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Old 23-01-2009, 06:01   #1
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Wood vs Foam core

Pros, cons, personal thoughts/preferences?

I woke up at 3 AM and this was the first thing that popped into my head.

Please keep in mind I am a complete novice, so you have to explain things a little.
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Old 23-01-2009, 06:54   #2
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for re-coring a deck? You need something strong, but lightweight. You want to keep the weight down above the waterline for stability reasons.
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Old 23-01-2009, 10:09   #3
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Quote:
Originally Posted by twisty View Post
Pros, cons, personal thoughts/preferences?

I woke up at 3 AM and this was the first thing that popped into my head.

Please keep in mind I am a complete novice, so you have to explain things a little.
I presume you are meaning balsa for the wood core.

The down side to balsa is, if there is any water intrusion, it can rot. Depending on the grain, it can also spread the water of a larger area before it rots. However, if there is no water intrusion, this is a null point. This is why you see builders use balsa with epoxy which is then covered in something like fiberglass. This is also the reason you hear about end grain balsa; there is the added benefit of impact resistance. It's sort of like grabbing a handful of straws tightly. It's light, it's pretty rigid and not too expensive. There are also precut sections that you use to make complex curves. Pretty good stuff overall.

But there is that concern about water intrusion and rot.

As for foam, it tends to be even lighter and faster to work. Some foams are pretty much impervious to water. You can make complex curves too. The speed of working gives it a huge advantage for production and semi-production builders because, as with so many other things, the real cost of building a boat is not the hull materials or the goodies but the labor hours required to put it together. Same same was true for fiberglass. Chopped strand mat is still pretty solid (but not as solid) as hand-laid glass. But it's cheaper.

The down side to foam is the volatiles that can off-gas and there have been lots of cases of de-lamination under heat and/or cyclic stress. The unanswered question is this: Are the delamination cases due to poor quality control and or design ... or the material? I would say since you don't see it on all boats the fault is in the building and/or design.

Both have pretty good insulation, at least when compared to steel or aluminum.
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Old 23-01-2009, 12:02   #4
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I presume you are meaning balsa for the wood core.
Yes that is exactly what I meant. Anything else I should know?
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Old 23-01-2009, 13:01   #5
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Foam is around 1/2 the density of balsa, balsa has more than 3 times the compressive strength and many times the sheer strength of foam.

Balsa is more resistant to fire.

You need to keep water out of both.
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Old 23-01-2009, 20:52   #6
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Also consider polypropylene honeycomb
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Old 23-01-2009, 21:03   #7
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Also consider polypropylene honeycomb
Just anecdotal evidence but I have seen lovely little hexagonal paterns all over a million dollar plus cat made from Polyprop honeycomb, it's the most noticable case of print through I have seen

I would also have some concerns over whether or not the air could expand enough to aid in delamination.

For internal (out of the sun) structure, I reckon its fine

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Old 23-01-2009, 21:05   #8
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Is this a new build twisty or a repair?
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Old 23-01-2009, 21:26   #9
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Is this a new build twisty or a repair?
Neither really, I am still trying to decide what type of boat to buy and what the best options within each category, be it sailboat, catamaran, trimaran or power boat. Although I got a look at a cat today that has me really doing some thinking, it wasn't for sale but it was more of an educate myself trip anyways.

Honeycomb Poly was dismissed as an option long before this thread was even thought of, even I could foresee problems with it.
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Old 23-01-2009, 22:27   #10
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Yes that is exactly what I meant. Anything else I should know?
Yes. There are also three other types of wood construction that are pretty similar. They aren’t used in production boats that I’m aware of, so this might not be anything of consequence.

The first is marine grade plywood sheet using standard chine construction. The second, constant camber, is the cut the (thin say 1/8”) sheets up from them around a curve which is a generalized hull segment and epoxy them together. The third, cylinder molding, is to keep the sheets thin and whole and form them around a ˝ hull form.

All of these coated in epoxy and then clad in fiberglass. You’ll often see them on the brokers’ boards listed as composite construction.

As for the honeycomb material, I think there is a great use for it for the bulkheads, floorboards, hatchcovers and table tops. If you aren't going custom or doing any of this yourself -- it's a moot point.
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Old 23-01-2009, 22:34   #11
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Yes. There are also three other types of wood construction that are pretty similar. They aren’t used in production boats that I’m aware of, so this might not be anything of consequence.

The first is marine grade plywood sheet using standard chine construction. The second, constant camber, is the cut the (thin say 1/8”) sheets up from them around a curve which is a generalized hull segment and epoxy them together. The third, cylinder molding, is to keep the sheets thin and whole and form them around a ˝ hull form.

All of these coated in epoxy and then clad in fiberglass. You’ll often see them on the brokers’ boards listed as composite construction.

As for the honeycomb material, I think there is a great use for it for the bulkheads, floorboards, hatchcovers and table tops. If you aren't going custom or doing any of this yourself -- it's a moot point.
A lot of the Kit Cats are composite construction right? A kit is not a direction I will ever head in, don't have the patience, but I don't really want to buy someone elses mistakes either.
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Old 24-01-2009, 00:32   #12
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A lot of the Kit Cats are composite construction right? A kit is not a direction I will ever head in, don't have the patience, but I don't really want to buy someone elses mistakes either.
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A lot of the Kit Cats are composite construction right? A kit is not a direction I will ever head in, don't have the patience, but I don't really want to buy someone else’s mistakes either.


I think what you are saying is that a lot of the owner-built boats are one of those last three materials. That's true. I would argue they aren't kits though but are instead plans by a designer or naval architect. They could be built by either a company or individual.

I certainly understand your desire not to build a boat. You really have to have a burning desire to make something beautiful or that there no way you could afford what you want. Surprisingly, this is true for lots of the designers. That alone should give you pause to reconsider a blanket statement about owner built boats.

As for the buying others mistakes, that’s what a knowledgeable surveyor is for. That holds for almost all boats. The truth is the build quality varies. Some companies have excellent reputations and can command a premium. Others are delaminating at the docks. The thing about owner built boat is while you might get a bad boat, you might also get a truly superior one (think rikki-tikki-tavi) built by someone who built with far greater care because there was no worry about the profit factor.

Again, the choice to go with one material (or construction method, designer, etc) is yours but you may be ruling out some outstanding candidates on an a priori basis.
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Old 24-01-2009, 06:15   #13
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I think what you are saying is that a lot of the owner-built boats are one of those last three materials. That's true. I would argue they aren't kits though but are instead plans by a designer or naval architect. They could be built by either a company or individual.

I certainly understand your desire not to build a boat. You really have to have a burning desire to make something beautiful or that there no way you could afford what you want. Surprisingly, this is true for lots of the designers. That alone should give you pause to reconsider a blanket statement about owner built boats.

As for the buying others mistakes, that’s what a knowledgeable surveyor is for. That holds for almost all boats. The truth is the build quality varies. Some companies have excellent reputations and can command a premium. Others are delaminating at the docks. The thing about owner built boat is while you might get a bad boat, you might also get a truly superior one (think rikki-tikki-tavi) built by someone who built with far greater care because there was no worry about the profit factor.

Again, the choice to go with one material (or construction method, designer, etc) is yours but you may be ruling out some outstanding candidates on an a priori basis.
I should have been more specific, yes owner built, with his or her own hands. Now a 'kit' by a builder I would probably consider depending on a bunch of factors obviously experience means worlds.

If you google RotKat there is a detailed photographic build of what turned out a beautiful boat but there are also statements of how when finishing up the fairing he had to make the choice between good and good enough. Those are the type of things that give me pause that I am trying to avoid. (And before I get flamed I understand that the final fairing can be extremely tedious and you have to draw the line somehere, I am just using that as an example because I can find no other.)
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Old 24-01-2009, 09:58   #14
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Twisty,

Here are some thoughts:

Solid fiberglass (no core). No core problems! But you want it thick enough that it hasn't fatigued due to flexing. Very early fiberglass boats built in the '60 and '70s were overbuilt because people didn't understand the material very well. These 40 year old hulls are as strong as new. Since the gas crisis in the early '70s it's become much more expensive to build solid boats so many "price" builders moved to cores - at least in the deck. Also, boats that need to be light (catamarans) need to use cored construction for performance.

Blistering bothers both solid and cored boats. This happens when water get into the fiberglass skin and reacts with the resin. Most common in boats from the '80s. This could be fixed but it was expensive. Boats built in the last 15 years or so have epoxy barrier coats and use more water resistant resins so the problem has pretty much gone away. The good news is that if the boat is more than 10 years old and hasn't blistered (or blistered but been well repaired) it's not likely to start.

Cores:

Very early or very cheaplly boats sometimes had cores of plywood or mystery wood. You're unlikely to find a boat with this core that hasn't either been repaired or been cut up with a chain saw.

Fiberglass with end grain balsa is usually fine if the builder was a quality builder who made sure that they got good adhesion between the fiberglass and the balsa. During the last 15 years most cored boats were built with a construction method that used a vacuum to push the fiberglass hard onto the core. This has helped a lot but the old hand method was fine if carefully done.

The biggest problem with cored construction is where a hole is drilled to mount something and the core wasn't removed near the hole. Unfortunately this is a very common problem with lower quality builders or when later owners weren't always watchful when someone approached their deck with a drill. Even a hole for a bimini snap can cause problems.

Having said that, the great, great majority of end grain balsa cored boats from reputable builders don't have a substantial problem. Surveyors are pretty good at finding core problems. This is probably the biggest "walk away from the deal" survey problem. You do not want to take on a major core repair!

Foam cores were viewed by many to fix balsa's problems but has problems of it's own. Again, a good builder will turn out a trouble free boat. Foam is less bothered than balsa by the "drilled hole" problem.

Here's a petty scary site with lots of pictures of badly built boats. Don't let it discourage you. Almost all of the examples are from lower quality builders and most are fast motor boats (where slamming puts even more strain on a core). Rule #1 - buy the builder not the boat.

Yacht Survey Online: David Pascoe, Marine Surveyor

Carl
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Old 24-01-2009, 14:14   #15
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OK, my preferred build method on home built boats is strip plank composite using Kiri or WRC.

The planking adds large amounts of longitudinal strength allowing less glass and epoxy to be used compared to foam and balsa.

The cost is also considerably less

Solid glass is NOT an option for a performance cruising cat

Dave
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