Selecting foam core

I need help selecting a foam core for a 25' runabout/offshore "racer".

The boat will need to sustain high impact loads from highspeed slamming. Toughness is therefore the main criteria when selecting core material.
Weight and cost and formability will be next on my list.

After reading around this and other sites I come to folowing conclusions:

Crosslinked PVC foam will be too brittle - unusable
PU foam will deteoriate over time - unusable

This leaves me to SAN and Linear PVC foams, and a little poaking around I've got these on my list, which, by a quick look at the datasheets, seems to have relatively similar properties:

CoreCell A
Divinycell H
Airex R63

I would very much like to hear peoples opinions on these core materials, how they compare chemicly and costly, and finally how to work them.
Sanding and fairing will not be an major issue, but I need to be able to cut it to shape and bend it into shape. It it will also have to be compatible with West System epoxies.

 

Tom,

Corecell and airex are the toughest ones, and the most expensive.

Toughness in this case is how much energy it takes to cause a fracture. One thing to remember is that especially airex is relatively soft, and the hull can dimple if not properly supported on land.

The makers of WEST system (Gougeon Brothers) also has a line of epoxies called Pro-Set for building larger structures with longer working times and better properties possible with post-curing than the regular stuff. (don't expose any resin to carbon dioxide while curing though) They might also have a toughened resin, get that if you can.

One of the later issues of Professional Boatbuilder had a good article by Bruce Pfund on the art and science of bonding cores, available online at proboat.com

Yoke.

 

foam

First time on what U need is a 8.0 foam it is high density for support more than buoyancy 2 gal. makes 2 c feet provides 54 lb of buoyancy per c foot i hop this will help ya.. westwind

This leaves me to SAN and Linear PVC foams, and a little poaking around I've got these on my list, which, by a quick look at the datasheets, seems to have relatively similar properties:

CoreCell A
Divinycell H
Airex R63

I would very much like to hear peoples opinions on these core materials, how they compare chemicly and costly, and finally how to work them.
Sanding and fairing will not be an major issue, but I need to be able to cut it to shape and bend it into shape. It it will also have to be compatible with West System epoxies.[/QUOTE]

 

yokebutt: You would recomend me Corecell then? I finally found the article and the time to read it. Very interesting - especially the way the core "breathes". I'm not soure that I liked the one-way-valve thingy. Wouldn't this "vacuum" simply accelerate water intrusion? What we would really want would be light pressuration with dried air (if we cannot have the core 100% sealed off). Just my 2 cents...

My gut feeling from just reading the respective datasheets points strongly to Corecell as well - they even lists their approvals (DNV, USCG ++). Gives the impression of a thoroughly tested and well thought out product.
Are my assumptions right? Is there any other coring materials I should consider?

westwind: around 8 in density leaves me at Corecell A600 or A800 (7.3 and 9.3 lb/ft³ respectively). This is a bit heavier than I was hoping for - I'm kind of on a diet... I'm considering core thikness between 50 and 60 mm, but I'm not soure if Corecell are avaiable this thick - at least the A300 was only avaiable from 5 to 35 mm in 5 mm increments. Laminating two layers of core is totally out of the question as this would give me a 50% bigger chance of delamination. Hopefully some of the higher densities will be avaiable in thicker plates.

 

Tom,

50-60mm and 8# per cubic foot sounds like way too much to me, I can't imagine you'd need anything that thick or dense on a 25 footer. If I were in your position, I'd snoop around to see what they use in current offshore race-boats of that size.

Corecell would be a good choice, and since ATC was bought by SP systems a while back, it might be wise to buy their resins and fibers too, they are very helpful with technical questions. (hint hint)

Personally, I've never put a vent on a cored laminate, just make sure that all gaps and scores are completely filled with core-bond putty, and no openings into the core, then there won't be anything to vent.

And finally, as usual the devil is in the details. It's tempting to do things like putting screws directly into the inner skin, instead of into something bonded to it. Consider making up little fiberglass brackets that you bond to the hull and then screw into those.

Yoke.

 

First thing that popped up on my screen that is a bit related is this one:
http://bassandwalleyeboats.com/output.cfm?id=943291

This 20' cat uses 1" for sides and bottom. The cat is not directly comparable, but was the best I could find in a hurry. As I'm going for a slightly bigger boat and higher construction speed I want a little more (and then some for good measure and peace of mind). How does 45-50 mm of 4.3# or 5# sound?

I'll have to contact SP again to check what cores are readily avaiable, and in what thiknesses. The A300 foam was not avaiable in Europe, but I could be lucky to find one that are.

Any other brands/products I should be considering?

 

foam

anny time U use foam be sure to seal it gel coat spray or brush just make sure it is seald

[/QUOTE]

 

Tom, I think 25mm would be plenty of overkill for your application, but I would encourage you to talk to SP systems about it. 50mm is is suitable for big boats, like 70-80 feet or so. After all, these materials are expensive, and you don't want to add any excess weight that isn't neccessary.

Yoke.

 

The scope of the project is to build a wooden boat as strong and tough as possible within my weight limits and construction abilities, and do it within a reasonable budget. I'm quite new to boat design and construction, and try to get advice where I can. Your last post is just perfect in that respect - real world facts that let me know that my thinking is far off, but in the "right" direction.

I have had some correspondance with SP Danmark (nearest dealer), and they are most helpful. I've been directed to one of SP's assosiated naval engineers, and am now looking forward to hear his advice.

 

Tom,

can you explain some of your thinking behind the fact that you've limited your intial question to selecting foam cores?
Since you're actually building a cored wooden boat, have you considered other materials - like balsa?

bye
Hans

 

To find the "best " just use what most military services demand ,

as they frequently really abuse boats , and are willing to pay for "the best".

To me thats AIREX, and has been the standard of quality for 35 years or so.

FAST FRED

 

Hans,

Endgrain balsa is a wonderful material, not the least for being lower in cost, in fact, a balsa-cored panel is stiffer than a plywood-cored one. But, for a bottom that gets a lot of slamming, a foam with better shock-absorbing qualities will have an advantage. As an analogy, imagine replacing the shocks on your car with rigid pipes, it wouldn't be a pleasant ride, and the chassis would break fairly soon.

Yoke.

 

Hans: The reason for just asking for foam cores are borne out of a process of elemination when selecting core material. Main criterieas have been closed cell structure and weight vs. mechanical properties.

Another aspect, that may soud weird is that I want to give the hull it's shape with wood, as opposite to make the core up first, and then laminate the skins to the core. This is a bit difficult to explain, and would probably be a good topic for discussion; but this "little detail" is what (for me at lest) will make the craft a true wooden boat. In more practical terms this means that I need a core that can bendt/shaped into the already finished outer skin. SAN and PVC foam are thermoformable and should work out well, and give me very uniform thickness throughout the hull, give me closed cells and the toughness needed (not X-linked PVC).
I do know that I have already selected a wooden core in my skins, but I belive that the thinner (1/4") stipplanking with epoxy resin will be less prone to wich in water than 1" endgrain balsa. I wold also more easily be able to visibly detect a failure in the skins than in the core.

Fred: Do you know why airex are preferred by military services, as opposed to Corecell and others?

Yoke: After a little more poking around it seems like Corecell A500 is the most popular density for "offshore" boats. This leads me to conclude that a 1" core of A500 (5.7 lb/ft) would be right for my project. Your thoughts?

I still have not head from SP's engineer, so I have given up on him taking interest in my project. Pretty much on my own again.

 

I am constructing a 2 seat Gentleman's runabout. All plywood. On a interlocking structure of 12" X 12" compartments. 1" X 1" triangular corner gussets. 100% on bottom skin support. 50% on all 4 sides of vertical intersections. Solid, full depth keel and engine stringers which are full length. This construction has vertical compessive strength of over 28,000 # per lineal foot. The weight of the 4.5' X 21' -- 28 degree constant deep V hull, cut out parts is 700#. Wood glues throughout, except in the powertrain diagonal bracing areas. -----------------------------------It will be completly assembled, ready to run and dropped 10' from a crane into the water. We do not see the slightest chance of structural failure. Unless we miss a bunch of glue joints in a large area. ------------ More than one way to build a light strong hull.

 

A 25' "high speed" racer probably incorporates a mounstrous powerplant. Motor mounts together with fuel tanks and their mounts, are probably where the highest stress points are in the hull and the hull is affected the most by slamming. So your choice of densities and types of core shouldn't be uniform and less stressed areas can be built with lower density and vise versa.
Imagine the boat slamming at full speed with out those weights I mentioned and then add the weights and their concentration.