Marine ply

[tobble]

I'm looking into building a little ply dingy as a tender to my sabre 27. It's looking like stitch and glue is the way forward, by having studied the subject, I'm confused as to why people always specify Marine ply, rather than just exterior, or even super cheapo stuff: if the whole lot is covered in epoxy, then what difference does it make?!

Discuss...

 

[PCUK]

Good quality exterior ply is as good as marine for the job you are describing. The only diference is that marine ply has no voids in the laminates while exterior may have some. Poor quality exterior ply is just that - poor quality, never buy it from a DIY "Superstore".
For good quality ply, as you say, once covered with epoxy it makes no difference.

 

[duncan]

it's what happens and how quickly when the epoxy sheath gots chipped or penetrated.

however anything but the extremely expensive A1 marine ply is going to suffer pretty badly and quickly from what I have been advised and exterior is as good as the basic 'marine' stuff as already stated.

 

[aztec]

dunno if this helps but...

i buy mine BS1088 marine ply from sydehams, only once had a problem (but i think it was down to the chap picking up the wrong stuff)

made many (oh so many!) cuts... never had a "void" issue.

for less quality jobs i use WPB glued stuff.. well cheap.

as you say as long as it's waterproof you could make one out of MDF.

 

[Blue5]

IMHO I would go for marine ply with the correct BS ref and from a reputable supplier like Silvermans/Timbmet or similar, although marine ply is more expensive you get what you pay for and the potential saving by using cheap lesser grade materials has to be weighed up against the overall cost when it all goes wrong.

 

[maxi]

Built my own sitch & glue tender from Exterior Ply in 1976 ~ still going strong !!

 

[tobble]

[ QUOTE ]
anything but the extremely expensive A1 marine ply is going to suffer

[/ QUOTE ]

I guess once the epoxy is compromised by either a) osmosis - probably won't be significant in the lifetime of the dingy, or b) chipping, then any wood will start to absorb water. The extent to which this is an issue depends on the ply, and the use of the dingy; If the glue starts to go, that shouldn't be a serious issue if it is a relatively small area, and one stems the absorbtion of water to stop it getting worse. If the wood starts to swell, I guess you could potentially have the epoxy lifting, which would make it more prone to chipping. And if the wood starts to rot, it would impair its structural strength, but again this would not be a huge issue if it is confined to a small area and it's progress arrested.

consider the following.

if I make a small ding hitting a submerged rock for example and remove, say, a flake 10mm across... the wood will gradually start absorbing water... I get back to the yacht, haul out the dingy and it stops absorbing water... some of the water penetrates throughout the wood, some evporates off, I see the ding, leave it till it looks dry then patch it... the small amount of water that didn't evaporate disperses thoughout the wood leaving it generally not much more humid than before the event, bearing in mind it will not be free of water as it has been kept in the air which contains some water...

please excuse my ramblings on a ficticious chain of events, but does this bear any resemblance to reality? I guess I probably will be too lazy to haul out the dingy evey time but none the less...


p.s. has anyone actually built a dingy out of 'cheap' ply, if so what were the results?

 

[tobble]

just another thought;

how about using cheap ply, or even MDF as suggested, and doubling or trebbling thickness of the epoxy? This would reduce the significantly reduce the chances of the waterproof layer being compromised, so you have even less to worry about water-damage-wise... guess the extra cost of epoxy would ofset the amount saved on wood, but which would ultimately last longer?

 

[aztec]

that's fine if you're building something special, but for the stuff I do, and for a cheap DIY tender i'd find it hard to justify IMHO only.

personally agree in principal, but for a ply project that will be painted, i can't see the benefits.

 

[johnlilley]

Just one final point, some thinner exterior ply has two thin laminates on the outer surfaces and a thicker centre core. If you get the alignment wrong, this can be very weak as the outer laminates don't add a great deal of strength in the wrong direction. However, it is possible to find good exterior ply with 5 laminates on thin sheets, which, I think would be much more reliable strength wise.
The glue on both exterior & marine should be the same.

 

[aztec]

Hi john,

just to confirm, i researched the British Standard for marine ply before buying the ply for the Osborne decks.

BS 1088 i think, gave WPB (water proof bond?) as the adhesive and varied with the laminations. AA gave two "good" finish sides, AB one good one not so good and BB.. which was exterior finish both sides.

sydenhams had some with multi laminates and some with fewer... for what i needed it didn't matter too much.

iv'e had the 1/4" stuff from them before which has a good finish (hardwood) and seems lo last well.... iv'e has a piece outside for over a year with no protection and no sign of deteriation.

just a "for what it's worth" kinda thing.

if you're about next weekend you'll have to pop in for that tea i promised you!

regards, steve.

 

[fishermantwo]

Tobble,
I have a small dinghy I made from exterior ply, stitch and glue using electrical ties and fibreglass tape on the chines. The ply is painted with oil based paving paint. I have made several of these over the years and I'm expecting about 10 years use out of them.
The first version I sheaved the whole bottom but that is a waste of time and just added weight. The latest version is a small punt I built specifically to fit in the back of my station wagon. Has two wheels on the stern and a decent handle on the bow. I just wheel it down to the waters edge like a wheel barrow with all my crap in it, in the water the wheels give some directional stability. The paving paint is as hard as anything, if it was to be penetrated it would be no different to a damaged fibreglass dinghy.
Very cheap to build, I always have a drum of fibreglass about, build one in a day easy. Epoxy taped chines would be stronger again and cheaper if bought in small amounts. Don't make it too flash or some prick will steal it.

 

[waterrat365]

I built a 11'6" "Cartopper" designed by Bolger about 15 years ago. I made it from 1/4 AC Fir plywood, available anywhere in the US at the time for about $10 a sheet. Best dink I ever owned. I used three sheets of plywood, stitch and glue construction. I took the simple precaution of coating both sides w/epoxy and cloth instead of the usual outside only. Result? Structural integrity is still intact and I should be able to get a few more years from it before I have to replace the gunnels, which are Phillipine Mahogony, which I do not recommend, BTW. The only difference between exterior grade and marine plywood is marine plywood has no internal voids. If you can live with a few voids and are willing to fill the voids in the C side with putty and coat the cut edges before covering w/glass, etc, you'll get a nice long life out of ordinary AC Fir. Cliff Moore, S/V PELORUS, New Jersey