Struggling with certain core concepts...

I keep hearing the hype about a skinned core being xxx times stronger than solid lay up "of equal weight" - I completely understand this is due to the distancing of the skin laminates and partly due to the core itself.

It is my understanding that when you core you would use the same amount of glass with a core as you would in a solid lay up ---- just say for arguments sake.

cored ----- 3 layers glass - core - 1 layer glass
solid ------- 4 layers glass - no core

3/1 ratio on the glass with core compared to solid - this is the basis of the equal weight comparison, correct?

Ok - I see a potential for abuse here on boat design and building - seems to me they will (because it is stiffer) use less on the skins with the core to cut costs because it will be stiffer than the original solid layup. The original solid layup will have less puncture protection but will it be strong enough - that is the real question.

for arguments sake, my thinking - and I am speaking in regards to no real project here - If a layup schedule calls for 1 layer each:
Cored - csm / csm / triax csm / core / triax csm

if I dropped the core would it be strong enough according to their comparisons. I understand this is relative to to them skimping on the design in the first place.
As well - due to issues such as design considrations, complexity and cost of equipment and processes, cost of coring material itself, difficulty - is it really worth it to core the hull of a catamaran below the waterline especially with complex curves.
What if (above example) you did away with the core and added a few more layers of csm triax. (as long as its strong enough) Would it be worth it to have solid below the waterline vs the core? I just don't see where the added weight will really be that detrimental to the project. I understand it will not be as strong as the cored and quite frankly none will be puncture proof.

There seems to be alot of hype about coring hulls but no one wants to talk about solid glass hulls and cored topsides. I do understand we are not comparing a 3/4 inch cored to 3/4 inch solid - I think the solid would probably win.
All sounds like a lot of advertising hype to me when it comes to coring hulls. Is it really worth it - taking everything into consideration?????

 

Ok lets get to basics !!
Why do you want to use a core ??
what are you making ?? '
Solid glass ??
Do you want the cored panel to flex at all ?
What thickness of core are you thinking about ?
A flexing panel makes for a softer riding boat .
Use the right glass and a thin core so it will flex could be more durable than you can imagine!!
Ridged panels will find the weak point and break from that area !! and you will have a boat that is really hard riding at speed !!.
3 layers of glass then a core and one layer on the inside ?? you got that wrong !!
The outside glass usually goes into compression and can wrinkle !! but the inside glass wants to stretch and needs to be really strong and one layer of glass is not a good idea depending on the structure and or frame work being used !!

 

you referred to making a solid laminate for the bottom of a catamaran hull. - many manufacturers and shipwrights already do this, ie, cored sandwich construction throughout the boat with a solid glass shoe from teh waterline down.

This simplifies the build by removing the need to conform the core material to a compound curve shape, such as trying to hold it in place and cut and fit it into molds etc. It also has a better puncture resistance which is great for below the waterline. The tradeoff is increased weight, but it generally only adds a small amount due to the heavier construction being confined to a small area below the waterline.

All in all, its a good way to go... i used a combination of both cored and solid glass below the waterline... i had a core where it was easy to mold and relatively little curvature, and went to a solid glass only laminate where i had tight compound curvature. The reduction in stiffness of a solid glass (but thinner cross section), is offset by the tight curvature, so the structure is still stiff enough.

Back to some of your earlier comments, its not just about strength. Far more often than not, a boats scantlings are determined by the required STIFFNESS, not tensile or compressive strength. By having a much thicker section, the stiffness is improved dramatically. The higher stiffness allows for greater separations between frames, stringers and other stiffeners and thus a further reduction in structural weight by having less of them.

And no, you cannot make any real comparisons by simply "dropping the core" and expect the scantlings to then perform like it was required to in the design. The most obvious reason would be a dramatic gain in flexibility. In general, you would need to add more glass to build up more thickness in order to compensate for the loss of the core material. Alternatively, you can use things like coremat or soric etc to build up thickness with less weight than solid glass alone.

 

I have been doing alot of reading and just trying to make sense of it all - at this point.

Eventually, I want to build a power catamaran.
Full Displacement, 14ft X 35 - 40ish ft. No fly bridge - just basic, not going to load it with a bunch of crap either. I want functional not pretty. The reason for the 14 ft width is for permitted trailering (hurricanes). I have drawing of what I would like it to look like but that doesn't mean a whole lot right now.
Nice clear tunnels and no real crazy design curves or hull apendages - just basic round bilge. I am thinking hulls around 3 to 3 1/2 feet. But again these are all just premilinary and I am just doing alot of research at this point.

I also know that it would be great to be able to do the latest greatest thing - fancy vacuum infusion fansy dansy stuff - which are without merit good but I want to be realistic here to. Corecell is expensive and can increase skill level and lessen margin for error - is it worth it for the weight.

I know this is all really preliminary at this point. Just learning and I do appreciate all the help so far.

 

I take it youve seen my build thread in the boatbuilding section...? sounds like its right up your alley, complete with all the fancy dancy stuff done right here in my 2 car garage in suburbia...

 

Yep groper read your thread on economy cat - seems that some is missing from the design section to the build section unless I missed it. Your original design was much like what I am looking for but after reading the build looks like there were alot of changes.
I guess I have alot of learning to do - that's why I am here. I am determined though, and have plenty of time to figure it all out.
I am a functional type person - I know what I want in it based upon what we do. No huge fancy galleys, bedrooms etc - open and functional - obviously light and economical. No need for "luxury" when you spend most of your time covering everything with squid and bait. No fancy counter tops - hard wood etc - just works and easy to clean and get around - not taking the whole house with us but want to be able to stay out as long as we wish and still be able to trailer it out when a hurricane comes.
I have looked at so many designs and love Jeff shionnings - prowler vt950 and the 10.4 - but I need to combine the 2 ---- want the displacement hulls for efficiency and stability - but the 10.4 (1040) is cramped for deck space for our needs (fishing diving) - sooooooooooo off to drawing up what I want that is functional for what we do.

I understand the principles of the coring and you confirmed, I guess, for me what I already knew in my head. solid down foam up...... Personal preference - I just think there is a huge learning curve when it comes to the cores and doing all of the fancy stuff with it - if I were a professional builder with all of the equipment it would be different. I just don't see myself thermoforming - vacuum infusing a whole hull when I have never done it before not to mention there is a cost relative failure factor and learning curve there as well. I like basic straight lines - easyyyyyy - except for what matters the hull - round or oval bottoms - I will get there. and any help I get from anyone out there I appreciate.

My next step is figuring - structurally where this all fits together with a cat - it would seem the weakest connection is the deck to the hulls....... and yet most deck/hulls are joined after the fact - I would think you would want that good ole chemical bond there. Even less of a structural connection would be the top enclosure - needs to support it self but not necessarily to hold the boat together. bulk heads and deck handle most of your structural integrity.

 

You might want to research Derek Kelsalls KSS build method, you infuse full hull halves on a simple flat table and then fold them up into nice round bilge hulls, its a very smart method ideal for what you want to do.

Steve.

 

Jpuseyjr, welcome to the forum, unfortunately there's no short cuts to the skill sets (yep plural) you'll need. For the craft you're looking to "create" you'll need to take one of a few paths. The simplest, most cost effective and fastest is a set of plans - you can start work with over night delivery. Next would be a correspondence course in yacht design and lastly would be home study. If you have a few years and some professional tutelage, you might be able to get to a point where a structure as complex as you propose could be self designed. It would be a very rare thing, but certainly possible. If you took a West Lawn or other correspondence course, you could be up and running in a year (if you where really good) or more likely 18 months to a couple of years.

Understanding the engineering across several disciplines, excluding yacht design, just for the material choices is staggering enough, but just wait for the math part of the equation, which you do have to understand, as it's not enough to rely on a bit of software to tell you something - you have to know if the results regurgitated by the computer, are in the range you desire (and why).

I'm not trying to rain on your parade, but your questions thus far about material basics and (I'm assuming) the engineering and hydrodynamic questions to come, suggest you're just way out of your league, with your current skill set. Again, this isn't meant as a personal dig, but an observation of your questions and replies, on the two threads you've participated in here so far.

Find a design that is close and strip it of all the none essential stuff, then work from there. This too would need to have a professional look it over at some point, but at least she'll float pretty much where you expect come launch day. Working craft plans of about every configuration abound. Pick a building method you are comfortable with and move on.

To start assemble a solid SOR and see which designs come fairly close. Of the designs that come fairly close, which can accept changes to make them closer and narrow the list. Of course airing out your ideas and concerns here will help, likely with other design options being offered, once your SOR is well established and voiced here.

Simply put, self designing an 18' center console, for working the local flats is one thing and possibly doable with your current skill set, but a class A - B vessels, not so much. This isn't to say, you can't put your mark on a 40' something or other, so much so that no one will actually know what you started with, which is pretty much the same thing, isn't it?

 

Dont be scared of infusion - i keep chanting this mantra, because it will make your life building a boat in FG sandwich, so much more enjoyable, and save you time. Its not fancy dancy stuff... arm yourself with a bit of knowledge and its quite simple... The reason i changed my design a few times, was to eliminate the compound curvature in all but the bottom section of the hulls - id already built them by that stage. Building with developable panels is much more efficient, and means you can make everything on a flat table rather than using molds and formas etc... Its not just the molds themselves, but flat table panels can be infused (or vac bagged) with both laminates at once - so roughly half the work as opposed to doing a single side at a time, and less time spent waiting for things to cure before proceeding. Flat table panels, infused or vac bagged, also dont require fairing, so less finish work later aswell.

So a little time changing the shape of my design, saved me alot of time building, once i figured all of the above out for myself that is...

Dereck kelsalls KSS method is great, although you would need to buy the plans from him if wanted to go that way, unless you know someone that can loft the dart cuts and profiles, as per the desired shape etc... it really is a system, rather than just a method you could use for anything. Once youve done it before, you could probably transpose the system to another design with a little technical know how, but from green, you wont know how to pull it off.

 

You have to differentiate strength from stiffness. They are two different but inter-related characteristics. A trampoline can take a lot of abuse and pounding, so you could say it has a high strength. However, it would not hold the shape a hull needs, which requires stiffness. The hull will most likely break if it received the same impacts a trampoline can absorb.

 

To dimension a layup, you have to consider two factors:
1. the pressure a bottom panel (ie between stiffeners) would have to resist to, as given by the standards, and
2. the puncture resistance, as given by the standards.

Generally, for a foam or wood core, the second factor will give the minimal thickness of the external glass layup.

As for the pressure calculation, the internal + external glass thickness for a sandwich will be much lower than for a solid layup. Plus the solid layup is isotropic hence flexible -so not very resistant to fatigue- so it will need smaller panels ie more stiffeners.

For a 40ft sailing glass epoxy sandwich cat, the gain for a cored bottom is >600 kg, typically 10% to 15% of the light displacement: only for these two small parts...

Indeed this is not the only factor to consider: never use cored bottoms with polyester (7% porosity) or vinylester (4% porosity) glassing, due to the evaporation of the ester solvant when curing . Even with an "anti-osmose" treatment which is basically two coats of epoxy over the gel coat, without glass the epoxy is brittle and will microfracture -same as the gel coat-, and eventually water will go through the poly/vinylester. Unless you drydock the yacht four months a year to totally dry the hull...

A 40ft cat with a sandwich bottom having an external layer of 1200 g/sqm eglass epoxy, is very safe and solid provided you do not intend to regularly beach the cat on the rocks.

 

You cannot just take a designer's drawing and replace the core with glass.
Apart from the fact that the designer's calculations don't take the additional displacement into account, if you don't optimize the size of panels between stiffeners for a solid bottom hull, then the needed thickness will be absurd.

Personally, I would prefer adding a 400gsqm glass epoxy layer for additional protection if I was worried as you are (with the agreement of the designer), than having a power cat with 10 to 15% additional displacement. Remember the energy to displace is the cube of the displacement!

Edit: There a thousands of cored epoxy bottoms sailing, they are not more expensive to insure. Problems occurred when boat builders thought that vinylester was a waterproof polyester because there were few epoxy elements added.

 

Here you speak about industrial polyester cats, where hull and deck are glued with polyurethane glue. Very quick process, as it is so difficult to bond together cured polyester...

Don't worry, no problems of bonding cured expoxy. It also allows easy repairs as long as it is dry.

 

Sorry for the hijacking but I would like to ask. How does camber or a curve affect this? Is the glass underneath now being compressed as well? ....

 

Please browse e.g. Diab Sandwich Handbook.pdf or Gurit Guide to Composite.