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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 02-09-2019, 12:11 AM
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I have a same sized Kevlacat. I replaced the timber floor a year ago with what they call Coosa board, a composite polyurethane foam reinforced with layers of fibreglass. I also filled all the hull compartments with Marine Buoyancy Foam POLYAIR EPE 5. Each sheet is 2000 mm x 1000 mm x 50 mm and 100 litres in volume. I cut the sheets and fitted them neatly into the cavities. The weight of a cubic meter of EPE is about 32 kg, so effectively providing 970 kg of buoyancy. I used around 1.3 cubic meters.I also fitted stainless steel inspection ports on the floor above each cavity. I might add, that when the floor was removed, the hull was still rigid. I personally would not remove any bulkheads.Specialized Structural Panels - Coosa Composites
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Old 02-09-2019, 01:04 AM
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The floor plays little part in hull integrity on those cats, it's simply to walk on, unlike a mono.
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Old 02-09-2019, 04:12 AM
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Originally Posted by keysdiver View Post
There is NOTHING structural about foam.
It is used as an inexpensive sound suppressor, an inexpensive way to fill spaces rather than bracing.
In a Boston Whaler, I would say the foam is very structural.
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Old 02-09-2019, 07:24 AM
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Originally Posted by 72barracuda View Post
In a Boston Whaler, I would say the foam is very structural.
agree. For a foam to be structural, it needs to adhere to a skin that has tensile strength. And to have a good measure of resistance to compression.
Its best to say that the foam is part of the structure, than to say it is “structural”.

If one understands how concrete with imbedded steel works, it is a similar argument.
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Old 02-09-2019, 08:26 AM
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- does the foam contribute to any structural reinforcement? NO
- will removing the bulkhead shown below affect structure? MAYBE. You have to assume the builder put the bulkhead in there for a reason.


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Old 02-09-2019, 08:38 AM
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Originally Posted by 72barracuda View Post
In a Boston Whaler, I would say the foam is very structural.
Not really. Boston Whaler hulls are two parts built in separate molds. The outer (hull) and an inner (liner) molds are mated together while the fiberglass is still uncured. The structure is created by what they call "shear ties". These are pieces of fiberglass that are inserted in between the molds. Their purpose is to tie the two fiberglass parts together and prevent them from moving in the horizontal plane relative to each other. (Shear) You need enough shear ties to prevent movement, but not too many or it will prevent the foam from flowing from it entry point at the bottom to its exit point at the top. The first article of each design is built with clear gelcoat so you can see inside and make sure the foam filled the void and the shear ties are intact. While the foam does provide some rigidity to the skins, it is not relied upon for structure.

Last edited by Island Marine Group; 02-09-2019 at 08:41 AM. Reason: spelling
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Old 02-09-2019, 08:53 AM
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Originally Posted by tunnles View Post
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 02-09-2019, 10:08 AM
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Originally Posted by Island Marine Group View Post
Not really. Boston Whaler hulls are two parts built in separate molds. The outer (hull) and an inner (liner) molds are mated together while the fiberglass is still uncured. The structure is created by what they call "shear ties". These are pieces of fiberglass that are inserted in between the molds. Their purpose is to tie the two fiberglass parts together and prevent them from moving in the horizontal plane relative to each other. (Shear) You need enough shear ties to prevent movement, but not too many or it will prevent the foam from flowing from it entry point at the bottom to its exit point at the top. The first article of each design is built with clear gelcoat so you can see inside and make sure the foam filled the void and the shear ties are intact. While the foam does provide some rigidity to the skins, it is not relied upon for structure.
Fair enough. But, let me ask this. If you were to remove all the BW17 foam, would you not find lots of oil canning? Lots of point stress on the hull at the shear ties? Eventually enough skin movement to cause a failure? My point, is that the foam is an integral solution, that does help the overall structure.
Not to make a direct comparison to a BW (I've done a lot with them and they have been fine); but have you seen a fairly simple structure, like an IGLOO cooler, loose its foam to skin adhesion? It's a mess.
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Old 02-09-2019, 11:35 AM
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Originally Posted by Island Marine Group View Post
Not really. Boston Whaler hulls are two parts built in separate molds. The outer (hull) and an inner (liner) molds are mated together while the fiberglass is still uncured. The structure is created by what they call "shear ties". These are pieces of fiberglass that are inserted in between the molds. Their purpose is to tie the two fiberglass parts together and prevent them from moving in the horizontal plane relative to each other. (Shear) You need enough shear ties to prevent movement, but not too many or it will prevent the foam from flowing from it entry point at the bottom to its exit point at the top. The first article of each design is built with clear gelcoat so you can see inside and make sure the foam filled the void and the shear ties are intact. While the foam does provide some rigidity to the skins, it is not relied upon for structure.
Ok, so foam cores filling and providing wider spaces between laminates is something that composite engineers are doing all wrong if you want to stiffen your structure using less pounds of fiberglass and hulls like cape horn and boston whaler have not proven themselves bullet proof. You are talking shear but you leave tensile (stiffness) strength out of your analysis.
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Old 02-09-2019, 11:42 AM
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Originally Posted by diver dave View Post


agree. For a foam to be structural, it needs to adhere to a skin that has tensile strength. And to have a good measure of resistance to compression.
Its best to say that the foam is part of the structure, than to say it is “structural”.

If one understands how concrete with imbedded steel works, it is a similar argument.
yes, but the denser the foam is the harder it is so structual cores range from 6-30 lbs per cubic ft and not the 2 lb crap that so builders try to get away with
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Old 02-09-2019, 11:43 AM
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Originally Posted by diver dave View Post
Not to make a direct comparison to a BW (I've done a lot with them and they have been fine); but have you seen a fairly simple structure, like an IGLOO cooler, loose its foam to skin adhesion? It's a mess.
The foam stiffens the panel for sure. The trouble starts if the panel is deflected too much and the fiberglass will spring back to its original shape but the foam can't. As far as the BW and Igloo cooler, I think they might have the same stylist.

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Old 02-09-2019, 11:51 AM
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Surely there is a difference between foam filling between bulkheads that are 400 mm apart in "Palers" original question concerning his Kevlacat, than core filling in the case of Boston Whalers. Is it wise to remove a bulkhead and rely on foam filling as a structural replacement?
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Old 02-09-2019, 12:05 PM
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Originally Posted by paler View Post
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
I have just had a closer look at your photo of the bulkheads. You have bigger fuel tanks than what I have and hence one less bulkhead.
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Old 02-09-2019, 01:52 PM
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Originally Posted by Danf View Post
I have just had a closer look at your photo of the bulkheads. You have bigger fuel tanks than what I have and hence one less bulkhead.
These accidental saw cuts thru the stringer tops are not doing any favors for bending resistance athwartships. If these are thru the horiz glass altogether, they need to be fixed. The deck helps, but...
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Old 02-09-2019, 02:09 PM
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Originally Posted by 72barracuda View Post
In a Boston Whaler, I would say the foam is very structural.
and been doing fine using it since the 1950s



the pictures in this thread look just like the noosa cat we worked on a few years, guess aussies all build very similar.
stainless tanks, no flotation foam, water tight compartment, all glass deck
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Old 02-09-2019, 03:07 PM
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Originally Posted by tunnles View Post
If YOU HAVE TO RELY ON FOAM FOR STRUCTUAL INTEGRITY THEN YOUR IN TROUBLE AND DONT UNDERSTAND BOATS AND BOATING !

Any wonder why nobody takes ANYTHING you say seriously, regardless of how much knowledge you think you have or actually have. Completely typical and useless response on your part, when you decide to add something of value to the threads you post in, maybe (but I doubt) you might get taken somewhat seriously. Until then.....TROLL

I feel bad for the people that use this forum to get answers and their threads are totally derailed by you. I cant say what I really want to say or I'll be banned, but then again it might be worth it
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Old 02-09-2019, 03:59 PM
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Originally Posted by jamesbfishin View Post
Any wonder why nobody takes ANYTHING you say seriously, regardless of how much knowledge you think you have or actually have. Completely typical and useless response on your part, when you decide to add something of value to the threads you post in, maybe (but I doubt) you might get taken somewhat seriously. Until then.....TROLL

I feel bad for the people that use this forum to get answers and their threads are totally derailed by you. I cant say what I really want to say or I'll be banned, but then again it might be worth it
be careful what you say about tunnels..I talked some shit to him and a mod messaged me and said keep your negative comments off this site. All hail tunnels.
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Old 02-09-2019, 05:00 PM
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Originally Posted by wizness View Post
be careful what you say about tunnels..I talked some shit to him and a mod messaged me and said keep your negative comments off this site. All hail tunnels.
Ahh yes. The mods are certainly on their knees for him.

He's great clickbait. It's all about the traffic
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Old 02-09-2019, 05:16 PM
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Yes, it contributes a massive amount of rigidity even if it's lighter density foam, assuming the cavities are filled with it. Is it necessary...can you do without it? Possibly, you'll have to figure it out. Thats the Y in DIY. How many longitudidnal stringers are there and how tall are they? I'm not seeing much structure there, and expect the foam is pretty damn important but could be wrong. The advice you received about repairing cuts in the thwartships frames is right on. Fix them, and if it were me I would add at least a couple layers of lamination on top of all them. Eliminate frames? If you do you need to provide a good load path to take the place of it.

Last edited by skypoke; 02-09-2019 at 05:54 PM.
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Old 02-09-2019, 06:03 PM
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What size are those new fuel tanks?
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