42.5 tons of ballast on a 70 footer?

Discussion in 'Stability' started by ldrumond, Nov 18, 2006.

  1. ldrumond
    Joined: Feb 2005
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    ldrumond Ya...I look like him

    70' x 24' x 8' steel ex-fishboat, tankage as follows, 1250 gal water tank in bow at waterline, 2 x 3000 gals fuel saddle tanks amidship, plus 1250 gal water tank in lazerette. If all tanks were full, assuming aprox 10 lb per gal, it comes in at around 85000 lbs give or take a couple of tons. How would this affect the seakeeping qualities, roll etc? Hull form is full keel with hard chines. Am exploring a vessel for possible liveaboard conversion and am wondering whether this would provide an exceptionally good platform from the standpoint of stability.
    Any thoughts?
  2. lewisboats
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    lewisboats Obsessed Member

    That is a lot of weight at the very ends...not a good place as it slows the ability of the boat to lift to waves, either fore or aft. Tankage is best kept in the middle of the craft and as close to the centerline as possible. Allowing that this isn't usually possible, I would say that the forward tank should be relocated to just ahead of the fuel tanks and split evenly as are the fuel tanks. Water weighs 8 lbs/gallon and fuel somewhat less so you are a few tons high on the calcs unless you were adding the weight of the tankage material too.

  3. Eric Sponberg
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    Eric Sponberg Senior Member

    A couple of thoughts. First, you don't provide enough information to give a full opinion. By the way, fresh water has a density of 8.34 lbs per gallon. Diesel fuel has a density of 7.13 lbs per gallon.

    The tanks are not located in good places for a liveaboard boat. They put lots of weight in the ends, which, as they are drawn down will cause the boat to trim adversely. Do the saddle tanks amidships extend up to the deck? If so, then you don't have the ability to put any light into the boat with portlights. So it will be dark inside, except for whatever light you can get through the deck. Also, midship tanks take up boat width right where you want you main living area. Beam is very important for roominess inside, and you are losing it to tanks.

    Stability wise, one cannot judge any stability without seeing the shape of the hull. Stability relates both to weight distribution and to hull form. In general, hard chine vessels may roll with less amplitude (smaller degrees of roll) but with faster rolling accelerations than a round bottom hullform (more degrees of roll, but slower accelerations). The accelerations are what make you uncomfortable in a seaway. How many chines are there? If only one chine each side, then this is the case--it will have snappy roll. If there are two or more chines per side, then the hullform is softened a bit, and the rolling accelerations become a little easier to take.

    With the limited information you provide, I would say that the boat might be made into a liveaboard, but it would not be exceptionally good from stability, motions, interior layout, and weight distributions standpoints.

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  4. Guillermo
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    Guillermo Ingeniero Naval

    Being an ex-fishing boat, it should have an 'stability letter', 'stability book' or the like. Posting here info from there would allow us to be precise. Have you asked for it?
  5. alex fletcher
    Joined: Jun 2006
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    alex fletcher Junior Member

    I agree with lewisboats, Eric and Guillermo on all points, if this vessel is a ex fishing boat under the USL code the stability characteristics will have been measured and recorded in the vessels stability records, these should be available to you, and under international law(USL code) a vendor must provided to the new owner the stability log! Please post these stats it will help us help you with your question. It sounds like you have what we in Australia call a wet deck trawler, If so you will have tankage at forward and aft with Large holds in the mid ships? Routinely the holds would be filled with water when thay do not hold the catch to maintain the stability of he vessel and to maintain the stability of this type of vessel once it is converted to a Expedition yacht or house boat it is generally nessercary to move the tank age to mid ships. The challenge is to maintain the stability when all tanks are empty so all compartments that contain high mass values such as the galley, crews quarters, game rooms that contain heavy items should be located mid ships also capital plant IE. the gen set, water making plants and refrigeration etc. will probably need to be moved to mid ships as well.
  6. Richard Hillsid
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    Richard Hillsid Senior Member

    I just put in 22 tons of solid unremovabal palast midships, not enough but i envision some heavy cargo later on, i have a hefty bow tank we keap full, we can pumb dry if we run aground and we actuly run aground on purpose every now and then, our diesel tanks are worth 7 tons a pieace wich gives a good rotational sift and we can inload 5 tons way at the stern to rase the bow to actualy do some repait on the stem, this all on a 81'
  7. longliner45
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    longliner45 Senior Member

    aer you guys calculating tensil strength ? for example ..yes one gallon of water wieghs at about 8 lbs,,but drop the same gallon of water from 8 or 10 ft ,,,,the differance is tremendous

  8. ldrumond
    Joined: Feb 2005
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    ldrumond Ya...I look like him

    Thanks for the heads up on some great points. Am still gathering info on the vessel, haven't seen it yet, it's a cross country trip. Planning on going out to see it, and others on my short list late January. Vessel has been converted (90%). House extension on main deck with holds converted to accomodations below decks. Haven't seen the hull form below water, owner calls it hard chined, don't know if one or more.
    Conversion included the placing of a considerable amount of concrete in the bilges. I mean a LOT of concrete, seem the recall a number around 20 yds. And still, judging by the pics I've seen, she sits a good 18" high on her lines. Searched a lot of posts in this forum on that issue. Still not sure. Hence the stability question. Thanks for your input. When I've had a look or have more info I'll update.