Structure Layout

Dear colleagues,

Lately I am involved in composite powerboat design (40-60 ft) with focus on GRP structure and boat strength.

Since I am quite new to composite boats could you please help me with your experience and knowledge on how to define hull structure geometry for preliminary calculations according to ISO 12215-5.

I am aware that there needs to be at least three bulkheads (1. between engine room and living space; 2. collision bulkhead at bow; and 3. somewhere in between engine and collision bulkhead to shorten the unsupported span of longitudinals.

Lead by the big ship structure logic, I tried to look at structure in this way:

- longitudinals should be continuous along the boat length (for longitudinal structure system). Span between bulkheads should be taken as unsupported span. My question is whether should the longitudinals break at bulkheads, or should the bulkheads go over the longitudinals.
- is it ok to consider beam (longitudinal) fully supported if bulkeads goes over it, but bulkheads is glued to longitudinal with structure glue.
- can the plywood or grp sandwich cross-beam (partial bulkhead between hull and hull liner) be consider as a fully support to longitudinals if glued with structural glue and over-laminated to longitudinals?

Please, any other advice, recommendations and literature suggestions as well as principle sketches are very welcome and will be of great help.

Thank you a lot.

Best,

Tomislav

(my contact e-mail is tprosine@gmail.com if needed)

 

ToMek-You must be able to identify the elements in the structure. Unfortunately, ISO manual did not define it clearly.

The primary elements are bulkheads, girders, transverses and web frames. Longitudinals can also be primaries if supported at its end by a bulkhead/frame. Primary elements are continous and unbroken. It is supported at the ends and heavily reinforced. If there is a need to terminate/break it, it must be bracketed or a transition taper is added.

Secondaries are generally called stiffeners, supported by the primaries, and is the smallest of the elements. Secondaries are to stiffen the plates. Secondaries are intercoastal and broken but is reinforced at end connection. Some prefer the secondary longi to be continuous for ease of construction. That is allowed but the connecting ends must be reinforced.

 

Thank you rxcomposite for the very nice sketches.

What I still don't find completely clear about the third sketch, who support who on this sketch?

It is obvious and clear that blue one and green one longitudinals are fully supported (fixed) at primary bulkheads (orange ones).
And also it is clear that those primary elements are able to support secondary elements.
But what is the criteria that allows us to consider blue and green one as fully support for pink ones? What are the unsupported spans that should be taken for calculation of blue one(s) and green one(s)? (is it span between orange or between pink ones?)

If pink ones are floors (partial bulkheads) going from chine to chine, is it possible to consider them as primary and what would be their unsupported span if keel can be considered as natural stiffener (deardise angle at keel is 122 deg)?

Can the chines and keel be taken as primary if they satisfy the natural stiffener condition?

Thank you very much once again

 

You must define first the boundaries of the plate. Use the bulkheads, centerline and the edge of chine (or bilge radius) to define. Adjust span/effective length later.

Decide whether it should be transversely framed or longitudinaly framed or a combination of both, then subdivide the panel.

Generally, wide beam vessels are transversely framed. There are more transverses than longitudinals. Slim vessels are longitudinally framed and have more longitudinals that sits underneath the transverses which are few.

Aim for high aspect ratio panels as it is more efficient and weight saving. The center girder can be ommited for small watercrafts but other rules apply, not ISO.