# Help with depth of hull frame and "lu" dimensions - boat calculations

Discussion in 'Boat Design' started by PeterSweter, Mar 4, 2022.

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### PeterSweterNew Member

Hello, i have questions for my drawing.

I design 6 meter plywood boat - bulding - gluing method.

I calculating boat usiq ISO 12215

1) when in calculation depth of frame (lu - top 2) equals me 145mm - can i use 150 mm - even i have cuts for longitudia stifeners (32 x 20mm - cedr) - or i should add 32mm to my 145mm dimensions and depth must be 177mm??

2) with "lu" dimenstion i should use in my (drawing) hull frame - for calculation - because in ISO 12215 - 5 theres no answer for my question. For all situatinons (bottom, side, deck and top)

Thanks form help me

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### bajansailorMarine Surveyor

Welcome to the Forum Peter.

Are you designing (and building as well?) this boat for yourself, or for somebody else?
Is there a requirement that it HAS to be in accordance with ISO 12215?
@TANSL is very familiar with ISO standards, so he can better advise here.

Re your frame depth of 150 mm, I would think that this should be fine, so long as the the longitudinals are properly bonded to the transverse frames - I presume that you will be using epoxy?

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### Will GilmoreSenior Member

To put this into US dimensions, so I can understand it better, the 118.52mm is equal to 4-3/4 inches of plywood construction. That is the minimum solid structural dimension.
What is the answer to that question.

I don't know, but that seems much more than adequate, depending upon the plywood thickness and properties. The lu deck2 dimensions would be just under 2 meters or 6 feet? If I'm understanding the question correctly.

-Will

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### PeterSweterNew Member

I desing using iso 12215 - because it better for me and i want to be shure - that everythink is ok - i just learnig of desing sailboats.

Now i desing for me - for sailing, but if it will be evetyhink ok, mayby i will sold cutting files.

For gluing longitudinal stringers i will use epoxy resin - for best strength.

For frame i want use 12mm plywood (pine) - 11 or 13 layers.

I hve different calculations for "lu top" and "lu top 2" but i didnt know - that can i use smallest diensions ("lu top" and "lu top 3") - then i can use smaller height than 150mm - or it is nessesry to use longer dimensiosn ("lu top 2") - because in iso 12215 - 5 example is very simple. (lu3 - in attachment)

my dimensions
lu top 2 - 964 mm
lu top - 550 mm
lu top 3 - 210 mm

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### PeterSweterNew Member

hello i drew new theoretical frame for second part of my question.

So, when i calculating frame for deck - should i use "lu3" (then i have long distance and it is nessesery to use more plywood)

or i can use "lu4" and "lu5" and "lu6" as separate calculations (then i have less plywood and ligher boat)

of course it is nessesery that all calculations will be according iso 12215.

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You have several load cases. First is keeping the boat sides apart while sailing, then keeping the panel from buckling. You use whichever dimension is needed for the specific frame section. For example if you have integrated floors, the frame will have more depth in the bilge area than elsewhere. There is no requirement for the frames to be the same depth or thickness, or to have regular spacing between frames.
Lastly you have local loads like attaching a keel or shrouds or an engine.

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### Will GilmoreSenior Member

+1
This means you use all of them. lu3 will tell you the thickness you need at the extreme width of your measurement. lu4, 5, & 6 will tell you the thickness you need at each of those points.

A force diagram would show that, like Rumars said, you have different forces in different directions to be concerned with. Your sections look like a sailboat, so you might have shrouds pulling up and inward, a mast pushing downward unless you counter that force with a keel stepped mast or a compression post. The arch of the cabin top requires less thickness over that span then the deck would over the span of lu3 where forces from the cabin top transfer and add to the cabin weight as well as the longer lever arm when flex is introduced.

I am not an architect, I am an artist, but I did take high school and college physics. Identify your different forces and build for the largest ones at each point.

-Will

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### TANSLSenior Member

As I interpret ISO 12215-5, notwithstanding that there may be other equally valid opinions, I would do the following. In the first place, clearly define the zones of the hull, since very different design pressures and load assumptions must be applied to each of them. Then I would place, at least, a side longitudinal that separates and serves as support for the bottom and side frames. I would take the lengths of each reinforcement as indicated in the figure.
It's all very schematic, and like I said, each designer will be able to add more details or look at things differently.

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### AlanXSenior Member

Hi Peter,
While I am not familiar with ISO 12215 I have used AS 4132 (retired) for a self designed and built plywood boats (up to 8m).
This still does not make me an expert however!
I have a couple of concerns however that you may want to look at:

Span Measurements
==============
The first is the use of measurements between "gussets" rather than the "effective length" of the structural member.
You can take the distance between major structural lateral members (that is the outside edges of the sheer, chine, keel etc.) but I would have taken measurements between lateral member centres at a minimum. Really I would have used outside hull measurements as this is the where the pressure load has been estimated that loads the frames. In each of these cases, the section modulus requirements increase!

Notches in Frames
=============
While I see this in aluminium frames (I have no experience), no doubt after welding the notch joint beam/frame is as good as an un-notched beam/frame.
For wooden/plywood beams I definitely think you cannot "just" use thickened-epoxy to join the structure members and expect the same result.
You should avoid notching the plywood and/or timber beams/frames where ever possible.

If you do notch your frames then you need to make the webs significantly deeper.
Based on my research (not based on any boat design standards I have looked at) I estimate your web would need to be 185 mm deep.
My formula is SM=60%*b*(d')^2/6
where d' is the remaining depth of the frame after the notch.
This the is the section modulus (remaining) after you notch your frame/beam.

Of course you could calculate the thickness of fiber-glass required to do the job and plaster the notch joint with that.

Joint Stiffness
==========
Something to consider is the frame joint stiffness.