: Well, thanks but I never said loading the boat on the car was my goal. I just
: mentioned it in passing that it would nice if my Niece was able to handle
: (load) her own boat.

I may have misread your previous post. It seemed to me that this was pretty high in your list of desirable design features, as you have mentioned trying to build a very light boat, but this is the only reason you've offered as to why you want to go that way. If I've guessed wrong, please forgive me.

If you have some other reason for trying to build a lightweight boat, let us know.

: And since I am designing the boat(s) I am guess that means I am writing the
: specs.

Fair enough. Where are you getting your info on what specs to write? It appears you are asking us. As your plans are still being finalized, about the best we can do is give generic opinions. Once the boat is built you can test it and see if your first one performs to your standards. If it fails in some way (and most first boats do) then you build a second one with some changes.

Other designers, based on their experiences or educations, use either a study of the history of components of other boats, a workup of mechanical engineering and stress analysis, a computer simulation, destructive testing of models, or they build several copies of a design and use them extensively for years to test them. Without some base to work from, you would be doing little more than pulling numbers out of the air. That is no way to make a safe boat.

: Maybe I am reading between the lines but it sounds like your saying
: 3mm ply isn't going work.

To the contrary. I'm the one who thinks it is more than adequate! Read my previous post on this. Also note that I insist the fillets be ample sized, as they'll carry most of the loads.

: Or last or last as long as 4mm. Care to explain
: why?

That may be someone else's opinion. Or, I may have mis-stated my views. I'm not sure that there is going to be much difference in longevity between boats built from 3mm versus those made from 4mm plywood.

If there was some reason for a boat made from thinner plywood to last a shoter time, I guess that would be from flex.

Eventually things wear out. Thinner (3mm) plywood flexes more than thicker plywood (4mm or 5.3mm) The good part of this is that in a sudden impact the material will flex rather than break. The bad news is that overall life expectancy is less. Eventually wood fibers break down form flexing, plywood glues delaminate, and so does glass and resin. But those things take a lot of years to happen.

On the water the boat will spread the stresses on the hull. Out of the water you'll have a much more fragile boat than one built with thicker materials. Tkae care of it as you would a skin-on-frame boat and it should last a long time.

You have mentioned carrying the boat on top of a car. You'll need to give it more support, and be more careful whien tying it onto the car. High wind speeds put a lot of stress on kayaks strapped to the top of cars.

How long of a boat are you planning? How wide? That is going to affect how thick you want your plywood, or compoosite of plywood and glass to be. Let me give an analogy. The floors of a house can be supported on 2x12's, or on trusses made from 2x4s. It is not just a matter of what the size of the materials might be, it is a matter of the direction and spacing of these materials, and how they are reinforced.

Again, a flat bottom which is 20 inches wide, and made from thin material is going to flex, and cave in somewhat under water pressure. A similar hull with a v bottom made from thicker pieces which are supported and only 10 inches wide, will flex less. But the thin material could be adequate if slight flexing is acceptable, and if it is supported by a strong fillet. Or, if the thin material is rolled into a portion of a cylindrical shape (round or oval bottom it becomes stiffer.

To be on the safe side, a lot of people go with overbuilding the first boat. I would advise that course. Sometimes the overbuilding is a something of an unexpected benefit from using inexpensive lauan (typically 5.2 to 5.5 mm) for their prototypes. If the prototypes work well then someone might build a lighter version with 4mm plywood, and then, they might build one with 3mm. The development process is then a series of steps where each new boat is made lighter and lighter. When the materials get so thin that they are hard to work with (and an 18-foot-long strip of 3mm ply is very flimsy) they accept what they have.

Sometimes the development of a design is spread over several builders working from published plans. They share their experiences on this bulletin board, or in similar discussion areas. The more boats built from a given design, the better the wealth of knowledge about that design.

First builds don't have that history. Unless ythe designer has a good background in structural analysis of boats, about the best they can do with a first build is to make it to the specs of an existing boat of similar size and shape. And most of those seem to be made from 4mm marine plywood. That makes 4mm a fairly safe choice.

From what I've seen, the big users of 3mm plywood have been in designs which required the plywood to be bent a lot (or "tortured") to get it to assume a rounded shape. After that it is reinforced with a full cover of glass cloth. That eventually increases the thickness of the material to nearly 4mm, and adds to its weight.

Over a SOF frame I'd happily use 3mm plywood instead of fabric. I'd cut it to size and glue it to the frame. amybe with screws or nails, maybe without. If I was to build a boat from scratch from 3mm, I'd make sure the fillets at the joints were as strong as the wood from a similarly sized SOF frame--'cause they would be doing the same structural job and I'd expect them to handle the same stresses.

Just my opinion. I keep learning more everyday, so that opinion can change tomorrow. If you build with 3mm I'd be very happy to hear how it works for you--and so would others on this board. The more data we get on builds with these thinner materials the better we know them.

PGJ