Will dispersed ballast help prevent snap roll and tetter tottering?

Discussion in 'Stability' started by ben2go, Oct 5, 2019.

  1. ben2go
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    ben2go Boat Builder Wanna Be

    I am about to start building a motor barge. The center section of the hull is somewhat flat. There is some shape with round chines. The ballast will need to be between 3 and 6 tons. I've been researching ballast location. I've read an interesting article about keeping ballast more dispersed and toward the outer hull to help prevent snap roll where the boat rolls to the side and sharply snaps back upright. I want to prevent this. The same article went on to say that weight toward the ends of a boat can cause excessive pitching in a rolling sea. If the ballast was added outboard from the centerline of the boat, along the center of gravity, wouldn't that make for a pivot point and cause the tetter tottering effect to worsen? I'm trying to nail down how I need to ballast my boat so I can add in ballast block compartments to contain the ballast and prevent their movement.
     
  2. TANSL
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    TANSL Senior Member

    The center of gravity of the ballast should be in the vertical of the center of buoyancy in the equilibrium flotation that you want get and located as low as possible and in the center line or so as to avoid a permanent initial heel.
    May I ask how you calculated the amount of ballast needed?
     
    Last edited: Oct 6, 2019
  3. philSweet
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    philSweet Senior Member

    This requires a proper dynamic analysis with accurate lines, a known displacement and trim, A known set of inertial properties, and some sense of the sea state you are operating in. This, coupled with a lot of experience with the vessel type, will provide the info needed to make an informed decision. You can't get there on a forum. If a type of vessel has a known problem with some mode of operation, it may be possible to say "don't do it this way, that is always bad on this type of boat" but that is about all. It's all numbers and calculations, there isn't any hand waving and thumb biting to be done here. There is a static issue where you may want to limit global bending moments by distributing the ballast where the hogging moment is high.

    Wikipedia article on Response Amplitude Operator. Response amplitude operator - Wikipedia https://en.wikipedia.org/wiki/Response_amplitude_operator
    Of course, you need to have some sort of goal in mind, expressed in terms of RAOs, in order to figure out what sort of ballast distribution to implement.

    Find an NA with the software and experience to help you. You will need the weight of everything and it's location to calculate the moment properties, and roll is one of the tougher ones in terms of sensitivity to displacement, above deck arrangement, and equipment fit out.
     
  4. ben2go
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    ben2go Boat Builder Wanna Be

    You guys over-complicate this. Everything I have is on the plans except layout,where to station ballast, and the actual amount of ballast required. The designer can't know what exact weight materials I'll use or how I plan the interior layout. This is a recreational vessel for coastal cruising. The most I'll ever see is 6ft/2m swells and I don't plan to see that if possible. However, in the event that I do, I don't want to be in a boat that snaps up or hobby horses. Michael Kasten has a good article on the subject "Beam vs Ballast". The boat has no real keel to support centerline ballast like a sailboat. When reading up on Dutch styled motor barges, they have ballast boxes located around the boat under the floor. Looking at pics there seems to be one just ahead of the center of gravity for the main ballast and one small ballast box just behind. The boxes are designed to disperse the ballast across the hull. There are other boxes located around the hull to use for trim ballast.

    I calculated the ballast by adding up build materials, mechanicals, and system weights until I got a rough dry weight. Then I added in fluids, stores, and provisions. Subtracted that from the weight at the designed waterline and got my approximate ballast weight. The final ballast blocks will be added when the boat goes into the water to get the trim and water line right.
     
  5. BlueBell
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    BlueBell Ahhhhh...

    ben2go,
    1.) Barges don't typically have snap-roll problems let alone roll issues.
    2.) What article are you referring to that scared you about snap-roll?
    3.) Have you spoken with the designer about your ballast concerns?
     
  6. TANSL
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    TANSL Senior Member

    A barge that needs fixed ballast from the beginning of its design is a poorly designed barge, unless there are very special reasons that justify ballast. Keep in mind that the more fixed ballast the barge carries, the less payload it can carry.
    If you want to build a ballast conveyor, you own it, but in my opinion, what you should do is maximize the payload of your ship.
     
  7. ben2go
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    ben2go Boat Builder Wanna Be

    1) This barge has soft chines not hard chines with a fine entry and carries her tansom above the water. Yes, it's flat bottomed but the bottom is fairly small in comparison.

    2) No article gave me pause. I've been aboard soft chined sailboats with heavy ballasted keel has problems with snap roll until the sails are set. This boats lower hull kind of reminds me of a keel-less sailboat.

    3) Yes, he says put in ballast as need to get her on her lines and under the floor. There is a space between the flat lower portion of the hull and the actual sole that is six inches deep, roughly. Not much room for ballast.
     
  8. ben2go
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    ben2go Boat Builder Wanna Be

    The boat is a modified Teign gravel motor barge design. The ballast is need because there will be no payload and to keep her on her lines and stable, also for trim.
     
  9. TANSL
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    TANSL Senior Member

  10. Ad Hoc
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    Ad Hoc Naval Architect

    You are conflating two separate issues here:

    The principle reason why these boats have ballast, you already know:
    Exactly.
    Ive designed barges like this before...they are sometimes so "voluminous" (as they are ostensibly a floating box) that the total displacement is often not enough to ensure she runs on the draft and sometimes trim required for general motoring about. Thus, just add ballast, that's all.

    With regards to "snaps"...these are: a hull form and weights and centres and hydrodynamic issues as Philsweet eluded too above.
    Not a one liner reply.

    So.. does your design exhibit poor motions?
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    My understanding is that a vessel that has weight concentrated at the ends, will have greater pitching motions, so I assume that if the weight is concentrated along the sides of a vessel , it will increase rolling amplitude, the opposite of what the OP is seemingly seeking. Beats me why you would want to build a boat designed to be on its lines under heavy load, when you have no heavy load to carry.
     
  12. bajansailor
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    bajansailor Marine Surveyor

    Is she one of Paul Fisher's designs? His Teign barges are 32', 45' and 65' in length.

    Motor Boats over 30' http://www.selway-fisher.com/Mcover30.htm
     
  13. Heimfried
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    Heimfried Senior Member

    It is the rolling acceleration (not the rolling amplitude) which is stressing if high and eventually causing seasickness. And the acceleration is damped by a high mass moment of inertia. Mass moment referenced to roll axis is greater if ballast ist placed as far as possible out of centerline.
     
  14. ben2go
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    ben2go Boat Builder Wanna Be

    Not as designed. I'm trying to get a grasp on using ballast to keep the boat steady and on her lines.

    Yes, weight at the ends will cause pitching. I experienced this on a fishing boat that had all the gear forward and all the mechanicals rearward. Very uncomfortable even in small 3 foot rolling seas.

    If I'm understanding what I have read, ballast along the outer hull in a motor vessel will help resist rolling.


    Yes, 45' Teign. I have the study plans and they show two different builds, plywood planked with three chines and strip-planked with rounded hull. I am going with strip planked. It's a better looking hull, IMHO. It also looks like it would move through the water easier.


    Thanks. That makes sense to me.

    My plan is to keep ballast in the center third of the boat and as far to the outside as possible. Then use trim ballast as needed in the bow. I'm hoping that won't be needed.
     

  15. Heimfried
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    Heimfried Senior Member

    Not in general. Things are more complex. There are no fundamental differences between pitching and rolling, although the axes are perpendicular to each other. The more apart from the respective axis the weights are placed, the higher is the mass moment of intertia. A higher mass moment is more resisting a turning moment caused by a wave.


    The frequency of this rolling seas is very important. A boat has a resonance frequency and if the seas (in relation with the boat velocity) pounding at the hull in the near of this frequency there will be a build up of oscillations, so pitching.

    Mostly, but it is also possible to come in resonance.
     
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