Cores in hull bottoms

This guy doesn't like them. Is he right, wrong or both? Sam
http://www.yachtsurvey.com/structuralissues.htm

 

David Pascoe is a well known and respected surveyor. However, he is like most of us, opinionated. But look at it from his perspective for the moment. What does he see? He sees all the problems. He doesn't see the boats that don't have problems. I have the same type of experience. For 25 years working for the Coast Guard's Boating Safety program all I saw were the horrendous problems with boats. This tends to color ones viewpoint.

But, I tend to agree with David that boat bottoms should not be cored. High impact, High sheer loads, flexing, and vibration are going to, in my opinion, soon ruin any adhession of the core to the laminate.

I will admit though that there are many people out there who will disagree with this, and who have successfully cored the bottom of hulls. A lot of drag boats, and other fresh water go-fasts have cored bottoms to get a very stiff, perfectly flat planing surface. I can't, off the top of my head, think of an offshore go-fast that has a cored bottom, mainly because of the types of loading they experience. High impact, heavy sheer loads and lots of flexing on offshore boats don't make a pretty picture for cored bottoms.

As I said, I think cored bottoms are going to fail eventually but then that's just my opinion. David is just a lot more vocal about it than I am.

On the other hand there are lots and lots of boats of all types with cored hull sides, decks and cabin tops. In these areas it is very common and works very well.

 

I have owned several fully cored boats ranging from klegecell/carbon/epoxy to balsa/sglass/carbon/epoxy to pre preg carbon/nomex honeycomb and they were all 18ft skiffs.They were all built between 1978-1991 by quality boatbuilders.I am no longer sailing the skiffs and these old boats are still in good condition.

I am planning to build a Farrier F82R and that will be core cell/ carbon epoxy with some kevlar.I think you really have to vacum bag any cored boat when laying it up to ensure a good bond between skin and core and it does remove excess resin.I have never had any problems with cored boats and those i have had no expense spared during construction and built by people with a lot of experience with these materials.
David Pascoe was very critical of the workmanship and laminate schedule on some boats with his report and i agree with him on that point and i am almost in disbelief that some boats have been built like that.

Cored boats can be strong,durable,light if it is well designed,engineered and manufactured and even david pascoe admits this.

Would i buy a second hand cored boat from an unknown builder? probably not

IKE
The Vor 70's and the vor60's that were racing all have cored bottoms.The Sydney to hobart winners for line honours would be all cored boats going back at least a decade from memory.Ellen's Trimaran and geronimo are cored so is that enough offshore go fast for starters or do you want more...lol

 

I'm all in favour of cored structures, provided that:
- The laminate is designed and analyzed by a competent composites engineer,
- The structure is designed with high safety factors and with thermal and fatigue loading considered,
- A high-quality core appropriate to the application is used,
- The layup is done by professional builders skilled in core construction, and is vacuum-bagged correctly
The trouble with production boats is that when cores are used, they're generally not well designed, there's very little actual analysis done on the drawings, the materials used are crap, and they're built by unskilled minimum-wage labour.
Modern racing yachts aren't built for longevity anyway. They're often retired after a year on the water, before deterioration has had a chance to set in. Major problems are certain when you try to take that technology and build it cheaply for the masses.
The 'hamburger-helper-for-fibreglass' stuff Pascoe mentions looks rather scary. Some of it resembles the pressboard cores in IKEA furniture, some looks like body filler. Whatever the hell it is, I agree that it has no place in boats. If I ever get the money to buy a new boat (wishful thinking...) I would certainly expect the builder to allow me to look over all the laminate schedules etc., and I'd be pretty leery of a builder that refused a request for a shop tour without good reason. I've spent enough time around carbon, epoxy, glass and glue to not trust what I can't see!

Baron- in North America, "go-fast" usually refers to a high-speed powerboat (60 mph +), most likely a deep-V or tunnel cat; unlike Australians, we don't apply the term to sailing craft.

 

I was refereing to offshore racing powerboats like Fountain and Formula. I am aware that some offshore racing sailboats have cored bottoms. They don't get the kind of slamming that powerboats do.

 

My apologies for being a smart *** Ike i wasnt aware that you were referring to power boats.

There are a few foam sandwich offshore tunnel cats in Australia,i recall ATL COMPOSITES used one particular boat in advertising their products.They may still be using it so if you like google atl composites and have a look.

Slamming is a problem with the faster larger maxi yachts and it is a much bigger problem for those who do not use high modulus fabrics like carbon and kevlar in the bottom.

I dont usually consider sailboats fast unless it is an 18ft skiff or larger multihull.

It was a good read with David Pascoes website and it is good to be informed of the questionable engineering and workmanship coming from some production boatbuilders.I am absolutely stunned at how poorly built some of those boats are.

 

Sportfishermen in the East Coast have been built with cored bottoms for more than thirty years. They have proven themselves in very rough weather and continuous use. I have built several of them. The normal cruising speed is 30-35Kts.

 

GonZo, what type of core is most prevalent and how did you install it.

 

I think only an experienced boatbuilder can use the sandwich in the bottom.

However today there is a large diffusion of sandwich bottom.

I personally projected the structure of Jaguar 24 sport (fp yacht),

and we have used pvc herex c70/200 in the bottom.

 

The name 'Pascoe' in the marine industry is a name which disserves respect. However I feel that David Pascoe's report on cored bottoms is very subjective.

The report concentrates on the failures which can happen, and not that successes other people are experiencing.

A properly designed cored structure can provide excellent specific properties (unit per kg) such as strength, stiffeness, toughness, etc etc.

This is all very well provided our quality composites are made with a quality manufacturing technique such as resin infusion or vac bagging.

Personally I wouldn't own a cored boat unless it was vac bagged or infused.

This is the 21st Century and its time the marine industry updates its manufacturing processes to suit the demands of todays products. Consolidation by hand isnt effective. We required tonnes of pressure over the whole laminate during consolidation for the duration of the cure, not a dab here and there. (1 bar = 10 tonnes/sqm)

Material science has developed somewhat with the use of PVC cores etc. Balsa is cheap and cheerful, and can rot. PVC is slightly more expensive and does not rot. It also compared in a range of density which different shear properties.

If only David Pascoe reported on the success of more suitable cores, things would seem different. Why are all the big players moving to infused, PVC hulls. Quality is the answer.

 

I perfectly agree with you.

 

Those boats are built with plywood cores. They usually have three layers with bi or tri directional glass and epoxy inside and out. Some of the boats still built by oldtimers are all juniper cores.

 

funny you say that

I learned a lot from Cuban boat builders in Florida, and for a tough boat they use WOOD. There I said it , the "W" word. I spent a lot of time crashing through waves in these boats, whether to go to work or to fish or to demo a boat. Big heavy comfortable boats. Nuff said

 

I used to have great respect for Davids opinions, then I read his opinions on marine engines. Who among us would rip the fuel injection systems out of our cars and replace with a carburator.(not Me). I find alot of his observations are based on bad manufacturing techniques. To build a 40ft go fast offshore of solid glass would be crazy.I only have a problem with balsa-core below the waterline.I love core materials as long as you know how to properly install them. Hi buckknekid, hows the project coming along?

 

Jim Virtually 90% done, glass work si done, now to paint the deck and go boating. Im on the lake anyway!