Coal Car 12. Narrow, deep rocker scow

Discussion in 'Sailboats' started by sharpii2, Nov 21, 2012.

  1. sharpii2
    Joined: May 2004
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    sharpii2 Senior Member

    Hi, everybody.

    When I started to design this boat, I thought of all the things I love and detest about sailboats.

    I love being out on the water with the wind moving the boat along as the water sometimes trickles and sometimes roars past. I love the idea of the wind doing the work muscle or petroleum usually does. And I love the challenge of getting to a destination in a sometimes round about way.

    What I detest is the centerboard or dagger board case always being in the way, the long set up time of a typical Bermuda rig boat with its pesky battens, which often slide out of their pockets, and or refuse to lay flat when the sail is furled, and I detest the mast being so tall it needs stays, which mandate putting it on the center line of the boat.

    I know all these things improve performance, especially upwind, and that's why they are on just about every sailboat I see.

    But if I'm designing, I can be boss. I can say I like performance too. But I also like comfort, short set up times, and better versatility.

    As always, everything is a trade off. More of one thing usually means less of another. As a designer I get to choose.

    So I chose.

    (see attachments)

    I went with an unstayed mast so I could move it to one side of the boat. This eliminates a few aggravations, one of which is the end of the sheet line having to be right over the tiller. I can also sit next to the mast, if need be, making raising and lowering the sail easier in some situations. This also keeps the boom away from my head on one tack.

    I chose a balanced lug and split it, so the mast could end up further aft and the halyard easier to reach.

    I kept the sail area relatively small for the displacement to keep the spar lengths within reason and for sailing when it's really fun to sail. A single short sweep, which can switch sides, in concert with the rudder, will provide propulsion when it's not. It is not as good as oars, but far better than a paddle. The smaller rig should also enable sailing during windy days and two reef points should further this advantage, so the boat can be used under a wider range of conditions.

    Being this is a small boat ( 12 x 3ft) the skipper is the heaviest component, so it is best to keep his weight where it does the least harm and offers the most help, near the mid length of the boat. For this reason, the helm is a bit too far aft for comfort. To solve this problem, there is an endless line, from the tiller to the splash boards, and back again, so the skipper can have the helm no matter where in the cockpit he happens to be. This also facilitates the skipper steering while facing either forward or aft.

    The Beam was kept small, so the boat would be easier to handle when out of the water. A long, narrow boat is usually easier to carry and car top than a short wide one. It is also easier to right, if it capsizes.

    The deep rocker, straight sided scow was chosen for ease of construction and for an experimental reason.

    The experimental idea being that, when there is little wind, the boat is nearly completely upright, making the Water Line short and the D/L high, which is good for drifting (if the water is flat, of course). When it breezes up, the boat heels and the Waterline lengthens, decreasing the D/L right at a time I would want it low, perhaps increasing the top speed potential of the boat. Whether or not I'm right about this, only experience will tell. (The skipper will also induce heel when using the sweep, which is why oars aren't used.)

    Though this boat has the proportions of a Garvey, it really isn't one. A true Garvey would have sprung, flared sides and a toboggan like bow. This is to maintain the Water Line length, while the boat is up right, and to put rocker in the bottom, without having to cut it into the planks. Planks with splinter like ends are a definite no no in plank on frame boat building, and that's what you would end up with if you went with a rocker like this, especially with straight sides. Being this is a plywood boat, I have no such limitation. And I, as this is a one off boat, don't want to have to build a strong back either.

    Though the platform in the middle of the boat adds weight and some complexity, it also keeps my butt out of the bilge water. Also, in the event of a capsize, it will temporarily trap water, making getting back on board the boat that much easier.

    The flotation system is intended to make the boat less likely to turtle, when capsized, and to make it easier for the skipper to get back on board, if it does. Since the flotation one one side is less than the weight of the boat, the boat will have a considerable amount of water aboard when it's righted. This can work to the skipper's advantage. The skipper can pull one side down and practically swim back on board. Once on board, the flotation on that side will pull it back up, so there is some freeboard and the skipper can start bailing the water out.

    The boat will be stored upside if kept outside, or on its side if kept inside, so it will use 12 sf less floor space.

    Attached Files:

  2. messabout
    Joined: Jan 2006
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    messabout Senior Member

    You will be better served by this boat if you increase the beam. Three feet is a bit too skinney. Judgeing only by the appearance of your drawing, you'll need to move the sail forward a ways.

    You have used a lot of rocker which will make the boat more than ordinarily sensitive to fore/aft trim. You said you do not want a centerboard but the drawing showns something sticking down below the bottom. Perhaps that is a lee board.

    Why do you believe that a garvey has pointed lumber parts?
  3. sharpii2
    Joined: May 2004
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    Location: Michigan, USA

    sharpii2 Senior Member

    Hi, Messabout.

    Actually the narrow Beam will not effect stability nearly as much as it appears.

    This is for two reasons:

    1.) the crew weight is kept very low in the boat (level with the WL) and
    2.) much of the righting moment comes from the ends.

    I do not believe Garveys have pointed planks.

    I said, or meant to say Garveys are design the way they are to avoid such an occurrence. That is why they have flared, sprung sides and a toboggan like bow. This is to avoid making oblique cuts into the side planks, which would have to be done to built my design plank on frame with straight planks. The flare of the sides, combined with the side to side curve, creates a fore and aft curve in the bottom, often without having to cut any curve into the side planks at all.

    You can prove this by bending a fore and aft curve in a three by five card with its edge, on a flat surface. Holding that same curve in the card on the same flat surface and tilting it outward will lift its end corners up while leaving its middle still touching the flat surface. If you look at it, then, from a view parallel to the flat surface, both the bottom and the top of the card will appear curved upward. This is one of the reasons why so many flat bottomed boats have flared sides.

    My design could be built plank on frame.

    It could be done, with the planks along the bottom edge of the side more or less following the curve of the bottom and the other side planks 'nibbed' into them, so all of them are more or less rectangular at their ends. Such construction would be more labor intensive than just building a Garvey, so it's little mystery to me why this idea was never tried.

    The idea is highly experimental and may not work out as well as I hope. So, if worse comes to worse, a Garvey hull could be built to replace my experimental one, and use its rig and appendages.

    I don't think that is going to happen, though.

    I'm at least 75% confident my idea will work and create a whole new type of scow, which will have a greater range of stability and be more sea kindly, as, when heeled, a much deeper 'V' will be presented to the sea, without immersing the bow or stern transoms.

    I actually did stability calculations and found well over 200 ft lbs of righting moment with the crew sitting on the platform. Heeled at 30 deg., the righting moment went past 300 ft lbs. At this point, it would be very uncomfortable for the fore and aft facing skipper, and she/he would be sure to let the sheet out to decrease the angle.

    If you look at the 3rd attachment of my previous post, you will see that the dagger board is side mounted outside the hull, held there with a rack.
  4. messabout
    Joined: Jan 2006
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    messabout Senior Member

    Tom Firth Jones book; Low Resistance Boats, has the plans for a 12 foot garvey. Offsets and sufficient building information included. The chine beam is 37 inches and sheer is 52 inches. He is using a 90 sf peak sprit sail. He says that it is a pretty lively little boat.

    Your boat looks a bit like a stretched and skinnied PDR. I am surprised that you can get 200...300 foot pounds of RM with so narrow a bottom. RM is, proportionally, roughly equivalent to the cube of chine beam. Adding a few inches to the chine beam makes a big difference.

    You are correct that heeling such a boat will change the shape of the waterlines to advantage. However, to maximize the shape and area distribution, reduce wetted surface, lengthen the DWL, promote favorable section centroid alignment, you need to pinch the ends together to some dimension that you discover by tinkering with the drawing. The parallel sided box shape does not do that very well. Ideal heel angle usually falls between 10 and 15 degrees with a cleverly proportioned little scow/garvey/punt.
  5. sharpii2
    Joined: May 2004
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    sharpii2 Senior Member

    I have that book and his Garvey is a wonderful design. But is more of a pure sailboat than I have in mind.

    What I'm trying to do here is highly experimental. Which is why the first hull probably won't be built to last (no epoxy/glass sheathing and cheap exterior plywood, with lots of roofing cement, PL premium, sheet rock screws, and porch and deck paint)

    Pinching the ends together will do just what you say. Such will keep the transoms out of the water as the boat heels, as the side to side curve becomes the new bilge, as the boat heels.

    What I have in mind will work nearly as well and have other benefits. Two of which are:

    1.) The boat will be able to turn quicker when upright and
    2.) the heeled chine will be much deeper and may be able to act as a keel itself, possibly allowing the boat to make upwind progress without the dagger board (it may even be able to sail without a rudder, as the fore and aft trim is so easily adjusted)(see comments below)

    The notion that Beam between chines determines initial stability is an over simplification. Like most over simplifications, it is more often than not true. But it is not always true.

    Initial Stability is more closely linked to Meta Center Height. And that is proportionate to the average Waterline half Beam, cubed, multiplied by two, then multiplied by the Waterline Length, divided by the volume of the displacement, with the height of the Center of Gravity above the Vertical Center of Buoyancy subtracted from all that.

    A very long narrow boat can have a higher Meta Center than a much shorter, but somewhat wider boat of the same displacement.

    As the CC-12 heels, the waterline lengthens and the half Beam increases.

    And even this is an over simplification.

    More accurately, the Center of Buoyancy shifts much further toward the lee side as the end immersed sections don't have much depth, so their individual center of areas are far more out from the center line than that of the deepest section.

    This, by the way, happens with all scows. The difference here is the Water Line increases significantly as well, making the CC-12 more initially tender, encouraging heel, until much of the lee chine is immersed. Then she should stiffen up noticeably. Hence she will then have a sharp 'V' bottom and a long Water Line, right at the time it will be most beneficial.

    One of the other major reasons for going with this idea is to avoid sharp bends in the cheap plywood. The toboggan bow would be the hardest to plank. Mr. Jones's Garvey was built plank on frame. A way of getting around this problem is to facet the sharp curve with multiple bow transoms. It will look ugly but will probably work. That is probably what I'll do, if I end up replacing the CC-12 hull with a Garvey.

    One possible problem I see with the CC-12 is its sensitivity to for and aft trim. Trim it too far forward and the Center of Area (CA) of the sail will move far forward of that of the dagger board. Trim it aft and you get the opposite effect. In the first case, it will fall off the wind, and in the second, it will round up. This could be a problem and is one of the reasons why I arranged things so the skipper can reach just about everything without having to move about the boat.

    Though it looks a lot like a stretched out pdracer, it is not the same concept. really, it is more like a Bolger 'brick' with the ends lengthened by about 50%, maintaining the same rocker rise, so the ends come out of the water.

  6. messabout
    Joined: Jan 2006
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    messabout Senior Member

    If you wish to consider a curvey bow like the T.F. Jones boat you can do it with cheap ply, no sweat. Use Home Depot 4mm luan bent across the sheet. Laminate two or three thicknesses. You can now brag about having cold molded at least part of a boat.

    In any case you are encouraged to get started with the build and keep us informed.
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