Cold Molded Hull Strength?

Gents,
Hopefully this hasn't been asked & answered too many times, but... I often read (and have read of) where Cold Molded hulls are stronger than Fiberglass.
When I look at the numbers for the physical properties of wood, vs. those for a fiberglass laminate of even moderate quality, it leaves me scratching my head.

So... I'd appreciate any & all enlightenment on this one. Including links to: articles on here, ones to info & wisdom elsewhere, and periodicals or texts which might be worth me taking a look at or purchasing.

Ah, also, if folks would be kind enough to share their resources in terms of veneers, & especially lower cost epoxies, I'd surely appreciate it. The big (epoxy) names, leave a sour taste in my wallet

I'm not 100% sold on cold molding as a building method, but would like to be able to make educated decisions when it comes time to pick a design, & it's build method.

Thanks in advance!
Andy

 

The "Hull Strength" depends on the material used but is not directly proportional to its mechanical properties. I mean, a steel hull can be very weak if miscalculated.
What you need to do is choose a method of construction, depending on our facilities, budget, final appearance, or many other things, and properly calculate the structure to have sufficient strength, depending on what you want to do wih the boat.
Needless to build the strongest but the most suitable boat.

 

To start with, wood properties vary enormously between species. Secondly, "stronger" is a rather meaningless term used colloquially. For the same weight, wood is stiffer than fiberglass (polyester/glass). Fiberglass has more abrasion and puncture resistance. Depending on what characteristic you are looking at, the results will be different. Also, as TANSL points out, the design has to be made for the materials used.

 

Uncivilized,
At the very least, you should download the free copy of the Gougeon Brothers book "The Gougeon Brothers on Boat Construction." Google that title, and you should be directed to the WEST SYSTEM website. I just tried it, but it seems to be down this morning. Try again later. This book is THE manual of modern wooden boat construction, and it includes cold molding, strip planking, and plywood construction. And, of course, it covers the use of their epoxy resins, which, granted, are some of the most expensive available. They are also the best.

As for strength and stiffness, wood-epoxy construction has lower strength, material for material, than fiberglass construction, and this is due primarily to the differences in glass weaves and wood grain, which are the determinants for strength. Stiffness wise, fiberglass and wood are about the same, material for material. What happens, however, is that wood-epoxy (cold molded) boats generally have thicker skins and more internal structure than fiberglass boats, but wood-epoxy is generally significantly lighter. So you can make up for wood's lack of strength by making the structures thicker. This also makes them stiffer just due to their structural geometry.

It is very difficult to compare apples to apples between wood and fiberglass because wood boats are built differently than fiberglass boats--different hull and deck skins, different internal structures, all of which define overall boat strength and stiffness.

So, as TANSL suggests, you really should not decide about building a boat based solely on strength or stiffness between fiberglass and wood-epoxy. Select the type of boat and method of construction that suits you best. You can make very strong and stiff boats with either fiberglass or wood-epoxy. The Gougeon Brothers book mentioned above gives lots of practical advice about how to set up a boatbuilding shop, how to estimate construction, and where to buy materials. Get a copy of WoodenBoat magazine or go to their website to find resources to buy veneers and resins. You'll get lots of help there. Link: http://www.woodenboat.com/. They also have a forum there with people with huge amounts of experience participating.

If you need to learn more about the techniques of wood boatbuilding, consider going to the WoodenBoat School and take some 1-week or 2-week classes to bone up on skills and collect information from other students and the instructors. Link: http://www.thewoodenboatschool.com/.

In Michigan (if that's where you are from), there is also the Great Lakes Boatbuilding School in Cedarville, MI, just east of St. Ignace. Link: http://www.glbbs.org/.

I hope that helps.

Eric

 

I dunno guys... I'd put up my Cedar Strip Adirondack Guideboats up against any Fiberglass hull for strength. I'm not going to argue with Eric on actual numbers strength, he is an engineer/designer and I am not. I wil say, however, that a Fiberglass boat has to actually be built correctly in the first place to call itself superior to a cold-molded wood hull. Having just been to the San Diego International Boat Show just a couple weekends ago and seeing the latest offerings of Sailboats & Yachts (Beneteau, Bavaria, Jeanneau etc). I was pretty dissapointed in the overall lack of quality control in the assembly of these $250 to $750K production boats... total lack of attention to detail, cutting corners, and cheap materials. Now this applies primarily to Deck fittings, rigging, and interior, its not like I could 'see' most of the hull itself or inspect a cross section, but when I find shitty workmanship on what I CAN see then I begin to fear what is lurking where I CAN'T see.
I'll admitt, I'm a bit of a wood freak... I think its the only material worthly of a Sailing Yacht. Of course, when we use laminate construction (ring frames) and cold molded hulls we are still using epoxy and putting on layers of glass on the outer hull and in the bilge, so its not pure wood contruction, its the best of both worlds. Also, both fiberglass and col-molded techniques benifit from vaccum bagging for optimum wet-out, reducing wieght and waste.
If you want to see where the 'pinnacle' of cold-molded wood contruction is happening today check out Spirit Yachts and Fairlie Yachts, both in the UK. Read the reviews on each of the Spirit Yachts, it will mention hull thickness on a few of them I believe. In the US, look at Van Dam from Boyne City, MI. They mostly do power but they have done sail in the past, but always wood. If anyone has any suggestions on more US builders who use modern wood techniques (not traditional, that is a different list) let me know because I'd love to see more myself.

 

CloudDiver, you've expressed your opinion and is as respectable as any other. However, the reasons you give, from the technical point of view, have no base.
My opinion, as valid as yours, is that no designer, experienced in the design of structures of ships may give you the reason. Because, well, let me ask some questions to focus the topic: What does it mean for you "hull strength", which means a boat is stronger than another?. Why you need a boat to be more resistant than another, for the same function?.
All vessels must be resistant as necessary, but no more.
There are mindsets that would lead us to the a solid boat to make it "stronger".
"Hull strength" does not mean thicker plates and larger reinforcements
A boat can have a large "hull strength" although the fittings are very bad.
It is difficult, I would say impossible, to know a boat engineering and quality workmanship of a laminate when the hull is finished. You should conduct some tests to evaluate characteristics that at first glance can not guess.
Engineers are used to check things out, although at first glance seem good. It is a grave mistake to assume that 10 parts are good (or bad) because 3 of them are good (or bad).
IMHO

 

As you guys can tell I love going off on tangents, lol! Anyway Andy, to answer your question more to the point...
Eric's suggestion of the Gougeon Brother's book is excellent... I wasn't even aware it was available for free download, so I will be looking at that myself.
After that utilize your public library. Hopefully you have a good size library in your city or county. San Diego just finished building and opening a new city library in 2013, it rivals the New York public library and is fantastic. I will try to pull my reading list from my history of check-outs, some really good books in there. As for Amazon, my shopping cart only contains books that are not carried by my library, I'll post those as well. Several of these books have chapters dedicated to various species of wood used in boat building, their strengths, wieghts, rot resistance, and practical uses.
For expoxies, there are a few other brands out there that are of equal or near equal quality to WEST system, but I have never found them to be that much cheaper. Epoxy is one of things you simply can't cut corners on. West System is simply the highest quality and has a variety of hardners, the slow and extra slow being very useful in laminate and cold molded construction. You can download a copy of thier full line product catolog which has detailed information and its quite educational. For comparison; I always keep a quart or two of cheap *** Bondo brand expoxy in my garage for general fiberglass repair (non-marine use), and I can tell you without a doubt that the difference between bargain brand expoxies and WEST System 105 is painfully obvious!
Also another note on libraries, if you don't have a good size (well stocked) one close to you, but you are near a college or universty the school will often allow the public to use their library services for a small membership fee... its worth it!

 

TANSL, I think some things are getting lost in translation here... My statements are not from the perspective of an Engineer because, as I said, I am not. My inspection of workmanship is quite a different perspective and based on experience not just in quality control but also in the functional operation of the end product. That is my 'base', so when I see poor quality work it is evidence that the same poor quality exists elsewhere... this is not an opinion, it is common deductive reasoning.
No offense to the Engineers here, but sometimes you have the tendancy to 'Nuke' the problem. To break it down to a simple yes or no answer for the OP; Yes, Cold Molded hulls (when built correctly) are very strong. For the purposes of the OP selecting a design and (I am assuming) building it himself, wood is probably the best material choice as I doubt he will construct a female mold for one hull, while a male mold for fiberglass is nearly the same process as a cold-molded male mold but more time consuming as easier to F up!
I was just on a wooden yacht a few weeks ago, a 40 foot Crowninshield built in 1910 named 'Fame'. Over 100 years old and still nearly 95% orginal wood, very well cared for and only major restoration in 2010. She is not cold-molded construction, she is traditional wood. I think this just speaks to the long term qualities of wood when done right, and she ain't hard to look at either!

 

CloudDiver, I still think you are wrong in what you say, I have explained why I think so, and I asked you some questions you did not answer. I do not care, I think it's a sterile debate that the OT is not interested at all
Going back to what the OP posed, that is:

Can not say without more, that "Cold Molded Fiberglass hulls are stronger than.". It is necessary to analyze many other things to say that the structure is or not stronger tan .... and, above all, explain why. I think this is what the OP requested from us. Precisely by being an engineer, I have to say it depends.
The physical properties of the materials do not ensure, at all, resistance of a structure. Affirm that's a simplification indicating little knowledge of shipbuilding, and can lead to OP´s totally poor decisions.
Once defined the type of boat, what to use, etc.. you can choose the material to use, not vice versa (I'm not talking about the case where you just have a material and want to make a boat). According to the tools at your disposal, and your experience, it may be more appropriate or less a construction system and, depending on such a system can be more or less suitable a material.
In summary, for our OP: Cold Molded Hulls do not have to be stronger than Fiberglass.

 

TANSL, I didn't realize I was supposed to be answering questions for you? I still think you are not getting the big picture and you are nuking the question.
I wrote a longer response but deleted it, it is pointless to debate this with an Engineer. Sometimes people get stuck on 'paper' and can't see the forest through the trees.
To the OP, if you are going to build your own boat, Cold Molded is the way to go. You will like working with wood better and you will make a far more beautiful end product than an ugly *** white plastic 'fat lady'. If plastic boats are your thing, better off buying one than building anyway.

 

If you want, we talk about girls, "fat lady" or stylized lady. It is also an interesting topic.
I answer exactly something the OP has raised directly:
1.- NO, you can not say without further study things, that the Cold Molded hulls are stronger than GRP hulls.
2.- A stronger material does not have to produce a stronger hull.
Not needed an engineer to know that.
By the way, my greatest experience is in metalic structures, hulls. I mean, I do not defend the GRP. What I defend is the right material for each case. Have I explained my self clearly?
Cheers.

 

TANSL, I never doubted your answers to the OP's question. What I take issue with are statements you make to me such as "the reasons you give, from the technical point of view, have no base" & "I still think you are wrong in what you say".
1. I never said Cold Molded wood was stronger than GRP, so there is no need to argue or try to prove me 'wrong' or convince me otherwise.
2. You need to go back and review my point about 'deductive reasoning', because you obviously have none.
3. "Affirm that's a simplification indicating little knowledge of shipbuilding, and can lead to OP´s totally poor decisions" The OP is asking the question and looking to learn more about different building techniques so he has not made any poor decisions, he has made the best choice by choosing to expand his knowlege first.
4. I think its plausible that you have difficulty understanding your written statements to me, as well as the stement above in regards to the OP, are abrasive and quite rude. Perhaps that was not your intent at all since English is not your first language. Trust me, my French is horrible so I refrain from writing in French. In all seriousness, the way you come off makes you sound like a jerk.
I think my 'base' is much broader, firmly planted in practical application and experience than you give me credit for. Your condecending tone practically screams that you like to force your education and experience on others without giving them the benifit of the doubt as to thier own level of of knowledge and experience.
As I said, that may not be your intent at all, some things can be lost in translation. So I'll just leave you with something my mother taught me as a child; "It's not what you say, it's how you say it."

 

The problem with using "strength" as a description, is that it is vague. An inflatable will take a harder pounding than a steel hull of the same displacement with no damage. A pocket knife can shred it into little pieces. You need to use strength in the context of a particularly characteristic for it to have meaning and be able to be discussed.

 

CloudDiver only intervene to clarify that I have expressed an opinion on the OP in no time. I just pretended that he had adequate answer to any of his questions and some answers not induce him to wrong decisions. To him, the OP, if I have done wrong, my apologies. To you, once again, and with all due respect, I think, in general, you're not right. I do not know if I've used a polite "form", but it's my opinion.
Maybe Gonzo is right to say that talking about "strength" is a vague term.
I will not continue this pointless discussion, I just wanted to pay my respects to the OP, if necessary.

 

Andy,
Let's get a little more specifc so us nerds can stop arguing over the word 'strength' in general terms... So what type of Boat are you most interested in?
Sail? General Cruising, Day-Sailer, Racing?
Power? Fishing, water-ski, casual cuising?
Do you have an idea of Length overall (LOA)? Beam?
Location and weather conditions; I am assuming Great Lakes(duh), but maybe you are planning to retire to the carribean on this boat?
Where you will roam and where you keep your vessel will make you consider draft restrictions. Skinny water? Keeping in a slip or trailer every time?
I am also assuming based on your post you are looking to build it yourself. Do you have a space large enough to work in? What is your general level of experience with Carpentry type work? Tool assets?

All these things can help people provide more focused suggestions... In general, for any application of a single hull you will be building yourself, I say Cold Molded. Based on a few more details you can provide, let's see what others will offer for advice.