Mast & Rig Design questions

anyone can tell me how to find the required inertia moment of the section of the mast that will be needed on a boat and how to tell where to place the spreaders,
is there a known calculation or a progrm for this?:!:

many thanks

 

Principles of Yacht Design by Larsson & Eliasson

 

unfortuanetly there are not much information on it about where to place the spreders according to what and what distance apart from each other does have to be equal or not ....

 

Whenever you add shrouds you modify the column once again so you don't analyze it as an unsupported column. It will depend on all the stay locations, the sail-plan the levels of fixity assigned . Stay and spreader locations can be dictated more by desired stay placement, minimum masthead to stay angles, sheeting angles and sail-plan than idealistic column theory.
An FEA package is useful.

 

Ege
I will answer your PM here since everyone benefits then.

Firstly it is going to depend on the RM of the hull since that is what produces the moment (static) that becomes mast compression, you need to know RM Max or at least RM30. Then you need to know your shroud angles, ; base fixity and maximum mast panel lengths, then once you have calculated your compression then you can use modified Euler column buckling theory (or FEA) to figure your maximum mast panel length then within that panel length you can figure where you want the supporting stays spreaders struts etc.

Use a factor of safety on the compression figure (on the load) before going into the calculations. 1.5 for a racing boat perhaps 2.5 for a blue water cruiser.

Can you do this ? I'm afraid I am rather busy theses days, once you have done it post your calcs and we'll review them.

Anyone else got some general info for this query?

 

Ege the usual position for single spreaders is 51% to 52% of the mast hight from its exit point, deck or cabin. Double spreader 37-39% lower, 69% upper.

 

in a search for something similar i came across attached

 

Good excel yipster,

I have a question for everyone: "water addict" called larsson to solve that scantling, but I want to remember that NBS is only useful for boats under 15 m. Then, If I want to calculate a mast&rig for a 30 m. sloop, how will I do it? (avoid FEM)

Thanks a lot

 

The mast/stays/spreader assembly behaves like a truss. The best way to analyze it for optimum design is to do a "free body diagram" for each component and determine loading, using a truss analysis as the loading basis.

For a conventional sail you can use about 1 pound per square foot of sail area to get the mast loading distribution (plus a typical 1.5 safety factor). If it is a foil sail or unconventional rig you will have to use an airfoil program (or a tedious long hand aerodynamic analysis) for the loading.

To optimize your design it takes many iterations, setting it up in Excel would be a help if you are trying a lot of different configurations.

If you know how to analyze a simple truss, determining the stresses on the components is a fairly strait forward process. If not, better get some help. In my observation, the "rules of thumb" often listed in boat design books tend to over design, and are not suitable for anything other than conventional rig designs.

 

Nast and Rig Design Questions

I would suggest to have a look at the Mast and Compression Loads thread as could be of initial help. Plus.....I would shortly add on that thread a post enclosing a spreadsheet with computational procedure used and relevant results for calculation of mast compression+mom of inertia on both axis and tension loads in shrouds for a 36.50 m loa alu ketch.

 

Sorry about my long absence... too busy last weeks. Thank you for your comment Petros. I'll be trying to put in practice your comments as soon as I can. Yades. I would appreciate a lot your contribution! Let's see what can be obtained from it!

Thanks to everyone!