covering plywood on carvel planking

what do you think about covering plywood on an old traditional carvel planked boat. some think that instead of caulking every year, glue plywood on planking with epoxy and apply the painting process on plywood.

cheers
ulas

 

Sounds like a last ditch effort to add a couple of years to an old hull rather than a way to reduce maintanance. Old hulls have been sheathed with glass/epoxy too, but usually when nothing more can be done short of replacing the original planking.
How is the plywood installed? Diagonally?

 

I have a question along similar lines but for new construction

If you were to plank the side and bottom with boards slightly spaced apart and covered that with ply, could you get by with thinner ply scantlings and use less marine ply?

epoxy sealed of course

 

Ulas,

Tetranora was originally not a classical carvel planked boat! She was already a composite build using resorcinol. Then as you well know she was sheathed in the mid 90ies.
Now, what is your goal? Are there issues in the outer shell? Have you ran her on the rocks?

 

Richard seems to have us at a disavantage here. Was this the boat you looked over last year Richard?

In any case, with all types of repairs like this, the basic hull structure, including the planking needs to be in good shape for it to work. Naturally, just nailing plywood over the hull can't work, unless you diagonally strip it. This will present if not just as many seams, very likely many more seams to the potential of leaking. This said, it can hold a carvel (or other type of build) together quite well, as the plywood is dimensionally stable and helps hold the original planking movement in check.

I'm not sure of the build method, but this can be an effective treatment, but it comes with the same warnings that other techniques of this nature, such as strip planking, diagonal veneers, etc. Usually, it costs more to do this, then to just make conventional repairs, but each project needs an evaluation and projected cost comparisons.

To give an idea. Several years ago, I had a 40' cedar over oak powerboat in for a similar treatment. I removed each plank to the carvel classic, then resawed each plank to 2/3's of it's original thickness. I restored the fastener holes in both the frames and planks, then rehung the thinner planking. This was followed by a diagonal veneer of 9 mm plywood over the whole hull. The seams ground down and filled with biax and thickened epoxy. The result was a really water tight hull, with no more caulking seams. Naturally, it cost a fair bit more to do this, but the hull will never need to be caulked again. I still occasional work on this yacht and it's bilge has been dusty ever since. In fact, I repainted a new, lower boot stripe on her a number of years ago, because she was always about 9.5 tons on the travel lift at haul out, but now she's about 7.75 tons.

It would have been a lot cheaper to just repair this hull conventionally, but it would have required occasional caulking, which the owner wanted to avoid at all costs.

 

Sure,

surveyed, bought*, and sailed to her new port.

*by Ulas

She was double carvel on frames, hot glued with resorcinol. Sheathed in glass epoxy in 1995 or so. (professionally done)

What I remember from last December, she was structurally in top condition, dry like salt and had no issues visible while afloat.

For that reason I don´t understand the request.

 

Something has "sprung" I think . . .

 

No it is not for tetronora.
it is for my friend's 8 metre old fishing boat. he doesnt want to make caulking every year.

 

actually,
i think this system is popular nowadays among old small boats in Turkey.
of course they don't do the job well.
usually their plankings are not in good condition and they dont make the covering process well, they left too much gaps under plywood and they don't use epoxy properly(sometimes they even dont use epoxy).
But I wonder if it is done properly does it function or not.
PAR gave a good example. It is a labor intensive job, i think he did it in that way not to add to much weight to the hull.
what happens if it is covered with 4mm plywood on the existing dry 2 cm planking with thickened epoxy? dont bother of adding too much weight. Does 4mm thickness create any problems?
And why should it be applied diagonally?

cheers

 

It needs to be applied diagonally because of the compound curvature on the hull. The plywood has to be ripped into fairly narrow widths to fit this compound, say about 6" (152 mm) is average, with 4" (101 mm) in tighter radius areas not unusual. Flatter areas of the hull can tolerate wider plywood strips.

Again this isn't something for the faint of heart, it's a lot of work, fitting these strips of plywood with reasonable precision and then filling and sealing the seams.

I can tell you this, 4 mm plywood over 50 mm solid stock planking isn't going to help much. The movement in the solid planking from moisture gain/lose and loose fasteners will easily over power the shear strength of the plywood and it's fasteners.

 

But would it work on 20 mm planking?...he said "2 cm". There is probably significantly less movement in the thinner 3/4"ish planking than stuff that is 2" thick.

 

Correct me if I'm wrong, but isn't 2 cm 50 mm? 4 mm plywood (3 ply) is fairly weak and I wouldn't use it in this application. The thinnest I'd use is 6 mm 5 ply and this would work on 3/4" softwood planking, though a hardwood planking might need 9 mm. 12 mm would be the bare minimum on 50 mm planking with well done Payson butt joint, three layer biax seams. As I said this is a lot of work. Most would be inclined to strip plank the hull, rather then diagonally plank in plywood, which is still painfully labor intensive. It's very rare to prefer this type of repair and usually reserved for boats of some significance. The USS Constellation had her hull strip planked recently (about 10 years ago) to save planking options.

http://en.wikipedia.org/wiki/File:USS_Constellation_Inner_Harbor.JPG

 

1 mm 1/1000 of a meter...1 cm 1/100 of a meter...1 dm (deci meter) 1/10 of a meter. 1 inch is 25.4 millimeters so I expect you are thinking that 2 cm is 2 inches (50.8 mm) whereas 2 cm is actually 20 mm or just a hair over 3/4" (19 mm). 19 mm and 3/4" are just a few thou (of an inch) apart and you can use the wrenches interchangeably...and they are pretty much all that overlap too until you get to to a multiple of either.

 

Nice that you slowly get a bit familiar with the metric units, thanks Lewis.

And I think just glassing would be the easier way. But of course that would have to be done professionally. Read: No task for a Turkish fisherman.

 

From what I can tell, 2cm = 20mm.

10 millimeters = 1 centimeter
10 centimeters = 1 decimeter
10 decimeters = 1 meter
10 meters = 1 decameter
10 decameters = 1 hectometer
10 hectometers = 1 kilometer

Not that I knew all that, but I knew it went in 10s. I would like to use the system, but it would take awhile to convert things in my head. I know about how long 1 foot is, but 30.48 centimeters just doesn't compute.