Modular Cruising Catamaran

[Richard Woods]

You may find my comments on bridgedeck slamming, unstayed rigs and crossbeam design on my FAQ's page useful

Richard Woods of Woods Designs

www.sailingcatamarans.com

[masalai]

Easy to do yourself - a pole and a wind speed thingy and a means to hoist it up and down, then sit below and measure the variance whilst sitting at the foot of the pole reading the instruments - Those tactic ones have self powered wind info senders - a good project for you....

[Autodafe]

Not a bad idea Mas, I'll see if I can overcome my chronic lazyness to do it some time - first to find someone with a 20+ meter mast who'll let me borrow their boat
I have found this handy wind gradient page, with formula and spreadsheet:
http://www.onemetre.net/Design/Gradient/Gradient.htm
There's quite a strong surface roughness component, so light winds and big seas is the worst case for short masts.

[Ad Hoc]

I.N.

Ok..sorry for the delay…had a 400km round trip yesterday and got caught up in a 45km traffic jam…long story!

So, attached is a simple diagram of the main loadings on a catamaran beam….as you can see three main load cases. I looked at GL,BV and DNV and compared to simple first principals. I didn’t bother with LR, since I’m familiar with these when compared to DNV’s. The DNV are the most easiest and consistent (based upon very simple first principal calculations too). Which is no surprise, since I’ve been happy with using DNV for the past 20+years. BV were odd that increasing the wavelength of wave encountered increased the loading. Which at first seems sensible, but if a wave of 300m is encountered, you’re not going to get this type of loading, only when the waves are near unity. Ignoring the mast loads for now, lets focus upon the main beams.

The transverse bending moment roughly 52kNm
The pitch connection moment is roughly 81kNm
The torsional moment is roughly 72kNm

There is also the combination loads to consider, such as 100% transverse bending and roughly 70% pitch connecting moment.

Assuming the bolted connection what are the vertical reaction loads?

Well, looking at the worse case and the combo loads, the reaction is 33kN.

Allowing for growth and being over weight add 50%, and, since this is a simplistic calculation, one needs to consider fatigue too, so a roughly FoS of 5 and then sea water environment another FoS of roughly 2. Therefore 33x1.5x5x2 = 495kN.

The connection of your box beams, via bolts, needs to take some 500kN. So, what does this mean, well, an M25 area = 490mm2. Assuming simple mild steel bolt, yield 235MPa, the number of bolts at each connect is 4.3, or 5 bolts.

The box beams must extend to the outboard beam and be connected at the outboard beam and inboard beam location. The beam being the demihull.

The structure that is supporting the beams and is being bolted, needs sufficient shear paths to take this load into the surrounding structure. But that is another detail.

So, once you have considered this, and if this is possible for you, we can look at the stiffness of the beams to take these moments/loads.

[masalai]

The nasty one (and I do not see it illustrated? is when in effect the diagonally opposite corners are suspended (2 waves) and the other corners have bouncing (panicking) weights (crew/passengers)....

[Ad Hoc]

that is the pitch connecting moment load case, or nasty case as you call it

[Autodafe]

Excellent summary on catamaran load cases AdHoc.

It is worth noting if unstayed masts are to be mounted in the hulls there is an additional case where the heeling moment generated by the sails and rudders has to be transmitted to the crossbeams. This appears as a moment between the inboard and outboard beam bolts.

Say,
max RM = 250kNm, hull beam = 1m, Total FoS = 10, then both inboard and outboard bolt groups require 2.5MN, or ~25 bolts each.
This force will be split between both beams, but still requires 25 bolts per joint.

Depending on the sail area and flexibility of the mast it may be reasonable to assume less than total righting moment on each mast, which would reduce this to perhaps half.

Alternatively, if the two masts are to be mounted on the cross beam directly this does not apply.

[Ad Hoc]

Autodafe

Thanks for the comments

Agreed, the sail loads have yet to be included. Since this is his first time, i can only go through the steps bit by bit, so first, the basic hydrodynamic loads from the hull displacement in a sea way...otherwise way way too much information to digest.

I haven't even gone through the stiffness requirements of the beams yet too..., whether with or without a mast, as noted.

So, as always in design, looking at every load case/scenario, and then see which is the worse case, never assume there is just one and its magnitude, must be 'proven' first.

[masalai]

316 to 346 is all the reward I can offer to you for your understanding, patience and compassion

[Ad Hoc]

masalai

That's jolly decent of you

Makes a pleasant change from the usual personal tirade and ad hominem attacks i get..

[masalai]

If a person makes a post that I reckon is praise-worthy, I give points, that is just the neighbourly thing to do, that does not mean I do not type cheeky responses if the cap seems to fit Personal attacks just display an immature and insecure personality, from someone who may feel you have got pretty close to the truth and made them feel very inferior... hehehe - think of that next time "personal insults" fly...

[Ad Hoc]

Indeed, and right neighbourly of you...well we are also in the same time-zone, just about, just different hemispheres!!

Yes i know how close i ahve gotten to the truth, and am happy to take all those immature hits just so the orginal poster can appreciate a different point of view and not a myopic one!

[masalai]

How is the land of the rising sun? seems out of recession officially announced.... - I am just waiting to see what the second wave of **** from the USA brings, how is the economic world from your perspective? perhaps you could post http://www.boatdesign.net/forums/ope...-26558-63.html instead of continuing this "piracy"...

[ImaginaryNumber]

Ad Hoc,

Like you, I was (am still am) on the road, so my responses are at times sluggish.

I understand the concepts depicted in your very helpful diagrams. I'm not sure what GL, BV, DNV and LR mean. I will try and look them up later, but I suspect they are some sort of engineering protocol that I don't need to know. As far as the numbers -- because I don't know engineering, I will certainly accept what ever you say, and hope that more knowledgeable forum members will catch any mistakes.

If what Autodafe is correct, I would like the calculations to include free-standing mast(s).

Is the process you used to calculate all the moments too complex also include here on the forum? I am grateful for whatever you wish to share, but am curious if I can follow the step-by-step math. It might then allow me to experiment with different size hulls, beams, sea states, etc.

Onward and forward!
i

[Ad Hoc]

I.N.

You need to decide whether you want free standing or not..otherwise lots of calculations for no reasons, other than, hmmm..just an idea. Hence you really need to decide what it is your boat shall be and what you want. Going around in circles of what if's...does just that, goes around in circles and no convergence is achieved.

Since if you want free standing masts, a shed load more work is required in your design before we can address the structural loadings..since where do these loads come from...yup..the free standing mast arrangement. Where is best place for the mast and what size sail plan etc etc..all these must be done first before we to get the structural loads....see my point?

DNV= Det Norske Vertias...these are all Classification societies.