Info Needed - How to Conduct an Inclining Moment Test

Discussion in 'Stability' started by UNCIVILIZED, Jul 21, 2014.

  1. UNCIVILIZED
    Joined: Jun 2014
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    UNCIVILIZED DIY Junkyard MadScientist

    G'morning Ladies & Gents,
    Hope that everyone had a good weekend. So... that said, to start the week off, I could use a little assistance with a brain twister. AKA "boat math" :)

    I'm wondering if anyone has done, or has info on how to conduct a proper inclining moment test on a recreational sailing vessel? Or has some ideas, specifically, in terms of where to dig in order to find said info.

    I'm needing to figure out the RM30 or something akin there to, as adding a keel bulb may be a not too distant project on the vessel in question. However, said figures are needed to make sure that the current keel bolts, & mast, aren't overstressed by the additional righting moment added by bolting on a bulb (and to some degree, ditto on the hull's structure).
    That, or to use said real world numbers in order to calculate what sized bulb might safely be added.
    BTW, She's a 40'ish footer, with a bolted on lead keel, with a good bit of fixed ballast in the bilge as well.

    Unfortunately both the designer, & the original specs & such for the vessel are MIA, or I'd just run the question(s) by him/them. So, given said state of affairs, I'm thinking that doing such a test will yield enough answers in order to take aim at the next step(s) of the perspective project.

    Any and all info, pointers, & poinent questions are welcomed. Ditto if I left out any key info bits needed in order to help me sort this query out.

    Thanks,
    Andy


    PS: I gather that I could simply crank the boom, or spin pole perpendicular to the boat's fore & aft axis, & add weight to it's end until the desired angle of heel is reached. However, I'm thinking that the sticky part in said simplified test, is at what height does one put said right angled lever? And what math goes along with conducting the RM to include this height?

    NOTE/EDIT: I forgot to mention that I'd prefer to be able to accomplish said test without hiring a crane, ORMA 60 style ;-)
     
    Last edited: Jul 21, 2014
  2. Ad Hoc
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    Ad Hoc Naval Architect

  3. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    Being a pleasure boat, stability is governed by the ISO standard. ISO 12217-1, 2 and 3 defines how to check the stability and the criteria that must be met. Not the same, although it is similar, to what is done with a normal cargo ship.
     
  4. Eric Sponberg
    Joined: Dec 2001
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    Eric Sponberg Senior Member

    Uncivilized, A stability test is going to give you the position of the longitudinal and vertical center of gravity of the boat. From that, you can determine the full righting arm curve of the boat. BUT, you need the hull lines in order to do all the NECESSARY calculations AFTER you do the stability test.

    The stability test and calculations tell you what the GM is. GM is the metacentric height which is the location of the center of gravity (G) below the metacenter (M). The location of M is known from the hull lines plan and the hydrostatic calculations taken therefrom--it is an imaginary point above the boat's center of buoyancy (B). That is, from the hull lines you can determine the location of B and then the height BM. Once you know B, M, and then G from the stability test, then you can determine the righting arms (GZ) at any given angle of heel. The righting arm, GZ, at any angle of heel, times the boat's displacement (also taken from the lines plan) will give you righting arm a any angle of heel.

    There are some procedures for calculating GM at 1° of heel, and then you multiply that by 30 to get the maximum righting moment of the boat. But this is just a rule of thumb, and is not considered to be totally accurate. RM30 is sometimes described as the righting moment at 30° of heel, but in reality, it is just the RM at 1° heel multiplied by 30, the 30 being an approximation to get to the maximum value of the boat's righting moment, which usually occurs at an angle of heel between 45° and 60°, not at 30°.

    For more information, I attach the ASTM test procedure for conducting stability tests, which is downloadable for free from the Internet. I also attach a copy of "The Design Ratios" which I wrote on this forum about 4 years ago. It might be helpful in understanding some of the hydrostatics calculations.

    That's probably a whole lot more information that you were expecting, but I hope it helps.

    Eric
     

    Attached Files:

  5. UNCIVILIZED
    Joined: Jun 2014
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    UNCIVILIZED DIY Junkyard MadScientist

    Eric, many thanks. And no, you didn't overwhelm me with info. I'm a bit rusty on some of it, but naught of what you're saying's new. I had plenty of Naval Architecture when I was at Annapolis... so we're talking about a quarter century of "corrosion" on said schooling. But it's "like riding a bike".
    Though for the moment, I'm just looking for the "simplified" version, as noted more or less in my original post. In an endeavor to be able to comfortably increase sail carrying ability, sans worrying about a "gravity storm"... or worse ;-)
     
    Last edited: Jul 21, 2014

  6. TANSL
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    TANSL Senior Member

    UNCIVILIZED, If what you want is to know a method for conducting the inclining test, I propose two readings:
    - Resolution A.749 (18) IMO, Annex 1: for all boats in general
    - ISO 12217: for pleasure boats.
     
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