Windage and choosing a mast - Alberg 30

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  • 02 Feb 2021 18:35
    Reply # 10056453 on 10035665

    Thanks Arne, I got the same numbers too. So I take it the sleeve would be unnecessary? Yes I've been looking at rigs similar to your sketches with the help of a sailmaker. 

  • 01 Feb 2021 14:56
    Reply # 10053071 on 10035665
    Anonymous member (Administrator)

    Ryan,
    now I went through the numbers (didn’t double-check...). If the boat has a

    Disp = 3970kg, Beam = 2.667m and I use a guess factor of 0.20, the 'guessed' righting moment should be

    Mr = 3970kg x 2.667m x 0.20 = 2118kpm

    When using the given max righting arm =1.352ft = 0.412m the righting moment will be

    Mr = 3970kg x 0.412m = 1635kpm ( = 77% of my guessed Mr)

    My way of guessing the righting arm tends to result in slightly high values of Mr, but I guess it’s better to be safe than sorry, and if I manage to make a mast which doesn’t add more than 3% to the boat’s displacement, I am happy enough.

    Now, if I didn’t screw up entirely, and the sigma of that 6061 alloy is 250MPa, the yield strength of that mast should come out at My = 4777kpm, which is over twice the MR, even using my conservative method.

    The lower part of that mast therefore appears to me to be strong enough.

    Arne

    PS: Are we still talking about one of these rigs?

    Last modified: 01 Feb 2021 20:07 | Anonymous member (Administrator)
  • 01 Feb 2021 13:45
    Reply # 10052942 on 10052350
    Anonymous wrote:
    Ryan wrote

    Hi Arne,

    I was looking at 25cm but was concerned about the windage which made me look into your hybrid technique. However it looks like at this size the difference in windage isn't very significant (if I were to use a 8" straight tube). Nonetheless the hybrid mast does look a lot easier to put together. What do people do for a step and does it need a sleeve if the tube is strong enough? I’m thinking it might be hard to find a tightly fitting sleeve tube at that size.

    It depends what alloy is in that tube and its wall thickness. With these facts in hand, one can calculate its yield strength (bending moment).
    If you look up p.4 on Chapter 6b of TCPJR, you have a method. I usually use mm in the formula and convert from Nmm to Nm, and then kpm in the end.
    (one inch = 25.4mm)

    http://goo.gl/WE36w3

    Arne


    Arne,

    Ive become quite familiar with your lovely guide. As mentioned in the original post I was looking at 8” 0.25 6061T6 which I’ve found in 24’ lengths. The data I have for righting arms comes out 20% shorter than what I got from your estimation method (original post first attachment). I’m looking into the carbon options for top masts as I heard people have used spinnaker poles at reasonable cost.

  • 01 Feb 2021 08:34
    Reply # 10052350 on 10050650
    Anonymous member (Administrator)
    Ryan wrote

    Hi Arne,

    I was looking at 25cm but was concerned about the windage which made me look into your hybrid technique. However it looks like at this size the difference in windage isn't very significant (if I were to use a 8" straight tube). Nonetheless the hybrid mast does look a lot easier to put together. What do people do for a step and does it need a sleeve if the tube is strong enough? I’m thinking it might be hard to find a tightly fitting sleeve tube at that size.

    It depends what alloy is in that tube and its wall thickness. With these facts in hand, one can calculate its yield strength (bending moment).
    If you look up p.4 on Chapter 6b of TCPJR, you have a method. I usually use mm in the formula and convert from Nmm to Nm, and then kpm in the end.
    (one inch = 25.4mm)

    http://goo.gl/WE36w3

    Arne

  • 31 Jan 2021 16:40
    Reply # 10050650 on 10050489
    Anonymous wrote:

    Ryan,
    from the general dimensions of the Alberg 30, I conclude that her righting moment must be pretty close to that of Edmond Dantes. Edmond, a Mikkel 32, had a 48sqm JR sitting on a hollow (dug-out) spruce mast. The mast's total length was 10.6m (I think it was) and the diameter at deck level was 25.0cm. I often spotted that boat when driven quite hard, but it never appeared that the mast was struggling. I therefor believe that a 25cm spruce-mast would be fine on your Alberg 30 as well, even if you have to make the mast a bit longer.

    Good luck!
    Arne

    http://goo.gl/KIOwkH



    Hi Arne,

    I was looking at 25cm but was concerned about the windage which made me look into your hybrid technique. However it looks like at this size the difference in windage isn't very significant (if I were to use a 8" straight tube). Nonetheless the hybrid mast does look a lot easier to put together. What do people do for a step and does it need a sleeve if the tube is strong enough? I’m thinking it might be hard to find a tightly fitting sleeve tube at that size.

    Last modified: 31 Jan 2021 20:25 | Anonymous member
  • 31 Jan 2021 15:28
    Reply # 10050498 on 10049325
    Anonymous wrote:

    Hi Ryan,

    it is always a balancing act between sail area and righting moment. I use a figure of one pound per square foot pressure on the sails for my calculations. This equates to a 20 mph wind or about 17 knots. This is about when most cruising boats start to reef. If you set more area then you have to reef earlier so that the sail pressure continues to equal the righting moment. Your boat has a sail area of 402 square feet (according to sailboatdata.com) so any more than about 500 square feet would be pushing it a bit.

    All the best with the project. David.


    Gotcha. I was taking the safety margin over my max moment which occurs at 60deg with an arm of 1.352 ft (per data in first attachment original post). Is it best practice to go with the max righting moment or the fully canvased heeling moment at 17 kts? Seems at 487sqft they become equal. I was looking at 460 sqft and considering adding more for a light air panel to be reefed at 10kts.
  • 31 Jan 2021 15:18
    Reply # 10050489 on 10035665
    Anonymous member (Administrator)

    Ryan,
    from the general dimensions of the Alberg 30, I conclude that her righting moment must be pretty close to that of Edmond Dantes. Edmond, a Mikkel 32, had a 48sqm JR sitting on a hollow (dug-out) spruce mast. The mast's total length was 10.6m (I think it was) and the diameter at deck level was 25.0cm. I often spotted that boat when driven quite hard, but it never appeared that the mast was struggling. I therefor believe that a 25cm spruce-mast would be fine on your Alberg 30 as well, even if you have to make the mast a bit longer.

    Good luck!
    Arne

    http://goo.gl/KIOwkH


  • 30 Jan 2021 23:51
    Reply # 10049325 on 10035665

    Hi Ryan,

    it is always a balancing act between sail area and righting moment. I use a figure of one pound per square foot pressure on the sails for my calculations. This equates to a 20 mph wind or about 17 knots. This is about when most cruising boats start to reef. If you set more area then you have to reef earlier so that the sail pressure continues to equal the righting moment. Your boat has a sail area of 402 square feet (according to sailboatdata.com) so any more than about 500 square feet would be pushing it a bit.

    All the best with the project. David.

  • 30 Jan 2021 21:11
    Reply # 10048979 on 10048843
    Anonymous wrote:

    Hi Ryan,

    I agree with Graham Cox's comment and would advise the 8"diameter 1/4" wall flagpole section as a good bet for your mast. I think it would be more than adequate for your needs. If you have a sail of about 400 square feet with a center of effort about 25 feet above the waterline you should have a safety factor of just under 3, which is fine for offshore work.

    All the best with the project, David.

    Thanks David. I’m looking at 460-550sqft to include extra sail area for light air panel. Will be higher aspect due to my rudder. I was looking at tensile strength times section modulus vs my max righting moment and was getting about 2.4 factor for the flagpole but perhaps I’m approaching this wrong. It seems like some people calculate based on the forces on the sails and others on the force of the righting moment. 

    Last modified: 30 Jan 2021 22:34 | Anonymous member
  • 30 Jan 2021 19:51
    Reply # 10048843 on 10035665

    Hi Ryan,

    I agree with Graham Cox's comment and would advise the 8"diameter 1/4" wall flagpole section as a good bet for your mast. I think it would be more than adequate for your needs. If you have a sail of about 400 square feet with a center of effort about 25 feet above the waterline you should have a safety factor of just under 3, which is fine for offshore work.

    All the best with the project, David.

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