Slender non-planing foil assist catamaran?

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

  1. Mr Efficiency
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    Mr Efficiency Senior Member

    It sounds like you would need to get more aggressive with the amount of available foil lift, which no doubt creates other complications.
     
  2. patzefran
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    patzefran patzefran

    BlueBell and Bmcf, you are right, a clear example is Vendée Globe IMOCAS, after 30000 nautica miles of race, boats with foils assist are marginally ahead of displacement/planing boats,
    300/30000 is 1% !
    Classical A cats are also in the same field versus curved daggerboards (foil assist), non foiling A cats.
    Last Jules Vernes Trophy, the two maxi foilers had to retire for breakage and they were at best equal to displacement Trimaran records.
     
  3. S V
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    S V Junior Member

    Ok, some quick and dirty numbers.
    River displacement cat, 2 amas, each:
    22.5m length lwl
    1.1m width/beam
    9.7 cubic m displacement
    44 sq m wet surface area
    0.6 m draft at max
    slenderness ratio ~ 1 : 20.5

    Total for ship:
    5m total beam
    2.8m tunnel width
    19.4 cubic m displacement
    88 sq m wet surface area
    0.6 m draft at max
    22.5m length lwl

    For example foil lift is 75%, at required operating speed ~18 m/s (35 kts, 65km/h)

    Cat amas at 75% foil lift faction:
    22.5m length lwl
    0.6 width/beam
    0.35 draft at max
    2.5 cubic m displacement
    23 sq m wet surface area

    Total for ship 75% foil lift faction:
    22.5m length lwl
    0.35 draft at max
    4.8m total beam lwl
    2.8m tunnel width
    5 cubic m displacement
    46 sq m wet surface area
    1 sq m wet surface area of 2 vertical foil struts

    Foil(s) is at 0.3 m in its deepest part, 0.05-0.03m from keel (so ~0.25-0.3m from surface) , their legth is 2.8m, so if we are thinking about two tandem foils, what 2 foils of total area, suction and pressure we need at 35 knots?

    In other words:

    What length (angle of attack, NACA profice, etc) of tandem foils for 2.8m tunnel witdth cat we need to lift 2 x 7500 = 15000 kg at 35 knots operating in 0.3-0.32m depth in their deepest part?

    EDIT: It may be possible to get a bit more tunnel width by the shape of the amas, so it may be 3.0-3.2m if it helps

    EDIT2: From quick and dirty formula from Hydrofoil calculations https://www.boatdesign.net/threads/hydrofoil-calculations.59313/ I get that 2.8m x 0.2m foil (just 0.56 sq m at CL=1) is able to lift ~8000kg at this speed if that formula is calculating only top or bottom foil surface. Those numbers look more than good.
     
    Last edited: Dec 31, 2020
  4. BMcF
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    BMcF Senior Member

    Except for the fact that a lift coefficient of 1 is way beyond reality.
     
  5. S V
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    S V Junior Member

    Ok, what CL should we use and what foil dimensions you are getting? If using 0.4 I still get 2.8 m x 0.5m (AR ~5.6) can lift 8000kg, which is still a good result.
     
    Last edited: Dec 31, 2020
  6. BlueBell
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    BlueBell "Whatever..."

    SV,
    What's your take off speed (lift off speed)?
     
  7. BMcF
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    BMcF Senior Member

    I went back and dug out the data for our rare success..the 80% lifted trimaran. The design Cl was 0.2.
     
  8. S V
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    S V Junior Member

    Dear BueBell, that is a very very good question! By using common sense and the facts that it is non-planning cat and it is foil assisted not fully foil lifted I "kinda" guess that the aditional drag by foils (wetted area of them is 1.2-2.8sq m) will not impact to reach service speed of 35 knots, at 70-80% engines MCR. So, there should be no "lift off" speed concept for foils or hulls, and there will be no other disadvantages related to fully hydrofoil or fully planning boats.

    DISCLAIMER: I am armchair naval engineer.
     
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  9. S V
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    S V Junior Member

    ...That is very very low number :(
     
  10. philSweet
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    philSweet Senior Member

    Nope, that's a normal average for an operating point. You'll run a bit more at liftoff for a full hydrofoil, and you need a bit of reserve in general. But the sea isn't smooth. The actual lift changes rapidly due to all sorts of perturbations, and the drag goes up as the square of lift, plus an additional penalty for the fluctuations themselves. So the average Cl which ends up minimizing drag is usually around 0.2 unless you have some very trick mechanisms and controls that let you adjust the foils very rapidly. Sailboat keels are sized for this Cl in performance boats, but the sail may operate at much higher Cls since not all of the sail aero drag is ship resistance - it's at an angle, and size is more of a penalty with the sails.

    So the most basic calculation is that the lift generated at a Cl of 0.2 needs to reduce the wetted area of the hull by four times the plan area of the foils. Two times for the top and bottom surface areas, and two more times because the induced drag under ideal circumstances is equal to the frictional drag of the foils. This gets harder and harder to do as the hulls get bigger and more refined. Scaling wise, the lift of the foils is proportional to surface area, but buoyancy of the hull is proportional to hull surface area raised to the 3/2 power. So you end up needing a high percentage of lift from the foils to get a lot of the hull's surface area out of the water as the hulls get bigger.

    So now you have really big heavy foils, and their structural weight scales about like size raised to the forth power, and pretty soon, you've added 30% to the hull's structural weight for the foil system. And they have to lift that additional weight as well. So your foil lifting 75% of the 30% heavier hull is lifting 97.5% of the original hull.
     
    Last edited: Dec 31, 2020
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    The OP's best starting point is a ship than runs on one main engine, that is the single most prospective thing to reduce running costs. That isn't a catamaran.
     
  12. S V
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    S V Junior Member

    I not mentioned the design object: it supposed to be fast and slim (~5meters wide) river boat for ~50 passengers. Sea state 1 at most, so only smooth and shallow waters. Ideally there should be one single operating condition: go fast at ideally flat water. I mentioned Twin City Liner 3 as it is a good starting point, although it is way too big. I am not sure how it will work to scale it down. The other boat from GCY, which is sea going cat just for the example of superslim amas.

    I do not see how in my example the hull will be 30% heavier. If the starting point is 20 000 kg, then 500kg for tandem foils made from steel and another 500kg for reinforcing is doable, 1000 kg total. If we are being cautious for a river boat - 2 000kg total. Lightweight material, such as carbon fiber sandwitch for foils will bring that to 1500kg. That is 10% at most. The foils could and should be designed to be cheap and replaceable, so if you hit smth, it will break at predefined points. This is asisted catamaran, so just 1 vertical strut at each foil midpoint is needed. 3 relativelly simple molds are enough for whole foil structure if they are made from composites.

    Mr E, your point is more than understood, but the point is that single engine planning boat for 50 passengers will not make into this efficiency and speed category. I do not see that. Lets assume that this is at least 1000 operating hours per year boat, so the fuel bill will be bigger than aquisition and maybe everything else combined. There are more or less 3 regions where simple constructions could work: Russia, Middle East and US. Fuel is very cheap there. So in most cases you build a big simple boat and attach most powerfull 2-4 shiny petrol ourboards - problem solved.

    Another huge advantage of simple planning or semi-planning/semi-displacement boat is that you can get naval engineer to help with design in every bigger city, even in my small country I can find some, it is a bit harder with catamarans. If we talking about single engine marvels such as stabilized monohull/trimaran I somehow feel that it is a very lucrative teritory. Another limitations is the ship width as this is river boat. Maneuvering and berthing of wide trimaran is another problem.

    Those not making it:
    1. Planning low deadrise boat - option number 1 if it was fuel efficient and maybe for 30 passengers max. Only advantages besides that.
    2. Semi-planning/semi-displacement - hmm... at 65km/h maybe even worse than planning???
    3. Planning cat - even worse fuel economy than planning monohull
    4. Hickman sea sled - noone knows how to design them, no live examples at all. Winninghoff boats have some examples, but the info is very scarce. http://invertedvboats.freeservers.com/sled35_388x174.jpg - this one looks like well designed one.
    5. Air cavity vessel - from all I know only one is promissing russian technology, mainly used for military applications. May be an option in future as basically this is simple slightly modified monohull.
    6. Howecraft/rigid sidewall SES...
    7. Cable&Wireless type trimarans - if it was narrower... Yep, could work with 1 engine. Not sure if suitable for 65km/h speeds

    And the most promising ones:
    8. Foil assisted planning cat
    9. Classic passive hydrofoil
    10. Super slender catamaran

    The wet dream of mine is to combine the best of the relativelly simple slender catamaran with simple foils. Classic hydrofoil is ok if the design budget is unlimited (state funded), the plan is to manufacture hundreds of them, the lead designer is genius who designs ekranoplans and friend of the president of big country. In attached image why. And this is from "Raketa" - oldest and simplest of them all. Passive foils.

    By chosing slender cat or planning cat you most likely need to design the foil profile. Every other solution may be off the shelf (read - cheap).
     

    Attached Files:

  13. BlueBell
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    BlueBell "Whatever..."

    I guess you didn't research Sea Sleds here on this forum...
    Check out DogCalvalry's thread.
     
  14. S V
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    S V Junior Member

    Thank you. On it!
    p.s. "Madness" word in the thread kinda worries me a little already...
     
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  15. philSweet
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    philSweet Senior Member

    For river operation of a boat this size, the usual issue that controls hull design and operating speed is the allowable wash. Near any form of shore development at all, we have a "zero wake" law and generally a 6 knot max on smaller boats. That's why our river boats all look like barges. They spend 80% or their time just creeping along.

    Here is the speed limit document for the Potomac, And it was the shortest one I could think of off hand - https://dnr.maryland.gov/boating/Documents/potomacriver.pdf
    Try designing a river boat for that:(
     
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