Designing a 9 meter sailing catamaran

Discussion in 'Boat Design' started by Pammie, Jul 25, 2018.

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rxcompositeSenior Member

The formula should read W * n or should simply be Wn (weight x load factor) but the author keeps referring to it that way. The formula appears to be complex. In Hollmans second book, he explained that it was formulated to follow Perry's equation, an industry standard for fixed wing aircraft.

As I have mentioned, it was too simplistic in approach. The distance used is the max height and the neutral axis is at 1/2 of height. The author does use tensile and compressive strength for top and lower caps. The compressive strength is lower in composites thus it needs to be thicker. In the calculations it was assumed that load factor is the same whereas in practice (or in FAR rules) there is a positive and negative g's. This is applicable in Cats. When the port side hull is lifting the stbd hull and vice versa. The load diagram reverses.

All this was corrected in his second book but the formula became complex. I'll stick to LR-ISO tabulations. The limitation is that it is still designed for a spar. In reality, it is the skin, outermost (with a greater width) that receives most of the stress. Just like the stiffener with plate analysis. The width of the plate is the effective width defined by the Rules.

You are correct that shear height is minus the top and bottom caps. Since the caps are thick, and consist of outer biax and inner biax cover, it cannot be lumped as one laminate. I use the LR-ISO method of tabulation to find the stresses on the individual ply, and the moment formula is changed from uniformly loaded, fixed end to a cantelever formula. The LR-ISO also uses a formula to determine the exact location of the NA. It won't be in the exact center. Of course, for negative and positive loads, I flick (reverse) the laminate schedules to determine which layer will fail first. For distance from the root analysis, I use 4-5 sheets of Excel. One for each distance. It will work, just patience. Or just use whichever has the highest value and use that for the top and bottom laminate.

You can change to metric as long as it is consistent. The trick is to convert certain outputs from Imperial to Metric and see how the spreadsheet goes. If you are arriving at the same answers, your spreadsheet is fine and you can proceed to change the input parameters.

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Niclas VestmanSenior Member

@Pammie: In your panel load calculation you used a sestate coefficient Kdc=1, and craft type coefficient Ncgnh=1,2 as a base for panel loads.
1) May I ask you what the definition for those values are. My guess would be that Kdc=1 is about equal to offshore use. Like CE category B. Is that correct? Craft typ 1,2. What does that represent?
2) is your hull width at waterline 0,55m per hull?.... thats very slender and possibly fast
3) What minimum safety factor did you use for determining laminating schedule vis avi panel loads?
4) Do you have a drawing, Delft ship model or similar, of what it will look like when it is ready? Would be very interesting to see. Not important. Just very curions....

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Niclas VestmanSenior Member

A) I read all posts agian and found your explanation for the craft type coefficient Ncgnh being 1.2. But still interested in the Kdc=1 explanation. Espescially since you found a large difference regarding Ncg in the other rule set you compared with.
B) To me your bottom laminate seems adequate. I probably would have used 3 or 4mm plywood on the outside to help resist point loads when beaching, and no wood inside. With your wood inside schedule, I would use a little more glass on the bottom outside to help resist small point loads or impacts. Just remember, I probably know less than you about this so don't take my word for it ;-).
C) I guess you already laminated sides and decks, since you mentioned current 150kg hull weight, with 220kg finished weight estimates, which seems very light. May I ask what schedule fiber you decided on? Core for sides is 10mm 80kg without plywood? Or do you use plywood in sides or deck?

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PammieSenior Member

Hmm, I compared my calculations with a symetric daggerboard for a 40 ft bridgedeck cat from a well respected designer. It has a crown of 11 layers 300 gr (total 4,2 mm) uni carbon tapered out towards the end. Even at 8 knots my calculation gives a higher number.

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PammieSenior Member

@RX, different sheets for boards positions seems a good idea.

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PammieSenior Member

@ Niclas,
Kdc is for design category. Kdc=1 is for cat. A.
Hull width at waterline is 0,55 m indeed... my foot is less wide...
Safety factor depends on the layer. Some layers have FS 1. My reason for this is that a safety factor is not specified and that there is propably a lot of difference between section 5 and 7. Propably because Section 7 is only preliminary yet.
Yes, I have a freeship file but not on this computer. Can tell you more about it by PM. I posted overview sketches in earlier discussions. You can also follow me on facebook.
In ISO ncg is used for calculating kL. Minimum for that is 3 and is about dimishing plate load on the aft side. It is difficult to compare Rules, but it is said results are more or less comparable.
Using ply at the bottom outside is good for protection but as thought then less good for bending stifness. Later on it seemed to work out fine as it balances the carbon uni on the outside. Calculation shows however that it still needs some extra glass on the outside. I design the boat for long distance sailing with underhung rudders so wasn't targetting on beaching her.
Yes, see FB. Laminate depends a little bit on the place. Side plates are most loaded (because of size). Laminate there is biG600-UDC375-foam15-triG830-UDC375 with more stiffeners than originally intended. Don't know or extra ringframes and stiffeners will balance a lighter laminate....

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Niclas VestmanSenior Member

@Pammie. Thank you for taking time and giving those detailed answers. They are very helpfull to me and exactly what I wanted to know. I will continue to follow your impressive work with great interest.

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BlueBell"Whatever..."

Happy New Year Pammie!
How's the outlook on the project feeling this year?
Cheers

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PammieSenior Member

Hi BlueBell, also for you a happy New Year!
Well, I have all kind of plans. No planning however. At the moment just focussing on finishing the daggerboards: first one is glued together, the other to be done in a few days. Good enough to make the daggerboard cases and finish the hull. No big design questions for the moment, besides decision on the outside hullcolour. Thinking about two colours green: RAL6038 RAL6037 Something lightgreen? First one is very brilliant: good for visibility, bad for my own eyes .

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Green is considered bad luck...for obvious reasons!

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BlueBell"Whatever..."

Pammie said: "Thinking about two colours green"

I like that.

I had an old Datsun 510 car that was two-tone green.
I called it "The Turtle".

My DN iceboat was dark green.

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PammieSenior Member

Well: she's going to be called Kermit after the most famous frog. That is: gecko... frog (kermit). So don't have much choice in colour .

But Ad Hoc: my first association on green is stopping lights: Green is GO! Which sounds OK for a boat. Green is also the colour of Ireland (I like to play Irish folk). I read about green being a bad-luck colour for racing cars? Any other??

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BlueBell"Whatever..."

KERMIT is excellent... I feel sure he'd approve.

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The "bad luck" aspect is based upon factual evidence too.
Since from a distance in bad weather, the colour green merges with its background - the sea. Thus much harder for a rescue ship/helo to spot a vessel in distress.