Full Hull Design Help

So you've never taken a FEA approach to hull design?

 

I've only designed boats on a private basis for my own use, and probably erred on the side of overkill, but you can't ignore what standard practices are, any major deviation from that is probably going to mean you either have something that will break, or be a slug because it is excessively heavy. Why not take advantage of all the experience of others, that is represented in those typical scantlings.

 

If you'd like to go through the bother of element analysis, on a small craft, than go for it, but this is more difficult than common yacht design techniques and only necessary on work, where you're standing way out on a design concept limb or with a racer, when every ounce has to be justified. Scantling rules can elevant much of this. Somethings need to be worked through like rigs and propulsion, just to be sure you hit your targets, but a fair bit is also can be common sense or within specific ranges.

 

Element analysis is always necessary. If you do not analyze the elements, how can you calculate their scantlings ?. The finite element analysis is another matter. It is one of several direct calculation methods that can be used to define the structure of a boat. And, even in small boats, it may be necessary to make some studies by direct calculation. Studies, for years, on non-homogeneous materials, have improved the calculations with them, and left completely obsolete many of the books featuring recipes practices for calculating scantlings.
Samdaman, many designers of large and small boats (I would say the majority) have never needed to perform FEAs.

 

The problem is the load definition. These are dynamic loads which we represent using static design pressures. Class societies developed these equivalent heads and refined them with a combination of calculation, observation and variation as failures became evident.

Loads are defined either by using class society equivalent static pressure loads and designing your structure to those loads which is called specific design, or simply using their scantling rules which incorporate a FOS

 

Ahh brilliant I will look into this! this is more or less the sort of information I was looking for. Once I have equivalent loads I can bring out my inner structural engineer! Thanks for the information.

 

It makes no sense to me to be designing boats and not availing yourself of the accumulated experience of the boating world in so doing, but rather start from a theoretical viewpoint. What you see in existing boats is the distillation of all theories, by experience.

 

What classification society rules would you recommend for boats under 10 meters / 33 feet in length and under 10 tons displacement? ISO? (Which technically is not a classification society.)

 

It really depends upon the application....and if one is willing to accept that the scantlings may be a slight overkill, owing to being at the extreme envelope of the validity of the rules. Since one is not paying Class for a revision of the rules to allow for the 'smaller' vessel to be designed using their rules; one must just adopt what has been calculated as such.

Having said that...we have designed vessels under 10m to Class ..but as noted the increases in scantlings from what most on this site would consider as acceptable, is not for us, as it is about fit-for-purpose.

ISO would be a suitable alternative.

It really depends upon the application and whether the designer wishes to cut too many corners simply to get a "design" to meet their requirements rather than what is fit-for-purpose..

 

David
I posted earlier that I recommended GL. Note that as of July this year they are changing all their scantling rules to the new DNV GL amalgamated format. I have not had a good look through their new offerings and whether they will keep their services for smaller leisure yachts and boats or not.

If they drop the craft covered by ISO as they were discussing it only leaves BV which is poor in comparison to GL. Or ISO ( and......yes as I posted earlier is a standard not a class scantling rule).

A quick search shows the GL docs are still current and available eg:

http://www.gl-group.com/infoServices/rules/pdfs/gl_i-3-3_e.pdf

From memory GL produced the scantling rules for the TP52 class of racing sailboats that just won the sydney hobart.

They are the only class society that stands behind rig design for smaller craft.

 

I'd suggest it's not a matter of a designer cutting corners on scantlings so much, as meeting the SOR goals without as much effort. Again, a highly loaded/stressed structure that's very weight/performance specific, will need a higher level of analysis, but if (the project) is less than this and some assumptions can be made, it'll is cheaper in time and trouble for the designer. Costs often have more to do with this than anything else, hence the reliance on a set of rules, instead of working through the load paths. In the end, you have to decide how much you can afford to spend on one compared to the other.

 

A typical 17' bowrider will succumb to damage and neglect. From a structural standpoint, a 3/32 solid CSM build (LOA/1000 is the original FG scantling rule) would handle design loads for approximately forever. But it might well fall apart on the trailer driving home. Little, neglect tolerant boats are designed for point loading and abuse tolerance.

Um, what sort of table are you going to feed the FEA stresses into - Notch impact, fatigue. In other words, what is the failure mode you are assuming governs the vessel life? If you can prep the following for FEA, go right ahead


Take a look at basic small boat fittings. The bow eye on my 350# dinghy had a 20,000# breaking strength, and it was going to break before the boat did. You should be able to rip the outboard off its bracket without damaging the boat. I've done that twice. A 3000# tiedown strap can take about 5000# before each hooks straightens out, and I have had a pair of them disappear skywards when I got rearended (bending the trailer frame in the process). That's 20,000 pounds of prying load and 10,000 pounds of girth tension on the sheer. Typical strength of 3/8 dock line is 5000#, but could run twice that. Deal with the concentrated loads and the rest ought to take care of themselves. Personally, I expect to be able to install a #8 SS bolt/washer/nut anywhere on a 17' boat and then pull it apart without any damage to the boat (call it 1500# over 0.15 sqin). I expect any flange joints to be able to withstand retorquing a 1/4 ss bolt/washer/nut annually for 50 years without undue thinning or distortion (you would expect to use a reduced torque after the first few years). Even so, this means starting with a pretty stout flange. And for crying out loud, make the stuff thick enough that you can run a DOT screw stud (snap stud) into the cap and liner without it stripping out. You really don't want to have to engineer special reinforcements to attach a boat cover.

 

"Yes. I'm looking to get the basic theories of design under my belt " Great statement for a new years resolution .The basic theories are the last 2 thousand years of boat design some which worked and some that didn't