Cheapest and easiest building material

I've spent a little time looking for scantling rules for plywood boats.
Gerr (Elements of Boat Strength has a little info, but suggests that the hull thickness should be the same as that used for conventional plank on frame construction, which would seem somewhat overkill if the boat is going to be sheathed in FRP / epoxy (Gerr doesn't go into this).
In fact there seems to be precious little info anywhere about scantlings for boat built this way - ply sheathed with FRP/epoxy.....:?:

 

The Lord plywood method of construction will yield a lighter and reasonably fast build then conventional, ply on frame or taped seam methods. Personally, I vote for Ashcroft with solid lumber planking and no sheathings as the fastest and easiest to build. You don't need cloth set in goo for a water tight hull or to prevent rot.

Mat-C, you use carvel planking scantlings for plywood hulls, because though plywood is more stable then the solid lumber it replaces, it isn't as longitudinally stiff. Since most wood skinned hulls require the planking bear the majority of longitudinal loading, the plywood scantling is about the same as it is for carvel. In fairness you can reduce the plywood planking by 10% with little harm, from Geer's recommendations in "Elements of Boat Strength" on taped seam compared to carvel and 20% in lapped hulls. By Dave's own admission, the scantling rules he developed are for wholesome craft.

 

par I am kinda intrigued

In your gallery you have lots of renderings
Can you show examples of your actual work? can you if asked back up your posts with references by happy owners Just wondering, you see , perhaps we can change virtual; to real
regards
Stuart , alias Lazeyjack

 

These Scantlings Manuals may help.

http://www.macnaughtongroup.com/publishingown.htm


Pericles

 

PAR, unlike some others in this forum, I not shy about asking a dumb question - 'cause I'll probably learn from it - but I have never heard of the Lord plywood method. Googling it throws no light either... Could you please elaborate a little? It would be greatly appreciated!

 

Trevlyns - Gerr gives an adaptation of the Lindsay Lord scantling system in Elements of Boat Strength though I've not come across the original published rules. It's an interesting system that allows the boat to flex. I'd be somewhat hesitant about employing it for reasons of longevity myself - though Gerr certainly seems to endorse it.

PAR - thanks - yes I understand that because only 1/2 the grain runs one way, that ply has only about 1/2 the tensile strength of the same timber in lumber form. But whereas Gerr gives quite thorough explanation of how to reduce core thickness when FRP/epoxy sheathing is applied to strip planking, he doesn't appear to do the same for sheathed ply.
Put it another way - surely there must be similar benefits in terms of reduced 'core' thickness, and hence weight, if you use the same sort of "heavy sheathing" inside and out that Gerr suggests for strip plank, but with ply?
Using the strip plank rules for 'ply core' construction would seem the obvious answer, but I suspect that the core thickness would probably wind up heavier than if calculated for carvel planking! Afterall, the timber used for strip plank is likely to be far less dense and probably of a lower grade than typical exterior or marine grade ply's...?

 

What About Steel?

I think steel would rate well for price & ease/speed of construction.

Tim

 

Pericles - How widely accepted are the McNaughton rules? And do they cover sheathed ply construction (they appear to cover sheathed strip, traditonal carvel, cold molded and FRP, but there's no mention of ply in any of the indexes...)

 

The Lord construction technique is pretty cool and is a true cored method. It's longevity is well established, though not well documented in the USA. Yep, it takes a different engineering approach, equally as sound as more typical rigid structures, but with an well focused eye on tensile modulus and elongation, which are the keys to the system. He wrote a book called "Naval Architecture of Planing Hulls" and was a professor at George Washington University. The book and the school may be a good place to find out more about his work.

In true cored construction, the core needn't be nearly as heavy or stiff as plywood. This places plywood at a disadvantage in cored construction and discounts its inherent physical properties, which frankly are hard to beat, pound for pound and dollar for dollar as a stand alone hull material, particularly in craft under 30'.

All materials have a displacement range, where it shines and one that is at the limit of cost effectiveness, when compared to other materials. In small boats, it's very difficult to beat wood and plywood. In large craft, metals stand out.

Trevlyns, if you need an answer to something, then it's only dumb not to ask.

I agree with Matt, that you really don't need to stitch panels together. I've used just about everything imaginable to hold things in alignment as the goo cures. Drywall screws are a favorite, as is duct tape, ratchet straps, rubber hose, etc.

You can torture plywood into compound shapes to a limited degree. This is especially true of thin sheets. For the most part you can make plywood fit just about any shape you want, but in compound areas you will have to rip it down and use it much like diagonal planking.

I'm not completely sure what you're attempting to do Mat-C. Can you clarify the reluctance in using more traditional plywood construction methods? Currently a double diagonal plywood planked hull can be very rigid, light and serviceable over a wide range of hull shapes.

 

I don't think that Lord's theory or planing hulls gives his scantling rules. In fact - other than Dave's book - I've never seen them published either...

As Paul suggests - every build style has its optimum applications. If low cost is yours, then (depending on boat type) glass over ply may not be the most economical choice.

 

PAR, Will etc - thanks for the input.
I'm looking at building materials for a 30ft trailerable planing hull - top speed probably in the mid 20's.
I understand that you don't need to sheath ply with FRP, but just wondered whether there's a similar advantage to be had as there is with other cored methods, like strip plank.
Also, surely a sheathed boat will be less susceptible to damage - abrasion, people dropping stuff on it , sctaches etc?

I can quite happily make the hull shape developable, so there's no great compromise to be had by building with flat panels. I guess I just like the idea of an epoxy/FRP, wood cored boat better than a straight wood/epoxy one.
I like the idea of a foam cored one the best, but from what you've all said, it won't be the cheapest option, and indeed may not be any lighter...

I would probably build to Gerr's rules, as they are obviously conservative. And whilst they won't produce the lightest structure, they appear well proven. I'm also (trying!!) to wrote a spreadsheet for the ABS Guide to Motor Pleasure Craft rules to check against (though I think these are not directly applicable to stitch and glue or glass over ply...)

Once again, thanks for everyones input. This is a great site with great people!

 

He may be new - but he's definitely wise!

 

If you read Geer's scantling rules carefully, he provides some areas to shave weight. He knows the rules in general will produce a "healthy" craft and offers a number of possibilities for reductions, which usually directly relate to speed, economy and build cost.

 

Mat-C,

To answer your question about using marine ply to cold mould a hull. I point you in the direction of Jarret Bay. http://www.jarrettbay.com/construction/why.php

"Solid Construction: Jarrett Bay uses a variation of the traditional cold-molding techniques on which the North Carolina custom boat-building business was founded. All framing is fir with three layers of marine-grade fir plywood on the bottom. On the sides, it’s three layers of diagonally planked Okoume marine-grade plywood. Everything is glassed on the inside and outside with 34-ounce cloth on the bottom and 18-ounce cloth on the sides using high-tech WEST SYSTEM® epoxy. This process produces a hull that is up to 25 percent lighter than a solid fiberglass hull, and one that has bonds better than foam-cored hulls."

From McNaughton.

Scantlings Rule for Cold Molded Construction

Cold Molded Construction consists of gluing up many very thin layers of wood in a "criss-crossed" way over a mold so that when everything cures you have a hull or other part that is essentially like a compound curved piece of plywood. If well done this is a strong light method of construction. However it is very labor intensive.

Cold moulding with Okume BS 1088 applied in 3 or 4 diagonal layers will give you as many as 28 very, very thin crisscrossed veneers. Personally I would not use the woods Jarrett Bay specify for the hull, only Okume or Meranti, because it is easily sourced in the UK.

Sheathed ply is a misleading description of the current position of epoxy ply composite boat building. First of all, both the outside and INSIDE of the hull have all joints filleted and taped with bias woven tape. Then all surfaces are glassed and this produces a laminated composite shell of incredible strength. The glasscloth skins are fully part of the structure and not an afterthought. As evidence of this, think how a foam core is used. The two glass skins strengthen the foam but it is the thickness of the foam that determines panel stiffness. The further separated apart the two skins of epoxied glasscloth are, the stiffer the panel and that principal also operates when the core is cold moulded ply. See also:

http://www.sculleyboatbuilders.com/const_superstructure.html and http://powerandmotoryacht.com/boat-tests/vicem/2007-vicem-85-classic/

There are also posts elsewhere on the forum about this subject. http://boatdesign.net/forums/showthread.php?t=18864&highlight=pericles

Pericles

 

Maybe you mentioned this, but MacNaughton also has a scantlings rule for sheated strip planking, based on Lindsey Lords.

I am waiting for a new set of ISO rules for plywood hulls.
They should be published soon.