Plywood thickness for skiff project

[BrianY]

I'm delevoping my own design for a 13.5 foot row/sail skiff in glue and screw plywood and I'd like your opinion on what thickness plywood (probably Meranti or MDO) I should use. I had thought to use 3/8" (9mm) for both the sides and the bottom because that's what I've seen other designers specify for similar craft, but I have also seen 1/4" (6mm) spec'd for the sides with either 3/8" (9mm) or 1/2" (12mm) for the bottom. The thought of using 1/4" sides with 3/8" bottom appeals because of the potential weight savings. Any thoughts?

[Bruce Hooke]

A lot probably depends on things like how close together any frames or similar reinforcements are, or, to look at it another way, how big the unsupported areas of plywood are. Also, some shapes and building methods are stronger in and of themselves than other shapes. For example, the basic flat-bottomed skiff, with a large flat bottom, is inherintly weaker than a glued-lap design.

As an example, my skiff, which I designed myself pictures here (http://www.bghooke.com/brucehooke/Skiff/Image2_1.htm)) uses 1/4" plywood on the bottom and sides, but as you can see from the second picture, there is a fair amount of reinforcing on the inside and the bottom is relatively narrow. There are also three rub strips on the bottom, so the largest unsupported panel on the bottom is probably about 7" x 14" on the bottom and about 15" x 22" on the sides. Of course my skiff is also an untested design so don't draw too many conclusions from it!!!

What I would do is take another look at the designs you have been looking at for reference and try to figure out why some designs use thicker plywood than others (if you can). That should help you figure out which way to go on your skiff.

[Steve Miller]

My Glen-L Utility Skiff used 1/4" ply for hull and sides. Plenty strong. Glue and screw over frames and lots of bottom support. One issue to think about is your boat shape - will 3/8 make the bends? It would not have in this boat the curved sides and bottom arc.

http://www.hevanet.com/kg7pv/Utility/whole.jpg

You can see the keel (1.25 x 3" ) and the bottom battens (3/4 x 2") and the frames, stem (barely) and transom framing. The chine is beefy too. This boat is only 11' so one more frame would get you to your 13'. Check out the Glen-L site for other skiff plans to see what they call out. Best bet is too use what the designer specs.

[StevenBauer]

I built my 14' John Gardner skiff with 3/8" sides and 1/2" bottom. Glued and screwed to white oak framing. She's strong but heavy. I wish I'd used 1/4" and 3/8".

Steven

[Wayne Jeffers]

Brian,

For a rowing/sailing 13.5-footer, 1/4" ply is plenty for the sides.

For the bottom plywood, a row/sail skiff should have plenty of rocker to the bottom, and rocker adds stiffness to plywood. If you were building frameless stitch and glue, I would suggest 3/8" ply might be better. But since you're planning glue and screw, I assume you will have at least two, maybe three, athwartship frames, which significantly reduces the unsupported area spanned by the plywood. In that case, 1/4" plywood should be plenty stiff for the bottom.

For a similarly sized powerboat, you would need heavier plywood and/or more framing to take the stresses/pounding.

Wayne

[mmd]

Aww. c'mon! A simple section modulus calculation followed by a indeterminate beam loading calculation leading to a stress-strain calculation with reference to published mechanical properties data on various wood species ought to be in order here! Where is all the mathematical obsfucation? tongue.gif :D

Seriously, and with the caveats mentioned above regarding unsupported panel areas, 1/4" should be fine for the sides. Unless you really need to reduce weight for some reason, I'd go with the 3/8" bottom just for the insurance against puncture when landing on a beach.

[Adamant]

Hi Bruce,
Nice looking skiff.
Don

[Cuyahoga Chuck]

On a ply over sawn frame construction ¼" should be enough. If British Standard ply is used the ¼" has 5 verneers which should make it fairly bulletproof. If you use temporate zone plywood the weight increase will be substancial, in any case, so going to thicker ply on a hull that requires a gazillion metal fasteners is asking for a heavy boat. Also, thicker ply, particularly domestic ply, gets less and less inclined to bend as the thickness increases.
Since you are building a design that has never been built before you'll probably encounter enough glitches to slow things down so it would seem prudent to go with the most pliant materials available to keep things moving at a modest pace.
In the S&G world there are dozens and dozens of modest sized sailboats that are built with ¼" hulls and even without dimensional lumber frameworks they are plenty stiff.
Charlie

[BrianY]

Thanks for the good information. I'll go with the 1/4" for the sides and 3/8" for the bottom.

Bruce - very nice skiff !

[JimD]

Twer it me I'd go with 1/4 inch all around and epoxy sheath the bottom at least, if not the entire outside. Meranti is about 20% heavier than okoume so it will likely add about 25 lbs to your boat.

[J. Blazy]

Totally agree with everyone, 'specially Bruce n JimD - and especially since my boat is exactly the same length as your proposed craft.

I went 1/4" superply all around (thick face veneers, fir core) and when I wanted a stiffer puncture-proof bottom, I sheathed in biaxial glass. The longest span between reinforcing ribs or bulkheads was about 18" and to this day 250 lb people board my boat with all their weight concentrating on the leading foot, and I never hear so much as a creak.

The money I saved from not using pricey marine ply went right into the extra gallon of epoxy and glass I used to sheath the hull, resulting in a hull exponentially more durable than unsheathed marine ply. Rocks just bounce right off - no repairs needed.
I can't understand why someone would spend mucho bucks on marine ply, have it shipped for more mucho bucks, then not glass it. It may be void free, but its still wood.
http://www.imagestation.com/picture/sraid139/pfbcc9bc73468518a7ee5d9f984b6796a/f700a25d.jpg

[Bruce Hooke]

Thanks Adamant and BrianY! Maybe one of these years I'll finally have time to finish it!!

[ 09-09-2005, 10:56 AM: Message edited by: Bruce Hooke ]

[pipefitter]

If lighter plywood needs more framing,where does the real weight savings happen? Especially if using white oak for frames? Doesn't that offset the weight saved by using thinner plywood?

[Bruce Hooke]

Originally posted by pipefitter:
If lighter plywood needs more framing,where does the real weight savings happen? Especially if using white oak for frames? Doesn't that offset the weight saved by using thinner plywood?One way to look at it would be to say that the weight savings comes from using thinner plywood were you can do so without adding more framing (i.e., where thicker plywood would be excessive). To put it another way, it all comes down to a coherent design that uses the right combination of plywood and solid wood to achieve a light but strong boat.

[Norske3]

MMD....would you say that again...a little slower...I think I almost got the drift of it.

Or just do what Vernon Langille did..."eyeball calculations"...his Tancook turned out wonderful.