Deadrise of trimaran

Discussion in 'Boat Design' started by Rounak Saha Niloy, Nov 23, 2022.

  1. TANSL
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    TANSL Senior Member

    This is just an opinion and as such I ask you to consider it. When some entities set out to regulate the design of small boats (less than 24 m in length) they realized that the position of the aft perpendicular, in many types of hulls, could no longer be established or, at least, could not be established a single criterion. So the Lpp was removed. On the other hand, the maximum breadth could often not be obtained in Lpp/2 but in a section located further aft. For all these reasons, the criterion of measuring deadrise in Lpp/2 was abandoned. The ISO 8666 standard states that the deadrise must be measured in a section located at 0.4Lwl. Lloyd's SSC regulations (not specifically developed for ships under 24m) establish a measurement point, more complicated to define, but also located further aft than the old Lpp/2.
    When I have had to use any specific shipbuilding regulations, in no case, none of them, have I referred to the ITTC Dictionary Of Hydrodynamics to define the main particulars of the ship, in all cases they have clearly defined how the dimensions required for their calculations should be measured.
    Perhaps, and this is once again just my opinion, the ITTC Dictionary has become a bit outdated in relation to current trends in small boat design. That is why all dictionaries, or almost all, carry out revisions from time to time. Is the world evolving too quickly?.
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Fine, that is YOUR opinion.
    But it does not follow correct nomenclature with respect to the hull and its performance.

    This where you are going off track.
    Rules, such as ISO and Class, in the case you have cited, case little about the hydrodynamics of what is what. They ONLY care about structure loads - nothing else.
    Because Class et al, are only concerned with safety of the vessel - nothing else.

    This is very very clear when using rules, as you cite, such as LR:- deadrise is measured - within LR Class rules - is only at the LCG.

    When you refer to deadrise, in the realm of Class rules, it is not the same as deadrise when referring to the hydrodynamics, as David has done, referencing the ITTC definition.
    Because Class et al, is only concerned with the structural loading on the vessel. Nothing else.

    The ITTC dictionary, is correct. This defines what is deadrise, in terms of the the hull and its applicability to hydrodynamics of the hull.
    It has no relation to the structure nor class rules....

    That is totally separate issue.

    Thus, if you wish to state your opinion, then you need to clarify whether you are stating structural or hydrodynamic definitions.

    It is a common error, sadly.
     
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  3. TANSL
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    TANSL Senior Member

    @Ad Hoc , I greatly appreciate your always positive reviews. It is very likely that if you read my post again, without prejudice, you will understand that I am always speaking in terms of regulations or norms related to ship design (not only with its structure, I have not mentioned that word). And it is in this sense that I say that, in some respects, the ITTC Dictionary has become obsolete.
    In any case, my opinions are worth what they are worth, as much as yours, as your posts, which are not dogmas of faith but only, and nothing more than that, your opinions. Nothing is true just because you say it is. Please learn that lesson and forget your hidden passions for a moment and try to be objective, before rereading my previous post.
    Although I appreciate your wise teachings for what they are worth, not one iota more, perhaps you can teach me more things without showing your contempt. Thanks.

    Edited : And you're right, Lloyd's SSC defines deadrise at the same level as LCG. My mistake.
     
  4. DCockey
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    DCockey Senior Member

    There are numerous locations used for measuring deadrise angle. When specifying a deadrise angle the location should be stated, such as "at transom" or "0.4 LWL ahead of transom". If a rule is being followed then obviously the location state in the rule should be used. My experience is in descriptions of small craft hulls, particularly planing hulls, if deadrise angle is given without a location specified it is usually at the transom. Deadrise angle somewhere around midships is sometimes used in descriptions.

    The ISO 8666 standard states that the deadrise must be measured in a section located at 0.4Lwl.
    LR:- deadrise is measured - within LR Class rules - is only at the LCG.

    Is the deadrise angle in ISO 8666 and LR used for in the calculation of slamming loads?
     
  5. Alan Cattelliot
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    Alan Cattelliot Senior Member

    Yes for ISO. Beta0.4 (deadrise @40% LWL) is used in the kDYN1, the dynamic pressure coefficient for planning monohull
    upload_2022-11-26_18-9-35.png

    Yes for LR, where betap is used in the hull form shape coefficient.
    upload_2022-11-26_18-14-43.png

    In general, if not to speak about pure hull shape looking, the deadrise angle is used to characterize the slamming pressure for a given hull shape, or its ability to go to planning mode. But I have to say that the context of this discussion is pretty obscure to me....
     
  6. TANSL
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    TANSL Senior Member

    LOL
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Indeed, as Alan as cited.
    When making a generalised statement is made without qualification the 0.4L rule, this would be incorrect, since "others", such as LR and DNV etc, define it differently.
    Definitions are everything in naval architecture.
    Only when it is referenced to the rule one is citing from, does it make any sense, that is is from a prescriptive sets of rules, and pertinently only to structural calculations. Nothing to do with hydrodynamics.

    But some seem to suggest that quoting rules, are only for those incapable of thinking too!

    The ITTC being a global body defines it differently, in respect to the hydrodynamics.
    It is hard to image the global body on hydrodynamics being - obsolete. o_O

    Indeed, the side show of the obfuscating dog and pony using google translate is hilarious, I must say :p
     

  8. TANSL
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    TANSL Senior Member

    Who has talked about the global body of hydrodynamics? Be honest and don't twist my words.
    You don't know how to talk without despising people. Insecurity in yourself, childish complexes...?
     
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