Draft and trim calculation

[Grifter]

In response to a post about how much the draft and trim changes with an additional load, I wrote a program to calculate it. I though I would post it here, in case it is useful to anyone in future. The script is hosted in my public dropbox at https://dl.dropboxusercontent.com/u/14846300/boat_trim_calc.m and can be run in MATLAB or Octave or online using http://www.compileonline.com/execute_matlab_online.php.

 

It uses Archimedes' principle and static equilibrium of forces and moments. Making a few assumptions (rectangular boat, small angles, no dynamic forces, etc.) and putting in boat dimensions, masses and the locations of those masses allows the draft at the bow and stern to be calculated.

 

For example, if I add 600kg of water 2m from the bow of my 10 tonne, 15x2m boat, according to these calculations, the front goes down by 64mm and the back comes up by 24mm, which seems to correspond to reality.

 

Also, by swapping the width and length values, I can look at how the roll angle changes with load. If I add a 60kg load at the extremity (e.g. me standing on the gunwhale) then the boat lists 12mm down at one side and 12mm up at the other.

 

Hope this might be interesting to a few people.

 

Regards,
Joe

[howardang]

In response to a post about how much the draft and trim changes with an additional load, I wrote a program to calculate it. I though I would post it here, in case it is useful to anyone in future. The script is hosted in my public dropbox at https://dl.dropboxusercontent.com/u/14846300/boat_trim_calc.m and can be run in MATLAB or Octave or online using http://www.compileonline.com/execute_matlab_online.php.

 

It uses Archimedes' principle and static equilibrium of forces and moments. Making a few assumptions (rectangular boat, small angles, no dynamic forces, etc.) and putting in boat dimensions, masses and the locations of those masses allows the draft at the bow and stern to be calculated.

 

For example, if I add 600kg of water 2m from the bow of my 10 tonne, 15x2m boat, according to these calculations, the front goes down by 64mm and the back comes up by 24mm, which seems to correspond to reality.

 

Also, by swapping the width and length values, I can look at how the roll angle changes with load. If I add a 60kg load at the extremity (e.g. me standing on the gunwhale) then the boat lists 12mm down at one side and 12mm up at the other.

 

Hope this might be interesting to a few people.

 

Regards,

Joe

 

Hello Joe, I am sure that your calculations will be of interest to many forum members and you have certainly taken a lot of trouble to arrive at your conclusions from first principles. You may be interested to take this further by having a look at how stability calculations are derived and used in the marine industry. There are many useful books available on the subject, some hightly technical, others of more general interest; one which is well know and has been available for many years is "Ship Stability for Masters and Mates" by D.R. Derrett, first published in 1964, ISBN 0540014036. This publication covers all aspects of Statical and Dynamic Stability, including loading and shifting weights, trim and list angles, etc etc etc. My copy dates from 1972 but the principles remain valid and applicable. There are many other publications which cover the subject and a visit to the library or online may find them.

 

I hope this is of interest,

 

Howard

[Teadaemon]

Just as an addendum to Howard's comment, the third edition, first published in 1972, is the first one to be written using the metric system, which makes life considerably easier.

[howardang]

Just as an addendum to Howard's comment, the third edition, first published in 1972, is the first one to be written using the metric system, which makes life considerably easier.

Unless you're ancient enough to have initially been brought up studying stability using Imperial measurements and then had to relearn it in metric,which is why my copy is in this new fangled system

 

Howard

[luctor et emergo]

The rest of the world calculates in metric. Get over it.

You gave us English as the international language for shipping, aviation, and pretty much everything else. Is that not enough?

[Sir Nibble]

And swearing.

[howardang]

The rest of the world calculates in metric. Get over it.

You gave us English as the international language for shipping, aviation, and pretty much everything else. Is that not enough?

Looks like a sense of humour bypass; I was joking, hence the smiley!:-)

 

Howard

[Teadaemon]

Unless you're ancient enough to have initially been brought up studying stability using Imperial measurements and then had to relearn it in metric,which is why my copy is in this new fangled system

 

Howard

Or you're American.

 

It reminds me that one of the things on my 'must acquire if you ever see one' list is a scale rule in Imperial divisions (8ths, 16ths, 32nds, 64ths, 72nds, etc), to go with my metric one. They're surprisingly hard to find, but essential for working with boat plans written in Imperial (or US customary) units.

[howardang]

Or you're American.

 

It reminds me that one of the things on my 'must acquire if you ever see one' list is a scale rule in Imperial divisions (8ths, 16ths, 32nds, 64ths, 72nds, etc), to go with my metric one. They're surprisingly hard to find, but essential for working with boat plans written in Imperial (or US customary) units.

I was standing by two RoRo vessels at Swan Hunters at the time of the change to metric and they were the first to have been built by the shipyard to metric plans. The shipyard workers were issued with metric tape measures, rulers etc and the first thing they did was to convert plan dimensions back to Imperial. This led to a bit of confusion until it was sorted!

 

Howard

[dmr]

Or you're American.

 

It reminds me that one of the things on my 'must acquire if you ever see one' list is a scale rule in Imperial divisions (8ths, 16ths, 32nds, 64ths, 72nds, etc), to go with my metric one. They're surprisingly hard to find, but essential for working with boat plans written in Imperial (or US customary) units.

 

This sort of thing demonstrates just how advanced us English are, whilst Europe was messing about with a metric system based upon crude decimal numbers, we had the foresight to anticipate the computer age and did everything in inverse binary.

 

............Dave

[Dalslandia]

The metric system is coming to USA too, inch by inch ... :-)

[Trento]

The Americans already have a metric rule.. They have a 10inch to the foot flexible rule. Used on their home build aviation industry.

Edited September 5, 2013 by Trento

[Dalslandia]

10 inch to a foot?,

It.s like 400 degree on a compass.

 

Jan

former "homebuilder"

www.jcpropellerdesign.com

[DHutch]

Or timber, sold in 'metric units' , which at 300*3000*25mm are to all intensive purposes, a foot by a foot by an inch!

Getting back to the topic, I would be very interested to look at and play with the program, but if im honest, having clicked on the second link, am not entirely sure what I'm looking at, and no longer have a copy of matlab on my computer or much desire to install it having quite not used it since graduation!

 

 

Daniel