What is the most stable boat design?

Discussion in 'Boat Design' started by Sundevil, Oct 13, 2012.

  1. Sundevil
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    Sundevil Junior Member

    Or can you modify an existing boat to be very stable in 2-4 foot waves? I'm sure it is different if you are trying to anchor out compared to actually moving through the water and waves.

    Hydrofoil
    http://www.youtube.com/watch?v=ORuDmzI48B4

    Swath comparison
    http://www.youtube.com/watch?v=m7LGUWYX9ho&feature=related

    Hydraulic Fins (do these have to be hydraulic, what if you had ridgid fins on a small sailboat, would it keep it from rolling as much?)
    www.youtube.com/watch?v=YakBY6zkOyA

    Gyros
    http://www.youtube.com/watch?v=-CpSL35Hxgs

    Torpedo Hulls (underwater hull or hydraulically moved pontoon that is balanced 2 feet under the waterline)
    http://www.boatdesign.net/forums/boat-design/torpedo-hull-25491.html

    Outrigger underwater stabilizer
    http://www.westmarine.com/webapp/wc...44&ci_sku=16907&cid=sc_googlepla#.UHkWimammnU

    How about a catamaran or trimaran with amas or outrigger floats? Could you add something to make the boat float like a longer hull boat by adding something to the front and back of the boat?

    Why don't we see more of these boats in the water (sub 35' / sub 12m)? Or am I just looking in the wrong places? What would happen if you merged a bunch of these technologies together?
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Define stable??

    With the referenced videos you have posted, it appears your definition of “stable” really means seakeeping.

    The motions of a vessel, for seakeeping, are primarily influenced by the amount of waterplane area. Lower the area better the seakeeping, as a general guide. The distribution of mass, longitudinally and transversely also affects the motions. By the motions, I mean its natural periods of motion.

    If you place a flat ruler on the end of your desk, flick it, it will twang and vibrate. It will always vibrate the same no matter how you flick it. But if you shorten the length or add a small weight to the end, it has changed. The “twang” is the natural period, in simple terms.

    So to modify a boat for better seakeeping is not so straight fwd. You can add appendages to dampen the motion, but it wont alter the natural periods of motion.

    And yes, the speed, or period of encounter with waves makes a big difference. You can “tune” a boat to the waves which are expected to experience, at a given speed. But this means it wont be as good when an anchor....and vice versa.

    Yes you can. But in doing so, you make it "stiffer". So the resorting force is greater. Unfortunately this usually leads to a reduction in seakeeping. And in this sense, I mean the motions. It is so stiff (the ability to restore itself after a disturbance), that the motions become too fast or quick or simply violent not soft and gentle. It is very stable, but, it's just uncomfortable.

    The objective of what "motions" you want, needs some clarification and also there is no magic bullet as such. Just small improvements, unless your radically alter a design.
     
  3. MrArkeveld
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    MrArkeveld Junior Member

    catamaran are usually the most stable of boats
     
  4. Stumble
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    Stumble Senior Member

    The quick answer is bigger. No design secret, just big.
     
  5. Sundevil
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    Sundevil Junior Member

    I don't have any problems with a radically altered design, if it feels like I am on an oil platform and the waves have no impact on the boat. But from what I have read, this leads to not realizing the waves are getting too big for the boat to handle and it will become a submarine or get torn apart instead of riding over the waves in a really big storm or big seas. I wouldn't mind going around the world, but would be just as happy going around North and South America following the coast and avoiding the bad weather seasons in certain places.

    I would think the most stable design would be a hydrofoil SWATH design. Have your underwater 'foils' provide floatation and the foundation. It would be like you were a submarine or a diver who didn't care about the waves on the surface. You would need to use an electric motor or one of those pod motors probably so it could be underwater all the time. It would probably be better if the battery weight was down low as well. The length of the boat underwater wouldn't matter too much if you weren't trying to get into and out of marinas

    And if it was small and light enough, it should fly right over the water. Well, in my imagination it would.

    I don't know if they have one of those gyros for small inexpensive boats (or how much power they consume). But if it were possible to have something that didn't need power, but would help stabilize the boat at anchor, that would be good.

    I'm not so sure about trying to sail it like the l'hydroptère, compared to generating electricity to run the motor.
     
  6. Sundevil
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    Sundevil Junior Member

    http://www.youtube.com/watch?v=3Gy1SIWiQpc&feature=related

    Here is an interesting video. It is the future now (probably 30-40 years which is older than me), but I'm wondering where all of the commercially available hydrofoil boats are? I don't need a boat that big, and computers and gyros can do the work of most of the men in that video now. And I'm not looking to get a 100 ton ship with hulls that can survive war. A 2-3 ton ship would be heavy to me.

    The advances in computers and material science haven't seemed to translate to basic or advanced boat seakeeping abilities yet. Both underway and at anchor. Well, the gyros are a good advancement, so I like those. But, I'm not sure they are on too many boats under $100,000 yet.
     
  7. keysdisease
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    keysdisease Senior Member

    What is the most stable boat design?

    A capsized trimaran, that's why within the multihull community rightside up is called stable 2

    Steve
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    This is simply because the natural periods of pitch/heave/roll are so far away from the periods of encounter of the prevailing waves that it remains unaffected. As for not “knowing”, this is owing to the main platform structure being far enough above the water surface that even the largest wave, the 1 in a 100 years statistical average, would pass under the structure and not slamm under the raft structure. There is no “magic” to it, just being very well designed and thought out to suit the prevailing annual conditions.

    A well designed and thought out boat is what you are after. Simple, no magic.
     
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  9. FishStretcher
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    FishStretcher Junior Member

    SWATHs aren't statically stable like a catamaran. But inside a certain range, they are largely undisturbed by waves. This same feature tends to make them unstable in pitch and roll, as there is little buoyancy location change with respect to pitch or roll- unlike a monohull or catamaran.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    I’m sorry, but your understanding of statically stable is flawed.

    To be statically stable refers to the restoring force only. Heel a boat and it shall return to its original positions. Keep heeling until it no longer can return…is a measure of a vessel’s restoring force. Whether a SWATH has more or less than a catamaran isn’t the point, the point, can it return to its equilibrium/starting position?

    A vessels reaction to a disturbance, a wave, is not only determined by its restoring force ‘ability’. It is primarily determined by the waterplane area and the location/distribution of the weights onboard.

    However for a freely floating vessel, only: heave, roll and pitch are affected by a vessel’s restoring force and thus influences its natural periods of motion.

    Thus, I think you are confusing ‘seakeeping’ with ‘intact stability’.
     
  11. FishStretcher
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    FishStretcher Junior Member

    Would you not agree that a SWATH has less restoring force than a catamaran, for similar sized vessels?

    The catamaran must submerge one pontoon and lift the other in order to effect a modest roll change. The result being very different forces to port and starboard, resisting roll.

    A SWATH has much less change in buoyancy in roll. The "pontoons" are already submerged fully, so they buoyancy essentially doesn't change until they breach the surface.
    But the resistance to roll can depend on design of the strut. A fat strut makes it behave more like a catamaran and a very small waterplane area strut near the LCG has much less resistance to roll and pitch.

    The phenomenon is similar in pitch, until the submerged tubular structure that provides the bulk of the buoyancy starts to breach the surface.

    Perhaps I should have better stated it as having less restoring force in pitch and roll compared to a similar sized catamaran when not moving. But I think that can be a rather severe understatement.
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    Similar size vessels, yes.

    No.
    By “pontoon” I assume you mean a hull? This is a function of the inertia of the WPA and its righting moment, (GZxDisplacement), at varying angles of heel.

    I am confused what it is you’re trying to say here. The LCG has nothing to do with it. The WPA influences the value of the GM. The natural roll period is inversely proportion to the square root of the GM.

    Again, I don’t know what it you are suggesting. You’re mixing up the concept of “stability”, in terms of statical stability, and seakeeping. The two are very different.
     
  13. Red Dwarf
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    Red Dwarf Senior Member

    Ad Hoc - Is there a chart somewhere that shows common disturbance frequencies for boats on the ocean. I know many things are happening but it seems that there should be a statistical distribution of frequencies and amplitudes for waves. For example a 50 ft wave is .0001% and a 2 ft wave is 10%. Some sort of bell curve or frequency domain.

    The reason I ask is on a car or airplane you design the motors mounts so their resonant frequency is below all operational frequencies. You can clearly see this when you shut off an airplane engine. Just as the engine comes to a stop it shakes like a wet dog for a second as the excitation passes through the resonant frequency of the mounts.

    So using the same concept can you design a boat so it does not resonate with the most common occurring excitations?
     
  14. jonr
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    jonr Senior Member

    > What is the most stable boat design?

    A submerged submarine.
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    Yes there is. Here is the online version:
    http://www.globalwavestatisticsonline.com/

    I have this in hard copy book.

    When you want a more "localised" data, you cab get this too. Of course it all costs money as this data is not easy to obtain. Since you want statistics records that go back many years.

    However with the advent of the Internet and real time data recording, you can now log into some sites and get this real-time data such as here:
    http://www.windfinder.com/wavereport/viking_platform_northsea

    and also obtain data direct from the buoy's such as this one at K4:
    http://www.ndbc.noaa.gov/station_page.php?station=62105
    or looking world wide their locations are shown here:
    http://www.ndbc.noaa.gov/

    Yes you can and do, or should do. Some boats are thus better than others, owing to the designers adherence to the local prevailing conditions.
     
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