stitch & glue: Strenght vs Weight

[jamoose]

hello--
This is related to the thread below on the duckboats. I'm trying to figure out how one of these boats could be built as light as possible, but with good strength and stiffness. Good would be defined as the ability for me (185#) and a fat dog to walk about on the hul and deck without the deck buckling and cracking. I had been thinking of 1/4" okoume with a 4oz cloth, but is there a better ply/glass combo?

thanks
Jamus

[Del Lansing]

I would think it would be the strength of the deck beams that would decide the load bearing rather than the strength of the coverings.

[mmd]

You cannot consider the strength of a structure by examining its components in isolation. You need to consider the structure as a whole. In this case you need to consider point loads (185 lbs balanced on the ball of your foot as you step aboard from a wharf), dynamic point load (your weight multiplied by the distance you drop as you step down from the wharf), panel load (how much weight the unsupported ply panel will support), and the beam load (how much weight the beam plus ply panel will support).

If you are willing to accept extra weight in the structure, you build to the scantlings of other similar boats. Obviously, they are strong enough as evidenced by their continued existance. If you want to flirt with the edge of the adequate strength/minimum weight situation, you have two methods available to you to determine scantlings: Trial and error, or engineering.

Trial and error - make up test panels of varying scantlings and materials and methods, test them to destruction, and tabulate the results.

Engineering - buy a copy of Schaum's "Strength of Materials" and calculate the strength and deflection of various beam/panel combinations.

Caveat - truly light & strong structures are usually either labour-intensive or expensive, or both.

My personal reccomendation for construction - look into foam-filled stressed-panel decks.

[LeeG]

wow,,mmd put it into words,,it's the structure/design, THEN it's the materials.
Once you get large expanses of flat ply things can flex around a lot. Flat stringers of 4mm with glass on 6mm can do a lot for panel rigidity as can external runners. One layer of 4oz glass is not very ding resistant when it comes to dropping heavy things like a tool box or bottle of beer onto relatively soft okoume but it can be perfectly ok if the structure is designed for it. Once you spend all that money for fancy ply and epoxy it would be a shame to skimp on waterproofing/durability with thin glass/epoxy where it'll wear a lot. Preglassing panels can help you control epoxy but with 1/4" ply in a 4'x8' or so boat there's a limit on how light it can be before you start sacrificing something.

[George Roberts]

I read the other thread so I guess you still want a 70# boat.

I am one who believes you can design each part independant of each other part.

Using 1/4" plywood may not get you down to that weight. But I suspect that it will be strong enough for your use.

[RodB]

To achieve the lightest construction using the materials you talk about, okoume ply, epoxy, and fiberglass... to meet your needs.... it must be engineered properly... I would think there are many very knowledgable sources on building ultralight craft out there.

I would suggest consulting a designer who has built variations of such craft. They know from theory and practical application what has worked and what failed. If you are determined to stay close to the edge of what these materials are capable of you will definitely need to be correct in your judgment on scantlings for construction.

Strong light deck beams such as David Nichols uses in his Lutra Flyfisher canoe (www.arrowheadcustomboats.com) would be a starting point.

This kind of structure , as others have said, can be copied from other similar craft or you can consult someone with the expertise to give you precise advice, hopefully the designer of the boat you select.

RB

[ 01-25-2005, 07:19 PM: Message edited by: RodB ]

[ssor]

Utilizing the technique used in hollow door construction, but not the cardboard, you could saw enough curved deck ribs of 1/4 plywood to allow you to place them on 2 or 3 inch centers and glue a thin skin on the underside and a thicker one on the top side. This type on assembly would be very stiff but I wouldn't allow anyone to dance on it. I'm sure that you would have to build a mould for it.

[ 01-26-2005, 10:56 AM: Message edited by: ssor ]

[htom]

You could approach Platt Monfort and see if he'd be interested in doing a Geodesic Airolite version of one of the classic duckboats.

[Bruce Hooke]

Originally posted by mmd:
Caveat - truly light & strong structures are usually either labour-intensive or expensive, or both.In addition, it seems like many things engineered to the edges of what's possible end up having a short lifespan!

[jamoose]

Thanks all. I appreciate the advice. In my journeys through the web I stumbled across the hulls program (what fun!) and have been tinkering with a couple designs. I'll post wire frame diagrams soon and then maybe could sollicit additional perspectives on weight a durability within the context of a particular design

regards
Jamus

[otterbfishin]

jamoose,
Please tell me how I can access the "Hulls" program.
Thanks,
otterbfishin

[htom]

hulls: http://www.carlsondesign.com/#Fun_Shareware

alternative tutorial: http://home.clara.net/gmatkin/hullstut.htm