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Perceived instability after adding battery bank

Discussion in 'Technical Discussion' started by cgoodwin, Nov 26, 2014.

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  1. Opcn

    Opcn Senior Member

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    My position is not that you have placed them wrong, but rather that you will face some reduced initial stability as a result of placing them in the boat above the COB. Just because it isn't optimal from an initial stability standpoint that doesn't mean it isn't worth the compromise.
  2. cgoodwin

    cgoodwin Member

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    Opcn- The COb of a vessel is typically just above center of the distance from the keel to the waterline, these batteries, as I stated are 8" above the bilge and below the COB. There would be no way to place them any lower without attaching them to the outside of the hull.
  3. cgoodwin

    cgoodwin Member

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    Having read on COB and COG all weekend I think the issue my be that the COG is very close to the COB creating a pendulum effect, that may change with the midships tank full or at least 1/3 full as is typically calculated when the stability is calculated.

    Interesting stuff.
  4. karo1776

    karo1776 Senior Member

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    A few comments but no solutions... but you can figure those out.

    A long time ago it was common with us sail boaters to add ballast inside the hull in the bilges usually in the form of some heavy concrete mix. Of course the designed waterline, if it was to be maintained, the solution was usually applied at build or during trials to bring the boat to the waterline. This was the days before CAD Engineering programs so some guess were made in displaced volume. Usually the idea was to design the boat to be slightly lower in mass than that needed to bring it down to waterline. The cement (with lead shot added to a thick mix) was usually had laid in the bilges to both bring the boat to waterline and balance the boat level due to the inevitable slightly unbalanced situation due to distribution of the mass from build. So there was a balance between external (keel) and internal ballast. Also, this technique was used and is still used with barges built for european waterways. Often the design approach also entailed using a proportion of the ballast balanced between the internal and external ballast... to effect the comfort or period of motion (rate at which the boat rolls or pitches). Usually the internal ballast was not totally concentrated in the bilge center but spread out more or less evenly. The higher the percentage of the internal ballast to the external ballast the longer the period of motion and initial stability. This made the boat seem to be more stable and comfortable in a seaway. However, the larger percentage of internal to external ballast also made the boat heel more for any particular wind condition on a sail boat. So cruisers used more internal ballast and racers used less. This proportion both affected roll, pitch and comfort.

    In European barges this is commonly used (cement or other ballast spread out over the bottom of the bilge) to add mass to converted cargo barges to pleasure or living use. If the ballast is not spread out and concentrated near the centerline this makes the barge feel tippy.... so that is to be avoided. Usually this is not an exact science to those doing the work. Nobody does other than some basic calculations to determine the AMOUNT needed to bring the barge down to preferred waterline. Preferred is usually results in less draft than a fully laden cargo barge because of draft considerations and the canals one wants to access.

    These are practical boat design, building and repair practices. Your boat was designed and built in the era before computer design and built by skilled people.... they gain a feel or a buildup common knowledge of what to do... that has been lost on you as you are not part of that culture... I would say... so you are working without that buildup expertise. It is like the crew operating the boat for a time learning how to balance the tanks and stores to the boat and the situation of operation.

    From your descriptions... the boat is in my opinion not less safe.... actually more resistant to roll and pitch in total magnitude and less resistant to initial roll and pitch forces... that is what you "feel." That is exactly what you have described and fits exactly with the physical modification situation of the boat. The solution requires some thought and trial and error. If the boat was at the original builders yard back in the day it was built this would have been recognized and adjusted in the build or refit process and they could get is very close. But you do not have the institutional and generational knowledge and confidence that creates what is called sense of "engineering proportion".... the most important thing in engineering that has been lost as the computer revolution as evolved... so you are stuck with learning by trial and error using your sense of proportion... learning as you go... exactly like you would have developed this sense of proportion in years past by perseverance and hard work. So take your time run the boat and adjust based on what you learn as you go... !

    Partly the situation is that you are just not used to the change in the feel of the boat... just like happens when someone un-familar with the boat balances the tanks or stores different and notices nothing but those having been on the boat for some time feel the change.
  5. jhall767

    jhall767 Senior Member

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    Although it was dismissed in the previous thread - The book "The Nature of Boats" by Dave Gerr is very informative for the layperson and is actually written by a well known Naval Architect with many years experience in the business. You can read his bio on Gerr Marine's web site. It is well worth the $20 or so that it costs.
  6. bernd1972

    bernd1972 Senior Member

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    Many reasonable aspects have been mentioned before here. But let me suggest a solution:

    Spreading the mass further to the outer sides of the hull increases the momentum of inertia and therefor makes the rolling period longer. By placing extra weight low into the boat you increased the stability, especially the initial stability by placing the weight near the centerline. This leads to a shortened rolling period, the boat feels more "nervous". Considering that any weight placed below the center of gravity (which is most probably about 2 1/2 ft. above DWL) is good weight for added stability you might split the battery bank into 2 groups placed further to the sides of the hull. This would result into adding momentum of inertia and therefor make the rolling period somewhat longer as you might prefer from what I read.
  7. cgoodwin

    cgoodwin Member

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    Excellent posts!!
    I had a 65' turn of the century (the turn before the last one) dutch barge in England, it was exactly as described and had cement and lead poured down the centerline fore to aft. Brigand was also ballasted with cement along the outside edges of the midships fuel tank. I have made two levels in the engine room by attaching rulers on each side and fore and aft butted to the floor joists of the salon which is over the engine room. I then purchased a roll of clear tubing and attached on end to the ruler on one side, ran it down into the bilge and up the opposing wall where it attaches to the other ruler. I filled it with water and red food coloring forming a liquid level side to side and another fore to aft. I am now preparing to fill the midship tank and check the balance of the boat.
  8. leeky

    leeky Senior Member

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    Hey, cgoodwin, it's been 8 months since you left us hanging.:( I would really like to know if you have solved the mysterious, perceived-instability problem.