figure is 114.3 o/d and 101.6 I/d (1404kpm). I am not sure how you would fill the gap between the tubes. I could turn a disc and shrink it to the bottom of the small diameter tube to keep things central but I am not sure about filling a 12.5mm gap. I also have the problem that if I allow 0.5 m bury I will be short of my desired mast height. I would then have to put 0.5m wooden base section. All suggestions welcome!!!

    Steve




  • 03 Sep 2019 21:40
    Reply # 7862265 on 7851186

    What aluminum alloy are you considering? I have read that the strength varies significantly with alloy and temper.

    Have you considered making the top section using a smaller diameter aluminum tube and not wood? 

    Last modified: 03 Sep 2019 22:46 | Anonymous member
  • 03 Sep 2019 20:15
    Reply # 7862190 on 7851186

    I have revisited my calculations and with the published displacement I will need an aluminium tube 6.5 inches outside diameter and a 6 inch inside diameter. The wooden section needs to be 6 inches diameter to have sufficient strength due to quite a high fighting moment. I find it a bit strange that the given displacement is 2545kg where the IF Folkboat is 2150kg and the Congress 25 is 2087kg. I did try calculating the mast stress with a 0.5m wooden base section but I still needed to up the diameter. Can anyone we an error in my thinking?

    Thanks Steve


  • 28 Aug 2019 07:03
    Reply # 7852629 on 7851186

    Many thanks to you all for replying. Arne, that is not the boat I am looking at. The boat is a Folksong  26

    FOLKSONG drawing

    Details are given on Sailboat data.

    I hadn't thought of adding a timber section to the base but it does seem pretty obvious now it has been pointed out. I will redo my calculations and see what happens. Once it has been confirmed that I have the boat I will attempt to post a picture.

     

     

    Thanks again, Steve

     

  • 28 Aug 2019 00:13
    Reply # 7852225 on 7851186

    I am no engineer, but it seems to me that you could use solid timber at the lower end of the topmast, where it is buried into the aluminium tube, and for the same distance above the join, then use hollow sections above that.

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