FE model of MONO HULL sailing boat

Hello all,

I moved about 2 months ago to the boat design world. I have been asked to develop a FE model simulating the first bending mode of a mono hull.

the force i was given are the mast precompression and the force at the stay, that i used to calculate the force at the runner.

Few questions:
1) where do i constrain the model?
2) shoul i model the cables stay and runners, and the mast?
3) if so, should i use a spring model or a beam model?
4) should i consider the effect of the hydrostatic pressure?

I should say the FE software in house is NEiNASTRAN..

Thanks for any help, suggestions, constructive critics and everything that you can come up with?

Fabio

 

1) where do i constrain the model?

Constraining at the mast foot usually works well for me.

2) shoul i model the cables stay and runners, and the mast?

What would be the reason for doing this? Those loads are the business of the rigging designer. Seems like unnecessary complexity if you are looking into hull loads.

4) should i consider the effect of the hydrostatic pressure?

This gets very compex, and on most "normal" hulls you will find point stress at the shroud endpoints and mast foot are the dimensioning factors. Your worst case is propably a broach, where the windward shroud has its maximum load. In this case the yacht is heeling over 50° and the hydrostatic pressure is rather unpredictable...

In real life slamming forces are propably higher than static pressure forces, but they are not easy to get at. (If you do want to go there, have a look at this paper).

Also don't forget the halyard forces, if the yacht uses the common arrangement of halyards going through blocks in the deck at the mast foot there is a significant contribution to mast compression from the halyard tension.

 

Hi JasperZ

Thanks for your reply. I have to say that i tried to constrain the mast foot, and i applied the forces, and i obtained very large displacement and i am talking of more than 100mm. That is why I thought to model the mast, the stay and the runners.

So i am puzzled, i have as reference a study done by a consultancy companym but it is on paper and does not give much details about the modelling techniques, and for a similar boat they get very reasonable displacement (50-60mm ) and stress along the main fibre direction of about 100MPa, which according to my boss these are the value that he is expecting.

Material are the same, the structure is very similar, and the laminate are very much the same.

I hope that these info could help you to help me.

Thanks Again
fabio

 

I fail to understand how modelling the mast and stays would make calculations more accurate. Unless you mean their stiffness is very low and the stay elongation will decrease the forces or something??

It's a little hard to know what can be wrong without seeing a detailed setup of the model.

 

Well modelling the mast and stays was an attempt, but i get the displacement but the stress levels are very low, so I cannot compare the 2 models.

what i can tell you that i am working on a 60footer, i have 10T at the stay, which gives a 5.2T at the runners. with this forces i get 400mm displacement measured at the runners and stay application node. The mast foot is about half way the lenght. I have also model the bulb as a mass element.

Thanks

 

Fabio
How did you get on?

It is usually better to break things up for more detailed analysis unless you are just looking for general skin stresses?
I have not used Nastran, However if you are modeling the hull and rig you will need to pre-stress the cable elements. Restraining on a central ring frame and part of the keelson and limiting rotation at certain nodes may be helpful, you also need to add the hydrostatic pressure if looking at skin and frame stresses . The problem with FEA is having enough experience to know that the results are sensible. If you make the model too large the run time will be many hours, better to calculate rigging forces and then add them to the hull model rather than modeling the full rig keep the model simple and run through a few times varying restriants etc and checking that the results all tally.

Cheers

 

Mike,

Thank you very much for your reply. First yes I am looking at general skins and structure stresses. Yes I figured that I would have needed a non-linear system in order to pre-stress the cables. Now I have a model set up and running (2hours ish), very much similar to what you described, I will also try what you suggested just to have an element of comparrison.

I have tried to split the model into main structure and rig, I got the forces from the rig (very similar to what i calculated by hand) and then applied to the main structure using a static analysis, but the model deform way too much, that is way i started looking for other solutions. But I guess that the problem was on the constrains set.

What do you reckon?

Anyway Thanks again for your very usefull post.....

Regards,
Fabio