GRP Barge

In my opinion, when the calculations are made correctly a cored panel does not have to be stronger than a solid one. A panel must be calculated to support a certain design pressure, which does not depend on the type of panel used, and therefore all the panel must do is adequately support those loads. Both types of panel will support the same load, with the same safety coefficients (which do not depend on the type of panel) and, therefore, are equally strong. Now, if the thickness of the solid panel is very large and you want to save weight, you should think about replacing it with a sandwich panel.
On the other hand, the impact resistance of the cored panel may be less than that of the equivalent solid one.
Regarding the insulation, also in my opinion, it would be best to place a solid skin and a thermal insulation, rock wool, in the inside about 100 mm thick. If you can place an air gap between both, a few millimeters, better than better (the best thermal insulator is air, if you can not vacuum).

 

"stronger" - is not an engineering term
there is Sheer, Compression and Tension. You probably need to add Impact effect to that, although that combines the original three.

A Core will always provide a better value ( less materials, lower cost) for the designed total forces.

 

"stronger" is a term that you have used and that is why I included it in my reasoning. I do not like this term but I wanted you to know what comment I was referring to.
There is also torsion, not everything is pure bending, you do not forget that.
"single layer" may be a very engineering term, I doubt it, but making a hull with a "single layer" seems very risky. I would always place several layers.
The impact is a specific phenomenon, very special, that should be studied in addition to the other loads.

Although I do not know the term "better value" how it is quantified, I doubt very much that it is possible to generalize the fact that a cored panel always gives less materials, lower cost.
"designed total forces" can be a very engineering term, I doubt it, but I'd better say "design total loads".

 

I have customers that build similar size items with a 4" foam core, and others that build shelters for arctic locations with thick foam and a few other things. I don't see any real issues with using a 6" foam core if that's what's required, but the thickness requirement needs to be confirmed.

The 4" foam is a low cost urethane around 2# density. Imagine a shipping container that holds water, so not that different from this application.

You can buy polyester resins and primers that are safe for use with EPS foam, they may not be easily available on a short notice purchase though.

All of this will be determined by the build options the professional he hires to design it feels will work.

 

I would like to explore a little more your idea "the best thermal insulator is air".

As previously stated one option is to use an existing boat and convert it to my design.

If air is the best insulator how would be best to achieve an air gap and what width?

Again, is it possible to create a vacuum in such a hull by using a system of battens and sealing the outer hull skin from the inner internal wall?

I have not thought of this idea but find it very interesting as there would be a saving in weight possibly, as well of cost.

 

Looking on the internet an air gap of 50mm would appear to be the optimal thickness. Not sure you could create a vacuum of that area.

 

Rock wool is sold in fairly rigid panels about 50 mm (2 ") thick. You can place small size spacers of, lets say 2 mm thick, glued randomly on the hull. The rock wool panels then supported on the spacers with what would be an air gap of 2 mm. You can do the same again with a second layer of rock wool, reaching an insulation of more than 5 "thick.The rock wool is also incombustible, not fire retardant but incombustible, which is always interesting for a house-boat.
PS : not vacuum at all.

 

I would not be interested in using Rockwool on a cruising boat. This material can hold a lot of water. I would only use a closed cell foam material or foil insulation.

 

Although sandwich construction gives the highest stiffness to weight ratio, there is a limit. Too much depth and you get a very thin skins, easily punctured. Too much depth and the weight of the core takes over negating the savings in weight. There is a proportion of skin thickness to core depth. Exception would be the use of Aerogel.

Like air, which is a good insulator, there is a ratio of depth to height void in glass panes. Too much height and it becomes ineffective. Search insulating glass panes. Vacuum (no air) is the best insulator but in relation to atmospheric pressure, there is an imbalance. More pressure will be exerted on the outside of the panel. You will have to build stronger and thicker panels. Everything can be explained by mathematics.

 

Thanks for all the input it has helped me to make up my mind. I will purchase an old Norfolk Broads cruiser and convert it into my home on the waterways.

There are plenty of experienced boat builders who can help me there in Norfolk.

 

My comments about water draft, air draft and head room have absolutely nothing to do with regulations. Instead they are about the basic physics of will the boat float with a maximum draft that is less than the minimum depth of water available while at the same time also not extending too far above the water to go under the lowest bridge which might be encountered, and have the desired amount of headroom inside.

 

Two possible construction methods with GRP and foam:

1) Cored construction with structural stressed skins on both sides of a foam core. This can be lighter but will also be more susceptible to damage from impact with solid objects. It may be more expensive.

2) Single structural stressed skin with foam insulation on the inside. This will be less susceptible to damage from impact with solid objects. It may be less expensive.

I can't see any functional advantage with cored construction over single skin construction with interior foam insulation unless reduced weight is needed or desired. As soon as ballast is added sink the boat into the water the advantages of lower weight are either significantly reduced or eliminated.

 

Certainly the least problematic method. I presume you "talked" to the GRP expert, and found out a few things too.