another scaling question - seaway

Discussion in 'Boat Design' started by philSweet, Sep 1, 2013.

  1. philSweet
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    philSweet Senior Member

    This is a spin-off from the Minimum-around-the-world-motorboat question.

    Basically, it is a question of how added resistance in a seaway and other seaway issues scale with displacement at a constant speed.

    If one had decent fuel consumption data for a 20 ton craft in open water, What would the expected efficiency gain be if you doubled the displacement? We are talking very low displacement speeds here and assuming nearly straight geometric scaling. In other words, if the 20 ton craft could run at .3 hp/ton at 4 knots, what could the bigger one do?

    I understand the relationship between the surface area and the displacement. What I'm asking about is the practical experience regarding moving through the open ocean. Particularly at low speed. A look at the practical designs from history, such as the Dutch butter-boxes that supplied the American colonies during the 17th century and were typically sailed with a crew of five, show a very different hull form than what Michlet/Godzilla might suggest. The riverboat of the time (galliots) were quite fine. The coastal traders were fuller (caravels for instance) but the ocean traders developed to look like bath tubs.

    It would appear that the trade-offs between smooth water efficiency and open ocean work on very low powered vessels will distort the hull form to one that has huge reserve buoyancy for a smallish free board. The Dutch appear to have made one loop per year in their boats - Holland, Africa, Azores, South America, The Caribbean, Savannah, Boston? and home. They didn't need to be in any great hurry to pull that off. They had about as much current as boat speed for much of the trip. I have a sneaking suspicion that the best small nonstop circumnavigator is rather boxy.

    A tug is a slow boat with a big fuel tank - just saying ...
  2. rwatson
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    rwatson Senior Member

    Considering the shape of ships back in 'those days' was also hugely influenced by material availability, shipyard facilities, cargo type and even political reasons, using them as a baseline.

    Nowadays, we have the unrivaled ability to design hulls for specific purposes, and amazing materials along with excellent performance predictions.
  3. daiquiri
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    daiquiri Engineering and Design

    I don't quite understand the relationship between your question and the historical examples you have mentioned.

    The traditional ships you have mentioned were mostly used for transport of goods, and hence their form was dictated by the primary requirement to embark as much cargo as possible into a given enclosed volume. Hence the full-ended hull forms. The river boats also had the requirement of shoal draft. The differences were mostly due to different levels of seakeeping capability required for their specific missions.

    The maximum cargo volume is still a primary design requirement for many modern ship types, and the resulting hull shape is quite similar to some of the mentioned historical examples:

    [​IMG] [​IMG]

  4. Ad Hoc
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    Ad Hoc Naval Architect

  5. philSweet
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    philSweet Senior Member

    There are a couple references to long skinny boats being the way to go for a small nonstop round-the-world vessel. As far a shape goes, I think that long and skinny are used where speed is a competitive advantage. You wanted to be on the fastest viking longboat if you wanted to pillage the good stuff. In commercial shipping, speed affects transport efficiency and the overall return of the vessel.

    I was trying to think of examples where speed paid no particular benefit. In the early settlements of the Caribbean, the fate of the islanders , all twenty of them, was entirely in the hands of the annual Dutch supply ship's visit. My contention is that these Dutch boats evolved with the understanding that they will make one loop per year and that all they can do is be as efficient as possible, they can't do any better by being faster. They completely displaced all other nation's shipping from the Caribbean due to their lower cost. (Mostly due to a small crew.) I can't find any lines drawings of these boats, but there are contemporary artwork and descriptions of them that show comically boxy shapes. They were the boxtrucks of their day - ubiquitous, efficient, and unremembered.

    I can't really think of any present day examples. Ocean tugs are closest in terms of size, endurance, and speed. But their drivetrains distort their aft shape.

    What I don't have any feel for is how minimum powering varies with displacement if you assume that four knots is an acceptable speed in fair weather and that you avoid landfall in anything less than fair weather.

    The early low-powered steamships don't really help here because they began by being applied to high value operations, not efficient ones. The last sailing ships to be displaced by power were the ones working the cheapest product - lumber.

    At present, what is the lowest value, least perishable cargo shipped in large quantities? green lumber? Scrap metal? Ore? Coal? If you compared the trend in hull forms as a function of value per ton per mile, you might be able to extrapolate a shape where the value is about zero. And I think it's going to be darn near a brick. The question is would a brick still be the thing sized at 30 tons and operating in the southern ocean?

    On the extreme end of the scale, micro sailboats that have succeeded in high endurance voyages are also pretty much bricks. I guess I could look at their powering and get an upper bound.
  6. daiquiri
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    daiquiri Engineering and Design

    You just have to look at the trends in the construction of supertankers. It is a perfect example where an optimum value of the so-called Required Freight Rate (RFR) is pursued (intentionally written "AN optimum value", not "THE optimum value"):


    As you can see - it is almost a brick, yes. But not exactly a brick because the hydrodynamics of a brick would make the optimum RFR value worse than the hydrodynamics of a streamlined shape like that.

    However, the RFR does not depend only on the hydrodynamics and the cargo volume. The costs of the environmental impacts, both during operations and in case of an accidents, are taken into account too. In case of crude oil tankers, for example, there is an optimum point beyond which the RFR starts to rise because the costs of the latter two variables become increasingly important with the size of the vessel. An oil tanker of 500.000 DWT is a much more costly environmental damage in case of an accident than an oil tanker of 100.000 DWT.

    So, in order to establish which hull form is the best for a given mission and tonnage, you need to define a function to be optimized by changing the design parameters (in the above case - the RFR, in your specific case - something else).

    All this to say that the answer is, as so many times - it depends. :p

  7. Leo Lazauskas
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    Leo Lazauskas Senior Member

    The hulls produced by a (calm-water!) thin-ship code are not useful for these sorts of comparisons.
  8. ancient kayaker
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    ancient kayaker aka Terry Haines

    Seems there is more than one question here. There's the expected efficiency gain be if you doubled the displacement of a craft with decent fuel consumption, then there's the whole question of hull shape for specific use like cargo, around the world, calm water, ocean etc. Looking only at scaling, my guess is as follows:

    Skin drag at the same speed and hull type for double displacement should go up as the 2/3 power. That is based on area varying as the square vs displacement varying as the cube of linear dimension.

    Not sure about wave drag but my guess would be straight doubling. However, at the low speeds you mentioned skin drag will predominate, since 4 kn is approximately hull speed for a 10' hull and you are speaking of several times that length.
  9. sharpii2
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    sharpii2 Senior Member

    Within limits, longer is better for any engine powered boat.

    The so called Dutch 'butter boxes' were powered by the wind, an un exhaustible power source. Hydrodynamic efficiency mattered, of course, but it was often trumped by other considerations. The chief among them is how well the vessel behaved sailing down wind on the open sea. The blunt bow more or less matched the volume of the stern and made a hull that was far less likely to round up into the wind when hard pressed.

    Later it was learned that one could make the ends sharper. That brought about the clipper ships and their like. They didn't last long, because the extra speed wasn't worth the extra man power and diminished cargo capacity it cost.

    A non wind powered vessel is limited in range by the fuel it can carry aboard. This includes rowboats. On a rowboat, the rower eventually becomes exhausted. If the rowboat has a higher cruising speed, it goes further before that happens. There are limits of course. If you make the rowboat too long, the rower is spending more energy controlling it than he is propelling it. Rowboats of the last days of sail tended to have waterline lengths of around 14 to 20 ft, for a single rower.

    If it takes 1hp per ton to propel a vessel at an S/L of 0.67, the longer vessel of the same displacement is likely to have greater range. This is because the S/L is based on the length of the waterline. If you have a 25ft WL vessel, its S/L of 0.67 is 3.35kts
    A 36ft WL vessel has a S/L of 0.67 which is 4.02 kts.

    Now, assuming both vessels have the same displacement and the same hp, the 36ft WL one is going to be 4.02/3.35 or 1.2 times as fast. That means, it will go 1.2 times further per given amount of energy consumed.

    So, if you double the displacement, it would be a very good idea to double the length, if you can.

    Yes, you will be adding more surface area and more friction, but not enough to to erase the huge speed/range advantage you will gain.
    Last edited: Sep 7, 2013

  10. jehardiman
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    jehardiman Senior Member

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