Calculating ballast for a motorsailer

Discussion in 'Motorsailers' started by Annode, Sep 22, 2019.

  1. bajansailor
    Joined: Oct 2007
    Posts: 2,820
    Likes: 1,084, Points: 113, Legacy Rep: 37
    Location: Barbados

    bajansailor Marine Surveyor

    Annode, do you have any photos of this 30 m trawler that you could post please?
    And a photo or two of her out of the water (so that we can see what she looks like underwater) would be useful as well.
    A full works stability manual on the vessel would start with manually taking the lines off the vessel, drawing up a lines plan, digitising all the offsets on the computer (if you drew a lines plan on paper) , calculating the hydrostatics, and then calculating the stability for various different typical conditions of loading.
    Along the way you also have to carry out the inclining test to determine the centre of gravity for that particular load condition - note that you will also need to measure the draughts fore and aft to calculate the vessels displacement from the hydrostatics tables. .

    The attached scan is a rough method of estimating the stability of a fishing vessel - but we have tried it, and it does work, for relatively 'small' vessels at least (eg under about 25'). If you can get enough manpower to try this on your 30 metre trawler, it could be an interesting experiment! But please note that it is not a substitute for a full works stability manual.

    In an earlier post you were worried about taking out many tonnes of fishing vessel stuff like fish, ice, equipment etc, all of which would probably be low down, and replacing it with a large heavy deckhouse and a sailing rig.
    Lets say that you know the typical loaded displacement and the draft of the fishing vessel when she was working in her previous life.
    And that you have done an inclining test to determine the KG.
    You could then do an exercise in 'removing' all of these fishing weights - on paper.
    Take moments about the keel such that :
    Load displacement x KG lever = (Light displacement x new KG) + (the sum of the moments of all the individual items that you are taking off).
    You would have to estimate the weights of all these individual items, and the heights of their Centres of Gravity above the keel.
    You can calculate the Light displacement from :
    Light displacement = Loaded displacement - (sum of all the weights that you have removed on paper).
    Put that back into the equation above, and you have your new KG.
    Now you can start to add the weights (and estimated heights of their centres of gravity) of the deckhouse, masts sails, etc, tenders stored on deck etc. The more the merrier. If you can manage to do it so that you arrive back at the original loaded displacement, then even better. If you need to add some extra weight to bring it up to this displacement, you could perhaps add it as a welded on ballast keel. If the centre of gravity of this ballast is below the keel, then the lever for the moment is -ve rather than +ve.
    Put all these into the formula, and the only unknown is the new loaded KG - which can hence be calculated.
    How does this compare against the original fishing boat KG?
    If you only have a very tiny GM compared to what you had before, then yes, you have a problem and she might fall over in a Force 3 with the sails up.
    But hopefully she will be pretty stiff, especially if you have added extra ballast at the keel as well.
    Please note that the above suggestion is a very simplified way of having an initial bash at finding out what your stability might be like, and that it does involve making various assumptions or estimates re weights and heights of centres of gravity.
    But I hope that you can follow what I am trying to say.
     

    Attached Files:

    Last edited: Oct 12, 2019
  2. gonzo
    Joined: Aug 2002
    Posts: 15,878
    Likes: 1,253, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    The inclining test does not need to take the lines. It is a simple way of calculating stability.
     
  3. bajansailor
    Joined: Oct 2007
    Posts: 2,820
    Likes: 1,084, Points: 113, Legacy Rep: 37
    Location: Barbados

    bajansailor Marine Surveyor

    I am puzzled Gonzo.

    In an inclining test the formula for GM is GM = w * d / W tan (heel angle)

    where w = mass of inclining weight, d is the distance of the CG of the weight w from the vessel centreline and W is the displacement of the vessel.

    As per Inclining Experiment- Determining Metacentric height of the ship https://www.marineinsight.com/naval-architecture/inclining-experiment-determining-metacentric-height-of-the-ship/

    And to get the displacement of the vessel we need the hydrostatics, and to get the hydrostatics we need to digitize the lines.......
     
  4. TANSL
    Joined: Sep 2011
    Posts: 7,006
    Likes: 563, Points: 123, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    LOL
     
  5. gonzo
    Joined: Aug 2002
    Posts: 15,878
    Likes: 1,253, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Digitizing lines is a waste of time, unless you took a cloud point with a laser scanner. The easiest thing is to hang it from a crane and read the gauge. Taking lines for displacement does not need the precision to copy the boat. It suffices to pick the rough shape and dimension of 7 or 8 stations, which can be done with a straightedge, some string and a measuring tape; even underwater.
     
  6. TANSL
    Joined: Sep 2011
    Posts: 7,006
    Likes: 563, Points: 123, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    Amazing and how simple it is !, right ?. I would never have imagined it. Anyway, why do all that if we already know that: "The inclining test does not need to take the lines."
     
  7. Annode

    Annode Previous Member

    >where're you going to sail, which determines how much heeling the hull must withstand. So you have basicly nothing.

    Sigh... I have to keep reminding myself that this is a public forum and obviously anyone can sign up and post stuff like this....

    Where I am going to sail determines how much heeling.... Really? I must have missed that conclusion in my years on the water. I though it was about the weather.... Thanks for the education <EYE ROLL>

    Gonzo, I agree, for a first approximation to get in the ball park, ie get some usable information without investing large resources only to determine that this particular boat is not as suitable as another perhaps.

    Bajan, posting pics in this forum is not going to happen given the level of egos, snark and push back this simple question seems to be generating. That would just add fuel to the fire.

    Despite the heavy weather in this thread, some of you are giving good information that is helping me think about this problem. The biggest problem is the amount of stability it has for small heel angles. I would really like to a more severe angle and take a measurement. This would require large static forces applied to the hull and the risk of it rolling over. I am trying to think of a clever way to use the tide to dry it out on some kind of inclined bed with a strain guage to measure the dry load and the load as buoyancy returns slowly.

    I find it very interesting that this topic has revealed a reluctance to post concrete information or explore ideas from even senior members because of the immediate challenges the specific information is subjected too and the amount of defending that member must do in order to retain credibility. This is just petty beyond belief and leads to fatuous answers that are not answers beyond hire a naval architect, do a full survey and make a complete computer model with enough calculations to choke a horse.

    I get the feeling that before it was adopted, if I had come into this forum and suggested a ski ramp for the front of a boat that launches aircraft as just a simple exercise in lateral thinking, I would have been ridiculed.

    I applaud the members that have made concrete suggestions and invite you to continue to post ideas in the full knowledge that most will not work, but one may lead to a discussion that reveals something very workable :)
     
  8. TANSL
    Joined: Sep 2011
    Posts: 7,006
    Likes: 563, Points: 123, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    I can understand your frustration when you check that nobody gives you the answer you are looking for. What happens is that, without knowing what your ship is like, its shapes and the current weight, it is not possible to give an answer. It is likely that with a simple sketch of the ship, with its main dimensions and knowing the current draft in the bow and stern, someone could give you an answer, which could only be approximated. But it would be something more concrete than what can now be said.
    I would like to help you but you have to give some information about your boat. If it suits you, send me a pm.
     
  9. bajansailor
    Joined: Oct 2007
    Posts: 2,820
    Likes: 1,084, Points: 113, Legacy Rep: 37
    Location: Barbados

    bajansailor Marine Surveyor

    Annode, please do post a photo - I am sure that it would help a lot, rather than add fuel to the flames.
    You will also receive (I hope) constructive comment re what will happen when she is heeled - one significant moment is when the deck edge goes under, another is when any apertures in the deck start to flood (if they are not sealed shut).
    Re the amount of stability that it has at small heel angles, you mention that this is the biggest problem (?)
    For small heel angles, the righting lever GZ = GM * sin(heel angle).
    For larger heel angles - this becomes more complex.
    There is a lot more to it than simply heeling her over to a large heel angle using some suitable force and taking measurements.

    For a 90' vessel it is probably not very practical to engage a crane to lift it in order to determine the weight hanging on the cable(s), unless you are planning on lifting her out of the water at some stage anyway (It sounds like she is currently in the water?).
    If there is no lines plan available, and no method of easily weighing her, are there any similar sisterships that you can use for reference, re an approximate weight for your vessel?
    Once you have an approx displacement you could perhaps do a rough approximation of where the centre of gravity of the vessel would be - but this would just be a wild guess really. The only way of getting it accurately is an inclining test.
    And this is the crux really - you really can't get any meaningful or accurate data unless you have a displacement and GM to start with. That is a simple fact of life in the ship stability world.
    Once you have these, please do have a go at the method I outlined in post #41 re removing weights on paper re the original vessel, and then adding weights (again on paper) re the vessel that you intend to convert it into. And see what happens. You might be pleasantly surprised re how much better the stability has become, especially if you have added extra weight (on paper) in way of the keel to compensate for the weight of the extra accommodation higher up(?) and the rig etc.
    If you don't like my thoughts above, then I am baffled as to what else to suggest. Maybe I am barking up the wrong tree totally?
     
    Last edited: Oct 14, 2019
  10. gonzo
    Joined: Aug 2002
    Posts: 15,878
    Likes: 1,253, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Where in the UK are you?
     
  11. TANSL
    Joined: Sep 2011
    Posts: 7,006
    Likes: 563, Points: 123, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

  12. Ad Hoc
    Joined: Oct 2008
    Posts: 7,324
    Likes: 1,218, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Ok...so, lets try another way then.

    Annode - what is your job, what do you do in your daily work or however you wish to describe your job?
     
  13. Annode

    Annode Previous Member

    [​IMG]

    Threadjacking is a deliberate attempt at steering a discussion away from the original topic by bringing up another, usually more provoking and completely unrelated, topic. Depending on the context, threadjacking can be used as a timely diversion in the event of awkward discussions or, more commonly, as a form of cheap amusement through trolling.
     
  14. Annode

    Annode Previous Member



    Perhaps the best way to calculate keel and ballast is a visual inspection :/
     

  15. Annode

    Annode Previous Member

    My next thread will be about calculating mast height :)

     
    DogCavalry likes this.
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.