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Question about ballast and dynamic stability

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  • Question about ballast and dynamic stability

    If you are adding lead (or other weight) as ballast in the bottom of your model and you have options on where to put it is it better to have it close together or spread out as far apart as possible? Does it make a noticeable difference? I'm guessing having the weight more centrally located would make the boat more nimble or twitchy in its pitch and yaw axis while having the weight further apart would make it slower to respond to pitch and yaw input and to forces affecting those axis such as waves and currents.

    being nimble in the yaw axis is a good thing of course but most models subs tend to be overly sensitive to pitch changes causing porpoising. I've always found it very hard to get a sub to stay at periscope depth for very long.

    Jason

  • #2
    Originally posted by Schmitty1944 View Post
    If you are adding lead (or other weight) as ballast in the bottom of your model and you have options on where to put it is it better to have it close together or spread out as far apart as possible? Does it make a noticeable difference? I'm guessing having the weight more centrally located would make the boat more nimble or twitchy in its pitch and yaw axis while having the weight further apart would make it slower to respond to pitch and yaw input and to forces affecting those axis such as waves and currents.

    being nimble in the yaw axis is a good thing of course but most models subs tend to be overly sensitive to pitch changes causing porpoising. I've always found it very hard to get a sub to stay at periscope depth for very long.

    Jason
    In heavy seas, aboard the TRUTTA, a diesel boat, we had to stay on the surface (or risk rolling over if the flooding ballast tanks had a big shift of free water during the dive)! Trapped topside in heavy swells it was considered good practice to fill both forward and after trim tanks to capacity (and sometimes the WRT tanks as well) to increase the moment of inertia to counter wave action. As the water was taken from the negative tank our overall weight did not change, nor the c.g., but by putting all that water at each end of the boat did increase the damping effect of wave action against the boat.

    Hard to keep the moment arms short on a free-flooding model. Easier on a dry boat. But I like 'nimble' and try to keep the fixed ballast weights as close to the c.g. as possible.

    David
    Resident Luddite

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    • #3
      Here's an article about how to trim the boat:

      V


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      • #4
        As David mentioned, the damping effect of a wider spread of ballast weight is negligible on small boats. It becomes more noticeable on bigger boats.

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        • #5
          Thanks for the responses. David, Great info on your real life experience with the TRUTTA. So in general not enough difference to be worth making an effort over one way or the other.

          I was thinking the damping effect would help with keeping a craft stable about its pitch axis but perhaps this is better dealt with with fine tuning the amount of travel of the hydroplanes and sensitivity of automatic pitch control.

          On a side note: those trimming instructions say to put a piece of foam half the size of your ballast tank above the waterline. Better have a giant ballast tank! especially for boats with a lot of freeboard when surfaced.

          Jason

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