Cold Molded Boats

Henning - Was that an expencive exercise having all layers Kevlar, to me the right way, but is it needed? Also, why did you use Carbon on the inside, why not more Kevlar?

SandEngXp - If you put the Kevlar on the outside layer, could the builder put a covering layer of cloth/CSM over the top?

Far

LightWeight

Yes. But normally when you go to Kevlar and/or carbon you are trying to maintain a very strict weight target. The glass (no CSM) will add weight but is an effective and usual way to protect the Kevlar on faired side.

Extremely expensive exercise but the guy wanted puncture resistance so the most protected and effective place to put it was between the wood strip layers. The reasoning behind the carbon inner shell was to add stiffness with the minumum weight penalty. I used plain S glass on top of the outside because that is where the majority of minor damage occurs. All in all, glass will absorb more damage in your typical sheering "rub", and it is the easiest of all the above to patch repair in a minor port of refuge. Plus it's a hell of a lot cheaper. I just couldn't see the sense in putting expensive material in a sacraficial position when a less costly option still met required spec.

Whether it was worth it is still unanswered, seems he hasn't run it up on a reef yet.

Does a Cold Molded hull flex at all? Or is it just as rigid as say a solid fibreglass hull?

Far

All materials flex but the degree is naturally relative to the inherent strength, design and construction. If you have ever spent time forward during a pounding (not recommended for taking a nap) you will see and feel that solid fibreglass flexs. A bit of 'give' is essential of course.

Well, the second question assumes that a solid fiberglass hull does not flex, and that is not necessarily true. All materials flex. How much they flex and in which ways is a matter of construction. For the weight, an engineered cold mold using advanced composite technique will flex less than solid fiberglass.

Assume I was, I hear of old Riv's flexing when they are in a pounding, but they were of a light construction. I though in general (for argument sack) e.g. A Bertram would be somewhat stiffer (older style solid glass Bert's), then a cold molded hull of the same size... Guess not. I'm also thinking it would also depend on the entry of the hull as well? Thanks guys.

In a pounding, I’m either steering, or next to the guy steering... I love watching the boat crashing through the waves… while everyone else is in the cockpit sea sick.

Far

Flex or Deflection in Engineering Speak

Deflection is inversely related to Modulus of the material of construction ( stiffness) and panel thickness and directly related to panel size and to a lesser extent panel aspect ratio. So I would need more information to answer directly. However, energy absorption requires deflection - so if it is within the design limit of the particular construction deflection is not a bad thing.

But excessive deflection may make you feel insecure.... ...kind of like a late 70's Ford Fairmont.

With the admin's kind permission, I would like to report on three cold-moulded hulls (over 60') built with a layer of 1200 Kevlar/E-glass hybrid cloth, covered by a layer of 1700 E-glass biax that are performing very well.

It is all epoxy construction, so the use of mat was not specified, and the hybrid cloth adresses the difficulty of Kevlar/epoxy bonding problems. All kevlar cloth has had problems as the Kevlar (Aramid) stretches and the fibers can seperate from the epoxy matrix without detection. The E-glass seems to bind the package together ans spreads out the loads much better.

A captain on one of the hulls reported hitting an 18" by 20' log at 25 knots in some rough weather that he missed seeing till too late. Only damage was a small fingernail-sized dent on the stem and some paint chipping. Log fared worse and broke into two pieces.

This is over a bottom of 3 layers of 1/2" AB marine fir plywood and sides that are Meranti plywood sandwiching a vacuum bagged layer of closed-cell foam (with perforations that bleed resin when pulled down properly).

Regarding your second question of flex, ABSOLUTELY! A cold moulded hull MUST flex to live. The strength of the hull is much the same as an eggshell (but not as fragile!). Impacts, loads and all are absorbed and transmitted around the entire hull, and any hard points will be stress concentrations and must be avoided.

As the late pioneer of cold moulding, Jim Smith once told me, he plans on his hulls flexing 4" over the length of the keel!

Henning, How much did it cost for just the hull to be constructed, including the carbon on the inside, and just main Bulkheads? Also, what size was the vessel?

Far

47' custom sedan. I didn't see the billing or negotiate for the materials on it, but I'd estimate there was $300k in the hull including labor. The Kevlar was more expensive than the carbon, I remember that. I do know I would do it differently now and have custom composite mat woven.

Gentlemen, please forgive me for chiming in to this discussion. Carl has graciously allowed me to remain on this forum and I have become more interested in the technical discussions after being drawn here by the Bertram hoopla. There are not a lot of people on the cutting edge of hull design, especially in the sportfish genre, and finding a network of individuals who are there is such a pleasant suprise. Without breaking any of the rules of the site, I would like to stay involved if possible and hopefully will be able to contribute positively.

I find the pricing of the Kevlar vs. Carbon curious, it must be a regional disparity. I buy a good bit of both, and the last rolls I purchased had the Aramid at about 1/4 of the carbon. Granted the Aramid was a 50/50 hybrid, and the carbon was unidirectional tapes but that is still a difference. Only recently have our suppliers resumed offering both for domestic comsumption, as our military was eating up all available mill output.

Your estimates for cost on a 47' Kevlar/carbon sheathed hull are almost exactly where we are here in the states right now as well.

Henning, would that make it more expensive, or around the same costs? Also, if you compared the costs to that of a standard cold molded hull, how much of a difference would there be (so leaving out the Carbon and Kevlar, and going for standard materials)?

Bill106, Welcome, more than happy to hear your knowledge on this subject, and any subject for that matter.
Also, you mention that there wasn’t much ‘cutting edge stuff’ in the S/F world, how would you, if you could, improve the technology? Would you leave cold molded hulls and get into cored vacuum bagged? Cheers

Far

Did you get that permission from Carl? Cheers

Far

It would leave it less expensive. If one would just glue it up and use E Glass, you might cut costs about 20%. Labor, capital costs and overhead are always going to be your greatest construction cost.

Thanks guys, I assume (and I know what that can stand for!) Carl "permitted" me by not deleteing (yet?). Thanks Carl!!!

I do believe the speeds "sportys" are going now are making it essential to switch from traditional methods to advanced composites but they can ONLY be used to their fullest advantage by vacuum bagging or infusion, just slathering on a layer of Kevlar doesn't do it.

As far as advancing the construction, why not combine the best of both cold molding and vacuum bagging? Both have different applications where they shine. (Here's where Carl may censor me but I hope not, I'm NOT advertising only trying to share ideas that have worked!) I have built four hulls now that are a hybrid of cold molding and bagging with "traditional" bottoms and cored sides. With each one I have incorporated more core in different locations and I keep trying to find new places it is "better" than wood. Built the first one for ourselves 12 years ago and it's still going strong!

As Henning said quite correctly, labor and overhead are the biggest costs, but what most people don't realize is you can vacuum bag many components and actually lower your labor costs, once you get over the learning curve, and build a far stronger and lighter boat.

You bet, not only does it make doing a lay up quicker, it also takes out a few skill/human issues and inconsistencies as well. Less man hours, and lower cost labor for the same level results = savings.

Just figured out how to load pictures, here are a couple to illustrate what I meant by combining cold molded with bagging and composites. The first is bagging a foam core between ribbands prior to adding the inside skin (also bagged).

The second is a honeycomb deck just after peeling the bag.

That was about five years ago and we have "discovered" many more places to incorporate composites since!

Nice Pics.... ...shop too.