trim, draft calculation for counter ballast calculator

[nrbt]

Hi there,


I am seeking the wisdom of the forum..


I am trying to develop an excel based, counter ballast calculator for a small workboat. The boat has a transom mounted, fixed lifting frame (lifting on the centerline). The lifted load and the counter ballast (at bow) lcg, vcg, tcg are fixed, only the magnitude can change. Assuming the boat has zero heel, the calculator could potentially have the following input data:

initial load case: displacement (without the lifted load and counter ballast), lcg, vcg, draft mark readings.

Interim load case: displacement (with the lifted load BUT without the counter ballast as this would be later calculated), lcg, vcg.

Hydro tables for the range of drafts, range of trim and displacement for the operational limits. (trim, draft AM, displ, LCF, wetted area, waterplane area, LCB, LCF, KB, KMt, TPc, MTc +whatever maxsurf can spit out).


My solution:

I developed a table where i include the displacement with the envisaged lifting load, lcg, vcg and in every row I correct the displacement, lcg, vcg with a 1te ballast increment from 0 to 8 te (8te is the swl for the lifting frame). My plan was to carry on with increasing the ballast until the point where the load case fails one of the criteria (freeboard, limiting KG, total displacement). Where it fails, I take the last pass and first failed load cases and do a higher resolution check between the two limits and interpolate out the final passing ballast mass. However..


My problem is, that in the interim load cases I don’t know the draft amidship, nor the trim. I am trying to get that out from the hydro tables but there the only data I can match against is displacement values vs interim displacement. I am trying to find a correlation between the hydro table and the actual lcg or vcg so by seeking for the lowest combined difference between the interim displacement

From the above data sets, would it be possible to calculate draft at midship and trim (by using only excel and no MaxSurf, GHS, etc.)?

I guess the change of trim could be calculated between initial load case and actual via MTc from initial load case combined with moment about initial load case lcg and lifting frame load? But that is relatively inaccurate as the lcg MTc change with trim.


Any idea folks?


Thank you.

N.

 

[gonzo]

Is the ballast something like a water tank where the weight can be adjusted? If the lifted weight is fixed I assume it means that there is only object that will be lifted and not used for any general use. You can use different methods with excel. For example, using Simpson's rule.

 

[nrbt]

Thank you Gonzo for getting back to me..
The ballast weight is adjustable, internal water tank.
The lifted weight is not fixed, it ranges between 0-8te. The lifting frame is fixed, meaning, just like the ballast, the lifted weight wouldn't change lcg, vcg, tcg.

 

[Ad Hoc]

...Any idea folks?...

What you are attempting to do is this:

A max KG curve. This is a standard requirement for workboats.

If you are using Maxsurf, then you need to establish the criteria it needs to pass, and then the software will dump out data, to produce a graph, like the one above.

You then need to create a simple spread sheet showing a loading calculation, like so:

And then once the new KG has been calculated, you can mark off the final displacement and KG on the graph, like so:

If you find that difficult, or unable to do so, then you can select a series of loading conditions and create the GZ curve, and plot each one, like so:

And then have a series of moment, from the lifting and plot them onto the GZ curve, like so:

Without have direct access to the stability software, there is no other way.
As there are far too many variables to consider.