Two choices for distributing weight - which is best?

Discussion in 'Stability' started by Mamaboo, Nov 4, 2009.

  1. Mamaboo
    Joined: Aug 2008
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: USA

    Mamaboo Junior Member

    I am not a boat designer but I have a question that someone here may be able to help with. I am adding approximately 800 pounds of equipment to a 42 foot hard chined planing powerboat that displaces about 24,000 pounds. I have two choices for the placement of the equipment. The first is to consolidate all the equipment on the centerline approximately 4 feet above the waterline. The second choice, and the more convienient from a layout perspective, is to split the equipment and place 400 pounds outboard near the rail on each side. It will still be the same distance above the waterline. In both cases, the weight will be roughly 40% forward of the transom. I am assuming (probably incorrectly) that the two scenarios would be equal in static stability tests or calculations, but in a heavy seaway with the boat rolling side to side, will the inertia from having the weight split out on the rails adversely affect the stability and comfort as opposed to having it all on the centerline? If so, is it likely to make a noticable difference? Is there a way to actually calculate the difference between the two scenarios? Thanks in advance for your help.
     
  2. Tcubed
    Joined: Sep 2008
    Posts: 435
    Likes: 18, Points: 0, Legacy Rep: 318
    Location: French Guyana

    Tcubed Boat Designer

    You guessed correctly.
    In both cases the added weight is at the same height so will affect static righting moments equally.
    At 40% forward from transom on that type of boat and considering it is about 3.3 % total weight you should be close enough to longitudinal center of gravity to affect trim very little.

    Now to whether out towards the sides or on the centerline-
    Definitely out towards the rails.
    This will increase your rotational moment of inertia about the longitudinal axis. (rolling inertial moments)
    This means your rolling period will increase or rolling frequency will decrease.
    Ie roll more slowly.
    It also means that the boat requires more wave energy to initiate a given roll.
    Much more information can be found in Marchaj's "Seaworthiness".
     
    1 person likes this.
  3. baeckmo
    Joined: Jun 2009
    Posts: 1,139
    Likes: 125, Points: 63, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    Problem is, that with the longer roll period comes a roll that is resonant with a longer wave period, ie a wave that has more energy. This means that the roll movement is shifted to a lower frequency with higher amplitudes. Wether that is good or bad for crew comfort is a question of what things were at the start!

    If we are talking safety only, then the mass should be concentrated because the boat will respond in resonance to low-energy waves; movements are fast, but maximum amplitudes versus wave slope is small. For comfort, too quick roll is uncomfortable, so as usual, there is a balance to be found.

    But, you are talking about a minor percentage of the total wheigt, it won't really matter!
     
    1 person likes this.
  4. Tcubed
    Joined: Sep 2008
    Posts: 435
    Likes: 18, Points: 0, Legacy Rep: 318
    Location: French Guyana

    Tcubed Boat Designer

    Yes it is true that weight spread out will create roll resonance at longer wave periods.
    However it is not so much long period waves which can cause a capsize but rather the steep upper portion of a breaking wave.
    And in that situation you will see that the curl in the wave's velocity vector field is very high. This will attempt to impart a large roll acceleration. With a boat that has a large roll moment of inertia, the tendency to roll right over with the lip of the wave is delayed so as to make it out to the other side safely.
    If you read the book i mentioned it goes into this process in quite a bit of detail.
    It also confirms the validity of traditional ballast placement in sailboats; as low down as possible (obviously) but also compacted for and aft and spread out athwartships.

    Of course the comfort issue is as you say , since most people do not enjoy high rates of acceleration.
    The same goes for the fact that it is not a large percentage of the total mass , although it is enough for there to be a small, yet noticeable difference.
     
    1 person likes this.
  5. JLIMA
    Joined: Oct 2009
    Posts: 123
    Likes: 4, Points: 0, Legacy Rep: 130
    Location: New Bedford Ma.

    JLIMA crazed throttleman

    Keep the weight to the sides it will be more out of the way and as said earlier will slow your roll. However the amount of weight we are talking about here is so small when compared to the capacity of the boat that it shouldn't matter much where it gets put, on a smaller boat maybe,but not one of this size. However in practice always put weight out to the sides and as close to midships as possible.
     
    1 person likes this.
  6. baeckmo
    Joined: Jun 2009
    Posts: 1,139
    Likes: 125, Points: 63, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    Gentlemen, your statements are oversimplifying the issue! You must consider the vessel's original status before making such a statement. If the original stability is such that there is a long roll period in relation to the beam/stability, and the roll damping is low, then such a statement is making things worse. The vessel will be at risk for resonant rolling in a seaway, ending up in a capsize. Not due to breaking waves at all, but simply as a result of ever increasing amplitude with each swing. The phase shift between wave and boat is coming close to 180 degrees in resonance, making the boat heel into the wave slope. Broaching in a following sea is often tre trigger.

    If, on the other side, the boat is originally stiff, with high damping, the increased roll period from mass distribution will increase comfort, without causing rolling with any significant phase shift between boat heel and wave slope.

    There are many examples of resonant roll incidents in the commercial field, and one of Marchaj's major issues is just that this is one of the "Forgotten Factors"!

    And yes, I have read (and reread....) Marchaj and some of his major referrences in addition to my participation in research on the working situation and safety onboard fishing vessels.
     
    1 person likes this.
  7. Mamaboo
    Joined: Aug 2008
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: USA

    Mamaboo Junior Member

    Great information guys, and I appreciate your replies. The boat in question is an express type sportfisherman so it doesn't have a big house or much weight high up. I couldn't tell you exactly where the center of gravity is or anything like that, but it has got to be pretty low. She is a deep vee with somewhere around 20 to 22 degrees deadrise at the transom, and has a fast, snappy roll. It would probably help comfort a bit if the roll were slower. I just didn't want to do anything that would really adversely affect ultimate stability in case I ever found myself in really rough seas. As far as resonant roll incidents, I have heard of that happening with big commercial displacement boats with a lot of weight up high. I have never heard of that with smaller planing boats, but then again I am not a professional mariner or anything. Is it really a concern that I just haven't heard about?
     
  8. apex1

    apex1 Guest

    To make it short: Yes..........
     

  9. Guillermo
    Joined: Mar 2005
    Posts: 3,644
    Likes: 185, Points: 63, Legacy Rep: 2247
    Location: Pontevedra, Spain

    Guillermo Ingeniero Naval

    Do you know what her natural rolling period is? What is the beam?

    As a rule of thumb, the roll period (seconds) / beam (meters) ratio should be close to a value of 1 (one) for safety and comfort together. Because of what you explain about the snappy roll of your boat, she seems to have a low T/B, so probably there would be no big problem in adding those 800 pounds, both from the point of view of initial stability and resonant rolling, in my opinion.

    I suggest you to measure her natural roll period in calm waters before and after the adding of the weight, with the boat in a 70% load condition (very easy to perform test), and find out what her T/B ratio is and how it varies, to quickly evaluate your boat's initial stability in both condittions. You can easily perform such test simulating the location of weight in several positions, using two old 210 litres lubricating oil drums filled with water, or the like.

    What also needs to be checked is her remaining freeboard and ultimate static stability (big heeling angles).

    You should contact a local naval architect.

    Cheers.
     
    1 person likes this.
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.