Plywood vs. Double Diagonal Cold Molding

It makes multi-chine hulls more rounded. The double curvature will produce a stiffer panel too.

 

There's nothing usual or unique about the radius chine method and Dix didn't invent it or modify it significantly. It's simply putting sheet goods to efficient use. Metal builders, have been doing similar for generations to soften bilges (any other stuff). Good engineering always seems to transcend applications and material choices.

 

Paul, I've thought some of the steel radius chine boats look really good. Been a long time since I looked at the process, do the builders add any compounding to the chine?

 

So it sounds as if, pretty much, there's little strength advantage of one over the other. Nor much else in the way of perks in terms of cold molding, aside, perhaps from what shapes one can develop with it.

Albeit there's a bit of disagreement over which method has the upper hand when it comes to resisting the primary loads on hulls. Where I, personally, am of the understanding that the biggest loads (in a sailing vessel anyway) are on the fore & aft, and transverse axis, with diagonal loadings coming in in 3rd place. Much of this due to rig loading in Bermudan & similarly stayed rigs.

So that with plywood's 0-90 fiber orientation it actually would be better at resisting these loads than if the primary structure of the fibers in the hull are laid on diagonally. Particularly if some reinforcements in the +/- 45 are laid onto the hull of a 0 - 90 (plywood) planked vessel. Such as a few layers of biaxial cloth when sheathing the vessel, in addition to any other sheathing on the outside of the hull. For water & or puncture resistance, to sheathing being added to increase a hull's stiffness, etc.

Kinda' makes one think that in order to resist the primary loads in a sailing vessel, there has to be a greater, overall, mass & thickness of structure in a diagonally planked vessel, than there does in one made of plywood. Given the nature of how the primary fibers are laid up, vs. the loads which they must resist.
Or in simpler terms, a diagonally planked vessel would need to be heavier than a 0 -90 planked vessel to achieve the same levels of strength & stiffness.

Comments & Refutations?

 

You're making some unjustifiable assumptions. First of all, there are significant advantages and disadvantages across the methods and this would include less tangible things such as, technique familiarity and friendliness, tooling, skill sets required, comparative build times, material costs, complexity, etc., etc. These are aside the inherent physical property coefficient comparisons, which are also differing among the usual applications.

In a nut shell, it's important to compare apples to apples, so the options to an ultra light, high performance speed demon will be relatively few, while a cruiser can explore and justify several build options, based on the previously listed other variables.

Taking general guesses at load paths of generic vessels, isn't a reasonable discussion, but orienting fibers can greatly reduce weight requirements, for given modulus requirements. Some generalities can be surmised to a degree, but for the most part, if really concerned about getting the most from the material and method choices, you have to do at least some of the math.

Of these generalities, you can assume molded veneers (double or more) will be lighter per modulus than plywood, simply because of the veneer orientation and the obvious differences between solid wood and plywood. Ashcroft would be the weakest of the cold molded veneer types, again because of fiber orientation, though as mentioned, you get a big build speed gain (more other variables crap again). A direct solid wood diagonal mold/plywood mold build comparison, would place the solid wood fibers in relatively complimentary directions, while still provided cross grain stiffness, while the plywood mold would have at least 1/3rd of the internal veneers in opposition to load paths.

The really lightest "wooden" build method would be the "Lord" method, which is really a wooden core/strip plank type of build, not molded and the resulting hulls are quite flexible, but strong and durable. It takes as much engineering to get the Lord method at it's ultimate level, as it does any high end cored composite, whereas general rules of thumb can be applied with some of the other, more conventional methods.

 

I made another visit to Jarrett Bay about a week past. The 90 foot sport fisherman is moving along and launch is projected at Spring 2017. There were a few new things to notice this time. The hull bottom is not laminated plywood as are all the other local boats. It is a fir planking/glass sandwich. The builder said that this came out as best when all variables were considered. The foredeck mold was set up in the shop and many of our houses would fit under it.

A most interesting feature that I have never seen on any similar boat is a foil. The foil which is calculated to decrease drag is a horizontal "T" configuration of about 6' span located just forward of the CG and about 3' below the keel. It will be carbon and fixed in position. I guess foils are finding the way into just about all watercraft these days.

Weight control is a major concern on many current planing boats. Non structural elements in the interior are made from very light open core hardwood veneer sandwich panels which can be made into curved shapes where desired. The final effect is beautiful but not waterproof. Speed projection for this boat is north of 40kts and cost will also be north of a big number.

Many forumites may be shocked that pleasure boats with building costs of $15M are being built of wood, and plywood at that. My amazement is that there are people with that kind of money to spend on extravagances. No matter, lots of local people are making a good living satisfying that desire and using their skills to good advantage. May not be the ideal situation but better than many outcomes.

 

Sam Devlin uses plywood for double diagonal planking on his largest vessels.

http://store.devlinboat.com/devlinsboatbuilding.aspx

Vicem Yachts are cold moulded.

http://vicemyachts.net/

https://www.youtube.com/watch?v=6TuXUyweVyE

 

Photos and other information about the Jarrett Bay 90 build are at
http://www.jarrettbay.com/carolina-construction/custom-yachts/hull-62/
and
http://www.jarrettbay.com/2014/10/j...ruction-its-largest-custom-sportfish-to-date/

The photos show the planking as several layers of plywood, cut into strips and applied diagonally. Bottom stringers are carbon fiber.

 

Let me say a couple things now & leave the rest until later. I'm going to assume we're talking BS1088 marine plywood here, either Lloyds certified, brand=Bruynzeel, or both. I'm open to the idea that brand=Joubert might be sufficient (I know boat-builders on both sides). Apart from Lloyds I don't know anybody that certifies BS1088 grade claims.

The glue is generally different:

• for plywood: phonol-formaldehyde
• for cold molding: epoxy
• I think resorcinol-formaldehyde can reasonably be substituted in either case, but it seems to have fallen out of favor.

It may be that phenolic & resorcinol glues do better when they can cure under direct pressure/clamping. Even pressure is more easily achieved when working with flat sheets.