Canting (swivel) keel…thoughts?

Discussion in 'Stability' started by Josh Smith, Oct 14, 2022.

  1. Josh Smith
    Joined: Feb 2022
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    Josh Smith Junior Member

    Posted awhile back about a grandbaby pirate ship design. Making a lot of design progress there. It occurred to me moments ago to attempt a variation of a canting keel underneath the hull to help address lingering stability concerns. If you find this idea useful, please steal the heck out of it.

    It works a little something like this: waterline length is 25’6”, so at CB just beneath full length keel mount two pivots to anchor two swing arms. Each arm would be steel tubing and end in a cap after 8’. I cannot give you steel tubing width or thickness dimensions because I haven’t gotten that far yet. Into both of the capped steel tubes add approx 850in3 of molten lead while the tube is oriented to allow the lead to flow to the capped end before freezing. Assume outboard portions of steel tubing have more internal volume and that the arms taper inboard closer to the pivots.

    Mount swingarms to pivots on keel, one lead filled capped end aft and one near the bow. The empty ends can be capped, or will be, but are structurally mounted so they swivel 90degrees from neutral (near the keel) to port or starboard on the same plane, parallel to the water surface. In this position there will be ballast fore and aft and the arms are next to the full length keel. One pivots to starboard from the bow while the other pivots to starboard from the stern position. Same same for port.

    Using a single line looped thru a winch on deck, running to the swivel point at the pivots, winch it one way to fully deploy both arms to starboard, and winch it the other way to go to port. I figure draft on this boat in the 2-1/2’ range, and my guesstimate puts the total canting (swivel?) keel ballast at 700lbs, 1ft3 of lead, 8’ out from centerline and 3’-6” past the side of the vessel.

    Deploy and retract forces cancel minus friction since the bow plane will swivel back from movement thru water so the stern plane will push forward with less effort. Retracting is the same in reverse. Water movement pushes the stern ballast plane back on its swivel to minimize effort expended winching the fore plane back to neutral in line with the keel. Assume a hydrodynamic NACA- neutral fairing for the tubing and that I get the structural numbers right for the swivels, the mount, and the span.

    Trim is easier, just don’t fully deploy or fully retract. I think the ballast planes will break the surface at a little over 25 degrees of heel. Voilà. Shoal draft canting keels?

    Tell me what I did wrong. Why won’t it work for a trailerable application? Where does it have to be careful?
     
  2. bajansailor
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    bajansailor Marine Surveyor

    This is all very clear to you, as you have designed it / come up with the concept of it.
    But when I try to read and understand your explanation above I rapidly lose interest, as I haven't a foggiest idea as to what you are trying to do.
    Can you post a drawing or two perhaps?
    As the classic saying goes, 'a picture can be worth a thousand words'.
     
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  3. Robert Biegler
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    Robert Biegler Junior Member

    I think this bit means that the rotation axes are vertical, not horizontal. Is that what you mean?

    In that case, why have two ballast arms? To maintain fore and aft trim? You could reduce the effect on trim by having a longer, single arm with the rotation axis nearer the bow, and swinging only 40 degrees or so to either side instead of 90. Then the keel in front of that axis could go a bit deeper to protect the ballast arm against grounding.

    That the largest forces on your ballast arms come from drag should only be true if you assume the boat is not heeling, which would need pretty quick and well-timed effort during a tack or gybe. Also, I would rather streamline those ballast arms. Round tubes will have very high drag. That would not only slow you down, but also give you strong weather helm.

    I don't see why it shouldn't. You could still look up the Defline 19, which has canting bilge keels. For falling dry, cant both out to the side. I don't see how that works for trailering, but given the size of the boat, there is a good chance it was intended for trailering, so you could see how naval architects solved that problem.

    High bending loads at the elbows of your ballast arms, and sand or mud getting into the lower bearings when you fall dry. And you will need good thrust bearings, so that the whole thing doesn't just drop out of the boat.
     
  4. gonzo
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    gonzo Senior Member

    What's a pirate ship?
     
  5. Milehog
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    Milehog Clever Quip

    He wants a cartoon boat with high topsides that doesn't heel and has no hope of sailing other than drifting downwind.
    Bowsprint, cutter bow and an even higher aft house are important. Look up his older posts.
     
  6. Milehog
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    Milehog Clever Quip

    Josh, have you ever sailed?
    It seems you are trying to reinvent the wheel with planks and angle iron.
     
  7. Josh Smith
    Joined: Feb 2022
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    Josh Smith Junior Member

    You’re absolutely right there. Small boats on lakes. That’s it. Just brainstorming and posted a bad idea that’s too complex and detrimental to performance.

    performance was never the idea though. Just something that sailed built by my hands that didn’t have to win any races except to stay afloat and make kids smile.

    I am building it off of what was state of the art for three hundred years, plus the improvements in materials that make it faster to build and stronger. Something along the lines of Indeavor, the first boat built in North America.
     
  8. Milehog
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    Milehog Clever Quip

    I'm not going to mince words here.
    The cartoon sailboat you propose has severely compromised sailing ability and is not safe.
    You don't know why it is not safe, yet somehow you seem to think you are ready to design a boat to put innocent children in.
    If you want them to like sailing, get-build them an established design that can sail. An ill-advised tub will turn them off to the idea.

    Save the cartoon carrack for a place to play pirate in the backyard.
    Please.
     
    Last edited: Oct 15, 2022
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  9. Milehog
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    Milehog Clever Quip

    Josh, listen carefully. You need to put your illconceived ideas aside and study modern boat design basics from the very start.
    Can you do that? I have my doubts but would like to be proven wrong.
    Study the modern basics, ask questions based on reality here, and learn.
     
    Last edited: Oct 15, 2022
  10. Josh Smith
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    Josh Smith Junior Member

    The cartoonish drawings you saw were what we drew during the beginning Alzheimer’s phase when mom moved in with us. She had a weird long Covid issue that turned her memory issues into a full cognitive reset where she knew names and nouns and verbs but had no memories and virtually no gross or fine motor skills. During that time, she did remember that she was a draftsman and it coincided with us watching a pirate movie, and since we were sleeping in shifts to keep her from wandering off or burning the house down, her and I collaborated on a series of drawings.

    Thos early drawings are ridiculous, but I can’t erase them. Since then, we’ve reached out to designers and builders to no avail. We have three sets of GlenL plans lurking in the house, but her heart was set on something like this. We purchased and read and reread several of the books you might want us to have studied. The original library included Hiscock’s Cruising Under Sail and Vigor’s Pratical Mariner’s book. I have worn out both of those and had to buy new ones since I first picked them up when I was stationed in Hawaii. I picked up Monk’s How to Build Wooden Boats. Combed through Rigger’s Apprentice. Couple of outdated fiberglass boat building books. Larsson’s Yacht Design. Was nudged by BDN recommendations to have a look at Colin Mudie’s Sailing Ships. That led me to look at a Reconstructing the Indeavor piece that really broke down the guidelines for how boats were built before they came with plans or modern tools and materials. Those led to Beuhler’s Backyard Boatbuilding, and The Art of Rigging. Picked up a couple of model ship rigging books for notes on construction and rigging accuracy.

    The Indeavor piece is really the focus of my current thinking, as it was 12 tuns berthen at 33’ with 10’ beam. I am scaling that down to trailerable with 8’6” beam and 25’6” length. Instead of fore and aft castles, we’re increasing the height of the bulwarks to give it the classic lines without the increase in topsides weight, and putting a crow’s nest on the bowsprit instead of running it up the ratlines to the top of a mast. Deck height is 4’6” so grandbabies are the only ones who will be able to stand up below deck. I’m trying to do something fun with that. 6500lbs, 2’6” draft, 1-1/2” thick plywood hull and I think pitch for hull coating.

    Three masts, loose foot lateen. Deadeye rigged.

    anyway I know that all the volunteer contributors here would saw off a pinkie with a butter knife before standing by and allowing someone to do something dumb. I will have good lines drawings and seek input before laying the keel or even taking any additional steps. I do have specific questions, like would you want a cubic foot of lead ballast concentrated near CB or do you want it poured all along the keel? Questions like hey, can I rig these lateens for dipping the yards when I tack, or just rig it so we lose a little bit of sail area on the port tack? Questions like, how does one get a motor into this thing?

    There were moments, I’ll admit, when I muttered “plywood and angle iron”. But I have yet to encounter someone here who didn’t care about the craft enough to give helpful advice. That is appreciated. I appreciate all the help and advice and I will Indeavor to be more on point with the questions
     
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  11. Milehog
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    Milehog Clever Quip

    Good luck.
     
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  12. gonzo
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    gonzo Senior Member

    What are you referring to? Ship and boat building developed for specific uses and localities. It was always dynamic and changed when needs changed and either new materials became available, or old ones scarce.
     

  13. Robert Biegler
    Joined: Jun 2017
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    Robert Biegler Junior Member

    I got so focused on reconstructing a mental image of the swivel keel from your words that I forgot this was your purpose. Shifting ballast is good for increasing sail carrying power. It is not good for addressing lingering stability concerns unless you can shift the ballast fast and appropriately. A 49er skiff has lingering stability issues, and a skilled crew can keep it level. Your ballast would have to be as responsive as a skilled crew. any system for shifting the amount of ballast you would need would be power hungry, and likely noisy enough to do away with the feeling you are aiming for. And when it fails, you will be in trouble. Accept that the boat will heel.

    That said, it should be possible to design something that, more or less, meets your requirements. First, get yourself Bolger's book "Boats with an Open Mind". Design 51 in there is Grandpa's Pirate Ship. A bit smaller than what you have in mind, but a start. Bolger's Birdwatcher and Whalewatcher designs, described in the same book, get decent stability at 90 degrees heel angle from the combination of water ballast and high freeboard, and a good view from having a good portion of that freeboard be plexiglass. They don't have the look you want, though.

    Bolger's Berengaria looks to be pretty close to what you ask for. The only image I could find online is this drawing, so I guess she was never built.
    [​IMG]
    As drawn, the boat has the full length glass house of Birdwatcher and Whalewatcher. A competent naval architect can tell you whether you could raise the deck aft of the main mast a little, put a centre cockpit before the main mast, lower the deck before the cockpit, and still get a decent stability curve. Or build it as designed, and your grandchildren will have a harder time to manage falling overboard. The gaff schooner probably needs less work than a three-masted lateen. I don't know how old you expect your grandchildren to be when you finish the boat, but if they will still be young enough to appreciate the pirate ship aesthetic, they are likely to be young enough to need one adult to focus just on them. The other adult will be singlehanding the boat. I don't think that is compatible with three lateen sails.

    As for the aesthetics of Berengaria, Bolger commented that "The general style was inspired by a galeass [a heavy, wide, high-sided development of a Renaissance war galley, used with great effect at the Battle of Lepanto] model that is (or used to be) in the Arsenal Museum at Venice, supplemented by, among others, a Swedish galley yacht design in Chapman's Architectura Navalis Mercatoria."

    Berengaria is larger than what you aim for. At Duckworks, I spotted what seems to be a smaller version, the Amherst Galley: Duckworks Indexes - Bolger A-E https://www.duckworksmagazine.com/r/plansindex/bolger/bolger.htm And I found a discussion of the design here: Amherst Galley https://www.woodworkforums.com/f33/amherst-galley-91157

    If you get those plans, you can save time on designing and start building before your grandchildren grow out of pirate ships.
     
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