Slender non-planing foil assist catamaran?

Discussion in 'Hydrodynamics and Aerodynamics' started by S V, Dec 28, 2020.

  1. S V
    Joined: Jan 2019
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    S V Junior Member

    Good evening everyone,

    After looking at some of the slender non-planing (!!!) ferry catamaran designs, like Twin City Liner 3 or Gold Coast Yachts - Caribe Spirit and many similar others it seems the common design feature is very very slender non-planing demihulls, 1 to 15+ ratio. From official data TwinCityLiner 3 looks like a marvel:
    Length : 39.9 meters
    Width : 11 meters
    Height : 5.9 meters
    Draft : 0.8 meters
    Dead weight : 70 tons
    Seats : 250
    Captain's Lounge: 35
    Top speed : 81 km / h (= 43.5 knots), according to other sources - ~77km/h
    Cruise speed : 60 km / h, according to other sources, 59 km/h
    Power : 4400 hp + 170Hp genset

    This data may be skewed, but I hope that not much. If it is true then we are almost in the hydrofoil teritory. If my calculations are correct this ferry is operating in Fn 0.8-0.9 range, from the pictures I do not see any planning hull attributes to it. In Hysucat, Hysuwac papers and some other ones mentioned experiments are in speeds Fn 2.5~3 and greater, but the example boats are smaller. If we are looking at speeds in knots, then 30+knots is the range where foils are installed in models or real ships. In one of the papers from Hoppe (Hysucat), he mentioned that when talking about hydrofoil or hydrofoil assisted boats, Fn becomes irrelewant, as foils lift boats to the water surface where hull length changes drastically or obviously there is no hull in the water at all. He states, that another metric, related to transport efficiency (engine power, weight of ship and achieved speed with that horsepower) better reflects how fast ship is. I personally totally agree with it, as this is universal dimentionless value, it can be applied to ships, cars, trains, planes and so on. Even better if we talk about Hp/passenger for the similar speeds. For example two diferent ships, Soviet era hydrofoil (Raketa, Voskhod, Polesje and other, they are similar):
    60 passengers,
    1000 or 1200Hp,
    70kmh top speed, ~60kmh cruise speed, lets asume, that the engine percentage load for cruise speeds is the same for both ships.
    16.7 to 20Hp/passenger


    TwinCityLiner3 or similar:
    250 passengers
    4570Hp
    80kmh top speed, ~59kmh cruise speed
    18.3 Hp/passenger.

    Almost exact match!!!??? How????

    My questions are:
    1. Is it possible, that this comparisson above is true?
    2. Will non-planning very slender hull catamaran of the similar size and type, of lets say wide range 15 to 70 tons, travelling at 30-32knots already as it is benefit from foil assist technology of partially lifting lets say 40-90% of the hull weight on foils to reach even higher speeds?

    Yhank you very much in advance :)
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    1. Not 100% sure what "comparisons" you are referring to?
    2. The speed are related to the foil area = lift, the drag of the foil v resistance (drag) of the hull, the amount of power you install, and the means at which the boat is propelled.

    But in simple one liner reply..yes, as anything is possible in design. It just matters which compromises you are willing to make, to achieve the SOR.
     
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  3. Mr Efficiency
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    Mr Efficiency Senior Member

  4. S V
    Joined: Jan 2019
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    S V Junior Member

    Mr E, yes, I looked there and many other Hysucat clones. All of them look like planning foil assisted cats or/and manufacturer states that they are planning foil assisted cat. I have not found combination of non-planing and foil assisted. Correct me if I am wrong.
     
  5. S V
    Joined: Jan 2019
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    S V Junior Member

    1. I am comparing transport efficiency of two ships at the same speed. Lets say that speed is 60km/h. It does not matter what technology, type, building methods are used.
    2. Ok, thanks for the clarification. The idea of mine is dead simple: take very very slender catamaran and lift it a little up. The question is if that will work for the speeds and ship sizes I mentioned as I have not much clue if the foil lift will offset the drag it creates (if the friction and wavemaking drag of the "lifted" hull + foil will be less that the bare untouched hull), it is way above my understanding of hydrodynamics. No need for the precise calculation, just a simple wild guess from someone who can guess pretty accurately :)
     
  6. DCockey
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    DCockey Senior Member

    S V, how did you determine those catamarams are non-planing?
     
  7. S V
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    S V Junior Member

    I did that with the help of photos and calculating Fn. Also, the size, weight, horsepower and slenderness kinda confirms me that. If the hull creates some lift - I think it is not substantial.
    Photo: TWIN CITY LINER (Passenger ship) https://www.fleetmon.com/vessels/twin-city-liner_0_70165/ and many others if you search.

    I am wrong?
    Does it matter to the exact question I have asked?
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    The main idea of foils is to reduce wetted area, nothing kills speed like frictional drag, I guess the question becomes whether a displacement cat is going to get similar reduction of wetted area, that a planing hull does, which is starting from a position of getting lift and wetted area reduction from the hulls themselves. Ad Hoc may know of examples of displacement cats with foils.
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    Wild guesses yield no conclusions other than.. there is no conclusion, as insufficient data or trend presented.

    And then you'll be in the realm of debating with the armchair designers, of which there are many, who like to 'design' with their gut, feelings and emotions....
     
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  10. S V
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    S V Junior Member

    Well... This is exactly the reason, why I asked this question here. I am armchair naval engineer, you, another wild guess, are not.
    In my work area I can make many wild guesses from past experience which will be more or less starting point until you need the very exact numbers.
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    Then you need to be more specific and exact in your question. Generalised Qs shall only elicit generalised replies - and this is not inferring that any are of value either, they are merely that, generalised replies.
    As it currently stands...your inquiry has far too many variables to offer any reasonable/qualitative comments.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    As I interpret his question, and in any case one I would also like to see answers to, are foils as prospective on displacement cats, as planing type cats ? The answer would appear to be "generally not", as there seems a lot less of them seen on displacement cats.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    You see figures quoted that 30% of a hydrofoil cat's weight being supported on the foils, on a displacement cat that might not result is a huge reduction in wetted area, or not enough at the speeds you would need to travel, to get that amount of lift.
     
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  14. BMcF
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    BMcF Senior Member

    We've been involved in at least two projects over the years that involved adding lifting foils to slender-hull displacement cats. Neither was a practical success and, in fact, one of the two, a cat around 45-meters LOA if I recall correctly, ended up being slower with the foils than without. We'd predicted that outcome...but the technical gurus above our pay grade disagreed with our prognoses and moved ahead anyway.

    The only project we've done that I would call truly technically successful was a foil-assisted super-slender monohull (ama stabilized when off foil); the foil lift fraction was 75-80% though. I would expect a similar result for a very slender cat with the same lift fraction...all other design considerations properly dealt with, of course.
    NorthWest Bay Ships also completed one foil assisted "trimaran" (again....a super slender monohull when operating at design speeds and a high foil lift fraction) and it's performance was...."OK". Better than bare hull but not sufficiently ground breaking that any more were built.
     
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  15. BlueBell
    Joined: May 2017
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    BlueBell "Whatever..."

    Wow BMcF, that's what I would have expected.
    Marginally successful, at best.

    Do the math: increased wetted surface area, cost, drag, weight, complicated, vulnerable.
     
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