: I have readily available dodgy quality 4mm plywood sheets (2440x1200mm) for
: around $20 australian a sheet to build a S&G guillemot. This plywood is
: completely random, i have no idea what grade it is or anything like that.
: It looks relatively knot free on the exposed surfaces which also happen to
: look quite ugly. My question is should i even consider using this stuff,
: it is around 3 times cheaper than high quality marine or exterior grade
: plywood of the same thickness. If anyone here is from australia, its
: bunnings plywood in 4mm.

You will be spending at least 100 hours building this boat. A lot of that time you will be up close to it, sanding. If you don't like what it looks like from a distance, you'll really hate it up close.

Will it work for a boat? Sure. Lots of people use lauan (about US$8/sheet) for their boats. Most of this has one good side and one side which is not so good. Some people use this to make full-size models, or trials of their plans. It gets knocked around and then it is either put out to pasture, sold to someone, of taken apart to be used as a template. Then they go back and cut up some nicer wood for the boat they want to keep. The time the spend on building this first boat has to be added into the total time spent to get one nice boat--but for some people this is well worthwhile. This is particularly true if they plan to eventually distribute additional copies of their original plans.

If you are planning to build just one boat for yourself, and expect it to last for several years, go with the better quality marine plywood now. It may cost more, but when you add up the additional costs of a complete covering with fiberglass and resin, the difference in price of the wood is not so big. If you don't expect the boat to last a long time, build it inexpensively and just use glass on the seams. You'll save some time and money that way.

A compromise would be to use the less expensive plywood and make it look better. You could cover the wood with a filler and then stain it. A dark stain would hide any unusual discolorations and ugly grain patterns. Check your prices on the stains. The cost of filling and staining might raise the price of the wood to nearly the price of better-looking wood.

Your lumber dealer should have some idea of the grade of the plywood you are buying. In the US, frequently they can just look at a panel and tell you whether it is A, B, C, or D grade just by the size and quantity of the visible knots or patches. I'm not sure how they grade plywood in Australia, but I assume they use similar methods.

: My 2nd queston, is 4mm plywood really thick enough? I have no doubt that for
: the general hull this should be more than adequate, but im curious how it
: will handle my entire body weight on one spot when i step one foot into it
: as i get into it off a jetty (i weigh about 98kg ~ 240lb?)

Yes. It is strong enough for most circumstances. Remember that this is jsut the core. You'll likely be covering it on both sides with fiberglass cloth and epoxy resin. That creates a very strong laminated material which is much stronger than the plywood alone. When you board the boat the bottom is supported by water. The wood and fiberglass will flex a little, but they will certainly take your weight. Imagine you are getting into an inflatable boat. The bottom on that would bend quite a bit, but you won't fall through. There are circumstances where this is not going to be strong enough. For example, Avoid girlfriends who want to jump up and down in the cockpit while wearing high heels. Similarly, other point loads, such as impaling your craft on submerged iron reinforcing rods (a fairly common hazard in areas where old concrete is dumped into shallow waterways to create new fish habitats) can cause punctures. Another problem might be if you encounter a dam which traps one end of your boat. Fortunately, most problems such as this can be easily repaired.

The nice thing about building your own boat is that the chooice of materials is up to you. Set some standards for this boat and stay true to them. If you go for an inexpensive, utilitarian boat--don't overspend on some later accessory. Similarly, if you go for museum quality, start with the best looking wood you can scrounge up.

Best of luck with your project.

PGJ