Hull Efficiency today vs. 25, 50 and 100 years ago?

Discussion in 'Hydrodynamics and Aerodynamics' started by AndySGray, May 15, 2015.

  1. AndySGray
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    AndySGray Senior Member

    A recent alcohol fueled debate about hull efficiency (which largely went nowhere at the time) was started when someone remarked about a claim made by a Dutch motor yacht manufacturer that their new hull was 30% more efficient meaning greater speed at a given power or better fuel economy at the original speed.

    The initial scepticism was that it was one of those annoying 'unquantified' marketing claims.

    "Aye, 30% more efficient than what? a Duck nailed to a length of 2 by 4" :p

    The inference was that the underwater shape was responsible for the massive improvement, and while it had a bulbous bow, that is hardly a new thing.

    Where I'm going with this, is how hull shapes have changed over time and was there a point at which things could have been considered the 'next generation'. It looks more like evolution than revolution - small incremental improvements.

    To the layperson a 1920's sailboat hull is, comparing similar dimension monohulls, not much different in shape to it's modern equivalent. Multihulls could be considered the next generation in that instance.

    But, for fast planing motor yachts is a 30% improvement a realistic claim and what (or when) would be the point of comparison if so.

    My feeling this is an overstep by a salesman, and it is the combination of lightweight materials and modern construction techniques, more efficient engines, better trim control, ballast management systems etc. which give rise to the "30% better" :?:

    :D
     
  2. motorbike
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    motorbike Senior Member

    Depends on how you measure it. But you do need to measure it otherwise the ducks win everytime:p
     
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  3. Ad Hoc
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    Ad Hoc Naval Architect

    Andy

    Oh dear, sounds like a lot of alcohol....you've been caught by that "old salesman chestnut" again. Your scepticism is rightly noted too.

    It can all be very simply explained once you have all the facts in front of you. But very rarely if at all, are such facts available especially for closer scrutiny.

    Since you rightly point out:

    The problem is that it can be simply explained by the length-displacement ratio - but do you have all the facts???. You can have 2 very similar hulls with very similar indeed exact dimensions for L, B and T. Yet one boat will be 30% better....huh...wow, hull shape has advanced so much....amazing!!

    And that's where simple numbers tells a different story. Since the important factor left out is...yup..the displacement.

    Below is a series of curves of LD ratio.

    L-D ratio-1.jpg

    So as you can see a hull with an LD ratio of 4 is very different from that of say just 6. If we look closer and assume a Fn of say 1.5, at an LD ratio of 6 the resistance is 0.09 Rt/W yet for an LD ratio of 4 it is 0.2, more than double!

    So a hull that is say 15m long, for both hulls an LD ratio of 4 is a displacement of circa 53tonne. But same hull same everything...but the LD ratio is 6, the displacement is just 16tonne. A massive difference.

    The same is true at high LD ratios but the difference in resistance becomes much closer and the gains/losses less.

    But I can guarantee you that the "sales pitch hull" is in the lower end of the LD ratio....ie heavy for its length. (Most of these boat usually are...). And as such and minor saving in weight can easily be seen to have a significant effect on the resistance. So...no magic I'm afraid.

    So, could it be 30% more efficient....er...yes....BUT..is it the exactly same hull...well no. Since if the principal dimensions have not changed the hull shape must change to accommodate the change in displacement with everything else being the same. In simple language the Cb has changed.

    And we all know what the Cb is:

    CB def.jpg

    And for those of you unaware of the effect on hull shape with simple changes in Cb, see below typical examples:

    Typ range of Cb hulls.png

    ERGO..it is NOT a like for like comparison. For that, the L,B, T and Displacement must all be the same. If the same claim of 30% is made...aaahh....someone has been smoking too much of the wacky backy :D
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Better fuel economy of modern engines would go a long way to explaining efficiency gains, especially with petrol engined boats.
     
  5. WindRaf
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    WindRaf Senior Member

    once hull forms was always conditioned by traditional construction technique. Today, new materials allow great weight savings and, most important, the freedom to specialize forms to every exact type of use.
    That performance is increased by 30% is normal.
     
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  6. gonzo
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    gonzo Senior Member

    Older hulls were limited largely by the low power available at the time. With modern powerplants it is a completely different set of possibilities.
     
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  7. TANSL
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    TANSL Senior Member

    I do not know if I understand correctly. You mean that before, not more efficient hydrodynamically speaking hulls were made because the engines were of less power than in our days?. Just when you have little power, a more effective hull is more desirable. Well, to be more correct, a designer has to find effective hull regardless of the available power.
    My question is: why most power increases the possibilities on the effectiveness of a helmet ?. I find that is a theory worthy of being developed in more depth.
    Thanks for the kind and expert responses that, without any doubt, I will get.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    Can you post a link/ref to a design, where 2 hulls are made both with exactly the same hull same displacement same engines same everything except one is made the "traditional" way and the other with "new materials"....that justifies that comment please.

    What does the power of the engine have to do with hydrodynamic efficiency?

    What does a helmet have to do with this?? :?::?::?::confused::confused::confused::confused::confused:
     
  9. gonzo
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    gonzo Senior Member

    The available power is the limiting factor. When engines of the power available today did not exist, designers had to produce hulls with little resistance. Today, floating bricks are common. They can be pushed through the water at great expense. Also, marinas charge by the linear foot. That is another factor that encourages short and wide hulls.
     
  10. philSweet
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    philSweet Senior Member

    One needs to carefully state ones point of view when talking about transportation efficiencies. If I lower the fuel consumption, there is a gain in efficiency in the sense of more service for less cost. But if this results in a greater volume of shipping or more miles (called rebound), then this could be an even greater efficiency gain, wrt service provided, or it could be mitigating the previous gains, if we are talking about costs such as pollution. There is no standard point of view. If a lower fuel cost resulted in the substitution of other modes for this one, then one needs to compare the options. That's harder to do than it sounds. The only thing to do is explicitly state everything that factors into the calculation, and how it is being treated. Efficiency reports, at least reasonably good ones, are about the most tedious reading material in existence.

    see table on page 10 in the following:
    http://www.trforum.org/forum/downloads/2010_42_Challenges_Measuring_Transportation_Efficiency.pdf

    by the way, "hull efficiency" is actually a propeller efficiency thing, not a complete vessel thing. Go figure.

    You should challenge the marketing guy by asking him to justify the expense in making it 30% more efficient. Tell him you'd probably choose the cheaper, less efficient version. See what he says. ;)
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    Indeed.
    Thus it is about the resistance and has nothing to do with what is driving it..being it sail, prop, waterjet paddles etc. That is a totally separate argument.
     
  12. WindRaf
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    WindRaf Senior Member

    gonzo, this time it's right PAR.
    For example, a modern displacement hull can have a perfect semicircular sections without keel and less weight, so with the same dimensions and same speed, has need half power
     
  13. WindRaf
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    WindRaf Senior Member

    the development of the hulls travel in parallel with the development of materials and modern computing capability with computers. but a modern genial designer must be an encyclopedia of the all traditional history of the boating
     
  14. TANSL
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    TANSL Senior Member

    Interesting question, as all yours. An error, not corrected, due to the direct, literal, translation with Google translator. The correct word is "hull". Sorry to have contaminated the thread with this error.:eek: :eek::eek::D

    Gonzo, never would have occurred to me that a boat designer change the shapes of his project in order to pay less money in a marina. It is a good argument that we should raise to ship designers (or anyone who knows anything about boats) to tell us what they think. On the other hand, ships that invest more resources in optimizing hull (whatever that is), do not dock at a marina.
     

  15. gonzo
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    gonzo Senior Member

    Boat designers, a boat sales people, use capacity as a main selling point. By capacity I mean how many people can sleep in it and how much deck space there is. I am used to my customers talking about buying a 30 foot boat because a 34 will have to pay for the 42 feet dock.
     
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