High-Speed Pentamaran Yacht

Discussion in 'Boat Design' started by High Life, Jan 23, 2013.

  1. Ad Hoc
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    Ad Hoc Naval Architect

    When an amateur comes onto this site they can respond to why they did X or Y or Z, to any critiquing. Thus understanding their rationale and/or providing assistance to changes for improvement, where necessary or pointing out errors. How is BMT going to respond, when they don’t even know it is being discussed here? It becomes pure speculation on your part.

    For your ref, this design has been discussed extensively, since it is not a “new” design, it has been kicking around for over 10years (1996). I see no need to repeat the same old discussions again here, when the SOR hasn't even been addressed, nor likely with such computer generated analysis.

    However, the part you fail to grasp is the SOR and the “marketing” of such vessels. The whole SOR is based around the objective of this hull form. That being it was “designed” for fast freight applications” across the Atlantic and other major shipping routes. For ref see “Optimisation of the seakeeping & performance of a 40knots pentamaran container vessel”, E.Dudson, N.Gee, Fast 2001.

    Existing “high speed” freight vessels such as container ships exhibit parametric rolling which can be serious, and in some cases capsize with the resulting loss of life. BMT were tasked, for whatever reason, to arrive at a new solution. Theirs is the pentamaran. The principal objective, apart from the high speed, is to eliminate parametric rolling, since to reduce resistance the L/B ratio inevitably gets higher which means stability becomes a serious issue. Existing container ships and Panamex vessels can attest to that. The 2 fwd sponsons/ ama’s, provide the stability to prevent parametric rolling. Thus the SOR, was satisfied using their pentamran hull form. BMT has spent huge sums of money tank testing this along with full Classification approval of the design in terms of structural integrity etc. It came close to being built….still collecting dust. The containership industry is as conservative as any other.

    That’s the SOR part.

    The marketing part.
    After spending huge amounts of money on a hull form is there any other application that it could be used for, why waste the good research? Well…why not fast ferries, or luxury motor yachts? The hull form remains valid…the only question is whether any other market will accept it, in its present form. They even say, and I quote, from another BMT published paper but presenting as a possible fast ferry:

    “…Firstly, it should be stressed that the data presented is for hypothetical vessel designs and alternative design features such as wave piercing or a semi-SWATH bow configuration, deadrise, etc, could have an effect on the comparison between the hull forms…”

    Thus they acknowledge it may not be perfect for such applications. But why waste good data.

    Which comes down to the critiquing of such a vessel with such large amounts of power for a yacht and 4 sponsons as you have been doing. Sure it’s fair game as any professional or amateur ‘presentation’ is, but don’t throw the baby out with the bath water. The whole raison d’être for this hull form is to prevent parametric rolling of large freighters at high speed.

    If it can be applicable to other markets, that’s a marking man’s dream, who has to justify the large R&D and marketing of global company such as BMT with their high overheads.

    Do I think this hull form is suitable for a luxury yacht, no, seems pointless to me. But perhaps the marketing man thought to exploit the current interest in trimarans. Thus it is more a marketing issue than a technical one per se.

    Your critiquing is based upon hydrodynamic resistance only. Not its raison d’être, prevention of parametric rolling of slender high speed hull forms.

    You tend to make all your judgements from the pressing of buttons into a program and reviewing the output, as if it is some absolute perfect magical answer to everything. Design is greater than the sum of its individual parts, especially when using absolutes from a computer program.

    As I said from the outset, you need to understand the SOR….what is the hull form ‘designed’ to do, then you’ll know why? The rest is all conjecture and supposition….
     
  2. tomas
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    tomas Senior Member

    I've been reading a lot of older threads here and I would suggest that AdHoc's perceived arrogance is just that, perceived (and subjective), on the part of many here, not anyone is particular.
     
  3. groper
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    groper Senior Member

    Thank you for a more detailed response.

    In regard to the above, its obvious that the amas are only there for stability, i thought it need not be mentioned. I had hoped we could have a more thoughtful discussion on, the difference in approach to the stability problem by using airborne sponsons or "pentamaran" as opposed to a trimaran with simple "reserve buoyancy" in the amas - like the austal ferry for example.

    Some put the idea down to reducing wetted area, for less resistance. I dont agree with it as i beleive in doing so, theyve simply traded length for draft or beam as the ama displacement is kept the same - for the same required righting moment.

    So the question i have, is why do it this way and not with reserve bouyancy in a tri configuration?

    The fact that this "pentamaran" design has never been built despite being conceived many years ago, speaks volumes IMHO - when many trimaran designs - satisfying a similar SOR - have been built and are successfully in operation.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Seems I’ll have to explain it again:

    How so…??...not being built is not a measure of whether or not is does what it says on the tin, that being eliminate parametric rolling for large high speed freighters?

    There are only 2 trimarans, in which to compare, for a like-for-like basis, not ‘many’. That’s the Austal and the NWBS’s.

    The NWBS has poor performance and technical problems. Vibration and seasickness and was pulled.

    Thus, you need to be careful with the “successfully in operation”….sales and marketing by the company does not make a design successful. I would hardly call 1 operationally successful; especially considering some of the bad press it also has got. Personally I am very surprised that the US Navy decided to buy 10, without even trying it operationally for several years as they seem to with everything else. But that’s a different story…The UK MoD built a 2/3rd scale 100m Tri called Triton. Tested it thoroughly for 1 year, and decided it wasn’t the Holy Grail. Thus, like every design, the clients SOR always trumps.
     
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  5. groper
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    groper Senior Member

    You failed, again, to explain why the parametric rolling is more favorably improved with this pentamaran configuration, compared with a trimaran configuration like the austal - still aft positioned sponsons.
     
  6. BMcF
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    BMcF Senior Member

    This parametric rolling thing...we solved that in other ways. That's why I've little more to offer to this thread. IMHO..the best/only thing amas are for on a tri are to keep it from falling over until the foils get enough bite to make a real performer out of it. I prefer they (amas..sidehulls..whatever..) be well clear of the water surface altogether..no matter how many there are.:D

    Like this....pay no attention to that ugly naval vessel we are passing with the trimaran.:p
     

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  7. Ad Hoc
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    Ad Hoc Naval Architect

    I haven’t failed in anything. It is you and your own personal aspirations in trying to understand what is the difference between the 2 hull forms by yourself, which you have failed to do.

    Firstly you need to understand what is parametric rolling. Parametric rolling is a roll phenomenon induced by a time-varying restoring arm. In general, this phenomenon is relevant to a ships hull form that has exaggerated bow flares and transom sterns which contributes to large changes in the restoring arm in certain conditions, such as modern container ships.

    The aft body of trimaran main hull is generally rather limited in draft owing to the waterjets installed (like the Austal), which as a monohull, like container ships is prone to large variations in stability. This is where the ama’s or outriggers contribute significantly to the overall stability aspect, which generally prevents this parametric rolling. However in large seas when the outriggers, become dry, i.e. out of the water, there is a large loss of initial stability, by this I mean the GM; traverse that is GM(t).

    For a trimaran, the non-linear phenomenon of rolling generally occurs in head seas, where the changes in GM due to pitching occurs. It is not an easy thing to establish when considering the large effects in the GM that the: length, volume, coupled with the transverse and/or longitudinal locations of the outriggers make on the initial stability, thus makes this “generalisation” even more tricky. However below a certain wave height the GM variations will be too small to initiate the roll motion, just like on a container ship.

    But it is more likely to occur when the natural period of the wave encountered is half the natural roll period combined with large pitch motions or loss of stability due to the outrigger emerging from the water. The natural roll period of the trimaran, is of course directly related to the outriggers, in its size location etc. Thus no two are the same, rather like a monohull. But a generalised estimate of roll period for a mono can be made, unlike that for a trimaran owing to the many variations possible of the outriggers parameters and layout.

    The pentamaran has been demonstrated by tank testing (which is all there is published and all anyone can refer to) that shows in the correct configuration of the 4 outriggers, parametric rolling is completely removed.

    Aaahh..that’s more like it :D
     
  8. groper
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    groper Senior Member

    You have succeeded in explaining parametric roll, but again you still failed to explain the difference this phenomenon effects the pentamaran VS the trimaran... In the papers ive read, about tank testing trimarans for parametric roll, some configurations will have tendencies to develop parametric roll, whilst other trimaran configurations do not.

    Therefore, I cant see a valid argument/explanation in terms of parametric roll alone because if your goal is to eliminate parametric roll, you can do it with either configuration, pent or tri...

    So thanks for the parametric roll diversion, but were still no closer to understanding the pentamaran, particularly in relation to the buoyancy distribution of the amas, their wetted area, reserve buoyancy, or potential resistance optimization...
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    Then you are in receipt of information that I do not have. Since all papers I have read indicate that parametric rolling is a factor to consider and at best has been reduced to "..not found to be significant.." in the configuration tested. But never seen anyone quote as being completely eliminated.

    Thus care to cite which papers you have where the authors have shown that they have successfully demonstrated the elimination/prevention parametric rolling.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    No, that "we" aspect would be you alone.

    Since any design must satisfy the SOR....you consistently fail to understand this.
     
  11. groper
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    groper Senior Member

    So now your turning to semantics...

    Well let me re-phraze, "either configuration can be designed without 'significant' parametric roll.."

    And your statement of me being alone, is your assessment. How do you know the number of people reading this thread and their level of understanding? You cannot...

    So let me summarize or para-phraze since youve still not explained things clearly...

    In order to mitigate parametric rolling, the pentamaran uses forward placed amas, clear of the static waterline, to provide stability where the encountered waves put the ship in hogging. In this hogging state, the rear amas could be significantly clear of the water so that their effectiveness in keeping the ship stable is reduced. So, the forward amas could become immersed when hogging, providing stability needed by the ship in this state.

    In a sagging state, the rear amas provide enough stability due to being fully immersed in this state. The forward amas are clear of the water and are not required for stability in this state, nor are they required in calm seas or conditions were significant parametric rolling could develop based on the vessels natural roll period.

    Now, where does all this fit in with an SOR??? clearly, parametric rolling is best avoided regardless of whether the ship is a container vessel or a opulent pleasure yacht, military vessel or a high speed ferry??? So in your design consultation are you going to say "now will you be having parametric rolling with your jet turbines, or just the jet turbines...?"
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    Hmm…so, you say:

    And now you say

    Well either these papers you have read exist and show configurations where there is no parametric rolling, or these papers don't exist. This is not semantics. You claimed to have read papers that showed this…so, please cite them for me, so I may read them.

    What has hogging and sagging got to do with it?

    If you understood what the SOR means in terms of design, you wouldn’t keep asking this question again and again.
     
  13. groper
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    groper Senior Member

    Firstly, It was you that referenced the "elimination of parametric rolling by BMT`s design" - and you pull me up on wording like "not significant"

    You know damn well how it works, you dont have to test me either... the ship effectively changes its beam from slim when hogged with the midships supported, to wide when the midships is in a trough but the bow and stern are supported by wave peaks. Since stability varies with beam, as the vessel drives through the series of wave fronts its stability changes significantly as the midship moves from crest to trough. When this pattern occurs together with a wave encounter frequency that is close to twice the ship’s natural roll frequency, the ship enters a condition of cyclically recurring minimum stability - parametric roll.

    This was one paper i skimmed through, whilst not a direct study on the point in question it makes reference to the tendencies toward significant parametric roll with regard to different trimaran configurations, specifically those with inward ama locations. When i made the previous comment in my earlier post, i had this quote in my mind taken from the conclusions section of paper below;

    "Only the inward side hull configurations have been found susceptible to parametric roll"

    https://www.navalengineers.org/ProceedingsDocs/FAST2011/A7-3.FAST2011.Onas_Datla.pdf

    Clearly, its a problem for stabilized monohulls with very small amas and generally located close inboard to the main hull, but with larger amas and a more true embodiment of trimaran, the parametric rolling consideration is of less importance - its difficult to even find a study which doesnt concern stabilized monohulls or container ships.

    This is the point ive been eluding to all along, why have the sponsons split in half, when you can just have a longer slender sponson - like a trimaran as opposed to a stabilized monohull... So, if BMT`s pentamaran design had the forward sponsons immersed, would they still not reduce parametric rolling? And if they were immersed AND joined to the rear sponsons, would they still not work in reducing parametric rolling? And if so, would it still not satisfy the SOR of a high speed luxury multihull?

    These are rhetorical questions btw... i know exactly what you mean by the SOR in this case. Thankfully, you agreed earlier that you dont agree with its ideology in a luxury yacht and neither do i. In an effort to save bandwidth, you might have simply gave an insight into why this design is the way it is, in order to satisfy which particular elements of an SOR, and saved all the drama... like, some rich ******* walks in and dumps an obscene amount of money on the table and says "i want a huge *** boat with jet turbines that will do 40kts with minimal parametric roll but i dont like trimarans or catamarans, i want something different"
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    Well, I’m still confused because you referred above to papers…plural, not paper..singular.

    Their conclusions are most peculiar. There is no mention anywhere of what is defined as an inward side hull. Thus no idea what they are referring too. And nowhere does it state that parametric rolling has been eliminated either, as this is the paper you indicated doing so?

    That’s an SOR. There is no magic.

    It always seems everyone over analyses a design to the point of stripping down the hulls into isolated hydrodynamic analysis, via a bit of software to claim XX or YY, or disprove the design as not possible…or something other. Citing odd selection or XX or YY could have been so much better because of, well.. blah bah blah.

    If the client’s SOR is “I want chocolate”…whether you give them a Mars bar, a Twix, a KitKat or another, thus, does anyone of them satisfy the SOR of “chocolate”…yes! Yet everyone is trying to either find a deep meaning behind the choice of a Kitakt when the Mars will do or be better - but that is YOUR opinion based upon a totally different set of parameters for arriving at said conclusion of Mars Bar, rather than KitKat. If the Kitkat satisfies the clients SOR…so what, job done. Is another going to satisfy the SOR..highly likely…but so what? And that shall always be the case, generally.

    If however the Kitkat is “claimed” to taste better and have superior ingredients to all others….then that is a claim that can be questioned about the chocolate, per se. But not forgetting the fact..it is still chocolate and still satisfies the SOR!
     

  15. groper
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    groper Senior Member

    Thanks for dumbing it down for me, cheers mate, chocolate is good for small brain...

    In regards to the inward side hull, i can only assume they were talking about the different positions they modelled the amas. You can see they tested the same main hull with same amas, but shifted their position laterally and longitudinally - i assume this was to verify their method worked for a variety of geometries rather than a single configuration. They didnt seem to bother publishing their results on parametric roll for the widely spaced configurations, only mentioning in the conclusion that only the inward side hull was found to be susceptible...

    As to the other papers, will we be having another exercise in semantics again? Or do you believe that ALL trimarans, not just stabilized monohulls, will exhibit significant parametric roll in realistic sea conditions?
     
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