Designing ballast keel for 1917 pilot cutter

Discussion in 'Boat Design' started by Alfie, Oct 17, 2023.

  1. Alfie
    Joined: Mar 2020
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    Location: Falmouth

    Alfie Junior Member

    Hi all,

    I'm working on what is supposedly a little inshore Pilot cutter called Doris, built in Milford Haven in 1917. I'm not entirely sure she worked as a pilot, she has been registered in the 30s as such with lloyds but her size is quite yachty. She's not been in the water for a long time. Anyhow, she came to me without any sort of ballast keel, the previous owners removed an iron keel of around 1200kg which came from another yacht - speculation is that she lost her lead in the war. The iron keel was apparently a poor fit, wider than the wooden keel structure. The bottom of the wooden keel is only 100mm thick, and more or less parallel, so it's hard to imagine she was designed for so much external ballast.

    I'll be making her a new lead ballast keel, and I would appreciate any thoughts on how best to approach its design. I'm assuming I will want a substantial proportion of internal trimming balance in addition to the keel. Questions I have are:

    - How best to calculate the mass required to get her to sit on her lines, and where to put it.

    - How much weight and lateral plane I could get away in a 100mm thick ballast keel - it seems like a nasty lever on the keel bolts if she dried out on her ear compared to the wide base of most ballast keels I have seen.

    - If there's anything cleverer than a parallel lead shoe midships with timber fairing pieces fore and aft I should be thinking about. Would adding some rocker make her handier? More drag aft?

    I've attached a couple of photos, and a lines drawing taken from a 3D scan of her hull. There aren't any dimensions on the drawing, but she's 8.4m on deck, 7.1 on the nominal waterline and 2.3 beam. She's going to get a little 1gm10 that weighs I think about 80kg under the companionway, and a fairly minimal fitout. The mast is hollow and very light compared to the original.

    All thoughts appreciated!

    Cheers,

    Alfie.
     

    Attached Files:

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  2. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Have you bought Doris for yourself, or is this a refit project being carried out by yourself for somebody else?

    Lead is very dense stuff - approx 11,300 kg per cubic metre.
    If the keel is 100 mm 'thick' (I assume you mean wide? ), then if you had a rectangular section ballast keel that is 4" wide and 8" deep (0.1 m x 0.2 m) and say 5 metres long, then it would have a weight of approx 1,130 kg - fairly close to the weight of the old iron keel.

    The hull lines in your PDF are rather squiggly above the WL, but pretty smooth generally below the WL - do you know how to measure section areas, and then put them into Simpson's Rules to calculate the volume of displacement?
     
  3. Alfie
    Joined: Mar 2020
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    Alfie Junior Member

    Excuse me, didn’t make that clear - Doris is my boat. Thanks for the heads up on Simpsons rule, I’ll look into that. My concern with the width of the keel was not so much getting the weight down there but that most ballast keels seem to have a wider top, and the short lever a narrow top presents the keel bolts could stress them if she was dried out on her ear, or run aground at some point. I guess oversized keel bolts are a straightforward answer.
     
  4. wet feet
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    wet feet Senior Member

    I suspect that most of the squiggles in the hull lines are showing the location of the wale strake.Depending on our poster's proficiency with Rhino,it isn't so hard to copy the lines to a new layer and trim the copied set of lines to the supposed waterline-once the other layers are turned off.If the trimmed lines have a new element added to modify them to a closed shape it is quite easy to loft through the modified sections to create a solid,Which does require locating them in 3 dimensions.Once this is done the Analyse function can be used to find the volume and hence the displacement.Additionally,the area of the top surface will tell you the amount of waterplane for the TPI calculation.

    The thicker top edge of keels may be for structural reasons or it may be to create draft for the casting process.Given a keel only a few inches deep the structural aspect won't be quite the same as with a fin keel four or five feet deep.I'm not sure how easy it would be to get a long keel cast in the UK these days in iron but lead isn't so hard and there are specialists in the West country.Correctly locating cores for bolts on a long keel might be a challenge if it is desired to use existing bolt holes.
     
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  5. Alfie
    Joined: Mar 2020
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    Location: Falmouth

    Alfie Junior Member

    Wet feet is right, the lines are from a 3D scan so include the rubbing strake and are very literal. I didn't do the scan and I don't have Rhino unfortunately, although I do have the rhino files if anyone felt inclined to have a play with them... I intend to cast the keel myself in lead. The keel is original, but I'll be putting a new keelson over it so I think I'll plug the old holes and drill through the lot.
     
  6. wet feet
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    wet feet Senior Member

    If you don't have the Rhino files,you need to apply a scale factor to make a decent attempt to calculate displacement. I was thrilled for days when I worked out that you could calculate displacement by plotting lines equal in length to the area of each section,and at the correct spacing and then drawing a spline through the ends off the lines to create a bounded area.Ask the CAD system for the area of the section and you have the volume of displacement.Comparing the area found to the area of the bounding rectangle gives the prismatic coefficient and we don't have to blow the dust off the planimeter or refresh our memory of Mr Simpson or his rule.I have a feeling that some of this could be done in Excel,but have never tried it.
     
  7. Alfie
    Joined: Mar 2020
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    Alfie Junior Member

    That sounds like a great method, simpson’s rule does look quite dense to the layperson! Much appreciated.
     
  8. BlueBell
    Joined: May 2017
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    BlueBell . . . _ _ _ . . . _ _ _

    I don't have much to add other than keep it simple.
    Straight lead between the fore and aft timber fairing pieces.
    However, should you find your keel weight inadequate, you could add a chord ( a straight line joining the ends of an arc ) of lead.
    And shazam! Rocker and more weight ( and at a longer lever arm).
    If you like how it sails you could make it more permanent next haulout.
     
  9. Alfie
    Joined: Mar 2020
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    Alfie Junior Member

    Thanks all! I worked out the displacement from the drawings I have using the method wet feet suggested, which gave a displacement of 6.3m3. Seems on the heavy side, but hopefully not far off. If it's even close to right, I'm going to need a fair bit more weight in the ballast keel than I though to get to a sensible ballast ratio. I managed to get in touch with the old owner who did a lot of the work on her, and as I suspected she had concreted bilges which I suppose made up the rest of the ballast. This is quite appealing in as much as I can just get as much lead as is practical (and affordable!) on the ballast keel, and maybe even wait until she's in the water to distribute concrete and punchings in the bilge to be sure I can get her nicely trimmed. I think this is quite well proven for this sort of boat. Bluebell - Is there a way you had imagined to add a chord to the keel after the event without drawing the keel bolts to go through both pieces?
     

  10. BlueBell
    Joined: May 2017
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    BlueBell . . . _ _ _ . . . _ _ _

    Yah, I realized when I posted the idea this may come up.
    This is two "if's" away: If it's needed, and then, if it works.

    Shallow lag bolts into the new keel with or without underwater curing epoxy.
    If it works then you're going to need to repour the keel with the chord built into it.
     
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