Deadrise of trimaran

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

  1. Rounak Saha Niloy
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    Rounak Saha Niloy Junior Member

    What does the deadrise of trimaran mean? Does it mean the deadrise angle of the central hull of the trimaran?
     
  2. gonzo
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    gonzo Senior Member

    A trimaran most likely has different deadrise on the center hull than the amas. Also, deadrise normally changes along the hull.
     
  3. ThomD
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    ThomD Senior Member

    It means the same thing, I suppose. Trimarans typically are designed to reduce the wetted surface area, and sometimes to inhibit leeway. So if we are talking plywood panels, say 4, then you are looking at the most efficient section. A section that could be circular in a molded case, you might have a 90 degree angle at the keel, with the chine immersed as the best compromise for wetted area, say 1/2 of draft.

    So I guess the deadrise is the rise from the keep to the chine. On the amas the hulls are acutely angled, and seemingly chineless. Even starting with the main section, today we would make the keel straighter, and the chine would probably be 120 at the transom.

    With fully formed shapes there really isn't going to be deadrise. There have been boats like Big Bandi...that used basically a powerboat hull shape. But the idea of deadrise, for me, is largely to do with trading off powerboat characteristics, like hole shot, fuel efficiency, ride, and how locked in the boat handles. most of that conversation rarely gets directed to multihulls.
     
  4. TANSL
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    TANSL Senior Member

    Any hull, whether motor or sail, has deadrise and its value is generally measured at a cross section located 0.4L from the aft end of the full load waterline. Another thing is the chine, which does not exist on all ships.
    I suppose, but I cannot guarantee it, that in trimarans what is taken is the value of the deadrise of the central hull.
     
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  5. Ad Hoc
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    Ad Hoc Naval Architect

    As I noted on your other thread...you need to read the rules you are using, rather than getting other people to do your work for you!
    Pt.3Ch.1 Sec.3, 3.2.3

    Seems you are either a lazy student, or simply way out of your depth.
     
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  6. Alan Cattelliot
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    Alan Cattelliot Senior Member

    In accordance with TANSL comment. Agrees with Ad Hoc.
    upload_2022-11-24_8-13-19.png
     
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  7. Rounak Saha Niloy
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    Rounak Saha Niloy Junior Member

    Dear Mr. Ad Hoc,
    I appreciate your effort to help the community members. FYKI, my this post does not have any relation with the other thread you are talking about. I am curious to know what happens with the deadrise of the trimaran. Is it the same thing of the central hull of a trimaran? Or is there any other way to measure it (considering the effects of outriggers)? I have not found any conclusive answers anywhere. That's why I posted it here.
     
  8. Rounak Saha Niloy
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    Rounak Saha Niloy Junior Member

    I know very well how to measure the deadrise of a monohull. That is not my query here. Thanks.
     
  9. rxcomposite
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    rxcomposite Senior Member

    Your question was,
    AH, TANSL and Alan answered you correctly. That should explain the deadrise. Question answered.

    Now in your other thread you asked,
    "Are rules of this book applicable to all hull types such as monohull, catamaran and trimaran?"

    Obviously, a cat has 2 hulls and have the same deadrise. a tri has 3 hulls (a central hull and 2 outboard hulls). You are asking how to measure it and that has been answered.

    And now you are saying,
    "I am curious to know what happens with the deadrise of the trimaran. Is it the same thing of the central hull of a trimaran? Or is there any other way to measure it (considering the effects of outriggers)?"


    If you are now asking "considering the effects of outriggers", that is design and no rulebook can guide you. It is all about righting moment (moment of inertia) and how the shape of the hulls, particularly the outboard hull ratio of displacement to the central hull affects the RM curve. It is all about the RM curve. Nothing to do with deadrise.

    And now, the question is..............?
     
  10. clmanges
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    clmanges Senior Member

    Off-topic, but just as a cautionary against sweeping generalities, consider such boats as PDRs and Triloboats.
     
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  11. DCockey
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    DCockey Senior Member

    What rule or rules does this definition originate from?

    For smaller motorboats deadrise is frequently specified as the "deadrise at transom".
     
  12. TANSL
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    TANSL Senior Member

    It is not a definition, it is simply an indication of where deadrise should be measured.
    What standard or standards do you usually use? I say this to focus our possible discussion on the one that is most familiar to you.
    Notice that in my "definition" I have stated that "is generally measured". Many designers of fast powerboats, once this value is defined, extend it, unchanged, to the transom, ie keep the deadrise constant throughout the aft area of the hull. Towards the bow, here it is very different, designers in general insist on increasing its value to frankly high figures, sometimes reaching 90 degrees. I point out again, because I want to be cautious, that I speak of in general terms. There may be other hobbies.
     
  13. rxcomposite
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    rxcomposite Senior Member

    That value you are referring to is a Series 62 hull, a constant deadrise hull whose deadrise remains the same all the way up to the transom. Published data is from 0.41 to 0.43. However, most of the testing done was on 0.475 to 0.488.

    Other series with constant deadrise ranges from 0.38 tp 0.52.
     
    Last edited: Nov 25, 2022
  14. DCockey
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    DCockey Senior Member

    I thought I might have missed a standard definition of where to measure deadrise.

    ITTC Dictionary Of Hydrodynamics provides two options on p 27: Search ITTC https://ittc.info/search/?q=dictionary

    upload_2022-11-25_21-3-23.png

    Donald Blount was co-investigator of Series 62. In his book Performance By Design he uses two locations for measuring deadrise angle. The primary one is "Beta"T which is the deadrise angle at the transom. The other is "Beta"mid which is deadrise angle at Lp/2 with Lp being the projected chine length (NOT length between perpendiculars). These are similar to the ITTC definitions.
     

  15. rxcomposite
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    rxcomposite Senior Member

    After the series 62 lines, there was succeeding study on warped (progressing deadrise) and hooked bottom. I believe DB is based on the series 62 lines on the chined "Destiero" which appeared in PBB.

    After that are tank test on bilged radius high speed semi displacement hulls. These hulls have minimally immersed transom whose deadrise is different from the midship. That us why they probably have two definitions.

    Back to topic of "Deadrise of trimaran" of which the OP should have "asked the right question".

    Attached is an image of a Tri I have designed based on several published CFD analysis. Does the outboard hull have a zero deadrise compared to the central hull? The hulls look completely different. Other design have deadrise in the outer hull but the study is based on hull resistance, interference, hull positions, and RM.
     

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