Crack in strip built dingy hull! need to repair.

[dmede]

Friend of mine has a 15’ compumarine dingy that he built. It’s WRC strip built and sheathed with cloth in epoxy inside and out. Inside is one layer. Outside is 2 layers overlapped so that the center 2 – 3 feet of the hull are double layered but the rest up to the sheer is single layer. The boat has only one full thwart at midship with two side benches in the rear and a bow seat up front.

The bottom from the center thwart to the bow seat has always deflected a lot in chop. Apparently it was flexing right along the line between the double layer and single layer in the outside cloth and over the weekend if finally cracked. There is now an open crack in the inner cloth and cedar about 2.5’ long from the center thwart angling up towards the bow right along the overlap line. The outside cloth is ok but shows stress along the same line.

The options right now appear to be:
a) sand down affected area; cover entire outside bottom hull in another layer of glass closer to half way up the sides to move the weak point between layers above the chop. Epoxy tape inside along crack.

b) same as above but lay a full sheet of cloth inside instead of just taping.

c) Same as a or b but also build in a false bottom. Several ribs of marine ply cut to fit the floor and make a new flat floor 2-3 inches above the old one; fill with foam; epoxy down a marine ply floor over the ribs; seal so that the foam filled compartment is water tight.

The builder is leaning towards c and after a harrowing ride in with that crack staring me in the face, so am I. Any thoughts on how to proceed?

Thanks,
Dave

[ 10-12-2004, 01:34 PM: Message edited by: dmede ]

[Buddy]

I would reduce the unsupported panel size. Do this by adding to the inside about three hat section stringers fore and aft, spaced maybe 12"to 14" apart, tapering closer together as they move forward in following the curve of the hull they "choose" naturally. Use cardboard tubes, foam strips, even hot glue hose but get a 1 1/4 to 1 1/2" section to it. Epoxy two layers of 17 oz biaxial glass over it to make your stringers. Leave the form inside. You could make "ribs" atwartship and reduce the unsupported panel sections that way, but fore and aft will give your boat more torsonial and longitudinal rigidity at the same time. That's what most lightweight dinghies could use the benefit os as well as long as you are adding meat.

[Cuyahoga Chuck]

First things first.
Thwarts in a canoe aren't for sitting. They tie the sides together to keep the sides from flexing outward. The wales and thwarts combine to form a ladder framework that stiffens the boat greatly.
A 15' canoe with only one thwart can't be expected to ward off flexing.
The boat needs either more thwarts or some other device to keep flexing to a minimum.
Kevlar racing canoes degrade the same way. The demand to keep weight down means thay have only a minimal structure to contol flexing. The lifespan of the boat is determined by how many patches the racer is willing to tolerate.
Charlie

[dmede]

Thanks for the reply Charlie, it's actually a dingy, as stated in the first sentence of my post ;)

I should have called the center thwart a center seat I suppose, as it is for sitting. But I wanted to illustrate the fact that it runs athwart ship unlike the other seats.

[dmede]

Where is everybody? You guys all on vacation?

bump

[Bob Smalser]

Goo on fabric is something I've never done in my life, 'cept in some small repairs...

...but seems to me if the cedar core is cracked crossgrain all the way thru for an expanse of 2 1/2 feet, the boat is totaled....more fabric and false floors won't fix it for long.

[George Roberts]

dmede ---

I will agree with Bob that it is totaled.

From your description I believe that a poor choice of scantlings was made.

[ 10-13-2004, 03:15 PM: Message edited by: George Roberts ]

[dmede]

Originally posted by George Roberts:
dmede ---

I will agree with Bob that it is totaled.

From your description I believe that a poor choice of scantlings was made.Well the boat was built per the plans and a good job was done. This is the result of too much pounding on a hull that wasn't meant to be in chop.

I disagree with Bob here. The cedar strips are only 1/4" thick x 3/4" wide and the crack runs across only 2 or 3 of these at an angle almost inline with the grain. They act primarily as a mold for the fiberglass. The crack can be opened a bit, epoxy can be squeezed in and they will be just fine to continue to support the shape of the boat as before.

The real issue is how to strengthen the hull after sealing the crack. I’m surprised to see two people think this boat is "totaled" due to such a small break. The boat is basically a fiberglass boat with a wood core. There should be no reason a well made repair wont leave this boat better than before.

[Bob Smalser]

Originally posted by dmede:

The cedar strips are only 1/4" thick x 3/4" wide and the crack runs across only 2 or 3 of these at an angle almost inline with the grain. That's not quite the picture I got from Post One:


...now an open crack in the inner cloth and cedar about 2.5’ long I'd fill the crack from the inside with epoxy under light heat til all the bubbling was done and I was certain the cracks in the cedar were completely filled.....feather in a fabric/epoxy patch for reinforcement in the interior....flip the boat....grind off the partial 2d layer of fabric....and add one or even two more entire layers of heavy fabric and goo.

This boat have anything that functions as a keel that can be built up for longitudinal strength? The stress in a chop is on the outside of the hull and that's where any mechanical reinforcement needs to be.....I doubt adding false floors and foam to the inside won't do anything but speed up rot. But if there were a structural member there, I suspect reinforcing it could negate a third layer of fabric.

[ 10-13-2004, 06:21 PM: Message edited by: Bob Smalser ]

[Todd Bradshaw]

Of course it isn't "totaled"! That's nuts. As long as you can get it to hold it's designed shape long enough to reinforce it, it will be fine. Depending upon how much reinforcement it takes, you may or may not want to clear-finish it, but as you mentioned, the wood is simply a core. The same type of cored construction could be made using much weaker materials (like foam strips) and the boat would work fine.

It's certainly possible to take a sabre-saw and planes to the cracked area if needed - cut out any bad wood, replace it over temporary forms, sand it, glass it and then have an "as good as new" core - but you probably won't have to. As long as the current core is dry and you can get it back into shape and epoxy in the crack, it isn't the least bit difficult to exceed the original grain strength of the core. Once the core is fixed, you can proceed to reinforce the hull to prevent future flexing.

Unless normal use is wearing the fiberglass thin on the outside, cutting through it, or you see examples of small whitish fractures spread around the areas which flexed in use, it probably doesn't need another layer on the outside. The break you have was caused by the inside being too weak, not the outside. The bouncing puts the inside glass layers in tension and they weren't up to the job. This left them unable to properly support the core, so it broke as well. The outside glass was damaged by compression, which is to be expected when the inner layers and core fail and the boat folds inward. The biggest mistake you can make on a stripper in my opinion is failure to properly reinforce the inside of the sandwich.

At a minimum, the inside could probably use a "bilge cookie" of extra fiberglass covering the bottom and up running up to a level slightly above the waterline. Buddy's idea of installing internal ribs would also be a good idea. On clear-finished, round-bottomed hulls, I've used either cedar or balsa pieces that are sort of a tall, half-roundish shape, typically about 1" wide. Their ends are sanded to taper-out and they are epoxied to the inside glass with weights to keep them down tight to the bottom shape while the epoxy cures. After that, they are glassed-over using bias-cut strips of fabric (two layers of six oz. makes a pretty tough rib, but one layer will usually work. Tape with woven edges doesn't drape all that well over bumps, so you're probably better off with cut edges). Cross-ribs tend to be stiffer than lengthwise ribs (stringers), though either should work. The boat obviously held together this long, so the scantlings can't be all that far off.

Once cured, the edges of the strips of glass can be carefully feathered out with a sander. Let the glass cure for a week or so first if you're doing a natural finish. Otherwise the green resin may tend to smear a bit and "bruise" the cloth, leaving whitish areas. The feathered areas wind-up smooth, in contrast to the weave right next to them, so if you want a really clean installation plan on a filler coat or two either right after applying the strips, or after feathering their edges. If you're a big believer in the whole "chemical bond thing" (I'm not) fill right after applying the strips. Otherwise, fill when the feathering is done. From certain angles, direct sunlight may "light-up" the feathered areas where you cut into the glass fibers slightly differently from the rest of the glass. It's really not very noticable though, even on a clear finish.

The toughest part of the whole project may be getting the paint, varnish, etc. off of the inside epoxy, which is important in getting a good bond and which may require fairly heavy sanding any place you intend to add fiberglass.

Forget the foam-filled plywood floor idea. They always leak and eventually you'll have water in the foam.

[JimConlin]

What Todd said.
Another possible 'thwartship stiffener counstruction would be a trapezoidal foam strip maybe 1/2" x 2" with 2 or 3 layers of 6 oz. glass fabric. This is often seen in kevlar or carbon canoes.

[dmede]

Sorry Bob, I should have said that the inner cloth had split open through the wood core to the outer cloth, but not through the outter cloth. The crack is thin buit 2.5 feet long in the direction of the strips (back to front) and crosses 2 or 3 of the strips. Hard to explain without a pic. but I think you got it now.

No keel really. It's all strip with a soild transom and stem. The idea behind epoxying in a floor over a series of epoxied in floor ribs was to give extra support to the sides along the waterline where the compression is occuring. Basically the boat has too much open space between the center seat (which runs across the boat) to the stem and it was flexing in (compressing the outter layer of glass and streatching the inner). You can see the boat here:

http://www.compumarine.com/images_jpeg/Ventura6.jpg
http://www.compumarine.com/images_jpeg/Ventura3.jpg

[Todd Bradshaw]

Who's idea was it to put bulkheads on the thwart? It makes for a monster stress riser. If you ever hit anything the hull is liable to flex until you come to the first thwart bulkhead and then break transversely at the bulkhead. In this case, I would absolutely build-in some ribs or stringers (and/or a keelson?)probably fore-and-aft, to ease the transition along the bulkheads and would butt the stringers up against them. In fact, I'd probably go bow to stern with them, butting against all the bulkheads.