The XPS I used is what was readily available.  It was the type usually used for insulation.

Two key tests for any sandwich laminate in a boat application are tensile bond strength and rolling fatigue or impact fatigue.  The bond strength test is ASTM C297 as described here:

http://www.intertek.com/polymers/tensile-testing/sandwich-construction-strength/

The rolling fatigue was developed by Boeing for aeroplane floor panels and is intended to replicate the damage caused by food-trolley wheels:

https://orbilu.uni.lu/bitstream/10993/9843/1/Paper2013_fatigue_ORBI.pdf

I consider this to be a good test for the hull skin and decks intended for a boat. It is similar to walking on the boat in hard shoes, rolling the boat onto a trailer,; lifting the hull with slings; grounding the hull on rocks and any other myriad of point loading the skins will see throughout their life.   

There are also standards for shear strength and flexural fatigue.  But the two I have given details on are highly relevant to a boat.

The primary difference between a boat and plane is that a plane usually contacts the ground with rubber wheels specifically designed for that.  Any part of a boat hull can contact the ground or something just as hard like rocks, logs, quayside, jetty, winch handle, errant tools and many more hard, unforgiving surfaces.  The other difference with an aircraft is that the critical part of a boat hull operates in an incompressible medium.  Any thermal expansion of infiltrated water can cause very high stresses on bonds when confined.

I know from experience that H80 with thin skins is durable.  I would not use materials commonly used in the aircraft industry for a durable boat without thorough testing. I would not use any open cell core on a normally immersed portion.  I would not use a core material that degrades in sunlight.  There are typically signs plastered in many spots on an aircraft “no step”.  A shoe is mild loading compared with a rock or a log that the hull will surely strike..

If XPS was used as the initial former to get a shape and then covered with a heavy skin that was structurally sound without the core then that would likely be durable.  XPS is suitable in block form as a bulkhead or solid buoyancy.   

Water will find a way in.  It may be possible to start with a watertight skin but there are many ways it can deteriorate and allow water in eg hull fittings, grazes from collisions and so on.  If you had skin that could take a good blow from a 2kg hammer (maybe more than 1000gsm) then it is likely the skin would remain water tight for a long time and it would not matter much how it was stiffened.  

The two test I have described should give good relative comparisons for boat skins.  Even if you could get XPS with the same tensile strength as PVC H80 and the same initial bond strength I expect the XPS would still not perform as well in the fatigue situation.  I expect this is related to fatigue strength.  There is some data here for PVC foam:

https://engagedscholarship.csuohio.edu/cgi/viewcontent.cgi?article=1113&context=enme_facpub

XPS does not seem to have the rebound or toughness of PVC core.  

I cannot find any fracture toughness data on XPS.  If you can then compare it with the data in the paper or perform your own comparative tests.

The other factor that comes into play with lightly skinned core is UV deterioration.   I have seen polypropylene honeycomb turn to dust in as little as 5 years sun exposure.  A light glass skin with only  a coating or two of paint will let sun in.  You would also need to check the resistance to UV radiation of the materials being compared.  PVC marine core darkens when exposed to UV but its mechanical performance does not seem to deteriorate.  UV performance of a core material on an aircraft should be one aspect relevant to a boat.  

On 3 May 2018, at 2:51 pm, '.' eruttan@yahoo.com [harryproa] <harryproa@yahoogroups.com.au> wrote:

Do you mean that was the value of the grade you speced in your prototype hull? You could have used a much better spec XPS. Why did you chose .4MPa?

>H80 PVC foam has a tensile strength of 2.5MPa. That is a massive difference as it relates to delamination.  

I have not seen a reference supporting delamination being related to tensile strength. Would you mind sharing? Typically delamination is related to over stressing the part afaik.

>I made a long slender prototype hull from a block of XPS. The skin was 200gsm carbon fibre. Over a few uses water found its way through pinholes and some surface scratches and that initiated progressive delamination through various processes like handling and thermal expansion of the water including a few periods of ice in cold weather. 

While i appreciate you sharing your experience. It seems like you are suggesting this is always true. Is that what you mean?

Do you think if you had taken care to not have pinholes, it would have happened the same way?

Because many different home built aircraft have been flying for going on 50 years. Many of these have been known to be stored, outdoors, in northern costal states, for their entire lives, with the freezing rain, snow, sleet, and hail one would expect a tied down aircraft to experience. Along with the sweat inducing tarmac temperature in the same day.

Delamination is unheard of. Unless builder error. Or violent accident.

>Once water can get in, it pumps the bond by forming a bubble under the skin.. The temperature of the water in the bubble could range from 10C to 40C in a matter of hours. The expansion works away at the edges of the bubble and extends it.
>
>A thin skin with pinholes on H80 PVC foam does not deteriorate the same way.

Your theory is that water works on XPS epoxy interface different than PVC epoxy interface? Why would that be? Usually these aircraft will sometimes notice a bubble if there is a water infiltration problem. Which is easily fixed. The shearing of a whole skin off a core does not happen.

Another explanation is the composite put too much stress on the core and it failed. And the XPS skin interface is where that failure shows up. This seems to agree with Robs observations too. Or perhaps the surface was contaminated. (with silicon?)

>The bond must be strong enough to prevent the thermal fretting of the bond.

Do you have a reference for this? Cause my experiences suggest that this characterization is not supported by the thousands of flying machines I am aware of. These see way more thermal cycling than a boat ever would. Its cold at 20k ft and 200mph. And hot parked on the tarmac. How many times a day they see that just depends on how many 100$ hamburgers one feels like in a day.

>XPS is a lower cost means of quickly building a disposable hull. PVC H80 is good for a durable hull that will give years of service.

XPS has given years of service to some. It serves in some pretty tough environments.