underlayment plywood

[sdakotadoug]

I have ready access to 1/4 inch baltic birch underlayment plywood in 4x8 foot sheets. This is made with waterproof phenolic glue and is advertized as void free. Is this an adequest substitute for marine plywood? I am considering building the Goat Island Skiff. Thanks Doug

[Boatmik]

HOwdy Doug,

The Goat is a very stylish boat ... so it depends on what you want to do.

It is best in Gaboon (sometimes called Okoume) ply. This is because it is so light, it results in a light hull that two people can move around on land by themselves OK.

You go for something heavier and the boat is going to be more difficult to move around and it will take a little edge of its sailing performance.

I prefer the 1/4 ply to have five veneers and have nice faces. If you are prepared to accept the higher weight ... that is up to you.

Maybe some of the US readers of this forum know this particular ply.

Best wishes
Michael.

[sdakotadoug]

It is a 5 layer ply. I should be able to find the weight for an individual sheet and compare it with a individual weight for the Okoume. Correct? Dout

[Boatmik]

yes .. should be from 50% heavier to 100% heavier - I haven't looked up the density of the Baltic.

MIK

[PAR]

Sweet birch specific gravity is .6 to .7, while yellow birch is .5 to .66, both quite heavy. Compared to fur at .35 to .45, birch is about the same as most oaks, Mahogany is .4 to .45, Okoume is .3 to .35, teak is .50 to .6 (okay sometimes more). All these specific gravity samples are with a stable substrate at 12% moisture content.

1/4" Okoume (Gaboon) is around 13 pounds per panel, while birch will be 22 to 24 pounds per sheet. This is for 5 ply BS 1088 compliant material

[sdakotadoug]

My sources here in the US quote 21 pounds for the Okoume and 27 for the Baltic Birch. Or 6 pounds per sheet, for about 30 pounds extra for the full sized 4x8 ft sheets. In the overall is this unexceptable for weight? It would involve a 300 + mile trip one way, to pick up the Okoume. And $100 a sheet versus $24 a sheet. Thanks for all your input so far. Doug

[PAR]

Okoume is 20 pounds per 1/4" sheet, Meranti (Hydrocore) is 25, Sapele (African mahogany) is 24, Poplar is about 19 pounds per 1/4" sheet.

Birch is a fairly dense hardwood and just about twice the weight of Okoume.

For an accurate look at all the important physical attributes of most commonly available woods, import and domestic, down load the "Wood Handbook". It's document FPL-GTR-113 from the US department of Agriculture.

Again, most birch is cabinet grade and employed in "decorative" applications only. Aircraft grades and something called Baltic birch marine, are available, though many have drop the marine versions, both have WBP adhesives. These aircraft grades are quite good, having equal thickness veneers and good construction practices. It has poor to none rot resistance.

The bottom line, especially if the panels are thin (less then 3/4") is to look at the outer veneers. If they're about the same thickness as the internal veneers, then you may have a good sheet, but more often then not, the outer veneers will be paper thin, which provides no longitudinal strength to the panel.

[Boatmik]

Howdy Doug,

It depends on you. The most important thing is that the ply be decent quality.

The second thing is a hull around 130lbs can be picked up by two adults OK, but 160lbs is getting a bit steep.

Then there are some differences for performance as well.

Just make sure that you will be happy with the increased weight too. Because ... Once it is built ... it is built.

Best wishes
Michael

[arbordg]

My sources here in the US quote 21 pounds for the Okoume and 27 for the Baltic Birch. Or 6 pounds per sheet, for about 30 pounds extra for the full sized 4x8 ft sheets. In the overall is this unexceptable for weight? It would involve a 300 + mile trip one way, to pick up the Okoume. And $100 a sheet versus $24 a sheet. Thanks for all your input so far. Doug

Doug - we built ours per spec. As it turns out, the 1/4" is a little light/fragile for the heavy-handed uses to which we put her (as beginning sailors learning and making lots of mistakes in a variety of locales). I've had to repair two cracks/holes in the hull. Also, she's a bit of a handful for a solo sailor when said sailor is one in name only. There are times now when I take along some portable ballast (a pair of 70# sandbags). For those two reasons - if I had it to do over again, I might be tempted to make the bottom heavier... maybe out of 3/8" material.

This may just be my blithe ignorance showing. I know that adding weight goes away from one of the core design goals of this boat. What I don't know is how far the extra weight would go toward solving the issues I mentioned. I also don't know how much degradation in performance would actually occur. Therefore, I don't absolutely know whether I'd do the 3/8"... but I'd be tempted. In fact, at this point, I'm planning on sheathing the bottom & chines only in xynole come Fall. I hope I don't muck up a really great boat

What I would not do - regardless of the concerns expressed above - is build the whole boat out of a heavier material. Mik is far too polite and easygoing to come out and call you or I a "bloody fool", but I'm sure he is cringing inside. Of course, I'm sure there's a part of him - as a designer - that's a bit curious. You might be doing a public service to make youself a test case: what happens when this boat is built double the weight intended by the designer? Does it actually make no appreciable difference, or does it turn the gazelle into a warthog? I'm kind of doing the same line of experimenting by glassing the bottom. My results, however, won't be in before you need to make up your mind


"If the people who make the decisions are also the people who will bear the consequences of those decisions, perhaps better decisions will result" -- John Abrams

[PAR]

I'm not speaking for Michael, but I'm frequently faced with "changes" by builders. The more skilled the builder, the more likely they'll call and ask what I think about the idea. It's the back yard builders, the first timer and novice builders that seem to fall into the "over build" pit. Their logic is that the boat will be stronger, but in the end they learn engineering lessons the hard way. Most times, making things "stouter" usually doesn't make the boat stronger, in fact more often then not it's weaker, because of added point loading or mass, etc.

You get use to it, though I some times get offended (not an easy thing for me).

I have a client that did just this. The bottom planks where to be 1/2" plywood, he elected to use two layers of 1/2". The topside planking was to be 3/8", he doubled it, ditto with the cabin roof. He applied a teak tongue and groove deck over the plywood I speced and installed furniture, built from 3/4" solid lumber. The boat, a classic shaped and rigged 18' LWL sloop. It displaced over twice (nearly three times actually) it's intended design and nearly capsized on launch day. He's since realized the comedy of errors and now is adding a 4" thick false bottom, so the boat can carry it's builder's burden. She'll never be right again, though maybe it can be sailed at it's relaunching this time.

If I took these things personally, I'd have been locked up for murder a long time ago. If you butcher one of my designs badly enough, I'll send a polite letter asking you the name of the "new" design you've created, as I don't want mine on any part of it.

[sdakotadoug]

Thanks all for your input. I've not made a decision, but at least now I can make an informed decision. Doug

[Boatmik]

Doug - we built ours per spec. As it turns out, the 1/4" is a little light/fragile for the heavy-handed uses to which we put her (as beginning sailors learning and making lots of mistakes in a variety of locales). I've had to repair two cracks/holes in the hull. Also, she's a bit of a handful for a solo sailor when said sailor is one in name only. There are times now when I take along some portable ballast (a pair of 70# sandbags). For those two reasons - if I had it to do over again, I might be tempted to make the bottom heavier... maybe out of 3/8" material.

Hey David, You never told me about the cracks! 5 years is not too bad though. I have cracked Beth maybe three times, but she never sprang a serious leak and the repair in each case was dead simple.

Is it 5 ply plywood or only 3 ply (ie layers)?

5 ply is very hard to crack or break, but some 3 ply makes it a bit easer.

(This next bit is for people reading the thread who don't know about the background of David and my discussions)
As you note the sandbags add 140lbs - and can be readily removed when there are enough bodies to make up the weight - nothing increases stability as fast as bodies or gear ... remembering the Goat is really designed as a 2 person boat and it can be reefed without changing the sail balance much at all when only one person needs to be carried.

Bolger writes a lot about the stability virtue of a thick bottom ... but it really only starts to be effective when the boat is quite large. Adding a greater thickness of bottom will not add stability.

However going up one extra thickness of ply for the bottom adds a huge amount of strength and stiffness

Actually TOO MUCH stiffness and strength in my book - which is why the bottom is not specified in 9, 12, or 15 but in 6.
(David, I am making the point for those who endlessly add weight to a boat thinking that they are making it "better" in some way).

But this is where I agree. If a boat is going to be in a rough use situation most people add a lot of glass. This is much heavier than making the ply a little thicker.

However I think you would have to toss up between the effects of making the bottom thicker or using a very light layer of glass - maybe 2oz (75gsm) or less.

I should calculate out the weight difference between the two.

But just to go back to the main point. And it is a philosophical one - so maybe you (the general reader) can see the strength of it or maybe you disagree.

I design boats where maybe in some unusual circumstances there will be some damage to the hull. Like the two cracks in David's 5 years of use (is it five David?). But the intention is that you just whack some duct tape over the cracks and keep sailing. When you get back home ... you fix. The fix is simple because the structure is simple and clean.

The other aspect is the arrangement of the structure means that any crack is restricted from going anywhere because the hull is criss crossed with so much structure. The tricky part is to make that criss crossed structure look simple ... it is a bit of a specialty - but I did pick it up from Bolger and OZ Designer Murray Isles who use those methods extensively.

But my own feeling is that ... "you can always hit something hard enough to break it" ... so you can build anything harder and heavier to avoid damage and very quickly it becomes like PAR's boat above. Each detail ends up that bit heavier and finally it all has its inevitable result.

So if people do want to build one of my designs heavier ... they can know what to expect. )


MIK

[arbordg]

Hey David, You never told me about the cracks! 5 years is not too bad though. I have cracked Beth maybe three times, but she never sprang a serious leak and the repair in each case was dead simple.

Is it 5 ply plywood or only 3 ply (ie layers)?

5 ply is very hard to crack or break, but some 3 ply makes it a bit easer.

(This next bit is for people reading the thread who don't know about the background of David and my discussions)

As you note the sandbags add 140lbs - and can be readily removed when there are enough bodies to make up the weight - nothing increases stability as fast as bodies or gear ... remembering the Goat is really designed as a 2 person boat and it can be reefed without changing the sail balance much at all when only one person needs to be carried.

Bolger writes a lot about the stability virtue of a thick bottom ... but it really only starts to be effective when the boat is quite large. Adding a greater thickness of bottom will not add stability.

However going up one extra thickness of ply for the bottom adds a huge amount of strength and stiffness

Actually TOO MUCH stiffness and strength in my book - which is why the bottom is not specified in 9, 12, or 15 but in 6.
(David, I am making the point for those who endlessly add weight to a boat thinking that they are making it "better" in some way).

But this is where I agree. If a boat is going to be in a rough use situation most people add a lot of glass. This is much heavier than making the ply a little thicker.

However I think you would have to toss up between the effects of making the bottom thicker or using a very light layer of glass - maybe 2oz (75gsm) or less.

I should calculate out the weight difference between the two.

But just to go back to the main point. And it is a philosophical one - so maybe you (the general reader) can see the strength of it or maybe you disagree.

I design boats where maybe in some unusual circumstances there will be some damage to the hull. Like the two cracks in David's 5 years of use (is it five David?). But the intention is that you just whack some duct tape over the cracks and keep sailing. When you get back home ... you fix. The fix is simple because the structure is simple and clean.

But my own feeling is that ... "you can always hit something hard enough to break it" ... so you can build anything harder and heavier to avoid damage and very quickly it becomes like PAR's boat above. Each detail ends up that bit heavier and finally it all has its inevitable result.

So if people do want to build one of my designs heavier ... they can know what to expect. :L-)


MIK

Hey Mik,

Our hull is 5 ply 6mm BS6566 Joubert Occoume throughout. The only exception is that - after a rather beefy friend lost his balance and put his knee through the center thwart - we replaced it with 9mm BS1088. We have abused poor Sisu rather dramatically, but she never took any damage till that one day at Timothy Lake. Winds 25-30 mph. Me reefed down to a handkerchief, and clenched down to knats-navel size... trying to land on the rocky, stump-filled, churned up shore with an on-shore wind. Just so you get the full flavor - just after I beached, a much heavier Michalak designed 20' cabin cruiser (with a very experienced skipper) also tried to land. He ended up pinballing down the lakeshore - ricocheting from rock to stump to rock. He didn't hole her (she has 1/2" plywood sides) but did end up replacing one exterior chine and one rubrail.

I cracked Sisu's bottom in two places - one on each side of the daggerboard case. They didn't leak, and I later repaired with a sort of modified Payson Butt Patch. That is, I sanded an oversized area (which included the fracture) of the plywood thin - then applied a lightweight patch of glass cloth in epoxy. Hotcoated, cured, sanded and repainted. I also punched a hole clear through each side up at the bow, mostly above the fore thwart. That was a bit more of a fix, but is all secured now (though not quite as perfectly fair as when new). The cracks came at the end of our third season of use... her first as a sailing vessel (and my first as a sailboat skipper).

I regard the sandbags as my training wheels. By the end of last summer, I was seldom bothering with them, and didn't (but should have) that wild day at Timothy Lake. It's still amazing to me how much more rock-solid-stable she becomes with a second body or the sandbags aboard.

So, I guess we can tack on an addendum to your theorem: "You can always hit something hard enough to break it... and Graybeal's just the daft bugger to do so"

All in all, I'd have to say that the Goat Island Skiff has taken far better care of us then we have of her. We all absolutely love her. I organized another outing for Saturday at a new lake with a bunch of the Coots. John K is even coming up from Eugene (100+ miles)... so there might be some fotos to share.

"Life is either a daring adventure or nothing. Security does not exist in Nature, nor do the children of men as a whole experience it. Avoiding danger is no safer in the long run than exposure" -- Helen Keller

[Boatmik]

Hey Mik,

So, I guess we can tack on an addendum to your theorem: "You can always hit something hard enough to break it... and Graybeal's just the daft bugger to do so"

All in all, I'd have to say that the Goat Island Skiff has taken far better care of us then we have of her. We all absolutely love her. I organized another outing for Saturday at a new lake with a bunch of the Coots. John K is even coming up from Eugene (100+ miles)... so there might be some fotos to share.

"Life is either a daring adventure or nothing. Security does not exist in Nature, nor do the children of men as a whole experience it. Avoiding danger is no safer in the long run than exposure" -- Helen Keller

I left the quote at the end because it is so perfect!

As far as a "daft bugger". Maybe you are ... but putting holes in boats is not part of the reason you are. Remember ... I admitted to cracking BETH's hull a couple of times. Once with the help of a gaff rigged keeled sloop that tried to sail over the top of her. By they way ... they didn't apologise or check if I was OK or anything ... just sailed off.

These are the things that happen when sailing ... and are part of the adventure. A boat with scars has its own story to tell too.

Like I said ... I love to see repairs in a wooden boat! Something like "patina" in antiques.

Fix 'em and keep going.

MIK

[b.o.a.t.]

"Life is either a daring adventure or nothing. Security does not exist in Nature, nor do the children of men as a whole experience it. Avoiding danger is no safer in the long run than exposure" -- Helen Keller

Thanks for a brilliant quote David.
That one is going straight onto the fridge.
Or maybe the back of the dunny door.

Probably not a good one for the fire truck dash-board though...

cheers