Designing small utility/work boat-skiff rocker question

Discussion in 'Boat Design' started by Murky Deep, Jun 8, 2020.

  1. Murky Deep
    Joined: Jun 2020
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    Murky Deep Junior Member

    I have built a few skiffs (8) including a successful one off based loosely on a free design. I have read hundreds of posts from this site, Sucher and Gardner's books. Despite owning a bunch of plans, I site haven't found what I am looking for. I've spend thousands of hours on the water since 1989, everywhere from California to Croatia. I enjoy small boats that can be moved with 1-2 people, low horsepower(5-15), and I rarely go more that a mile or so from shore. Any boat that I own needs to do duty as a light fishing boat, family beach boat, and clamming skiff. Hannu Vartailia has values of efficiency with materials that I admire. Unlike many others, I do not enjoy high speed deep v hulls, and have friends that take me fishing on such boats often enough anyway. If the weather is bad I simply stay home.

    Now, what I plan on doing is building a serious strong back and 3-4 molds (2x6) that will produce a 14-17 foot skiff with a flat bottom out of ply or pine planks. Target weight is around 220-275 pounds. I want to build setup that can be disassembled and put back together, and produces a quality boat that would work well with 8-12 hp, yet still could be rowed home if needed. At least 1 friend and a family member will want one as well. I would build a 14 ft Brockway skiff, however, I am tired of one mold skiff building. Fighting the boat into symmetry takes up far too much time.

    The amount of freeboard forward is easy, as is the beam (just under 4 ft at the chines, 4'8-5' at the shear). Where I am struggling is the amount of rocker aft. In most of the pure rowing skiffs, I see 4-6 inches or so, in many power skiffs, none. I am happy with 6-10 mph under power, so as I try to decide on the height of that last mold to determine the rocker.

    Right now I am thinking 3 " of aft rocker, as it seems a good compromise. Any input?
     
  2. Will Gilmore
    Joined: Aug 2017
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    Location: Littleton, nh

    Will Gilmore Senior Member

    As I understand your intent, you want a skiff that is stable and easy to move with a small outboard and oars. Something that can handle fishing with a friend or two without requiring a ton of materials to build.

    I'm not familiar with your terminology around the use of the word molds (1, 3-4). Is this removable framing? Anyhow, the lines and Web description for this little 17 footer seem like what you are describing. It looks like just enough rocker (I'd guess by the drawing about 3") and 'V' to make achieving hull speed easy with a good rowing setup. The build parameters sounded efficient in both cost and weight.

    Good luck with you hunt. I hope to see the results soon.

    Spira International Inc - Ablemarle Wooden Displacement Boat Plans https://www.spirainternational.com/hp_able.php
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    -Will (Dragonfly)
     
  3. Murky Deep
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    Murky Deep Junior Member

    I was looking through Sucher and found this. It’s not all that different than the one Will posted other than the v.
     

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  4. Murky Deep
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    Murky Deep Junior Member

    Does v make it easier to row?
     
  5. Will Gilmore
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    Will Gilmore Senior Member

    It depends on how the bow and the rest of the hull are designed. The less and more gradual you make the water bend around the hull, the less energy is needed to glide through the water. Hard chines can contribute to drag where the direction of water is forced to make a sudden change in direction, but flat bottoms can be better than round ones, where transition areas, like bow, stern, chine and rocker don't have to be accounted for. 3d movement offers more options for water to get out of the way than 2d movement (read here, configurations like plumb bow and no 'v'). Bring that flat bow above the waterline, it might be a different story.

    -Will (Dragonfly)
     
  6. messabout
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    messabout Senior Member

    A general idea for the aft rocker dimension depends on the displacement of the boat. When the boat is loaded at its intended all up weight, the transom should rise a bit above the static waterline. The boat may be a tiny bit faster if rocker curve is drawn gradually from the lowest point of the bottom upward to the aft end of the boat.

    Forward rocker is also an item of some importance. The forefoot should not be deeply buried for a simple skiff like this. If it is allowed to be too deep when in normal trim, the boat will be inclined to "hunt" when in a chop or when quartering waves or wakes. The term hunt is another way of saying that may not follow the helm steadily. Put the forefoot down into the static waterline but not by more than an inch or so for the small boat that you have described. If you raise the bottom of the stem ( forefoot) above the waterline the boat will be noisy and have more of a tendency to pound.

    The first order of business is to predict the all up weight and then draw the rocker curves to accommodate that figure.
     
  7. Pericles
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    Pericles Senior Member

  8. Murky Deep
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    Murky Deep Junior Member

    I have built an LYS a few years ago-too much hp, weight, and cannot be rowed. zero aft rocker.
     
  9. Pericles
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    Pericles Senior Member


  10. messabout
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    messabout Senior Member

    Murky, you asked, above, whether the vee bottom would improve rowing or require less effort. The answer is Yes,no, and maybe. Will Gilmore correctly told us that the the bow and curvature of the waterline is of some importance. It is then obvious that a long narrow boat will be better because the acceleration of the water particles can be less severe. Newtons second has F=Ma. If mass is constant then the force input is dependent on the acceleration etc.............Lets call this form drag or wave making drag.

    But at very low speed like when rowing, the form drag is not very large. Another important factor is involved. That is the surface friction drag. It should be apparent that to reduce total skin friction drag we should try to minimize wetted surface. The shape of the immersed cross section of the boat is a determinent. A flat bottom boat will have the most wetted surface of the several alternatives. A rounded chine shape will have less surface than a hard chined section for a given section area. < Plus one. The vee bottom has a bit less wetted surface than the flat bottom. So far so good. Another section shape is the so called trapeze bottom. That is a narrower flat bottom with sloped (beveled) chines. It is generally better than a vee bottom in terms of displacement versus wet surface.

    More.................We could have a narrow boat with round chines and it would row very easily if it had a very smooth skin. A smooth canoe shape would be an example. If we make the boat narrow there is the matter of righting moment to deal with. How tippy will the boat be? A too narrow boat with a deep vee will be uncomfortably tippy. A boat with the same dimensions that has a flat bottom and hard chines will be more stable than the vee bottomed one. An Adirondike Guide boat is an example of a good set of compromises. A wide flat bottomed boat like the Albemarle would be very stable but not row as easily as a narrower boat. With all that to deal with it is clear enough that you can not have it all. You have to pick a design that best satisfies your priorities.. There are "horses for courses".
     
    Will Gilmore likes this.
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