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Old 25-01-2021, 13:07   #1
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Sailboat building and hull worthiness myths

Hi,

So I've been reading these forums quite a bit as there is a wealth of knowledge here regarding any number of things to do with the world of sailing. As someone looking to buy a boat in the next couple months, this knowledge has been invaluable. However, one thing I have continually come across is the myth of hull thickness corresponding to the boats strength, especially in comparing old boats to new ones. In particular, I have seen 'for a blue-water boat with a thick hull' or any variation indicating that a thick-hulled boat is inherently better. From what I understand, this is misleading at best, and flat out wrong at worst.

A little background; I am a young engineer, so I don't have the experience of many on this forum. I see this as having both strengths and weaknesses, as I am more accepting of new ideas since I don't have personal experience to delineate between new and tried and true. That last part there is the weakness; I don't have experience , only theory.

So now that I have established that, I this article is the best source I have found on hull construction: https://www.boats.com/reviews/fiberg...ng-a-laminate/

In this, I would really like to highlight the comparison between uni-directional fiberglass vs mat, woven roving and chopper-gun built boats. Obviously vacuum-bagging would result in a much lighter and stronger boat than one built without it, so I won't go into that. Anyway, the danger I mentioned earlier between saying a heavier boat being inherently stronger comes into full focus here, as I believe many if not all older boats were built using either chopper-gun, woven-roving, or mat fiberglass. Though in certain instances I imagine a mat etc. built boat would be stronger, by and large a lighter, thinner boat using uni-directional glass will be much stronger as its strength is equally distributed across all impact angles. Without going into the math of it, as admittedly my materials science is a bit rusty, I'll give an example. Think of Mat fiberglass as the cinderblocks people are able to break stacked on each other like this guy:
he is able to break all of those blocks as their resistance against a perpendicular force is very low, and as they are uniformly stacked on each other that force is linearly passed onto the next block. If these blocks were instead stacked across each other with the centers of each block varying at each layer, only the first block would have(probably) broken. I understand this is not a perfect example, but I hope it is accurate enough to illustrate the point.

I fully expect I am wrong on some aspects here, and am very open to learning other peoples inputs. However, I wanted to put this out there as I know there are many new and potential sailors who wouldn't know to consider this complexity. Please let me know what you think
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Old 25-01-2021, 13:30   #2
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Re: Sailboat building and hull worthiness myths

If you want to know how tough a boat hull is, haul the boat out, and smash it progressively harder with a hammer until it breaks. If it takes all your strength to punch through, and no water comes out of the hole, I'd say that's good enough for me.
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Old 25-01-2021, 13:33   #3
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Re: Sailboat building and hull worthiness myths

Don't sweat it.

Unless you are building a boat from scratch, the details of laminate schedule and such are really not worth the analysis effort. No two boats will be so similar that the most significant difference between them will be the laminate schedule.

My thoughts:

If you are racing, then the most important thing will be light weight, and stiffness.

If you are cruising across oceans, the most important property is toughness or impact resistance.

If you are coastal cruising, the most important thing is likely to be cost.

Each of those have different balance points based on how you are using the boat, and your need for price vs. safety vs. performance.
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Old 25-01-2021, 13:38   #4
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Re: Sailboat building and hull worthiness myths

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Originally Posted by Catalier View Post
If you want to know how tough a boat hull is, haul the boat out, and smash it progressively harder with a hammer until it breaks. If it takes all your strength to punch through, and no water comes out of the hole, I'd say that's good enough for me.
I don't think this test is included as part of your sea trials
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Old 25-01-2021, 18:48   #5
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Re: Sailboat building and hull worthiness myths

You might enjoy the Gougeon brothers' book on yacht construction. They go into some detail about various layers and strengths of different materials in hulls and other structures. https://www.westsystem.com/the-gouge...-construction/. I believe that it is now available online for free.
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Old 25-01-2021, 19:07   #6
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Re: Sailboat building and hull worthiness myths

Very few boats are built of only chopped-strand mat. Even the oldest cruising boats I've ever looked at were mostly woven roving, with mat only in the outside layers and occasionally between layers of roving. But even with a panel that is only mat, a thicker one will be more impact resistant, stiff, and strong in tension than a thin one. This is because there are more molecules holding hands.
For boatbuilding purposes, a strand of fiberglass doesn't have much reinforcing power until it's 3 inches long. Or, you could say that at 3 inches it becomes better enough than chopped strand mat to make it worthwhile. So to tab a bulkhead, you would use 6-inch cloth, giving you 3 inches of surface area on bulkhead and hull each.
But glass fibers are only part of the equation: available resins vary widely in physical properties, and lamination techniques and quality of labor also must factor in. So two panels made with identical layers of cloth can be completely different in strength depending on the other three variables.
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Old 25-01-2021, 19:46   #7
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Re: Sailboat building and hull worthiness myths

Quote:
Originally Posted by Catalier View Post
If you want to know how tough a boat hull is, haul the boat out, and smash it progressively harder with a hammer until it breaks. If it takes all your strength to punch through, and no water comes out of the hole, I'd say that's good enough for me.
Even with multiple cuts from an angle grinder on a relatively light layup like below, punching a hole in the laminate is tougher than most would expect. There is really nothing really ‘wrong’ with a heavier layup, you may sacrifice performance and speed but so will a poorly designed hull with a light layup. Many things to consider and as mentioned above not worth worrying about too much outside any coring what/where etc.

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Old 25-01-2021, 21:34   #8
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Re: Sailboat building and hull worthiness myths

Quote:
Originally Posted by Notyetsinking View Post
Hi,

So I've been reading these forums quite a bit as there is a wealth of knowledge here regarding any number of things to do with the world of sailing. As someone looking to buy a boat in the next couple months, this knowledge has been invaluable. However, one thing I have continually come across is the myth of hull thickness corresponding to the boats strength, especially in comparing old boats to new ones. In particular, I have seen 'for a blue-water boat with a thick hull' or any variation indicating that a thick-hulled boat is inherently better. From what I understand, this is misleading at best, and flat out wrong at worst.

A little background; I am a young engineer, so I don't have the experience of many on this forum. I see this as having both strengths and weaknesses, as I am more accepting of new ideas since I don't have personal experience to delineate between new and tried and true. That last part there is the weakness; I don't have experience , only theory.

So now that I have established that, I this article is the best source I have found on hull construction: https://www.boats.com/reviews/fiberg...ng-a-laminate/

In this, I would really like to highlight the comparison between uni-directional fiberglass vs mat, woven roving and chopper-gun built boats. Obviously vacuum-bagging would result in a much lighter and stronger boat than one built without it, so I won't go into that. Anyway, the danger I mentioned earlier between saying a heavier boat being inherently stronger comes into full focus here, as I believe many if not all older boats were built using either chopper-gun, woven-roving, or mat fiberglass. Though in certain instances I imagine a mat etc. built boat would be stronger, by and large a lighter, thinner boat using uni-directional glass will be much stronger as its strength is equally distributed across all impact angles. Without going into the math of it, as admittedly my materials science is a bit rusty, I'll give an example. Think of Mat fiberglass as the cinderblocks people are able to break stacked on each other like this guy.


He is able to break all of those blocks as their resistance against a perpendicular force is very low, and as they are uniformly stacked on each other that force is linearly passed onto the next block. If these blocks were instead stacked across each other with the centers of each block varying at each layer, only the first block would have(probably) broken. I understand this is not a perfect example, but I hope it is accurate enough to illustrate the point.

I fully expect I am wrong on some aspects here, and am very open to learning other peoples inputs. However, I wanted to put this out there as I know there are many new and potential sailors who wouldn't know to consider this complexity. Please let me know what you think
First things first. All other things being equal, generally speaking, that "a thicker-skinned hull is 'stronger' than a thinner skinned hull" is not a myth.

It is a fact.

The myth, if there is one, is that hull skin thickness is the only thing that matters in determining the strength, generally recognized as 'seaworthiness', of a particular hull or design.

I've never seen that proposed by anyone who has a basic understanding of the factors that have to be considered when building/evaluating a boat's hull, or any other structure.

If the referenced 'article' "is the best source I have found on hull construction", I suggest you look further because what I read when I went there was one of the most brief, though reasonably accurate, cataloging of some basic FRP construction materials and techniques that I've ever seen. With all the easily-available information out there, I'm sure you can find stuff an order of magnitude better by searching a little harder (I'm specifically not sending you somewhere for personal ulterior motives.)

I don't exactly understand the point that you're trying to make with the poor block-breakers (guess no-one's ever told them about the future), but some points hidden in the analogy can be drawn.

To begin with the main reasons the blocks are easily broken.

The first (obviously) has to do with the forces involved in resisting the (ideally) centered and vertical force impinged on the block by the unfortunate martial artist's elbow.

Those would be tension and compression; concrete (which those blocks, according to the colloquial definition [more on that below] aren't) excels in resisting compression loads, but fails miserably in resisting tension loads. Which brings us to why the slabs break so easily, and also the first part of the analogy with FRP.

When the block is struck (actually, when it's just sitting there as well) the top surface is in compression, the bottom surface is in tension. When the tensile force that the bottom surface can withstand is exceeded, the block 'fails' from the bottom upward. What should be taken into consideration is that the 'opposing forces' are not confined to the upper and lower surfaces, but exist in decreasing amounts to, roughly, the midpoint between the upper and lower surfaces. It should therefore be obvious that the thicker the block is (there's more of the strength-giving 'compression side' of the block to resist the breaking force), the stronger it is.

Here's that part of the analogy.

Given the above, imagine how strong the block would be if all seven slabs were mortared together. It's almost exactly the same with the FRP. The 'slabs' have the same function in the block as the layers (whether they're mat or cloth or roving or even individually blown layers of chop-strand) glass, the resin performs the same function as the mortar; fusing the layers into a composite whole (yes that why it's called a 'composite' structure) which obtains its strength primarily by increasing the distance between opposing sides, i.e. making it thicker.

Another part of the analogy regards the constrution of the blocks themselves. Concrete generally means a mixture of portland cement, sand, gravel and water. There are many relatively new additives available now, but we'll leave them out for this discussion.

The blocks we see so easily broken have no gravel, AKA aggregate, which is the primary strength-adding material. What they're actually made of is closer to mortar, though I assume the sand screened size is substantially large than would be used for, say, brick or block mortar.

Anyway, the point is that the aggregate, gravel, has the same role in making the concrete strong as the laminate does in FRP; within reason, and with the same binder (resin or cement/sand), the size and distribution of the aggregate/laminate within the composite has a crucial role in the strength of the parts made from it.

Whew.

This of course leaves out tons, from core and core materials to bulkhead and stringer spacing to incorporating rig forces into hull design to the expected range of hydro and aero dynamic forces within the designated life of the hull, but it should give an idea of the complexities involved in designing, building or repairing a 'seaworthy' hull.

The important thing to remember is that hull thickness is only one part of the system involved in constructing a boat, and while important, is hardly the most important. As noted in the word 'system' there can't be a 'most' important; they all are.

There are some principles that are most important though, and it is, of course, harder to rank them, and much harder for the individual to verify them after-the-fact.

In no particular order, quality control, adherance to design and designed parameters, actual design, material quality, adherance to construction techniques (really a subset of quality control), builder philosophy and I'm sure others I've missed are some of what need to be considered when judging any constructed thing's fitness-for-duty.

What is most important is that that duty be accurately judged and specified, a huge stumbling block in today's (often falsely) self-assured world...
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Old 25-01-2021, 23:16   #9
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Re: Sailboat building and hull worthiness myths

The comparison to concrete blocks is highly in appropriate to the situation. Wood planks would be better.

A thicker section will not only be stronger, it will have greater fatigue resistance which in the long run is more important.

Regarding the thread title, “worthy” of what?
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Old 26-01-2021, 09:31   #10
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Re: Sailboat building and hull worthiness myths

Currently 40 years old, my boat was designed in 1973, built in 1981 & is still going strong. I am confident it will continue to do so for at least another 10 years & probably a lot longer - can't see why not. I wish I could say the same about me.
How strong do you want your boat to be??
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Old 26-01-2021, 09:48   #11
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Re: Sailboat building and hull worthiness myths

Not to long ago as an experiment I took a scrap piece of Lowe’s blue foam, about 8”x12” x 1” and coated it with one layer of about 10 Oz glass fabric and polyester resin each side. Didn’t bother with the edges much. Amazed at the strength and rigidity of this sloppy fabricated test piece. Propped on the floor I could stand on it and by hand I could not easily flex it. Tension and compression, wow.
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Old 26-01-2021, 11:19   #12
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Re: Sailboat building and hull worthiness myths

To many variables. I think you would be better off asking about specific manufacturers than a strength of materials forum.
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Old 26-01-2021, 11:20   #13
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Re: Sailboat building and hull worthiness myths

I'm an Engineer fairly knowledgeable in composite design.. not a naval archy though.
1st "Strength" is not an engineering term and has no real value in an engineering discussion. For example, Kevlar is "stronger" in impact and shear resistance than Carbon fiber, But most layman would say carbon is "stronger" than Kevlar because it has higher tensile strength. You must define what strength you are talking about, tensile, impact, shear, bending, fatigue, etc. when discussing blue water boats, I think boaters are most concerned with impact resistance.

A common error is thinking vacuum bagging makes a composite "stronger" as in stiffer and able to take more bending and/or tensile loads, which it does not for a given number of layers of reinforcement (glass fiber typically). Hand layups result in less compaction and a thicker layup for a given number of layers of glass and thus results in a stiffer panel more resistant to bending (taller I-beam affect), though the tensile strength may not be much different. The hand layup will be heavier and more resin rich though and has more of a 'human' impact on repeatability.

For a given thickness, a vacuum bagged lamination will have more layers of reinforcement than a hand layup. So it would be stronger in both bending and tensile... there is a point where a thinner vacuum bagged layup becomes stronger in bending and tensile than a hand layup. Vacuum bagging is used now to improve repeatability, reduce emissions (infusion is required by law in some locations), produce a lighter less resin rich composite. So yes, there is a case for saying a vacuum infused boat of say 12 layers of 1708 thickness (~0.53in) is stronger than 11 layers of a hand layup which may be 0.6 inches thick... splitting hairs IMO

Uni-directional cloth is another can of worms. It's only used in specific areas where high directional loads are known such as in foils and masts and load attachment points. It is rare to see much unidirectional in production boat hulls. Uni directional tapes are not going to improve impact resistance. That's not what they are for.

Back to the "blue water boats" and thick hulls. In general, I think the concern is sinking or damaging your boat because you run aground or hit a dock/rock or someone drags into your boat and bounces against it repeatedly.. In all these situations it is the impact resistance of the hull panel that is important. Thick solid glass layups will always be better in impact resistance than a thinner layup or a cored layup, assuming equal chemical resin and reinforcement properties. Yes a thinner Kevlar/epoxy boat will be better at impact resistance than a thick polyester FRP boat, but nobody is building cruising boats with Kevlar because of cost. Tartan uses Epoxy resin, so may have an advantage there... you pay more. Coring adds stiffness, but with point load impacts, like a corner of a dock or rock, the outer skin can be pierced and impacts can result in de-lamination... then there's the wet core issues...


And yes, the boat structure is a system, as said above.

hope that didn't muddy the waters even more! haha
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Old 26-01-2021, 13:50   #14
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Re: Sailboat building and hull worthiness myths

Think about this in historical terms. The early fiberglass cruising boats were built with thick layups throughout, allegedly to maintain a similar rigidity to wood, but also to deal with the loads that are tied together through the hull. In those days they were not doing an engineering analysis - they just overbuilt. The result is a hull that resists flexing and impact, and should be very durable. Also, these old hulls were frequently built before the 1973 oil crisis that resulted in a change of polyester resin and the consequent blister problem. The caution here is that thickness does not guarantee quality of layup or design. For example, some friends had a boat that they learned had been laid up in two halves, then glassed together. Under the compressive load onto the mast step in the bilge the two halves parted - oops. I doubt you would see this in first-world construction but these things do happen.

Modern boats are highly engineered and put strength only where it is needed. Often an insert that provides a box structure is used to connect the keel loads to the hull and rigging. The result is a much lighter boat, which can be very fast while using smaller mast, sails, winches, etc., and also costs far less to build in quantity (for the length). The downside is that they are not nearly as impact resistant as the older thick hulls, and hence at least one manufacturer boasts of using kevlar to resist penetration. Also, they may not do well with unusual loads in unexpected places. But for 99+% of the time they are really superior in performance and cost. Personally I would not do world cruising in a boat that I can "oil can" the side of the bow with my hand, as some do (seen it), but I'd take the kevlar-reinforced ones.

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Old 26-01-2021, 14:05   #15
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Re: Sailboat building and hull worthiness myths

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Originally Posted by captlloyd View Post
Not to long ago as an experiment I took a scrap piece of Lowe’s blue foam, about 8”x12” x 1” and coated it with one layer of about 10 Oz glass fabric and polyester resin each side. Didn’t bother with the edges much. Amazed at the strength and rigidity of this sloppy fabricated test piece. Propped on the floor I could stand on it and by hand I could not easily flex it. Tension and compression, wow.
polyester is loaded with styrene which dissolves XPS foam.

Not sure how you achieved this.

Maybe you used epoxy?
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