SibLim update

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  • 10 Mar 2021 18:16
    Reply # 10182919 on 10065452
    Arne wrote:

    Thanks, David for the numbers, most interesting.
    I notice that she shares some main dimensions with the Alo 28, Johanna. She have about the same length and beam in the waterline, and almost the same displacement.
    The difference is that Johanna has 1400kg finkeel and the SibLim design has about 40cm wider beam over all than the Alo.

    From the numbers it appears that the outside keel is 791kg and then there is 400kg inside ballast. Was FanShi actually built this way?

    Another thing; the vertical CB of the whole underwater body is given as 0.41m.
    Then the vertical (z plane?) COG of the whole boat is 0.761m, or well above the waterline. Does that mean that she is unstable within the first couple of degrees heel? Practical sailing has already shown that this is not a problem. I am just curious if the description ‘corky’ could have something to do with this.

    Arne


    Now I'm back into Freeship, and I can update these numbers.

    The draught of Fanshi is 71cm and the all-external ballast is 1218kg. The vertical CG is given as 58.7cm, but this is measured from the baseline which I've set at the lowest point of the hull, so that actually, it's around 20cm below the DWL. 

    And another point: this calculation was without the mast, sail, battens, parts of the fitout, equipment, personal items and stores, etc, which will have raised it to be at or just above the DWL, as is normal for sailing vessels. A motor vessel with no rig and no need for stiffness under sail will have its CG designed to be much closer to the metacentric height, so as to slow down the roll and make the motion more comfortable and less 'corky'. 

    1 file
  • 13 Feb 2021 16:06
    Reply # 10092357 on 4315719
    Annie, FanShi is a beautiful boat and you can be more than proud about what you have accomplished. Really inspirational. A big thank you to both of you for documenting the entire process so detailed and making SibLim open source!
  • 07 Feb 2021 14:33
    Reply # 10067306 on 4315719

    Annie, I only just caught up with the launching blog entry - congratulations on a fabulous project.

    We (Louise and I) can also add our weight to the fact that Arne's sail making instructions are perfect, and make the process clear, even to people like us who have never owned a yacht or made a sail - he deserves a medal!

    We (Lou) also did all the sewing on the boat (sails, covers, cushions etc) with a 30-year old domestic Brother machine, and as you discovered, it really made a great job of it.

    We just need to get out of lock-down now, and we can get someone with a bit of experience on-board to help us understand how to better sail her, and evaluate what we made...

    Either way, well done! and thanks for all the blogging.

    :-) 

    Last modified: 22 Feb 2021 11:54 | Anonymous member
  • 06 Feb 2021 16:15
    Reply # 10065822 on 4315719
    Anonymous member (Administrator)

    Thanks, David,
    and thanks to Ueli as well. The metacentric height is a bit in the fog to me, but I will read myself up on it. There is so much to learn...

    Arne

  • 06 Feb 2021 15:15
    Reply # 10065678 on 4315719

    hi arne

    Arne Kverneland wrote:

    …the vertical CB of the whole underwater body is given as 0.41m.

    Then the vertical (z plane?) COG of the whole boat is 0.761m, or well above the waterline. Does that mean that she is unstable within the first couple of degrees heel?

    the center of gravity doesn't need to be below the waterline to keep the boat stable. it needs to be below the metacentric height (initial mc of siblim is 1.791 m).

    ueli

  • 06 Feb 2021 14:08
    Reply # 10065596 on 4315719

    Ah, no, this can't be the latest version, but I'm unable to go back into the Freeship model and check it at the moment. One thing that I'm sure of is that Annie let me add 50mm more draught,  3 layers of steel plate instead of 2, so the ballast is all external, a bit lower, and its weight is 1200kg.

  • 06 Feb 2021 12:45
    Reply # 10065452 on 4315719
    Anonymous member (Administrator)

    Thanks, David for the numbers, most interesting.
    I notice that she shares some main dimensions with the Alo 28, Johanna. She have about the same length and beam in the waterline, and almost the same displacement.
    The difference is that Johanna has 1400kg finkeel and the SibLim design has about 40cm wider beam over all than the Alo.

    From the numbers it appears that the outside keel is 791kg and then there is 400kg inside ballast. Was FanShi actually built this way?

    Another thing; the vertical CB of the whole underwater body is given as 0.41m.
    Then the vertical (z plane?) COG of the whole boat is 0.761m, or well above the waterline. Does that mean that she is unstable within the first couple of degrees heel? Practical sailing has already shown that this is not a problem. I am just curious if the description ‘corky’ could have something to do with this.

    Arne


  • 06 Feb 2021 08:16
    Reply # 10065218 on 10063585
    Arne wrote:

    ‘Corky movement’
    I think that is a result of the detail I like most on FanShi. It is my view that David has chosen just about the best mid-section you can have on a shallow-draught boat. It ensures a narrow boat in the waterline (dimensions, please) for good light-wind performance, and then, as the boat heels, the wide ‘cheek’ ducks under and the centre of buoyancy literally jumps sideways and stretches the righting arm. Excellent! In addition, this section helps to lower the cabin sole for better headroom. The slightly ‘corky’ movement is a little price to pay to get a seaworthy boat with shallow draught.

    Congratulations, both to the Builder and the Designer!

    Arne

    Hydrostatics results are attached, for all the dimensions and ratios.
    1 file
  • 05 Feb 2021 19:03
    Reply # 10064089 on 4315719

    The motor is a 4-stroke, 6hp Nissan/Tohatsu.  I wouldn't have a 2-stroke motor.

    I cleaned the carburettor very carefully before using the engine.  I also filled the tank with new petrol.  Anyway, it now seems a lot happier.  I guess like all of us, it just wanted care and attention!

    David has drawings of the self-steering gear, on the website.  At this stage, I cannot praise it too highly.  It is far better than any servo gear I have lived with.  I wonder if 2 rudders and 2 trimtabs help with this.  There is a lot of power from those big trimtabs and they are always fully immersed.   However, I haven't used it in heavy conditions, so cannot say that i have thoroughly tested it.

    Yup.  I am perfectly happy with a bit of lee helm in very light conditions.


  • 05 Feb 2021 15:33
    Reply # 10063585 on 4315719
    Anonymous member (Administrator)

    Lots of interesting stuff by Annie and David, below.

    Lee helm:
    It appears that Annie has sorted this out well enough. Since tacking goes well and the wind vane copes, it is probably no big deal. I suggest you leave it as it is, Annie until David have designed SibLim 2 and 3 etc. and tested them. My own Johanna had lee helm in the same conditions as FanShi, and I hardly noticed it. Some noted it when sailing her in the JRA rallies in Stavanger  -  but they didn’t blame Johanna for being slow or difficult to sail.

    Asymmetric helm
    Only on my Malena have I noticed this. My guess was that on starboard tack, with the sail undistorted, the CP was sitting further forward of the geometric CE. This was expected to happen on Johanna as well, but I was unable to detect any differences. My hunch is that the (welded, distorted metal section) rudder, which pulled slightly to starboard, neutralised such tendencies when tacking under sail.

    Directional stability.
    To me there is no such thing as too good directional stability as long as tacking and harbour manoeuvres go well.

    ‘Corky movement’
    I think that is a result of the detail I like most on FanShi. It is my view that David has chosen just about the best mid-section you can have on a shallow-draught boat. It ensures a narrow boat in the waterline (dimensions, please) for good light-wind performance, and then, as the boat heels, the wide ‘cheek’ ducks under and the centre of buoyancy literally jumps sideways and stretches the righting arm. Excellent! In addition, this section helps to lower the cabin sole for better headroom. The slightly ‘corky’ movement is a little price to pay to get a seaworthy boat with shallow draught.

    Self-steering gear.
    I hope Annie will produce a real write-up, with sketches, photos and a few numbers. I bet many more than I are impressed that the brand new FanShi was launched with a brand new, home-made wind vane, which worked from Day One.

    Motor:
    Is that a four-stroke or a two-stroke? Since it appears to behave better and better, it may well have had problems with sludge in the carburettor from old petrol. I have seen coughing, spluttering outboards getting a new life after the carburettor has been taken off, opened and given a good rinse. And a new spark-plug, of course. My own 4-stroke, which I start some 30-60 times a season, but which only runs long enough to burn 2-3 litre, is kept happy by running on alkylate petrol.

    Congratulations, both to the Builder and the Designer!

    Arne


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