What prevents shady timber-merchants from selling cheap plywood as marine grade?

[dancrane]

Thanks for all recent contributions.

I would have added this thread to an earlier entry I started a while ago, about making a flat draining deck in the Osprey, so that everything from chocolate bars to small passengers don't end up inevitably rolling into the deep areas either side of the centreboard case, which are often awash...

...but that thread was hijacked by chaps concerned about other matters, and their unsolicited hubbub left the deck-question barely addressed.

I can still see that plywood is vulnerable to water ingress...although water contact would only be very temporary - there'd have to be an unwelcome weight of water in the bilge before the ply deck was continually immersed. I have this year applied several coats of epoxy to 12mm marine ply sections which I had planned to use as supports on my trolley...

...and while the support principle wasn't a huge success, the resistance of the epoxied plywood to moisture seems to have been absolute. So I confess I'm still open to the idea.

I wouldn't even be considering ply over hardwood slats, except that if I use ply, the drainer-grooves I cut needn't separate the sections - so overall, the deck would retain rigidity.

Whereas creating individual slats, each mounted on carefully-carpentered support-pieces to spread the load over irregular curves in the deck below, will challenge my ability.

I must visit my local timber merchant. One look at nice iroko planks and I may rise to the challenge. I'm aware that slotted ply sections can't possibly look as good as real wood.

What thickness of iroko would suffice, assuming I'm able to fashion "risers" every 12 inches or so to lift the slats off the cockpit floor below?

 

[Seajet]

Dan,

now I have a better idea of your needs, I would definitely check out those Homebase jobs I mentioned; even if they required a bit of resin or varnish I think asking the type of wood an idea.

Re plywood, unless you're absolutely sure your idea and shape pattern are ideal, I'd go for external grade ply and treat it; proper marine ply is extortionate !

As a possibly practical but less aesthetically pleasing idea, I have seen dinghies use plastic stuff like a 1" slice of a milk crate, it's not actually that but looks like it; no idea what it is or where from, but it would keep feet and stuff off the bilges and conform to the hull shape.

 

[scottie]

Try looking under " duck boards"

 

[ccscott49]

So you have checked the wood type of the actual things I mention, or tried them ?

Or is this by assumption ? I agree it's a long shot, but unless you have actual knowledge of the Homebase things I mention, it's worth checking out just in case !

I have bought and used one as a rope locker, fine, for a while then started to shrink, warp and generally fell apart. Great as a washing basket! Dunno what timber, but maybe some type of gum, by the way it warped.

 

[Seajet]

Thanks, much as suspected but at least you've tried it...

 

[dancrane]

Thanks, that's all interesting and instructive. Proper gratings certainly look terrific. I wonder how difficult it can be to construct them from some robust but affordable hardwood?

I suppose the necessity to cut numerous 'crenellations' in every piece, greatly reduces the overall strength of the timber, relative to the same planks left a consistent thickness?

Is it just a matter of using a router or saw & chisel to remove half the thickness at each crenellation, such that the open surfaces of two or more similar sections can meet without exceeding the thickness of the wood? Is there a minimum advisable thickness, given that it must bear the weight of an adult?

 

[Seajet]

Dan,

I'm no carpenter but making gratings looks a right pain to me, I'd think some sort of machine saw on a setting would be the way to go, something like a saw attachment in a horizontal drill, pre-set to a certain depth if you see what I mean.

I just had an idle glance at Amazon under ' duckboards ' as Scottie suggests; didn't stay long but it would seem plenty of options with proper wood at not bad prices...worth a look if you haven't already.

 

[dancrane]

The duckboard idea is a nice one, although such things which I've seen in bathrooms were seriously heavy for their small area - I dread to imagine the weight of nearly 30 sq ft!

The more I think about it, the more I suspect that the most difficult part of creating a flat deck in the boat will be creating the wide variety of slender athwartship triangles, all exactly gauged to support planking at the same height and the right angle, which will vary gradually from the steep sections at the front of the cockpit to the flatter sections aft.

That accomplished, whether I make a deck using nice straight iroko planks or a whacking great 8' x 4' of treated marine ply with slots cut in it, I reckon the hard bit will be done.

 

[Seajet]

I wonder if, instead of all the hassle of loads of transverse webs ( if going that way cardboard for templates works well but I bet you'd thought of that ) how about some sort of nautical bean bag / soft buoyancy bag to take up the hull shape and allow plonking things & people on top ?

Just thinking out loud really...

If you do fit webs they'll need limber holes at the lower corners to let bilge water go to the drains, in a perfect world with dinghy RWO bayonet fit bungs so you could shut areas off.

I'm already wondering if a small electric bilge pump would be an idea...might double up as a feeble dinghy / mattress pump ?

 

[dancrane]

Thanks Andy, I had thought of drain-holes to prevent water accumulating and overflowing the partitions in the new raised deck, making it no drier than at present, but I hadn't imagined that the partitions would be watertight anyway...short of Sikaflexing anywhere the wood meets the bilge, I reckon it will just wash around freely, down there...

...I'm supposing that the woodwork will just sit on the GRP floor, with a couple of stout forward and aft securing points so the wood deck won't drop out if the boat rolls over.

I'm thinking that it will be best to have port and starboard halves, because the centreboard case capping and the thwart prevent anything being lowered cleanly over them...

...in fact, it'll have to be made in quarter-sections - port aft, port forward, etc - because the position and low height of the thwart already prevents easy insertion of my 8ft oars onto the cockpit floor, and putting a full length decking panel in there is likely to be impossible.

The bean-bag alternative is pretty intelligent, but I like the idea of all that nice decking looking smart on the floor, and of engrossing woodwork occupying dark winter days ashore.

Count on some photos hereabouts, of my endeavours and the final effect. There may be quite a few more silly questions before I get there.

PS...electric bilge pump...very exciting thought. I'd been thinking of a Whale stirrup-pump style, but hadn't made up my mind.

 

[Wooden Boat Fittings]

Dan, if you do decide on gratings for your burden-boards, a properly set up router would probably do a better job than a saw. And you can cheat a bit with the timber thickness -- 1" is considered a suitable depth for a grating with pieces spaced at 2" each way (ie 1" gaps between them), and as has been said this requires full housing joints at each intersection. But if you make all the longitudinal pieces 1" deep and the cross pieces only ½" deep, then you'd use half-housings in the 1" sections only, and you'll only need to cut half as many of them. Moreover, that way you wouldn't need limber holes, as bilge-water is free to run to the lowest point for pumping out.

I'm not familiar with your Osprey, but my fifteen-foot clinker dinghy Aileen Louisa had solid burden-boards, in five sections -- a small one full-width under the sternsheets, a second large one full-width aft of the c/b case, a half-width one each side of the c/b case, and a full-width one forward. They were at all slightly different levels (usually changing under a thwart) to make best use of the available beam, and this is something you might also consider if it's a possibility in your case (whether you use gratings or not).

The last thing about gratings is that they normally have wider perimeter pieces (often curved to fit the available space), and having the wider edges not only looks better but also delineates the different adjacent gratings so you get a visual cue as to where the level changes, if it does and if you need it. (See http://www.teakmarinewoodwork.com/images/Grate - 1 - VSuarez.jpg for an example.)

Mike

 

[dancrane]

Thanks Mike, for that detailed and interesting description. If I could pick a grating for my raised floor, I would...but if in order to be strong enough, the timber needs to be an inch thick, I think the weight of all that wood (about 8ft x 4ft) is likely to be more than I'd hoped to add (the Osprey is under 140kg in standard form).

I'm still not clear what thickness of planking will be adequate to support an average adult tread, but I tend to think once more that an arrangement like this is my best bet:

 

[FWB]

Howells will make you one to fit, if it's within your budget.
http://www.kjhowells.com/teak_grating.html

 

[dancrane]

Beautiful gratings...but I think budgetary and weight-considerations will limit me to a simpler, planked set-up.

Are there any wise words about the spacing of individual sections? I don't want to lose tools down the gaps, but too narrow would seem to encourage puddles gathering on top.

 

[Wooden Boat Fittings]

Perhaps the first thing to determine is your frame spacing. From the photo, and if you don't mind a bit of spring, then ½" could well be thick enough. If you want gaps, then ⅜" or even ¼" would probably be enough. But you can have no gaps -- ie solid boards -- if you want. As I said, Aileen Louisa's were solid (that is, built up of narrow boards on cross-pieces, to make solid pieces that sat on the frames), and I never found that a problem. Only a slight heel causes any surface water to slide off the edges and run into the bilge. Alternatively, you could have just have one or two gaps down the centreline if you wanted, for additional drainage.

Mike

 

[dancrane]

Thanks again Mike. Not sure which photo you were referring to...here's the aft end of my cockpit; it's only a shallow "V" but it also slopes forwards substantially, longitudinally...

...and below, the view forward from the thwart, pre-repaint. You can see how a gallon or so of water seeping up gradually through the self-bailers, makes that a pretty nasty place to sit, or even to park the feet or anything else. And the steepness isn't ideal, even when it's bone dry. May not matter to most Osprey owners, but I want to level mine.

The idea of using a single unbroken deck-surface instead of carefully-aligned planks, is tempting...mostly because I could just buy a sheet of marine ply, cut it into sections which will fit into the cockpit's four 'corners', then round off the curves where the ply meets the cockpit bulkheads, leaving a slot for run-off. Good idea? Here's my cockpit from above:

Still begs the question, (I apologise if anyone answered already), what thickness of ply will suffice? The 12mm I bought this year is very rigid...would 10mm be satisfactory?

 

[Wooden Boat Fittings]

I was referring to the photo in Post #32, but I now take it that that boat is a different type of vessel to the Osprey?

I confess I don't properly understand the Osprey's construction. Are those 'frames' and 'stringers' readily visible in fact sold members encapsulated in fibreglass, and is there just thinnish sheathing between them? If so, were you considering just laying the ply pieces over them and allowing the ply to conform to the curves, or were you proposing to build up underneath the lower edges of the ply with tapered pieces each way, so as to keep them more-or-less horizontal?

Aileen Louisa http://www.woodenboatfittings.com.au/boats/aileenlouisa.htm had no decks, and hence more in the way of burden boards. Were you fitting her type of boards to the Osprey, you'd have one shallow piece extending horizontally from just under the self-bailers to the frame just aft of the c/b case, full-width, then another piece at a lower level each side of the case either to the frame visible between the thwart and the foredeck -- where the level might change upwards again -- or else running all the way to the forward bulkhead at the same level. Aft of the thwart they'd probably extend full-width, but forward it might be better to have them narrower in order to reduce their height a bit.

The problem with doing that in a vessel like the Osprey might be that it raises the helmsman's CG more than you would want.

As to ply thickness, if you're proposing level pieces as I've described, then ⅜" would be fine. If you were not building up under the lower edges but letting the ply sag to conform to the frames and stringers, then ¼" would probably do. (The ½" I referred to earlier was assuming solid timber.)

Also, you're right that a grating would be too heavy for that hull.

I actually like the arrangement shown in the photo in Post #32. The boards look attractive; and they don't attempt to stretch the full width of the hull, hence keeping them lower.

 

[dancrane]

Thanks for your interest Mike. My Osprey is a 40 year-old Mk2, so I reckon the GRP is fairly solid compared with more recent lightweight dinghies, but it must be on the insubstantial side, compared with traditional wooden dayboats such as the unidentified example whose photograph I put in post 32, swiped from the internet.

The Osprey is an unusual boat to be adding a 2nd, technically superfluous layer of decking to...but my use of this old race-boat is fairly unusual, so I'm happy to ignore convention.

I was thinking that the solid, GRP-encapsulated areas (the frames?) would provide the best location for tapered supporting timbers below the ply. I'm relieved that you think 3/8" ply would be stiff enough - I can save quite a lot of weight and money if I go for 10mm instead of 12mm!

Raising my centre of gravity won't be a problem, I think...I generally hike on the gunwale but will single-hand her using the trapeze in moderate winds, once I've mastered it.

Raising the floor will improve comfort and usability of the space within the cockpit, where at present dropped items roll into the flooded 'gutter' either side of the centreboard, and can be hard to retrieve. In light winds the sidedecks are too far from the centre-line to sit on - doing so makes the boat heel exaggeratedly when singlehanding downwind...

...so it can be temptingly comfortable to sit on the floor, leaning against the cockpit bulkhead...except that at present, the floor slopes away from the bulkhead and is often awash.

I'll update here when I've bought the materials. :encouragement:

 

[Wooden Boat Fittings]

It all sounds good Dan, and I'll be interested in learning how you get on.

Mike

 

[AntarcticPilot]

I may be looking for some soon...but there's such a price-disparity between the ordinary grades and the presumably much more durable marine ply, I'm wary of shelling out.

Hard to believe that stamps or 'brand' markings on wood can invariably be trusted...faking them must be tempting to the fraudulent.

Is there any such thing as a list of thoroughly reliable timber merchants?

If you come across an example of plywood being fraudulently marked as marine ply, with the appropriate BS 1088 markings, then your local Trading standards will be very interested. Reporting such things is the only way of reducing the problem of mis-sold stuff. Trading Standards will certainly follow up goods mis-marked with a BS number, especially if they carry the "kitemark".

That said, "marine ply" is not the same as "decorative ply". I have some perfectly good marine ply (BS 1088 marked from a reputable supplier), but although it has the right qualities, the veneers are bland and uninteresting, with a white colour and no strong grain pattern. Doesn't matter for what I'm using it for, but it wouldn’t look right if it was to be in a visible location.