zoidberg
Well-Known Member
I'm looking for professional expertise in composites. Someone to tell me how many layers of what bi/tri/quadraxial glasscloth to bond in, for a specific reinforcement task.
Any pointers?
Any pointers?
Composites expert
- Thread starter zoidberg
- Start date
coopec
N/A
Surely reputable retailers of fiber glass resins and glass products would have experts on their staff who could advise?
zoidberg
Well-Known Member
I assumed that.... but apparently not. I've asked several, as you might imagine, and they've all backpedalled, citing concerns about 'liability insurance'.
coopec
N/A
I don't know the exact circumstances of your situation but that sounds overly cautious. Nine times out of ten it is a matter of opinion.
A resin supplier in Australia would very quickly lose business if they would not give advice on how to use their products!
HissyFit
Well-Known Member
Wing Mark
Well-Known Member
I have a mate who designs things in metal, plastics and composites.
I think the problem will probably be that you can only guess at the loads on the reinforcement.
And then you won't know how much it's acceptable for the thing to distort under load.
For most bits of boats, there are scantling rules and designers' 'rules of thumb' which have proven adequate over the years.
If it's something like a simple beam, you can find basic calculators online to work with the sectional moments of inertia and the materials properties.
The properties of glass are fairly well known, just work with the fibres which are aligned in a useful direction.
It's simple enough for easy problems like 'how much glass do I need for my centreboard not to break when fatboy rights the dinghy by standing on the end'.
For more advanced questions, you may need a 'finite element' model of the whole structure with all the loads and accelerations modelled.
People in this line of work tend to run away from vague questions or offer to do a lot of work and charge you a lot of money.
F1 teams and aeroplane designers are prepared to pay.
I think the problem will probably be that you can only guess at the loads on the reinforcement.
And then you won't know how much it's acceptable for the thing to distort under load.
For most bits of boats, there are scantling rules and designers' 'rules of thumb' which have proven adequate over the years.
If it's something like a simple beam, you can find basic calculators online to work with the sectional moments of inertia and the materials properties.
The properties of glass are fairly well known, just work with the fibres which are aligned in a useful direction.
It's simple enough for easy problems like 'how much glass do I need for my centreboard not to break when fatboy rights the dinghy by standing on the end'.
For more advanced questions, you may need a 'finite element' model of the whole structure with all the loads and accelerations modelled.
People in this line of work tend to run away from vague questions or offer to do a lot of work and charge you a lot of money.
F1 teams and aeroplane designers are prepared to pay.
AntarcticPilot
Well-Known Member
Very many years ago - when it was something state of the art, not a routine add-on to engineering drawing software - I worked with FE modelling of glaciers. I didn't develop the FE bit - that was code we got from elsewhere; I just made it work in our environment, and wrote modules to automatically generate meshes so we could run it for a time step, then generate a new lattice at the next time-step and so on. But it uncovered one of the limitations of FE modelling - it's very difficult for it to handle changes in the regime. We were hoping to model glacier surges (which result when the glacier becomes decoupled from the rock beneath) but were unable to use FE modelling to model the decoupling mechanism.
zoidberg
Well-Known Member
I've done a bit of that, relating to the bolt/composite design task. It's rather complex, especially as the expert composite engineering academics have yet to agree what is meant by 'strength'.
( ShieldSquare Captcha ; Simplified procedures for designing composite bolted joints - NASA Technical Reports Server (NTRS) ; https://apps.dtic.mil/dtic/tr/fulltext/u2/b108123.pdf ; https://www.matec-conferences.org/articles/matecconf/pdf/2017/17/matecconf_iscee2017_02031.pdf ; and more )
I looked at this arrangement which 'looks strong enough', then thought there may be a better way - with answers to the questions 'why...?'
I can think of no good reason why the reinforcement cannot be on the inside. I'm looking to secure a pair of padeyes to the quarters, like this, but not using sticky tape.
These titanium padeyes are rated at 'in excess of 20,000lb' in shear. The 12mm BUMAX 88 bolts each can manage that. The Max Laden Weight of the boat will not exceed 9000lb. That's my target spec. Don Jordan's calculation of 'Design Load' onto each padeye is not more than 5040lb.
How many layers/laminations of what spec of cloth/tape, with some numbers? Shouldn't be this difficult....
Wing Mark
Well-Known Member
Some problems are just not amenable to computer models.
Tiny changes in the boundary conditions create big changes in the outcome.
Reminds me of my uni tutor who had a habit of writing complex equations involving factors like 'the viscosity of granite'.
AntarcticPilot
Well-Known Member
At what temperature? Mind, granite melts are pretty viscous, unlike basaltic ones.
Pete54
Well-Known Member
It is difficult to be sure - but I suspect that the titanium padeye needs to be rotated by 90 degrees to actually have the strength you quote. AS mounted it would suffer both shear and bending stresses - which padeyes ae not generally rated for.
Wing Mark
Well-Known Member
The compressive strength of GRP is about 120N per sq mmI've done a bit of that, relating to the bolt/composite design task. It's rather complex, especially as the expert composite engineering academics have yet to agree what is meant by 'strength'.
( ShieldSquare Captcha ; Simplified procedures for designing composite bolted joints - NASA Technical Reports Server (NTRS) ; https://apps.dtic.mil/dtic/tr/fulltext/u2/b108123.pdf ; https://www.matec-conferences.org/articles/matecconf/pdf/2017/17/matecconf_iscee2017_02031.pdf ; and more )
I looked at this arrangement which 'looks strong enough', then thought there may be a better way - with answers to the questions 'why...?'
I can think of no good reason why the reinforcement cannot be on the inside. I'm looking to secure a pair of padeyes to the quarters, like this, but not using sticky tape.
These titanium padeyes are rated at 'in excess of 20,000lb' in shear. The 12mm BUMAX 88 bolts each can manage that. The Max Laden Weight of the boat will not exceed 9000lb. That's my target spec. Don Jordan's calculation of 'Design Load' onto each padeye is not more than 5040lb.
How many layers/laminations of what spec of cloth/tape, with some numbers? Shouldn't be this difficult....
9000 lb is presumably about 40kN. (you'll need a septic expert if you use pounds).
So the bolts need to bear on 40k/120 = 340 or so sq mm of GRP
4 bolts each 12mm suggests a bit over 7mm thick to take the pure shear load.
In reality, the load is not pure shear, the fitting will lever two bolts into tension and distort the panel.
So you need to look at the bending strength of the panel local to the fitting.
There is no basis for assuming the 9000lb as a load, you should probably work on a multiple of the breaking strain of the rope.
It's much easier to get Gerr's ''The Elements of Boat Strength' and use scantlings for a comparable fixture (e.g. a chain plate), then add some.
You can get a lot of glass and resin for the price of the time it takes to explain your problem to a consultant.
The strap with the hex bolts is visually offensive BTW!
But if little round fittings were a good way of feeding shear loads into a flat(ish) GRP panel, why would people make chainplates as long strappy things?
Wing Mark
Well-Known Member
I did make the stoodent error of asking similar, or 'how do you measure that' and got trapped into a 45 minute monologue.
Wing Mark
Well-Known Member
I assume it's for one of those drogue thingies which holds the boat back enabling overtaking waves to smash the rudder.
I saw one on a boat for sale the other day.
A bit OTT for a small family coastal cruiser?
Yellow Ballad
Well-Known Member
When I asked about my JSD plate design Thinwater seemed to have an idea about things...
One thing that I remember was somebody mentioned about loads from rigging being taken by the bulkheads rather than just the GRP which made sense and I don't want to under estimate spreading loads when installing backing pads for mine. My hull deck joint is bolted so I was thinking of using an internal plate to spread the forces this way.
The other thing I took from is was there was no reports of drogues ripping/pulling hulls to pieces, but then maybe they didn't get chance to report it...
One thing that I remember was somebody mentioned about loads from rigging being taken by the bulkheads rather than just the GRP which made sense and I don't want to under estimate spreading loads when installing backing pads for mine. My hull deck joint is bolted so I was thinking of using an internal plate to spread the forces this way.
The other thing I took from is was there was no reports of drogues ripping/pulling hulls to pieces, but then maybe they didn't get chance to report it...
WoodyP
Well-Known Member
"You can get a lot of glass and resin for the price of the time it takes to explain your problem to a consultant."
My thoughts exactly. Don't overthink it,
The engineer can build anything to a minimal spec., If it's over spec, will it matter?
My thoughts exactly. Don't overthink it,
The engineer can build anything to a minimal spec., If it's over spec, will it matter?
penfold
Well-Known Member
It will if overengineering it transmits the overload from where it arrives to an unreinforced part of the hull; consideration of where you'd like the failure to be is as important as making things strong enough.
WoodyP
Well-Known Member
That's why I aren't an engineer
zoidberg
Well-Known Member
I'm grateful to Wing Mark and WoodyP for their inputs. No, I don't want to commit to forking out several £hundred for a pro's consultation that won't be any more definitive than what I can determine for myself - 'getting by with a bit of help from my friends'.
FWIW, the Dyneema rope-rode is good for >20000lb/40kN - insh'allah. Devoid of other input, I've taken the boats' Max AUW as the largest, biggest, no-shit 'Mega Wave Strike' load that could be imparted. DJ's calculations - he was an aviation engineer - reckon the maximum 'Design Load' overall to be 80% of the MAUW, or 7200lb. He reckons the distributed load into each leg of the bridle, and their attachments to the hull, to be 70% of that, or 5040lb. I've decided to go with 9000lb as my datum for reinforcement, in the absence of any better candidate figure.
The ocean isn't interested in the slightest that we call this a 'small family coastal cruiser'. If we're in the wrong place at the wrong time, we will get whatever's coming to us. There are plenty of places where breaking seas of that size/energy occur. The Celtic Sea is one of them. This kit is my Fourth Reef. It must not fail.
Oh, and I do have a copy of Gerr's 'Boat Strength' on my desk.
FWIW, the Dyneema rope-rode is good for >20000lb/40kN - insh'allah. Devoid of other input, I've taken the boats' Max AUW as the largest, biggest, no-shit 'Mega Wave Strike' load that could be imparted. DJ's calculations - he was an aviation engineer - reckon the maximum 'Design Load' overall to be 80% of the MAUW, or 7200lb. He reckons the distributed load into each leg of the bridle, and their attachments to the hull, to be 70% of that, or 5040lb. I've decided to go with 9000lb as my datum for reinforcement, in the absence of any better candidate figure.
The ocean isn't interested in the slightest that we call this a 'small family coastal cruiser'. If we're in the wrong place at the wrong time, we will get whatever's coming to us. There are plenty of places where breaking seas of that size/energy occur. The Celtic Sea is one of them. This kit is my Fourth Reef. It must not fail.
Oh, and I do have a copy of Gerr's 'Boat Strength' on my desk.
Wing Mark
Well-Known Member
Dyneema rode?
You cannot be serious!
I once saw a fairly substantial cleat ripped off a yacht by towing a small boat using a bit of dyneema rope.
Because the towed boat hit a little bow wave of a few inches high.
Dyneema stretches about 1% at its breaking load, so 15m of rope gives only a few cm and can jerk the tow at several g.
You don't just have to shift the boat at however many g, but also the water that's in its way.
Maybe yur boat will be surfing at 10 knots when the rode goes tight? It then stops in 10cm if that's all the dyneema will give?
That's a lot of g's.
I think you want nylon.
You cannot be serious!
I once saw a fairly substantial cleat ripped off a yacht by towing a small boat using a bit of dyneema rope.
Because the towed boat hit a little bow wave of a few inches high.
Dyneema stretches about 1% at its breaking load, so 15m of rope gives only a few cm and can jerk the tow at several g.
You don't just have to shift the boat at however many g, but also the water that's in its way.
Maybe yur boat will be surfing at 10 knots when the rode goes tight? It then stops in 10cm if that's all the dyneema will give?
That's a lot of g's.
I think you want nylon.
