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Does foam contribute to structural integrity on catamaran hull?

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Does foam contribute to structural integrity on catamaran hull?

Old 02-07-2019, 12:22 PM
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Default Does foam contribute to structural integrity on catamaran hull?

My boat, a Kevlacat 2000, was completely filled with foam at the factory. It has a sealed deck with no compartments underneath.

After I ripped the floor out to replace it, I also removed the foam. I am considering to make some below deck storage with access hatches.

At the moment, I am concerned with the following:

- does the foam contribute to any structural reinforcement?
- will removing the bulkhead shown below affect structure?

I have been advised that the only reason that boats under 20 feet are foam filled us due to a USCG requirement. I cannot attest to that. Regardless I am overseas and do not need to comply with such regulation.

In case of emergency I have a Viking rescue raft. So the issue at hand is regarding structural integrity.


If I remove this bulkhead there will be room for dry storage or kill box.


If it is removed the unsupported span fwd to aft is almost equal to the fuel tank´s area span, around 44".


The bulkhead is quite thin, around 1/8
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Old 02-07-2019, 01:33 PM
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The bulkheads are there to hold the deck up. The foam has zero structural value. Maybe a little sound deadening. Zero value and huge downside on resale if you put it back in Some KVs get waterlogged.
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Old 02-07-2019, 02:06 PM
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Take out what you want and replace with 4lb foam. Then it IS structural. Just cut out small areas to make storage areas. If you want no foam to hats fine as long as a hollow boat won’t bother you.
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Old 02-07-2019, 04:17 PM
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Foam is just foam and to try and use it as structural YOUR ANIDIOT !!
And the reason NEVER BUY A SECOND HAND BOAT !
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Old 02-07-2019, 04:26 PM
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Originally Posted by tunnles View Post
Foam is just foam and to try and use it as structural YOUR ANIDIOT !!
And the reason NEVER BUY A SECOND HAND BOAT !
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Old 02-07-2019, 04:54 PM
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And Boston Whalers don't work
Foam most certainly can be structural. Some catamarans have had issues, the structural loads aren't necessarily intuitive and I would suggest that there was nothing in the original construction it would be considered unnecessary. The only people that can answer your question are the people that built the boat or somebody that's capable of reengineering it.
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Old 02-07-2019, 05:14 PM
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American boats no wonder THE Chinese don't have many here ! they want boats that last !
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Old 02-07-2019, 05:56 PM
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If the sponsons were filled with 2 part expanding foam there is no doubt it stiffened the laminate more than just air would.
2-4 lb/ft3 is usually used for flotation, and 4 will be more resistant to water, chemical and fuel absorption
6# is what is used to to make wider bulk heads about 3 " thick
and you can find the 13# stuff if you want even more stiffness and strength.
So if you take it out, add some more mat to the rounded tunnel radius to beef it up and some on the bottom and other side too.
The deck is one thing, but the weight of the bottom on the trailer is also reinforced by what you want to remove so I would not cut deeper than box bottom depth on the bh in view of that.
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Old 02-07-2019, 11:40 PM
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If YOU HAVE TO RELY ON FOAM FOR STRUCTUAL INTEGRITY THEN YOUR IN TROUBLE AND DONT UNDERSTAND BOATS AND BOATING !
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Old 02-08-2019, 12:17 AM
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And if you have to type in capitals and bold you must be talking shit.
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Old 02-08-2019, 12:22 AM
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The bulkheads don't hold the floor up, the crossover beams do that, the bulkheads do a couple of things, provide multiple watertight compartments, and add strength to the hull in both sideways and and vertical planes.
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Old 02-08-2019, 12:49 AM
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OH YEAH ANOTHER ONE FROM GUESS WHERE ! now whats that age old saying about ozzies and sheep ??? !!
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Old 02-08-2019, 02:54 AM
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There is NOTHING structural about foam.
It is used as an inexpensive sound suppressor, an inexpensive way to fill spaces rather than bracing.
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Old 02-08-2019, 03:03 AM
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As a non-boat builder but reasonably logical thinker, my thought would be this: that bulkhead has taken time and materials to build - therefore why would it have been there if it wasn't required? I wouldn't remove it anymore than I would remove part of my car chassis without being a chassis engineer.
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Old 02-08-2019, 03:46 AM
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....- does the foam contribute to any structural reinforcement?
Kevlacats were never foam filled, foam in this case sounds like some meaningless bureaucratic misunderstanding

....- will removing the bulkhead shown below affect structure?
Don't touch that bulkhead, a smart person put it there for a good reason
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Old 02-08-2019, 03:49 AM
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Originally Posted by tunnles View Post
OH YEAH ANOTHER ONE FROM GUESS WHERE ! now whats that age old saying about ozzies and sheep ??? !!
You are probably a little confused and don't understand where NZ is relative to Oz, as for your apparent need to raise your voice, I think you are talking shit as well, there's nothing wrong with buying second hand boats, probably a lot smarter than buying Chinese mudguards
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Old 02-08-2019, 05:59 AM
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Foam can most definitely be a structural component. As an example Boston Whaler builds small boats with low density urethane foam filling the gap between two relatively thin glass skins. If the foam wasn't there, they most assuredly would fail to hold up as long as they do. Many boats have cavities filled to support the bottom panels which helps reduce deflection. Deflection can lead to fatigue failure. This is not to say that the hull bottom can't be supported by other means (frames, stringers, thick laminates, etc). The foam could be doing the job of supporting the hull or it could just be there for flotation.

I don't really know much about Kevlacats but I don't recall them being high speed craft. With a middling speed (up to maybe 40 knots or so) in mind, you could probably get away with adding some frames to support the hull and still leave yourself room for storage. Look up Prisma Preforms. You could use that type of product or make up your own. Either way it could be an answer for you.
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Old 02-08-2019, 06:01 AM
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The original poster is correct about what he has heard regarding foam on boats less than 20ft. USCG requires any outboard powered boat under 20 ft to have upright level flotation. When the foam used is a 2 part poured or injected foam it will add nothing to the boat structurally. It will create some sound deadening, but that is not its primary function. There are some high density foams that can be used as laminate cores for structural items but you have to remember that the core of any composite that involves glass/carbon fiber/kevlar/etc does not provide the strength, it only acts as a form to create the structural member. The cloth combined in the resin matrix is what provides strength. Coring materials are used to create thickness without adding the extra weight that a solid material would, because thickness = extra rigidity. Companies like BW have used foam in their stringers and even in hull cores because when you market boats as "unsinkable" you need a lot of flotation. Putting foam in those areas cuts down on the amount of poured/injected foam needed elsewhere to achieve "unsinkability".

As far as removing the bulkhead, i would not do so without very careful research, which would probably conclude that if you remove that member, you would need to add or reinforce the next crossframe to the one you want to remove.
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Old 02-08-2019, 06:55 AM
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You will have to reinforce the thin gas tank bulkhead if no foam is replaced there.
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Old 02-08-2019, 07:29 AM
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A lot of comments from people who think they know more about boat construction then they do...
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