scantlings for a fiberglass cored molded 28 cat?

28 foot cat
with 1708 on both sides. Is 3/8 balsas core sufficent?

 

I would go with 1/2", maybe 5/8"

 

Scantlings

Really depends on the panel sizes, smaller panels or more bulkheads can have thinner core.

 

You don't say what catamaran, which will make a difference!

I wouldn't use balsa at all, I'd use 10-12mm foam.

I don't know what 1708 is, US measurements I suspect. I have done a number of 8-10m catamarans using one layer 600g glass each side. Not just on racing boats, but also ocean cruisers. High loaded areas have extra glass, areas prone to flexing have thicker cores

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Not so much the size of the panels as the relationship between the dimensions, length / width, of the panel. The number of bulkheads has nothing to do with the thickness of the hull. A hull without bulkheads, with adequate separation between frames, and longitudinals, can have very optimized thicknesses.

 

Gee, I thought length and width determined the panel size ?
If you want to split hairs over frames/bulkheads on such a vague question be my guest.

 

Yes but a panel of 2 x 2 is much stronger than an 8 x 0.5

There is not a vague question but are two well-defined and very different things. I do not try to explain it because, I fear, you are not able to understand.
Cheers

 

More of a description of the design I am thinking about . Basically a design of a 28 foot long hull that is very flat sided with lets say 4 feet from the sheer to the very bottom. Length to beam ratio of 17 to 1.

I repaired my buddies 44 foot Lagoon and its was one layer of 1708 in side with a 3/4 inch balsa core and a single layer of 1708 on the outside and this is on a 25 000 pound boat.

I have a Baltec Duracore scantling book but it if for the strip plank not a balsa core .

Balsa is much more affordable than foam and also is very reasonably priced until you go over 3/8 and than the price starts to skyrocket.

Thinking maybe run some longitudinal stringers maybe 3 on each side with the 3/8ths core double up with 2 layers of 1708 over them.

I think the 3/8 will provide the thickness just not sure about the shear strength.

Thanks for any input

 

Balsa is heavier, so you need a bigger rig/engine for the same performance. And you use more resin bonding the core down

Richard Woods

 

'Scantlings' do not live without specs of boat, particular location and dimensions of panels and stiffeners, particular materials. Please remember! Just changing the weight of craft or its speed, or design category will cause change in design loads and different laminate.

I am often approached with such questions, and the guy asks why we have to make complete structural analysis and cant we 'just give a layup schedule'? No serious designer or professional will 'just give' anything because he clearly understands the complexity of the issue, and the level of responsibility.

I believe one of such topics should be attached in this forum as FAQ, so there is no need to explain the basics again and again.

 

Alik, I very much agree with you. I stand amazed when people in this forum, without knowing the boat says "put 5/8", that has been OK for me". But there is no doubt that this works for many people.

 

Well non the less I was totally floored that my buddies 44 foot 1/2 million dollar Lagoon hull was nothing more than a singler layer of 1708 biaxial cloth on each side of a 3/4 inch balsa core.

We are talking about a boat that is 10 times the mass as the 28 footer I am thinking of'

Weight savings in a 28 foot hull I can appreciate Richard and do appreciate you coming on this forum and giving advice . Big fan of your designs , just got off Youtube to see Lightwave getting cut up and lengthend a 2nd time.

So what thickness in a foam core would you recommend than for a1708 biaxe on boths sides.

 

tommymonza, after what Alik and me have said, I thought you would not do that question. Clearly, someone is going to give you a specific thickness, so the responsibility to listen him or not is only yours.
You should give some more information for someone, sensible, give you a logical answer.

 

Tommy,
The NA's and designers will just tell you what you already don't know unless you provide full design details about the boat, it's intended use and how much each passenger weighs and what they had for breakfast. Being professionals they don't want to get sued for incomplete advice.
For some ballpark figures talk to your balsa supplier, they may be able to give you some broad advice based on what their customers use it for, if you are serious they may even design a laminate for you.
Alternatively look at existing designs and see how close they match your intentions.
I would say yes you can use 3/8 core if thats what you have already but the structure and laminates will have to be designed around it.
Lastly, personally, I don't like balsa either, cut into too many sodden cores in my time to have real faith in it. Building yourself, knowing you've done it all correctly helps, but, one incompetent installer and it's trouble.

 

I don't know what you do in "real life" but suppose you were a doctor and I your patient. If I email you and say "I am not feeling well, can you give me some antibiotics?" what will you say?

I suspect you will want to know my sex, age, medical history before you even say "come in for a consultation and we'll see what is wrong with you and then maybe I will prescribe something"

When you ask for scantlings do you mean round the rudder stock, the daggerboard case, the beam attachments, the hull bottom, the stem, the forestay/chainplates? They will all have different laminates, none of which will be the same as the basic hull skin.

And you say 1708 - I still don't know what that means. Is it +/-45biaxial glass, quad? tri? 0-90? Are you using epoxy, vinylester, polyester? vacuum bag, autoclave, standard layup? Again those will all change the scantlings

And I don't know what your boat is used for - power? sail? cruise? race? commercial? inshore? offshore?

Richard Woods