Stiffness vs Tensile Strength in Foam Sandwich Construction?

It always helps top reply to technical questions with actual facts and values for the OP to make an informed decision - rather than anecdotal hearsay information.

Typical Composite fatigue strengths values, GRP noted - at THAT million times:



As you can see, it losses approximately 70% of its static strength.

 

That doesn't gel with the fishing rod example, does it now ? And there are plenty of glass boats that have done a power of work without need of glass repairs, and alloy by comparison plenty of visits to the welding shop. I don't buy that as indicative of the practical situation. If it was, glass hulls would be shot well short of the very long time spans seen. And in any event, with super-stiff high modulus sandwich construction, not worth worrying about.

 

Boat Strength by Gerr isn't terrible if you just want a basic understanding. The problem is what I alluded to earlier. It leaves a lot of questions unanswered. Perhaps one of his other books delves deeper into the derivations on the 'scantling number', or how he arrives at constants against the frp thickness to arrive at core thicknesses, etc.

 

yeah, but at least laugh at yourself a bit when you just circled back to super stiff...I mean no disrespect

 

If you understood how structural DESIGN works - yes it does!

However, your endless hearsay anecdotal comments without any evidence to support such ascertains - then of course not. Because that destroys the narrative you wish to maintain.

Indeed it does, hence needing some 'spiritual' guidance for enlightenment with regards to it rationale!

 

In the field, the last thing that goes is a fatigued laminate, prior to that rotten stringers and transoms and osmosis and a few other things stop the music before fatigue does. Some high performance outboards had fibreglass reeds, plenty of cycles there before fatigue was a problem, in short fatigue is well down the list of things to be concerned about. Just about all the common damage to glass boats is cracking and fracture from localized loads around hard points, and that can happen early on from excessive operation in rough water

 

Well, I never heard of a glass fishing rod breaking in old age from too many bending cycles, most end their days when they foul a tree branch when driving down the track to the beach, or someone runs over it while it was lying on the beach etc, in other words, an overload beyond its limits.

 

It surprises me sometimes how some old glass boats survived the punishment of wave impacts so well, for so long, when the poor crew were not so fortunate ! (Complaints about bad backs and chiropractors and what not, aplenty).

 

I think a fair indication of the gravity of fatigue in glass boats, is that people invest large sums into hull restorations of old "classic" boats 40+ years old that are of superior design, like the Formula 233. You wouldn't see that if the meat of the things had gone awry from fatigue.

 

That's interesting yeah. Not sure if you can buy S-2 glass or S-glass at acceptable prices. Have you found any good prices? The other limiting factor besides weight is always cost
And availability. And a plan or kit designed for it.

I still have to learn laminate theory properly myself, but this pdf is often linked: HexWebTM HONEYCOMB SANDWICH DESIGN TECHNOLOGY.

 

His formulas are based on empirical data. For wooden structures they parallel old formulas like Herreshoff's and give almost identical results. For fiberglass they are based on previous successful designs. Gerr's system results in a medium weight boat. An engineering approach is needed for really light structures though.

 

I would say that any study or calculation that does not try to optimize the weight of the structure results in a boat, not of medium weight, but of exaggerated and useless weight (and therefore very expensive).

 

Very light boats have low inertia, and that isn't usually kind to the human frame.

 

Purely semantics here, but I want to build a small dinghy or two and had hopes to use some of my available core that is surplus. I used Gerr and came up with some numbers and had one or two forum members and the designers scantlings and they were all basically the same. A tiny boat mind you. When I wanted to really push the weight down with carbon or glass skins; it became a bit more difficult to use the Gerr method. And I'm sure the small boat was driving things to be less variable as the factors were all small.
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Madonna mia! A large vessel with a 'light' core construction is not light. Sorry. I am a bit mad at myself for glassing up my hulls with mat tapes. I think I added about 300 pounds to her. Do you realize how silly your comment sounds or are you just trying to get under the 'skins'?

 

It isn't a silly comment, although your style of boat, with a low waterplane area for the weight involved, is less affected. Something that is bobbing like a cork is uncomfortable to be aboard.