How many layers of fiberglass?

[rowann]

I am currently working on building the hull for a robotic catamaran, and I am looking for advice on how many layers of fiberglass to use. Each hull will be 5.25 feet long, 0.7 feet wide, and 1.48 feet tall. The fiberglass will be the only structural material. I am planning on using a 1708 biaxial cloth with polyester laminating resin. Any advice is appreciated!

 

[ondarvr]

With that limited information, and assuming this doesn't need to survive in harsh conditions, one layer of 1708 would be more than sufficient. This also depends on the exact shape, flat panels would flex a great deal, curved surfaces won't flex in the same way.

Look at any fiberglass kayak or canoe, they are much larger and still have thin laminates when weight is a concern.

 

[TANSL]

How can one claim that laminate is sufficient when the conditions in which the catamaran will sail are unknown? Furthermore, the stresses to which a catamaran's hull is subjected have little to do with those of a monohull.

 

[ondarvr]

A

How can one claim that laminate is sufficient when the conditions in which the catamaran will sail are unknown? Furthermore, the stresses to which a catamaran's hull is subjected have little to do with those of a monohull.

Maybe it's because I own two, an 8' and a 10'

The laminte schedule is one 1.5oz csm and one 24oz roving.

I was also involved in building an 11'(?) catamaran rowing shell, remember that friend that was a marine engineer, he was involved in designing it. The project was started by the son of the owner of a very well know company that built powered catamarans from 22' to 34'. At which that friend was employed by for a very long time.
The hulls were very thin and light, and we're plenty strong.

And as I stated in the first sentence of my response "With that limited information, and assuming it doesn't need to survive in harsh conditions".

I used that rowing shell quite a bit in Lake Powell at a dealer event put on by the catamaran builder. I spent most the time fishing with the lead engineer from Yamaha outboards during that week. They supplied the outboards and I was the resin and gel coat supplier to the builder.

 

[kapnD]

1708 will be quite a heavy layup for such a tiny boat.
I’d be looking at some 6 oz fabric

 

[Ad Hoc]

...Maybe it's because I own two, an 8' and a 10'....

And the fact that his cat is merely:

... Each hull will be 5.25 feet long, 0.7 feet wide, and 1.48 feet tall. ...

Hardly an ocean going yacht!

...And as I stated in the first sentence of my response "With that limited information, and assuming it doesn't need to survive in harsh conditions"....

Indeed, that's the caveat to cover all too.

 

[TANSL]

Indeed, that's the caveat to cover all too.

Exactly, I completely agree with you.
What I would say is : "With that limited information, and assuming this doesn't need to survive in harsh conditions,......, it's very risky to give an answer and, therefore, without more data nobody should advise anything", (an honest expert wouldn't, in my opinion).

 

[rxcomposite]

I am currently working on building the hull for a robotic catamaran, and I am looking for advice on how many layers of fiberglass to use. Each hull will be 5.25 feet long, 0.7 feet wide, and 1.48 feet tall. The fiberglass will be the only structural material. I am planning on using a 1708 biaxial cloth with polyester laminating resin. Any advice is appreciated!

Is this a Uni project or for commercial application?

 

[fallguy]

I hate to disagree with ondarvr or any other contributors here.

One layer of 1708 as ‘the only structural’ material won’t be sufficient to keep the hull shape unless the hull shape is really contributing and even then one layer is pretty flimsy at about 0.040-0.050” thick.

I have built a few moulds using layers of 1708 and the most common minimum for me for moulds and for the mould to maintain its shape is 3 layers. Of course, I wanted a fairly stiff mould, but nothing super strong either.

We have no idea what the hulls are, but if we assume a canoe body with dev surfaces; you also need some way to bond half shells. For the mould to be done well; you want to relieve tabbing and you cannot do so in a single layer; so the half shell bonds are plus one layer; done properly.

Anyhow, I don’t quite understand how you intend to build the hulls, but if a half shell mould, then I’d layup at least two layers, but probably 3. You can also do some weight calcs to help you decide if it’ll make the lines at 3 layers because 3 layers would be less likely to move and easier for any bridgedeck attachments as well. Assuming your bridgedeck attachments are fairly simple, only one layer of 1708 on the 0.7m beam of the hull would probably allow movement from the opposing hull, so again, I’d go two layers min., leaning to 3.

Also, two layers gives better assurance of waterproofing it..

 

[TANSL]

@fallguy, I still think it's unwise to offer solutions to a problem that, with the available information, we can't analyze and therefore can't properly solve. However, I must admit that your advice is much more realistic, more professional, and demonstrates real experience in the field than other previous suggestions.

 

[waikikin]

How can one claim that laminate is sufficient when the conditions in which the catamaran will sail are unknown? Furthermore, the stresses to which a catamaran's hull is subjected have little to do with those of a monohull.

It's a robotic catamaran, no lives at stake, merely a toy unless it's loaded with something evil.
Surfboards are pretty tough with very thin laminates over foam, they'reused in the break zone, hit rocks, cop a few dings..
The post looks like a brain fart anyway, no need to get worked up..

 

[wet feet]

I'm guessing that the contents of the proposed vessel contain equipment that has some value or scarcity,due to it being novel and thus in need of preservation.There may well be power sources as well and if recovery is part of the plan there may be some kind of position sensing and transmitting equipment.I'd like to think we don't have a poster seeking construction advice for an unmanned destructive drone,but you never know....
I'm amused at the thought of using 1708 for mould construction as I would normally regard mat as a better bet,due to better conformability,or even pointing a chopper gun at the plug after an initial ply of mat.
I know of quite a few racing dinghies that are only around a 1200 gsm laminate,some with a core and some with inserts for stiffness and internal mouldings to reduce the unsupported span of the panels.The stated dimensions of the vessel are also a bit odd with a height greater than the beam,but it is a catamaran and consequently may need additional strength around the structure linking the hulls.

 

[TANSL]

It's a robotic catamaran, no lives at stake, merely a toy unless it's loaded with something evil.
Surfboards are pretty tough with very thin laminates over foam, they'reused in the break zone, hit rocks, cop a few dings..
The post looks like a brain fart anyway, no need to get worked up..

I agree, there's no need to get upset.
Since it's a toy, and I'm also speculating about things not mentioned in the first post, any kind of fabric would be too much. Why not just make it out of resin? If the hulls tear where they join the crossbeams, so what? It's just a toy......

 

[fallguy]

Well, @TANSL , resin is really hard to work with as it runs down the mould, so not sure if you are wagging the bot or stirring the pot here, but no.

My hunch is we never hear back from the OP, so we are only posting for thirds.

@wet feet the only reason I responded to 1708 is that OP proposed .. assuming internal works are attached to the hull; csm is certainly a good choice .. I would not know the weight or layers

 

[ondarvr]

Anybody that has built kayaks and very small boats knows the laminates are very thin. As in my two cats at 8' and 10', they are one 1.5oz csm and one 24oz roving, many thousands of these were made, mine are 40+ years old. My kayaks are even thinner, and I used to build those too, one 1.5 csm and one 10oz cloth, they were 13 to 16' long.

Now if this is a mine sweeper or something for industrial purposes where its going to get beat up, yes it needs to be built tougher. But for a robotic (RC) craft at less than 6', the single layer of 1708 would be sufficient for it to work for years. It may not be the best choice, but would work.

This in the RC boat size and use range, they are very thin laminates.

And surfboards were mentioned, and yes I've worked in that market. They are very thin with a foam core and get beat up while in normal use, and yet they survive.


You can easily make up scenarios where it wouldn't be sufficient, but they weren't given in his question, you need to make those up on your own. I'm thinking some comments are just being fabricated in an attempt to be disagreeable.

So if you have no experience building, owning, and repairing this size craft, you are just guessing at what may be required.

The person only asked about the hulls, not brackets, beams, supports, or other structures. It's common sense that attachment points may need additional strength.

And remember shape was mentioned my original reply, shape has a huge influence on panel stiffness.

Mold building is a completely different scenario with very different requirements.