Why are submarines poor sailers?

Discussion in 'Boat Design' started by misanthropicexplore, Jul 30, 2018.

  1. upchurchmr
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    upchurchmr Senior Member

    To equate Speedream with a submarine is just silly.
    Someone doesn't understand subs - or boats.
  2. portacruise
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    portacruise Senior Member

    No one wrote about "equating" SpeedDream and subs, and I bet everyone reading these posts understands the difference between the two.
    But I can see where you are coming from, no disrespect implied to anyone.

    The marriage of sailboat and submarine - Ocean Navigator - January/February 2010 http://www.oceannavigator.com/January-February-2010/The-marriage-of-sailboat-and-submarine-0/

    "SpeedDream is part sailboat and part submarine" according to the author, FWIW. I wouldn't write it like the author did, but I can see how subs might have given an inspiration for SpeedDream.

  3. Wavewacker
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    Wavewacker Senior Member

    well, gotta say, all submarines have a sail on deck but it's too hard to use it to sail.
  4. Rurudyne
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    Rurudyne Senior Member

    This thread, the thought of sailing a submarine, kinda reminds me of a classic Dr Who episode where immortals from a void beyond time raced sailing ships through space to gain enlightenment. One of the immortals fell for Tegan hard. The human crews wore space suits to work the sails and climb the rigging.

    That said, IIRC a sail for propulsion while not being used militarily was a feature of Fulton's sub design.
  5. High spirit
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    High spirit Junior Member

    The idea of using a submarine type hull may not be so bad.

    Note that Marlin fish can swim about 100 km/h (= 27.8 m/s).
    Given that they are some 14 feet long (4.2 meter),
    their top speed corresponds to a Froude number
    Fr = U / (g L )^0.5
    of about Fr = 4.3
    (Here SI units were used and gravity is g = 9.81 m/s2)

    Had the Marlin been a surface piercing swimming fish in displacement mode, the top speed according to Froude would have been about Fr = 0.4
    which is equal to 9.3 km/h or 2.6 m/s. Clearly the Marlim is outperforming displacement boats by a factor of about 10.7.

    To me, that is very interesting.

    The question on submarine sailing hulls should therefore be rephrased as:
    Is is possible to design a ship that
    1. Has a hull that is submerged (and deep enough not to make appreciable surface waves)
    2. And has a superstructure above the water (where the sailers can sit)
    3. With an interconnection between hull and superstructure that is so slender that it hardly makes waves (the shape of knives).

    If you analyse this system, as I have done, you find that in principle this is possible.
    The key problem to be resolved, is as was pointed out above, transverse stability.

    The stability problem is this:
    A fully submerged hull has constant buoyancy force (making it go deeper in the water does not increase the buoyancy force).
    That is an unstable configuration. Note that for a hull floating on water, if it is pushed deeper in the water, the archimedes force (or buoyancy force) will increase, thus you will find some equilibrium. The fully submerged hull has zero form stability.

    This argument shows that to make a stable submerged hull configuration, you have to do something smart.
    Of course you can cheat by adding also buoyancy above the water line in which case the ship will look like a monohull or a multihull above the waterline and like a submarine under the water line.
    That way of cheating is a bit clumsy, as the superstructure will now generate the surface waves that were trying to avoid.

    Then there is the possibility of adding very heavy ballast below the submerged hull. This would work in principle, but has the penalty that the submerged hull must be designed much bigger to compensate for the mass of the ballast with extra buoyancy.

    Is there a better way to solve the stability problem?
    That would have the potential of very fast sailing boats.
    I hope I have intrigued you by this question.
  6. upchurchmr
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    upchurchmr Senior Member

    You have just defined why this idea is completely useless.

    Why don't you believe your analysis?
  7. portacruise
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    portacruise Senior Member

    HS wrote: Is there a better way to solve the stability problem? Answering your question, some wild ideas below. I'm sure others will punish me on merit, but I will be intrigued by the responses anyway.

    My opinion is there are several ways, but there will compromises to sub or sailing function- so not absolutely better. Maybe the trimaran idea using foils or in combination with inflatable outrigger hulls? It might also be possible to divert some amount of prop power to the sides to produce enough form stability while using the sail, but the controls might be complex (Segway on steroids concept). Yes, I know the righting force needed for a Segway are minor compared to a Segway with sail. Converting the sub to full time sail is one thing (remove the heavy batteries and use ballast), but doing so while retaining the sub function is more complex.

    Hope this helps with your question.

  8. TANSL
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    TANSL Senior Member

    If the hull is the same and the displacement is the same, the righting force due to the hull is the same in both cases.
  9. portacruise
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    portacruise Senior Member

    Tansl, thanks for your comment. I see your point about the righting force being the same from the hull standpoint. But the sideways force exerted by a strong wind while under sail is a much greater challenge to transverse stability, compared to that using conventional propeller power for propulsion? What am I missing?

    Transverse stability can be solved with SWATH designs, if you don't need the sub function: Small-waterplane-area twin hull - Wikipedia https://en.wikipedia.org/wiki/Small-waterplane-area_twin_hull
    SWATH is not particularly fast from what I understand.

  10. Magnus W
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    Magnus W Senior Member

    The reason why modern (military) subs behave like they do on the surface is because they are designed to perform at their best below.
    The hull has the best shape for the purpose which is stealth when submerged.
    Naturally you can build a sub that performs as any other surface boat while not submerged. But it wouldn’t be a very good sub.
    And having sailed on Swedish subs (ie rather small to operate well in the Baltic Sea) I can attest to them being rather unpleasant on the surface in high seas. Pitch is ok but the roll is as wild as it is unpredictable if the sea is coming from the side (pounding the sail).
  11. High spirit
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    High spirit Junior Member

    All, thanks for your helpful remarks. Especially the SWATH hulls are an interesting lead.

    These hulls seem to be sailing at larger than hull speed without having to be in planing mode.
    This is especially clear if you look at a you tube video's

    If you look at the wave patterns behind the stern, it looks like the wake pattern of a fast-planing dinghy, except that there is no planing surface.
    What seems to be the case here is that the submerged buoyancy (for this vessel it is a central submerged shark-like structure) does not make a wave pattern, and
    only the surface piercing elements that have a sharp trailing edge leave a wake in the shape of a planing ship.
    To me this suggests that the stern wake pattern is minimized by having a narrow wake.
    For those of you that are familiar with aerodynamics: this resembles the Kutta condition for an airfoil. The Kutta condition implies the requirement to have SHARP trailing edge at at airfoil, this ensures that the pressure stagnation point at the airfoil rear has a FIXED position; this ensures smooth flow off the wing at both top and bottom of the airfoil.
    Translating what this means for a ship: at the stern of most ships there is not a sharp line where the Kutta condition is satisfied. Rather, it is a broad stern that promotes the formation of a wide and unstable wake, with clearly a high wave resistance. Often, only the rudder trailing edge approaches a sharp line, and the rest of the stern does not.

    Now for the SWATH vessel, the sides (these look like ultra-thin catamaran legs), the training edge is indeed a sharp line that resembles the trailing edge of an airfoil, hence kutta condition seems to be satisfied.

    This video demonstrates the benefit of the SWATH hull for creating comfortable conditions in rough weather. Something that would be valued by many sailors.

    Let me rephrase my original question.
    Can a SWATH type hull be used to design a sailing boat that can sail at larger-than-hullspeed conditions?
  12. jehardiman
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    jehardiman Senior Member

    First of all there is no such thing as "hull-speed". It is an archaic concept, especially with respect to SWATHs. A hull will go as fast as powering will allow it to based upon the "humps" and "hollows" of it's powering curve as well as any lift/sinkage effects.

    So the short answerer to your question is yes, but probably sideways to the way you are thinking. Because a SWATH has minimal waterplane inertia and needs to be flat (i.e. 0/0 WRT roll/pitch) to achieve resistance efficiencies, it cannot support a classic mast or wing sail. It needs a kite sail and depressors. There are also a lot of obscure proportion, structure, powering, and stability issues with SWATHs with do not lend themselves to a "all weather/all ocean" solution for the hull type.
  13. Ad Hoc
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    Ad Hoc Naval Architect

    Just to add to JEH's excellent summary.

    A Swath is lightly damped and very sensitive. Thus, a sudden gust of wind would create a large heeling moment. as there would be insufficient restoring moment until a greater angle of roll/heel is encountered. This may be considered to be welcomed by many. Thus a Swath uses a combination of ballast tanks to maintain its fixed draft and trim, in various load scenarios, as well as automated control surfaces, such as a motion control system which would be unsuitable for a sailing Swath.
  14. waikikin
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    waikikin Senior Member

    Question- So beyond the adjustment of ballast a sub uses variable control surfaces to function and maneuver whilst submerged... what's to stop these being further developed to apply a righting moment to counter the heeling forces induced by a sail... sub under water- sail above water or even a kite above the surface(deployed by subsurface missile??) and controlled/flown by wire & suspended camera. Seems that some form of DSS or similar with some awesome reactive logic applied to the stabilising system could well be in the realm of possibility given budget and the desire however it may defy the usual logic applied. Remember there's no bad ideas in the think tank(just don't let escape to reality..).

  15. Ad Hoc
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    Ad Hoc Naval Architect


    That's what the automated motion control system does.
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