Instability after adding batteries...

Discussion in 'Metal Boat Building' started by cgoodwin, Dec 8, 2014.

  1. essenmein
    Joined: Dec 2014
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    Location: halifax

    essenmein Junior Member

    It was more of a point to consider rather than being the only plausible explanation, esp considering I have/had no idea what your displaced volume looks like.
     
  2. Tad
    Joined: Mar 2002
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    Location: Flattop Islands

    Tad Boat Designer

    It doesn't really follow that the boat has become less stable, unless there has been a significant change of trim. We can only go on what you state, as there are no facts in evidence.

    First step is establish some facts. Measure the floatation (sheer to water) very carefully (to at least 1/4" accuracy) fore and aft, port and starboard. Take the batteries out and re-measure. Do an inclining experiment with and without the batteries aboard, and estimate GM in both cases.

    Does the boat have any fixed ballast?

    At a guess, without batteries or fuel aboard G was pretty high and GM quite small. The boat was not very stable and roll recovery slow. Adding the batteries lowered G, increasing GM and thus stability, and increasing roll speed. Could it be that what you perceive as less stability is just a snappier roll?
     
  3. cgoodwin
    Joined: Mar 2011
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    Location: Seattle

    cgoodwin Junior Member

    I agree that the roll is snappier rather than actually being unstable at all, one might say that she is less "sea kindly".

    As for facts I can tell you that the stern did not drop at all, before the batteries the bottom of my swim step brackets just touched the water on a flat day, they are the same. The bow could not have dropped any more than 1/4" if at all. Brigand was welded above the waterline and riveted below and there is a seam just below the waterline which has not noticeably moved. We will see what happens when I fill the midships tank this weekend.

    Interesting stuff really.
     
  4. DavidJ
    Joined: Jun 2004
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    DavidJ Senior Member

    I was reading this thread wondering when someone was going to point out the obvious. Finally Tad did just that. The boat being "tippy" or "rocking noticeably" are not signs of less stability. That sounds like an increased roll speed and that's usually a sign of an increased GM and hence greater initial stability. Which is exactly what you'd expect if you lowered the centre of gravity.
     
  5. cgoodwin
    Joined: Mar 2011
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    Location: Seattle

    cgoodwin Junior Member

    Right, I think that is very true and I should have chosen a different word than stability. The real question is how to mitigate the tippyness. A really stable boat is nice but if everyone is sick or scrambling to keep the plates on the table, its not very fun.

    Part of the issue with Brigand is that she is long and narrow and has a round bottom as was the design of the day some 60+ years ago.

    I wonder what options I have to make her less tippy.

    A weeks work on the midships tank paid off and it has now been holding 475 gallons of water for a day without a single leak or issue, fitted with all new valving it will be pumped, dried and filled with diesel next week after the inductive fuel level sender arrives. I may keep the teak dipstick, but I hope to never have to use it - what a pain.

    The additional 4000 lbs definitely brought the bow down noticeably and while it seems to have only slightly effected roll, it will surely make a huge difference in porpoising.

    I find it interesting that these old boats had no level gauges and that no one added any in the past. Brigand has two 200 gallon water tanks, a 475 gallon midships tank and 4 linked tanks in the lazerette totaling 250 gallons, none have gauges...
     
  6. AndySGray
    Joined: Jun 2014
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    Location: Cayman

    AndySGray Senior Member

    Any chance of moving the batteries towards the side? 2 banks of 8 outside the motors?

    What you have is 'moments' issue - basically leverage

    a 200 pound man steps on the boat, there is about 7 feet of leverage (at right angles) - equivalent to 1400 pounds at 1 foot from the center line. but if the batteries are 3 foot below the CoG a 15 degree roll will only move them up by about 1.25 inches and won't generate a large righting moment to fight that.

    your battery bank on the center is probably only a couple of feet below the CoG - the torque from stepping on will overcome that inertia very easily and then as an easlier poster mentioned it will pendulum...

    Think of it like the tightrope walker - he carries the pole to slow rate of any turning force the forces are the same (slightly more because of the weight of the pole) but he has much longer to react.

    If you move your banks to the sides it may be marginally worse for the CoG But you now spinning a 13' 1,600lb flywheel verses a 3' one... as the former is MUCH slower to react the opposing forces can damp out the rocking instead of allowing it to oscillate.

    Was the boat designed with detroits in mind or did they replace earlier heavier units?
     

  7. cgoodwin
    Joined: Mar 2011
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    Location: Seattle

    cgoodwin Junior Member

    She was designed with Chrysler Royal Straight 8's
     
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