Choosing wood for a wood/glass Chris Craft Coho

Par is right you shouldn't go around bonding and tabbing things to make the boat stiffer or stronger unless there is a specific weakness that has shown up as a result of the past life. Tabbing the sole to the sides of the hull will cause a "hardpoint" and as the hull flexes it can crack where the sole constrains the movement.

If something is breaking or cracking, you need to understand why and how to fix it, but unless you know that you should put it back together the way it left the factory.

 

F14, keep in mind that PAR has used the expression "epoxy encapsulation" a number of times here.

If part of the structure wants to move and we've epoxied/glassed up an adjoining part and it cannot move, well something's got to give. Eventually it will and you'll have a failed part somewhere.

In the case of my old Silverton I probably went over the top but I encapsulated everything. Everything. However, there are a number of vertical bulkheads inside the cabin. These bulkheads (you could also think of them as partitions) were not tabbed to the inside of the hull by the original builder. Neither were the walls at the rear of the cabin. Years ago as I was taking the boat apart I wondered, "why didn't they tab these?"

Eventually I realized that the builder had designed the boat with some clearance between the bulkheads and the hull so that when the hull topsides flexed inward there was maneuvering room and the bulkheads wouldn't break or the cabinets get wracked or the rear cabin walls fail. The fiberglass topsides can flex inwards and if they were tabbed the loads would be transferred right into the bulkheads. Not good.

But, the cabin sole joins the hull down by the chine. The glass work is very heavy in this area and I don't see how the boat is going to flex much down there. So the cabin sole was tabbed to the hull in the original build and I also tabbed the new sole in when I built it.

You have to look at what was done and think through what you want to do as you restore. If you see something that doesn't seem to make sense, it's time for some pondering.....

MIA

 

Thanks guys.

I can see what you mean by something being bonded or tabbed when it wasn't originally, and forces being transferred to something else that wasn't designed for that (and all that being bad).

When I restored my Bayliner I tabbed, bonded, and filleted everything. I don't have experience rebuilding anything larger than 19' and I can see how flexing is a different story on a 33' cabin cruiser.

There are a couple bulkheads that are tabbed to the hull sides in the case of my Coho.

Just wondering...how much flex does a boat like this exhibit, in good shape? I know how a convertible, pickup, or trailer with a tractor flexes but in my eyes a fiberglass boat like this, with a thick keel, chines, and hull sides just can't really flex. I'm not saying this to go against you guys, just something I haven't been able to comprehend yet.

Maybe part of my thinking is from when I was following my Coho while being transferred by road and seeing the cabin structure move side to side. With how rotten the structure is I probably just need to calm down and not try to overbuild it like the iBoats forum crowd when buying a rotten $400 bowrider or cuddy from craigslist

It's wood and screws, and Chris Craft along with tons of other companies built boats that way, and it worked fine...I need to remember this.

I'll definitely be posting back as I dig deeper and replacement actually takes place. I'm sure there are cases where epoxy and glass would be good to use. As always, thanks again

 

The amount of flex in 'glass structures is astounding. It's not a stiff material at all.

 

While this is not my primary project forum, you guys are definitely worth of consulting especially on the wooden parts of my Coho, so I'll try to keep you guys updated.

The boat is finally inside and the flybridge is off (which makes it look kinda cool), so the teardown will be progressing throughout this fall, winter, spring, and hopefully not into the summer, but we'll see.



It's sharing barn space with like a 30' bullnose Constellation with some bottom work in progress, a nice 35' or so Constellation, and a 35' Commander Sports Fisherman

Back to the exterior wood, which you can more easily see now without the bridge...

I've heard arguments both ways regarding whether wood is "waterproofed" by the epoxy or the glass, which leads to the question of whether it's ok to put a few coats of epoxy on the exterior wood of my cabin without using a layer or two of glass. From what I know, epoxy is brittle without glass, but epoxy itself can waterproof fine, but glass adds lots of strength and impact resistance. But in this case, this wood was not epoxied nor glassed from the factory so I don't think it needs additional strength.

Just trying to get my options kicked around

 

Some mis-conceptions in your last post.

" . . . but epoxy itself can waterproof fine, but glass adds lots of strength and impact resistance . . . "

Yes epoxy will waterproof (completely), unless thinned (biggest mistake wooden builders employ), though this should be qualified with two essential things: film thickness and encapsulation. Both must be complete or it's not waterproof.

'Glass does improve abrasion resistance, but you need quite a bit of it, to improve strength. A light 'glass sheath over a plywood panel, does literally nothing for it's strength, adds only slightly to it's stiffness (depends on fabric type and weight) and impact resistance isn't significantly improved either (again, depends on fabric type and weight).

This all said, there are fabrics that are far superior to 'glass cloth in regard to abrasion resistance, such as Dynel and Xynole, which are 3 and 6 times superior respectively, then a cloth sheathing of similar weight. Impact is a different story, but this too can be greatly supplemented with different fabrics, instead of plane old cloth and their location in the laminate.

" ... I've heard arguments both ways regarding whether wood is "waterproofed" by the epoxy . . . "

No argument if discussed by educated folks. There's no question about it and tests have long proven it. In short, the jury has been long in on this question. An encapsulated piece of wood, with sufficient film thickness (about 10 mil) will be waterproof. The real question is "what is waterproof". Waterproof is generally accepted as being a state where the wood is stable, doesn't experience moisture gain and lose dimensionally and rot doesn't form. Is epoxy 100% water proof, nope, but it's in 97% - 98% waterproof, which limits the amount of moisture gain a hunk of wood can receive. If the wood is encapsulated properly, it will have sufficiently low enough moisture content, to not dimensionally change shape and more importantly, will not have an environment, in which little wood eating beasties can survive.

It all about numbers really. About 17% moisture content and the wood will harbor a place for the beasties and will suffer moisture gain movement. If you lock the wood down with epoxy at 12% moisture content (the usual recommendation), then it might gain a percent in moisture content from immersion, but it's still well below the levels necessary for wood to move or have beasties grow in it, so it's stable and safe. If you encapsulate wood with a higher moisture content, say 15%, you risk getting near the threshold of movement and beastie hang out. I've done a few projects with 15% moisture content without issue, but I was really just pushing my luck.

On your boat, epoxy coating aren't as important as you might think. A coating on the outside of a cabin side, isn't going to waterproof it. It'll still be able to suck up moisture vapor from it's other edges and faces, so nothing has changed, except a real costly coating that limits how the moisture can get back out. As a coating, epoxy needs to fully encapsulate wood to be a truly effective waterproofer.

There can be occasions where you might want to do this anyway, such as hardening up a softwood surface or improving impact and abrasion protection under a varnished finish, but mostly, traditionally built boat don't need, nor is it desirable to use epoxy as a coating.

Epoxy can be considered the save all and end all by some (not me), while other still buy into 20 year old marketing ploys about the wonders of this goo. Facts are in, tests repeatedly preformed and it's not a magic goo (though it does taste pretty good on toast, once you get used to the twitching), but is a fabulous adhesive with no rivals and a wonderful coating if properly done.

 

Thank you again PAR. I have an idea of what's going on when it comes to boat restoration but by no means do I want to portray myself as being experienced or an expert.

And thanks for correcting my misconceptions. I do not mean to give a short reply to your extensive reply but know that I am taking all you've said into consideration.

From your knowledge and knowledge gained from others and my own reading I'm thinking to skip "encapsulating" the exterior cabin wood with epoxy or glass, which sounds like the thing you would do or support.

 

I've done a few restores on boats just like yours and in all cases, the cabin sides were not encapsulated. Again, in traditional builds (like yours) encapsulation isn't as often used, as it is in new construction. This is usually because the part isn't removed, but just refinished and since it can't be completely embalmed in goo, it's just cleaned, fixed and refinished.

 

Another thing: will my primed/painted plywood check after a while?

 

I'm interested in MDO again as well...

 

Yep, primer usually absorbs moisture, amazingly enough, so it need to be over coated soon after.

MDO is very suitable for many things aboard, though as previously mentioned, it's quality is well down from a decade ago. Cabin walls, furniture partitions, cabinetry, etc. all can life with MDO. The veneer count is minimum, so structural applications should have thicker pieces employed, compared to marine grades.

 

Would the painted exterior marine plywood not check?

 

Generally, it's not the type of plywood, so much as the species used. Exterior APA grades are all Douglas fir in the USA, so this is the problem. Douglas fir will check if you just mention the word moisture around it. It'll check under 'glass sheathing in many cases. Most professional don't use this type of plywood because it's just too hard to finish (turns into a washboard if sanded and the checking issue).

MDO is made from Douglas fir, though skinned with rosin paper. The rosin paper helps a bit, but it still can check under it, which shows through the paper.

The only way to insure Douglas fir will not check is a complete encapsulation, using a little more film thickness then normal and a fairly heavy sheathing. Even with this, there are no guarantees, but it's as good as it'll get. If the sheathing is at least 10 ounces, you have a good chance of controlling the stresses in the plywood that cause checking. Two layers of 6 ounce will work better.

These types of plywood are typically reserved for bulkheads that will be skinned with something, interior furnishings, partitions, etc. Things that will be veneered, skinned with Formica, out of sight, etc.

 

Would you have to go with something I've heard of being called maranti? Or similar? (to avoid checking)

 

Some folks say Okume and Meranti don't check. That is not entirely my experience although I would say they are far better than Douglas Fir in that regard, but if you bright finish them you should use at least a 6 oz cloth over them to prevent checking if they are in the sun a lot.

My 30 year race old mostly meranti boat was a garage queen and it still had some checking on the flat surfaces that were exposed to the sun. It was coated with a polyureathane varnish and it checked around some places were water got to the ply underneith some fittings, and (I belive) as a result of getting wet from time to time, it didn't rot, but it did have some checking.