Question about marine plywood

Agreed, for a relatively inexpensive build, you don't need BS-1088 grade plywood. What you need is plywood that has a true WBP glue. A meranti BS-6566 grade (Aquatech) is about half the cost of 1088 and built to nearly the same standards. Also the AB-Marine grade can be made to work, though is much lower in quality, compared the BS grades, it'll likely serve fairly well anyway.

I wouldn't bother with PT plywood at all. It's really not very good stuff structurally, though it does have a WBP adhesive. I disagree with Ilan, in that a more traditional surface treatment, such as linseed oil will help much. This treatment works on solid wood, but is about useless on plywood, mostly because the oils can only get in as far as the first glue line, leaving the remaining internal veneers helpless. Oil treatments also need lots of oil, several times a year to be effective, which simply put are not. In fact, these treatments are the absolute worst at keeping out moisture, compared to more modern methods, particularly along the end grain areas.

Lastly, if building on the cheap, use paint. The best paint you can afford, preferably an oil base. Porch and Deck enamel is a popular choice, as is regular house paint. Use a high gloss, as these are more water proof then the satins and flats.

 

Im going to throw in other options into the mix. First off I agree with everyone about not using the vinyl stuff. Now, when it comes to choosing your plywood while BS 1088 rules you should be aware of several things, the only reason to use okoume is if you really need the lightest plywood available and that this comes at a cost of extra $ and no resistance to rot. On a small boat like a duck boat you will not be using many sheets and if you are willing to accept the penalty of 4lbs per sheet for example for 1/4" meranti you will get a stronger, more rot resistant panel for a lot less $. If you want to save more $ you can buy BS6566 which is perfectly suitable for your project. I just looked at noahs site and they list 6566 meranti at $26 and okoume at $42 and 1088 meranti at $39 and okoume at $55, that's a big difference. The only advantage to okoume is lighter weight and easier surface finishing(meranti has a more open grain)
the disadvantage is more $ and a "non durable" rating as a species.
The advantage to meranti is cost, strength and a "moderatly durable" rating as a species, the disadvantage is its heavier and takes more finishing which is probably not an issue on a duck boat.

Steve.

 

If it is just meant to be used during hunting season on inland lakes and stored the rest of the year, just use the cheapest ply you can find and make sure you do a good job epoxy coating. It will last longer than a Benelli shotgun and who cares if it doesn't have as smooth a surface as the fancy stuff. Fancy doesn't shoot ducks, being out there on the water gets the birds and the sooner you are there the better.....

 

This is very good advice.

 

Just a thought, but you may like to include in your experiments, a 'stringerless' boat. You can probably experiment on a 2 metre toy boat, but the age of stringers on small craft is over with epoxy and 'stitch and glue' construction.

Why people still insist on problematic fasteners and the extra bother of long bits of timber for boats under 20ft, eludes me. There are so many plans that give you the precise dimensions of side and bottom panels, you dont have to be a rocket scientist to work them out.

I cant emphasize the benefit of light weight for any small boat too strongly. Both in ease of transport, and moving on the water, every kilo you save adds a whole lot more pleasure and usability.

 

Light weight and low inertia aren't always friendly to boat users, though, even if the speed and ease of propulsion are better. For any design there is an optimum weight range, and in a small boat the operating (sea state) conditions have a lot to do with it. The boat might not care how rough it is, but humans are a different matter.

 

You can always add weight to a boat for conditions - you cant take it off when its too heavy.

 

Good point, and one I will take to heart ! Water ballast, of course, is what makes that practicable. The snag there is you have to make provision for it in the design, which might complicate the build.

 

maybe - but in a small dinghy it could be some scrap steel, etc. Water ballast can be problematic.

 

Well, scrap steel is hard to find out on the water, you'd have to take that kind of ballast with you for the duration of the trip. Water ballast is all the rage for maritime shipping, I expect it can be adapted for most boats, successfully. Spoils an unclutted interior a little though.

 

I am not suggesting picking up scrap at the waters edge.

If you have a heavy boat - you have to tow it, lift it, power it as a heavy boat always.

If you want to go out in heavy weather for a fishing trip, have some steel billets or concrete blocks in the ute to fasten to the keel. If you are going for a row on the river, leave the weights at home.

 

You could build a deep-V runabout from advanced composite materials that weighed much less than standard boats, but it would be pretty useless in all conditions, unless permanently ballasted. Too tippy at rest, and riding along high out of the water, very hard to keep on an even keel. Lighter is not always better, but, too heavy, never much good.

 

rwatson, would you like to elaborate on what you see as "problematical" with water ballast ? There is certainly no shortage of it to be used, it never runs out !

 

Fair enough. As an amateur student of water ballast, the problems I have encountered are.

1) Intake and exhaust of water in normal operation.
2) Location and righting effect within small hulls ( especially part way between filling and emptying).
3) Amount of room taken up for the required weight.
4) Structural adjustments in the hull design.
5) Building for a waterproof and easy maintenance structure.

None of these are 'easy' - and depending on the design of the hull, can create various headaches.

For example, on small boats, just the problem of getting rid of the ballast easily can cause dramas. Pumping is a pain, but a hull with rocker may not allow the water to drain easily under power. If your trailer for a dinghy doesnt have a winch, you cant easily drain it during retrieval. etc etc.

Apart from all that - its a great idea

 

I recall a discussion somewhere about a chap wanting to add water ballast tanks to a boat, but being discouraged by either a NA, or a shipwright, or some-such, on the basis that water ballast isn't restrained, and was therefore unsuitable. Seemed like the wrong advice to me, but I guess a poorly designed sytem could exhibit such flaws if partially filled.