foam cores and core materials

Marc,

Getting to your question. You wouldn't want a 14 foot john boat to be solid glass. Way (weigh?) too heavy, pardon the pun. This is why many boats in that size range use good marine grade ply with a fiberglass coating for abrasion resistance. They're built with epoxy. The reason for the epoxy coating is that the chemical structure of epoxy, once cured, is a three dimensional matrix. The "esters", polyester, vinylester and such result in a two dimensional structure. I'm not a chemist but it was explained to me that epoxy is waterproof while the esters are somewhat permeable. There are other reasons as well

You might want to look at the boats that Chesapeake Light Craft makes. Not necessarily to buy one but to see how they design and construct theirs. They have a good reputation. I own a little Eastport Nesting Pram and am always amazed at the strength of this little dinghy, considering how light it is. It uses 1/8 inch marine ply (okoume) for most of it's structure.

As for keeping water out of a composite build, that's what sealers are for. Screws and machine screws are a necessary evil. Sometimes you just have no alternative. When installing screws you can predrill the holes and then dip the screw in a cup full of sealant, that will keep the water out unless you hit something hard enough to damage the screw and break the seal. As for larger holes you might make for machine screws you can use the "epoxy grommet" trick that I learned about on this site many years ago. I'll post a photo of one below. You can make these grommets easily on a horizontal piece or use thickened epoxy and a little packing tape to make one on a vertical surface.

This thread looks at poly versus epoxy: Vinylester vs Epoxy on new plywood? https://www.boatdesign.net/threads/vinylester-vs-epoxy-on-new-plywood.58107/

 

You have to keep in mind what they say about opinions...... I do know that not all cores are equal and not all the people applying them are paragons of diligence.It gets worse if they are chasing a productivity bonus and cut a corner or two.I would even accept a polyester laminate on either side of a foam core if the job is done well and I know of some that were good after thirty years and some that caused problems before their third birthday.You can't tar all foams with the same brush and slabs of polyurethane insulating foam are very different to a premium foam such as an Airex or Divinycell product.You also can't expect to just throw a sheet of foam at a job and get a good result and I have a strong preference for vacuum bagging to ensure a solid and uniform bond.Obviously you need a suitable bevel in all areas where the foam transitions to single skin and decent solid inserts where large local loads are applied.Put simply,its a job for professionals not just anybody.

 

Epoxy is not waterproof. It is less permeable than polyester though. Both are cross linked polymers but epoxies also have nitrogen and a very different molecular structure. Epoxy absorbs much less moisture than wood and other cores. However, with enough time, there will be water diffusing through any resin. That is the reason that epoxy water/moisture barriers use additives like graphite and ceramics to slow the diffusion of water.

 

You couldn't sell premium core materials, that cost plenty, if they did not confer advantages. The biggest advantage is stiffness, which GRP inherently lacks, compared to other common boat building materials at the same weight. Vibration gets soaked up, you have in-built positive bouyancy, good insulation, and allegedly, greater resistance to the shell being breached in collisions. It is plausible that the foam can soak up collision forces, it certain soaks up sound, cored boats are quieter as well. The biggest demerit, or the one that gets most air-play, lack of damage resistance to sharp point impacts. In practice, not a huge issue if the places where it is likely to occur, are designed and built correctly.

 

things like rod holders work best in higher density cores

Certain other higjer load areas require 12# density cores.

transom cores typically around 25# densities

instead of mounting a bilge pump by screwing into core; get a weld mount kit and acrylic adjesive and mount studs, or use 24 pound density aquaplas and make a base for something that screws down

lots of options and variables and ways to deal with foam cores

simple things like a bow eye are best mounted in either denser cores or other ways, but you cannot mount a bow eye in 4-5# density core with backer only because any latersl force will tear sideways, for example

 

5 minutes into reading and I say he does not know what he is talking about. Reminds me of a guy who has more air than him and posted an article in Linkidin. Well, most of the composite experts rebutted his claim and asked him to remove his article. He did. He has to. That is what happens when ego becomes inflated more than knowledge.

 

Not entirely true. Pascoe ran into poor workmanship, poor early products; poor workmanship. His posts are out of date, but offer fair warning.

Here is one of his last posts.

ATC Core-Cell: A Foaming Solution? by David Pascoe, Marine Surveyor https://yachtsurvey.com/ATC_Core-Cell.htm

 

He makes a lot of claims that are not true. For example "If water gets into the core cavity hydrolysis can occur, thereby releasing styrene in the same manner as blister fluid (which contains styrene) is created. Styrene will soften PVC foam."
Where is the styrene being released from? If it was in the core already, according to his claim it would soften the foam. Further, if that claim was true, the core would soften and deform during lamination before the styrene evaporates or reacts by creating cross-links. He goes on to say: "The interesting and disconcerting thing about foam is its general weakness. Take any type of foam core material and one can easily tear, crumble or shred it by one’s bare hands. You can dig your fingernails into it and easily crush it. When you see a demonstration of weakness like this, the first question that comes to mind is why anyone thinks this stuff is suitable for building boats with". This goes against the evidence of hundred of thousands of boats built with cored laminates that are not having structural failures.
In short, his opinionated articles are not based on fact but only evidence of an inflated ego and a profusion of ignorance.

 

Which one isn't?

From the link you posted and it is getting worse. This just shows his ignorance.

"The naval architect recommended ratio of outer skin to inner skin thickness is 3:1. It is not necessary, nor even desirable, to make both skin thicknesses the same for reasons I won’t get into here. Unfortunately, we surveyors are finding skin ratios at 4,5, even 6:1. I’ve seen numerous boats where the inner skin was only a single layer of light fabric or even chopper gun. That is so thin that one could easily just push a screwdriver through it; I know because I’ve done that."

"The naval architect recommended ratio of outer skin to inner skin thickness is 3:1." That is guessing. No self respecting NA will do that.

Clearly he does not understand about design and yet he keeps talking,

 

He also doesn't provide any evidence to his claims. For example, he could name the brand and model of the boat he is referring to. I would like an explanation of why a balanced laminate is not desirable. I an a surveyor, and the only way to find what the ratio of the laminates is would be to make a hole. Unless there was a structural failure, destructive testing is very rarely done.

 

And again, this.
"Testing for water absorbency involved a simple exposure test as shown in nearby photos where the foam was simply floated on a pool of ink for a period of time. In a more elaborate test, we submerged the foam under pressure. Tests for creep rate and capillary effect were also conducted. The last form of testing was for heat distortion where we placed the sample in the hot sun, overhanging an edge with a weight on the end. We then measured the amount of deflection that occurred as it heated up. Most types performed fairly well while only a few didn't."

When the foam is cut some cells gets opened. If water can get in, so will the resin first. Which makes for a good bond. Not all foam absorbs a lot of resin (or water). It is too easy to generalize.

"Some foams absorb more water than others. This one with large cells and double kerfs takes up significantly more." Of course. This is very obvious. Kerfs is an empty space. It will always fill up with resin (or water according to his test)." I guess saying something obvious makes the article lengthy.

 

Right. In production, holes cut in any part of the boat is kept by the quality control to verify if the skin thicknesses are to specs. This is standard practice.

If suspect, the surveyor can always ask to remove any thru hull fitting to verify laminate thickness. That is if he is surveying for damage. If it is forensic, he should always ask for original samples. If in doubt, cut a hole on the damaged area.

 

In my research on using foam, I read the Pascoe articles and my take from that reading was workmanship and material quality still matters. All I meant with my comment. Generally, I found, as a newbie to foam boat building, that Pascoe was a bit over the top in his criticisms of foam. Things like his comments on skin thickness were not anything a newbie would pay attention to, but about 5 years ago or so, I read the corecell article and it cpuntered many of his other claims.

Let there be little doubt, I'd ask rx for advice on foam about 1000 times vefore Pascoe.

The other unforgettable Pascoe article for me I will link and would like to hear rx take on it.

Bad News for Bertram - Massive Hull Failure on a 2008 Bertram 63 https://www.yachtsurvey.com/bad_news_for_Bertram.htm

I read this not as a negative for core et al, but as a workmanship warning.

 

For the record, the kerf comment is especially frustrating because the foam is not used in the open fashion.

 

Yup, seen that failure a long time ago. That is the time when the Unis are greatly misunderstood. Unis, even the stiched variety are very "strong" in tensile, sometimes 3-4X than WR but it is good only in one direction, the zero degree or along the fiber direction. Whack it in an angle and it loses its strength tremendously. Sometimes 9-12% of the original strength is remaining at 45 to 90 degree off axis load.

The most common mistake in computing is that the designer relies on the tensile strength and places it outside the laminate where it is most needed. Testing for 90 degree load will yield a very low result.

The second mistake is laying it up in a 0-90 degree layup. The designer is fooled in relying in the calculations that it is "strong" in either the 0 or 90 degree axis because that is what the properties shows and the laminate passes the calc test. What is not evident is that when you place that laminate in the most stressed portion, The in plane bond will fail first, that is the resin. Poly resin usually fail first relative to the strength of the uni. Epoxy is a better alternative. ISO has this in plane bond calculation.

"Precisely what that was I cannot tell, but the overall failure pattern tells me that the laminate simply fell apart under what was probably low stress due to overall weakness." That is an incorrect conclusion. It has too much strength relative to the matrix. The picture shows the fibers are still intact.

"a condition usually due to inadequate framing resulting from over-reliance on the core for strength" Not enough proof unless calculated. It is not "strength" for the core, just shear.