Junk Rigged shallow draft boat

Discussion in 'Boat Design' started by Robert Hegarty, Nov 14, 2021.

  1. Robert Hegarty
    Joined: Nov 2021
    Posts: 4
    Likes: 3, Points: 3
    Location: Queensland, Australia

    Robert Hegarty New Member

    Hi everyone. This is a project I am working on at the moment and after trying to draw my design I went and bought some balsa wood and am making a scale model. This is working a lot better for me and after quite a few alterations I have a general hull shape I like. The biggest problem I have at the moment is working out how to place the centre-board, mast and sail size so I get the CE and balance of the boat correct. I had thought about using twin masts but would prefer to have only one as I am trying to keep her as simple as possible. I will be building her using plywood, clear pine, epoxy and fibreglass. Also is there much advantage in making the bottom with a slight curve ( across rather than rocker as I have enough of that ) rather than flat ( Sharpie style ). IMG_20211113_180624_885.jpg IMG_20211113_180624_999.jpg IMG_20211113_180624_908.jpg
     
    Will Gilmore likes this.
  2. Robert Biegler
    Joined: Jun 2017
    Posts: 59
    Likes: 24, Points: 8
    Location: Trondheim

    Robert Biegler Junior Member

    That is a problem that also comes up when converting a boat from Bermudan to junk rig. I recommend joining the Junk Rig Association (junkrigassociation.org), where you find the largest number people who have either converted boats to junk rig, or designed from scratch.

    One question they are likely to ask is which kind of junk sail you intend to use. A cambered junk sail is likely to have its centre of effort a bit further forward than a flat sail. A split junk has more area before the mast, so a centre of effort closer to the mast, so the mast can be further back. The split junk doesn't allow much adjustment of the balance area, the others allow you do adjust how much of the sail is before the mast by where you attach the halyard to the yard, and by using parrels.

    You could set up the boat with a board in front of the centre of effort, and some lateral area aft, either a long keel, or a large rudder. (If you use a rudder, it helps if it is balanced.) Then you can adjust the centre of lateral resistance. When you sail on the wind, drop the forward board to get what degree of weather helm you desire. When you sail off the wind, raise the board, giving the boat as much lee helm as you want. That may give you inherent self steering over a good range of courses to the wind. I once saw a 10 metre long shallow draft junk schooner in the harbour, and spoke with the owners and designers. Mudskipper had three boards. Two in the bilges, far enough aft that lowering only one or two of them gave lee helm, and one forward, which could give weather helm when lowered. While the owners were designing Mudskipper, they tow tested various models in a river. Their main criterion was control. They did not want a boat that would broach. The multiple centreboards were the solution, along with a canting rudder.

    Length to beam ratio also matters, though perhaps it is more hull length to mast height ratio. Mast height tends to be proportional to beam. So, for a given heel angle, the centre of effort moves out to lee by a distance proportion to rig height. So if you have a short hull relative to rig height (and beam), then your rudder has a short lever arm to counter the weather helm that you get. I had a Benford Dory 28, and it developed very strong weather helm when pressed hard with full sail. I am sure that had nothing to do with the junk rig, and everything to do with the boat being rather short for its beam, with a modestly sized rudder. In a 15 knots wind with full sail and perhaps 30 degrees heel (I think a boat should be under control at that heel angle), I needed 30 degrees rudder, which seems excessive, and was probably slower than if I had reefed a panel. But I wanted to see how the boat can handle being pressed.

    Your design seems to have similar length to beam ratio as that Benford Dory, so I would recommend either a large, balanced rudder, or that forward board so you can adjust the centre of effort to where you want it. My dory had a fixed keel only.

    Phil Bolger said that the first time he designed a sharpie that had the same curve in the bottom and side panels (having no curve in the side panel is, I think, specifically a trait of New Haven sharpies), he did so only to make the boat easier to build. To his surprise, he found the boat was easier to control downwind, and generally faster. He came to think that is because equal curvature in the edges gives equal curvature to the panels, and so equal pressure either side of the chine, and therefore fewer eddies across a sharp chine. I never came across an explanation how that works once you have leeway. Perhaps having eddies of the same intensity all along the chine helps? Whatever the explanation, Bolger says the boats sail better. The drawback was a stem that is out of the water, giving a flat overhang that can lead to slapping at anchor. And with the flare you get from the angle of the side panels, you would get a lot more rocker, so a lower prismatic coefficient. That tends to be an advantage at low speeds, a drawback at higher speeds.

    Also, Bolger says if you can round the chine, that can also prevent those eddies. The Dovekie (I think about 5.5 metres or 18 ft long) has no rocker at all, but a 10cm (4 inch) radius chine takes care of the eddies, at least with the curvature of the side panels it has and at the speeds it can reach. So if you want to go with Bolger on this, you could put in a chunky chine log that you shape to be round on the outside, or you strip plank just the chine.

    If you have a river nearby, do tow tests with a model. Set up two masts, and a longitudinal spar in between to which you can tie your tow line, so you can move the tow line fore and aft. Attach the other end of the tow line to a pole. Give the model a radio control, then steer it so that the tow line pulls at angles ranging from directly forward to as close to abeam as the model can handle. Do so with different heights of the longitudinal spar over deck to simulate reefing, and with different attachment points along the spar to simulate different locations of the centre of effort. If you can find a longitudinal placement that lets you control the boat at all attachment heights and all reasonable speeds through the water, that is where you want the centre of effort to be. If you need to move the towline fore and aft to balance at different flow speeds, different courses, and different heights, then your hull would benefit from an adjustable centre of effort.

    Also, that hull does look good.
     
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  3. Robert Hegarty
    Joined: Nov 2021
    Posts: 4
    Likes: 3, Points: 3
    Location: Queensland, Australia

    Robert Hegarty New Member

    Thanks Robert. I was going to put a large rudder on there and your explanation on placing the centre-board forward would explain why they are like that on the sampans and junks I have looked at. I will join the junk rig association, I have been watching some of the videos they are putting on youtube lately. I have also seen the video of the launch of Annie Hills new boat FanShi, which looks great and read a lot on the old Chinese Junks, Sampans etc and have always had a bit of a soft spot for the Sharpies.
     
    Will Gilmore likes this.
  4. bajansailor
    Joined: Oct 2007
    Posts: 2,382
    Likes: 813, Points: 113, Legacy Rep: 37
    Location: Barbados

    bajansailor Marine Surveyor

  5. clmanges
    Joined: Jul 2008
    Posts: 500
    Likes: 93, Points: 28, Legacy Rep: 32
    Location: Ohio

    clmanges Senior Member

    @Robert Hegarty
    Do join the JRA. It's cheap, and you get access to all their resources. You can download a free PDF copy of Practical Junk Rig, by Hasler and McCleod, as well as tons of other stuff. PJR is methodical and steps you through all the calculations needed for a design that will work.
     
  6. Robert Hegarty
    Joined: Nov 2021
    Posts: 4
    Likes: 3, Points: 3
    Location: Queensland, Australia

    Robert Hegarty New Member

    Hi bajansailor. she is going to be about 8.2 meters ( 27ft) I am building her to replace my TopHat 25 full keel boat so I can cruise some of the beautiful rivers and bay we have here in Queensland Australia. the Tophat 25 has a 1.2 meter draft which limits you a lot as to where I can go.
     
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  7. Robert Hegarty
    Joined: Nov 2021
    Posts: 4
    Likes: 3, Points: 3
    Location: Queensland, Australia

    Robert Hegarty New Member

    Hi clmanges, I have joined the JRA and downloaded Practical Junk Rig and a couple of other articles. It is certainly helpful and I am looking forward to learning from the other members.
     
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