1/4 or 3/8 ?

[Leon m]

I'm going to start building a 20ft.sharpie,
Bolger's "Zephyr".Today I picked up some plywood
at an auction,some Supraheckt (mahogony)10 are
1/4"and 10 are 3/8".
So my question is this:The plans call for 3/8"
and paint.But if I am planing on fiberglassing
the entire outside of the hull can I use the 1/4"
to cut back some weight ?
Your insight would be most helpfull!
Thanks! Leon

[Jack Heinlen]

I think quarter is too light for a twenty foot sharpie with one piece sides. The glass doesn't add much strength, mostly abrasion and puncture resist. Why not build with the 3/8, and glass only the bottom to save weight?

[imported_Conrad]

In my experience, 1/4" stiffens up considerably when glassed on one side and coated on the other. Make a test panel, say 1' by 3' and see what you think. Another possibility is to run an additional stringer or two down the sides or bottom, reducing the panel span- this will really increase stiffness and apparent strength. Ask yourself what your priorities are; light weight, long term durability, carrying capacity/strength, etc.- pick the trade-off that makes sense for you!

[ken mcclure]

Probably best to ask the designer.

[Leon Steyns]

Hello Leon,

If you were able to pick up some Bruynzeel Suprahecht® (which is without any doubt the best plywood in the world!), you are a lucky man. When properly coated, you get 20 years warranty on them. Besides that, the high quality Mahogany is very strong (but also heaver than for example Okoumé).

You say the plans call for 10mm ply (3/8"). In my personal opinion, I think the 6mm ply (1/4") with epoxy and glass is stronger than the 10mm ply (3/8") with paint, but you should consult the designer or a naval architect to re-calculate the scantlings, like Ken suggests. I doubt the weight savings will be huge (epoxy and glass are heavy, not including the heavier Mahogany!).
Good luck on building the boat!

Greets, Leon Steyns.

[Dave Carnell]

I built ZEPHYR of ¼" B-C pine, which runs a little fat. It worked fine; epopxy used only as glue.

[Dan Cavins]

I'm wondering if you want to save that weight anyhow? Being a sharpie, an inherently light and tender boat to begin with is lightening up the hull a positive thing? I'm with Ken, call the designer. I'm building an NIS 23' sharpie which is ballested and has a cabin. I would be nervous with 1/4" sides. Not familiar with the Zephyr. Now Dave did it but still I'd call the designer. Good luck, Dan.

[JimD]

I have a 15 foot sloop with cabin, 1/4 inch sides. Fiberglassing the outside I considered mandatory, but not convinced I fiberglassed the inside, too, which made a big strength difference as far as I can tell. But I was adding strength, not trying to save weight. I echo the question as to whether you should be trying to save weight in the first place.
jimd

[PeterSibley]

Please correct me if I'm wrong but I always compare material strength,stiffness, as a square of its thickness,so

3/8" or 10 mm as 10x10= 100 units
1/4" or 6 mm as 6x6 =36 units

So we have got a very big difference in stiffness between 1/4" and 3/8" !!
Peter

[ishmael]

Brother Peter knows what he's about.

[Dave Carnell]

Stiffness varies by the cube of the thickness, so 3/8" is 3.375 times as stiff as ¼" ply. A layer of fiberglass adds so little thickness that it is little stiffer than without. A coat of epoxy adds essentially no stiffness, what a Ch.E. professor I had used to call "a fart in a whirlwind"

[Leon Steyns]

Well, well... adding 2" of foam wil add a lot of stiffness then?! What was that again about density way back in physics class? Hmmm, wait... there's more: I vaguely remember something about tensile strenght, shear strenght... but then: I'm not an engineer... smile.gif )

Greets, Leon Steyns.

"If only the world was that simple..."

[NormMessinger]

Right, Leon. When one glasses a piece of wood there is more to the stiffness it adds than the thickness, at least in tention. Glass isn't much force in compression however.

--Norm

[Stephen Hutchins]

Go with the 3/8. Fiberglass is the worst temptation to ever happen to a boat builder. The boat can be built quicker and more eficiently with wood (Especialy rotary cut Plywood) Your time in labor to glass the 1/4 inch stuff will be far greater than the savings in sheet thickness. Plastic resins damage our living environment, and our lives in general. They also give the builder a temptation for shoddy workmanship. ie. "I can cut that bulkhead as fast as I want cause it's gonna get filled with poop anyway. By the way that's was the glass builders call it-"POOP" Don't build a boat that will last to much longer than the lifespan of the trees used to build it. If we work with nature rather than against it. We could have a sustainable way of life.

[On Vacation]

Sandwich foam comes in many densities. The separation of laminates on foam creates stiffness. The compression strength is only as good as the density of the foam. The ideal combination is skined foam and glassed.

[imported_Conrad]

By coating the wood with glass or resin, you increase its stiffness not by increasing its thickness so much as by changing the modulus of elasticity of the surface fibers- the cloth/resin matrix has very different mechanical characteristics than the wood fibers, stiffer and stronger. Since the surface material is initially most critical in absorbing the tensile and compressive strains of flexure, the total composite (coated plywood) is indeed now stiffer and stronger. There are two things going on here- the first is that by increasing the thickness of a homogenous material the stiffness does increase, by the cube. The second is that by changing the nature of the material, we've altered its' modulus, making it indeed stiffer. Were we to then continue to increase thickness, it would rapidly get stiffer still. Gotta keep track of your apples and oranges- there are more variables involved!

[ion barnes]

Contact the West System guys. I remember them having data about the various strenghths of composite layups. It was either in their Epxoyworks bulletins or their book on boatbuilding.

[On Vacation]

Increasing glass thickness is more weight and cost and labor which in many cases creates an inferior layup for what you are attempting to gain in a small craft.

[ 10-23-2002, 03:31 PM: Message edited by: oyster ]

[Leon m]

Originally posted by oyster:
inferior layup for what you are attempting to gain in a small craft.Oyster
Please elaborate above ?
Thanks Leon

[On Vacation]

Increasing the amount of glass to obtain stiffness decreases performance or the need for more power, either by human or machinery. In so many cases we have seen the wood acting as a mould left in the encapsulated glass outer shell.

A veener will weigh less for the stiffness obtained as in the example displayed than structual glass. In small craft building, cost is a big factor associated with bulking up.

[ 10-23-2002, 11:00 PM: Message edited by: oyster ]

[Leon m]

I have already purchased the 3/8 ply
so that cost is not a factor.The reason I want to use glass is for abrasion resistance(beaching),and to preserve the wood.The Suprahecht
ply tends to run heavier wihch leeds me to wonder
is this boat going to end up much more heavier
than originaly intended,and will I lose some
boyancy.I also hoped My wife and I might be able
to car(truck)top it,but I can put it on a trailor
if need be.But what about the boyancy does
any one think I,ll lose performance if this thing
starts dragging
in the water too much,or am I splitting hairs?
Thanks! Leon

[On Vacation]

If the plans called for no glass and 3/8", IMHO opinion with or without glass, 1/4" will not be the ticket for the boat. You surely don't need a structual glass for 3/8". Have you asked about it from the designer? A layer of glass, 4 oz finishing cloth will do fine on the outside. I would not think you need glass on the inside. This will save you APPROX. [ESTIMATE] 25 lbs. of materials in a 20 foot boat.

There is an alternative to glass for the inside that we have used in many work boat applications. We have coated decks and interiors of many wooden hulls with a product called Gluvit. It is a thick epoxy. Works great and last a long time under heavy use conditions. You will need to paint it. Just a side note. I didn't wish to leave you with the opinion that this was a finished product.

[ion barnes]

I dont think you will car top this 20 ft puppy. I build a 9 ft all glass dinghy that I can car top but prefer to put into the back of my truck, as they weigh 110 lbs. People ask me if Iwould build a light wieght one for them and I say "thats it". When you have pared all the weight out of a boat, there is nothing left. Everything weighs something, it has to!,and a lot of people just can not understand it. For example, my wife wanted a light weight canoe that she could pick up herself. She found one,16 ft long, and bought it home. I was not with her when she got it but I had to help lift it off the car and to be the one to show her the damage that had been pooly repaired because of its light weight, and defects in its assembly. The point is I guess, What is the first priority? Using the boat or being able to cartop something? I know its using the boat and I am sorry for the crude question. Go ahead and build the boat and let the transportation question answer itself later. That canoe I spoke of, well I rebuilt the gunwales which is where it all started from, and neither one of us would dare move that shell without help. Its thin glass and a gust of wind will overpower a single person. As for the comparable strenghts of ply and glass you really should ckeck with West System, they do have alot of info. Personally I am in progress on a little kayak of 7.5 ft built of doorskin ply with 6 oz glass inside (stitch and glue) and it weighs 18 lb right now. It will have a coat of glass outside too but the weight wont be much more. My test panel proved to me how much more rigid the ply was after just one layer. The second layer on the opposite side did not improve the stiffness but does make it moisture tight at the cost of being a little heavier. Also to be considered is the shape of the panels. Curved panels do resist flexing, whereas panels with a long straight run need framing to maintain its shape.

[Leon Steyns]

When it comes to hull thickness, let's make a clear difference between fiberglassing a plank-on-frame design and a composite material boat (where the glass is structural, not just for abrasive resistance).
In that case similar designs will ask for radically different scantlings. Again, I'm not an engineer, but I do know I'd have a certified NA to do this for me (wear your PFD for a different reason! smile.gif .

Greets, Leon Steyns.

[Steve Paskey]

Just because Bolger specified 3/8, one should not assume he thought 3/8 was necessary. A case in point: Bolger also specified 3/8 for Teal, a 12-foot double ended skiff built from two sheets of ply. He has said that he picked 3/8 over 1/4 because he believed it would be easier for the novice boatbuilder to find the thicker ply, and NOT for the additional strength.

That said, it's worth looking at Bolger's Light Schooner, a sharpie skiff that's a bit longer (23'6") than Zephyr. The Light Schooner is built entirely from 1/4 inch ply, with a single layer on the sides, and a 2-layered lamination on the bottom to finish 1/2 inch thick. However, LS has considerable framing and a bridge deck amidships, which would add enormously to the strength of the boat.

For what it's worth, Bolger's 31-foot Folding Schooner also uses 1/4 for the sides, but the FS is essentially two 15 foot skiffs joined by a hinge. Again, the use of 1/4 ply is made possible because there's a substantial structure amidships to maintain the strength and shape of the topsides.

If you really want to build the boat from the 1/4 ply, I don't think you could go wrong by doubling up the bottom and adding strength amidships. You won't save weight, but you would be able to save the 3/8 inch ply for another project.

[On Vacation]

I have never figured out why people buy plans and then try to reinvent the wheel or second guess what has been drawn. The only 20 foot cartopper that I ever seen is a rowing shell. One cannot compromise weight for safety. In a bulkhead boat, you may well get by with lighter sides as long as the longituals are there. Trying to create a boat that is durable in rocks with finishing cloth is almost impossible. Good luck. Plywood thickness and total number of plys along with types of plywood will play a roll in compression and fracture with or without just finishing cloth.

[Leon m]

Well Guys
I talked to MR.H H Payson on the phone today.
he said stick with the plan (3/8).He also said
glassing the outside was a good idea,and as far
as the weight issue...well anybody got a trailer
for sale?cause this thing darn sure aint going
on top of my truck!But thats ok I still love
the design and I,m still going to build it.I
realy am glad you guys are here to share your
knowledge with me or I probable would have wai
sted alot of time and money just to find out the
thing was too heavy to through on my truck anyway
then I,d be stuck with a boat thats less than it should be.
Thanks a bunch !
Leon

[ion barnes]

The info I spoke of from West System was in their bulletin "Epoxyworks" #10 ,winter 98. Bare 1/4 cedar plywood .10.9 oz/sq. ft. deflected .63in. with a weight of 225lbs at failure. Same plywood with one skin of 4.0 oz cloth 12.8oz./sqft, deflected .79in. with 429lbs at failure. doesn't take much eh? The eqivilant in cedar strip? Would you believe 1/4 in. with 2 layers of 6 oz cloth?! Sorry but they did not test 3/8 plywood

[Don Maurer]

In the unbalanced construction, with fiberglass on one side only, the data is for a sample with the fiberglass on the outside. If the fiberglass was on the inside, the numbers would not be much higher than the plywood by itself. What this means to the boatbuilder is that 1 layer of glass on the outside will decrease torsional (bending and twisting) failure, but won't do much for you if you broach on a rock. To increase resistance to holing, you need the fiberglass on the inside as well (or a thicker core, of course).

[Jack C]

Leon,

If you like the Zephyr but are concerned about the size, check out the Crab Skiff/Surf. Same boat only a little smaller. Both are an adaptation of the Maryland Crab Skiff found in H. Chapelle's works.

Anyway, the Zephyr is too long for my garage (plus I don't like the sailing rig) so I'm opting to build the Surf. At least after I'm done with the J. Michalak Mixer (80% done).

Jack

[Leon m]

Jack c
What do you think about the placement & angle
of the dagger board on the Zephyr?Is this a big
differance in performance over a centerboard trunk?
Leon

[ion barnes]

Hey Don, I would have added that detail but it was an abreviated excerpt.The author was JR Watson who I believe works for West System or at least writes frequently for them so should be approachable there. I want to add that Epoxyworks is a FREE mag and has a lot of info. It is published four times a year and I have found them very useful. Just go to www.westsystem.com (http://www.westsystem.com) and sign up. I stated out with nothing and I still have most of it left