Stringer repair and improvement

[adam aunins]

I read and artical the other day and in it the person used 5/8 garden hose and covered it in glass to form extra ribs to help stiffen up the hull of their boat.

[Phill]

Foam core, Great idea.

20 Years ago, I had a similar problem, altho my balsa wasnt totally wasted.

Since you have aleady removed the stringers (I left mine in place)......

I used Diviny-cell, Closed cell 2lb foam, prescored for easier shaping, 1/2" (wud use 5/8-3/4 if doin again), and foam cored the entire floor area of Lollipop, between the seats (seat removal would violate class rules, and make the hull perminately non-raceable). Under the cuddly I cored the floor to just above the waterline.
Complete old paint removed....miserable
40 grit sanding, to give good bonding tooth for resin.
Shaped 12x12 to 15/15" pieces of foam and "tiled" areas of the floor with the Divinycell. Used Polyester resin with fillers, micro ballons and wood dust to trowel on paste to fasten the tiles of foam to the floor.
After an area was tiles, one side bow to seat front edge for expample, I then fiberglassed that area with chopped mat, 12oz i think. Then repeated above for other side of bow to front of seat tank.. Two layers of glass mat where crew would be stepping. Then repeated above for each side from seat front edge to CB rear edge. The rest of the floor to the stearn as one tiled area and glassed as a whole, again two layers of chopped mat whene stepping on would happen.

Took all winter in my tiny garage, Trailer tongue under my workbench, and transom 3" from garage door, starboard coaming against wall, and 15" of walk space on port side.

Hull properly stiff now.

Just one idea that takes awhile, but my DS was worth it.

For those who are less OCD, the half round tubes glassed in place of the original stringers work pretty good too. They do not need to be formed over PVC. (in fact, you would probably have difficulty getting bond with any kind of resin and pvc). One fleet mate here used cardboard tubes for the forms. Once cured, all the strength will come from the glassed tube shape, not the origianl form. My main problem with the tubes is footing issues. I was setting up Lollipop to sail with my kids and family. If I tripped over the tube stiffener, cud I really trust them to fish me out ?? Vanguard 470's used to have low profile tube shapes that didnt trip me. Maybe you could find a form, or shape foam to a larger radius than a pure half round and get the best of both ideas.

phill

[K.C. Walker]

Phill,

Your timing is perfect! My OCD had just about put me into analysis paralysis! LOL

I kept going back to foamcore being my preferred method. If you are happy after 20 years, that's good enough for me. I have already removed old paint and a lot of the fiberglass that had gone gooey. Yes, a miserable job! I just went out with a magic marker and marked the remaining places that I still need to grind away with 40 grit. Thankfully, it shouldn't be much more than another hour or two of grinding.

Thanks for the detailed description of your technique and also your experience and recommendations. This helps a lot! I like the "tile" method. I was trying to decide how I would be sure that I had proper bonding without vacuum. I think with the smaller pieces it will be easy enough to weight them down and see the squeeze out.

So, maybe soon I will be tiling the floor of my DS! That should be classier than balsa wood parquet.

Thanks again!

KC

[DS 1858]

I assume carbon is not class legal in the DS, something to consider.

I replaced wet wood core stringers before. Used foam hotwater pipe insulation from Home Depot for the core. The foam made a a good lightweight non-water absorbing form. The real strength comes from the glass and the core acts as the form. These stringers are much stronger than my original ones.

[K.C. Walker]

I assume carbon is not class legal in the DS, something to consider.

DS 1858

I'm not sure that carbon fiber is prohibited. You say that you are assuming, so I'm wondering if you are actually sure if this. I don't see anything in the bylaws that prohibit carbon fiber. It would appear that the relevant section of the bylaws would be this.

5.4. The hull and/or deck may be reinforced or stiffened on the inside. The interior of the
hull and deck may be altered in the location and type of running rigging, additional
ballast installation, conveniences for sail or equipment storage, and wooden seat
replacement, to the extent that the changes are not specifically prohibited elsewhere in
these Specifications.

If it is prohibited I would like to know that so that I don't disqualify my boat.

KC

[Phill]

I know for sure, that any exotic fibers, carbon, kevlar etc, is forbidden in the building of the CB and Rudder.

For stiffening, shoot a question to the Measurer, Bob Damon is still 'IT' i thinik. I have been see'n carbon fiber tiller extensions and spin poles.

For stiffening, I would question the value of the extra stiffness that an exotic might add. Given that the boat has some flexing as a whole, I wonder if maximizing stiffness in one area, might force another area to carry more of the flex and.....fail ???

just about .02 cents worth.

[K.C. Walker]

Phill,

I will shoot an e-mail to Bob to get the word on carbon fiber.

I think your thoughts of maximizing the stiffness in one area only to increased stress in another is certainly something to consider. GreenLake suggested that stringer ends should be tapered out at the ends as well as tapered on their edges to distribute stress for this very reason. This of course is a very good engineering concept to keep in mind. Hard spots in fiberglass hulls are known problem spots. Things like bulkheads and in our case seat flotation tank edges and centerboard trunks are places of high stress, because of hard spots.

My thoughts regarding carbon fiber reinforcement would be to try to minimize these hard spots in my hull as much as possible. Because I have the inside of my hull stripped and sanded down I can see a lot of the stress points that have damage in them from 35 years of use. I don't know if it's overkill to use carbon fiber, but I don't think so. It would not take very much to make a considerable difference in the strength.

I would consider the keel to be the most beneficial place to use carbon fiber. Extending the keel reinforcement from the centerboard trunk to the transom would make the most sense to me. Therefore you would be minimizing the hard spot at the aft end of the centerboard trunk and tying it into the transom.

Thanks for your thoughts.

KC

[GreenLake]

K.C.

I always thought that the benefit of carbon is that you can reach the same strength with less weight (and volume). Because certain structures have effective weight (or volume) constraints, carbon can be effective in allowing those structures to be stronger. Somehow, I'm not convinced that these benefits are crucial in this case. Can you estimate some numbers that would make your case?

Also, to go back to stringers for a moment, I've had a question. I've wondered about the placement of stringers in the lengthwise direction, given that the hull is reinforced by the (shallow) V shape of the keel, which should resist flexing front/back and by the seat edges, which also resist flexing front/back. Neither of these supports directly resists flexing where the middle of the floor is higher than the sides, so a lengthwise stringer would seem to be placed where it simply rides up and down with the hull. (Should be true fully for the middle of the stringer, less so for its ends).

Coring the hull would seem to add that missing sideways support, so I'm not surprised that it appears to be successful. But I was speculating that short sideways stringers might work well for those of us not contemplating a full cored hull. I wonder if anyone has tried that, or has some compelling arguments why this line of thinking is heading in the wrong direction.

[K.C. Walker]

GreenLake,

Okay, nothing like requesting numbers to bring things into perspective. I am now of the opinion that you are correct about carbon fiber versus fiberglass in this case. I do not think that it is crucial and may even be a problem. My experience with carbon fiber is in making guitars and model airplanes. In both cases my goal was maximum strength and stiffness with a minimum of weight. In neither case did toughness play a factor. Where as in boating it is a definite consideration.

It seems that carbon fiber mixed with fiberglass in a matrix needs to be carefully engineered. What seems to happen is with the significantly higher tensile and compressive strength of carbon fiber (as opposed to fiberglass) it takes on a localized load and so you have to be very careful not to engineer in stress points. So, I think that I will just stick with fiberglass in this case. If I want more stiffness I'll just add a stringer on top of the core.

I did a test of 2 layers of 12 ounce carbon fiber cloth epoxied on each side of a quarter inch square piece of straight grained mahogany. The strength and stiffness increase is astounding. The piece was 18 inches long and I decided to see how it would fail. I suspended it on each end from blocks of wood and stood on the middle. It took a surprising amount of weight to get it to fail. When it did fail it was the mahogany that failed first in sheer. It split lengthwise towards the middle of the mahogany.

If I didn't have "tired" fiberglass for the bottom of the hull of my boat, I wouldn't be considering coring. It just seems a good solution to my problem. I think that coring will take care of both lengthwise and widthwise stiffness. I think if you're stringers are in good shape and the fiberglass skin is in good enough shape and you just want to add some stiffness, going with ribs makes a lot of sense. A lot of powerboats use a grid pattern of stringers and ribs.

The West System manuals have pretty good information in them and they have PDFs online. Here's a link to their fiberglass repair manual http://www.westsystem.com/ss/assets/How ... enance.pdf .

Thanks again for your thoughts!

KC

[K.C. Walker]

I know for sure, that any exotic fibers, carbon, kevlar etc, is forbidden in the building of the CB and Rudder.

For stiffening, shoot a question to the Measurer, Bob Damon is still 'IT' i thinik. I have been see'n carbon fiber tiller extensions and spin poles.

For stiffening, I would question the value of the extra stiffness that an exotic might add. Given that the boat has some flexing as a whole, I wonder if maximizing stiffness in one area, might force another area to carry more of the flex and.....fail ???

just about .02 cents worth.

I got a note back from Bob Damon (the class measurer) regarding the use of carbon fiber to reinforce or stiffen the hull. The class rules are pretty liberal regarding stiffening the inside of the hull. Although he was not aware of anyone using carbon fiber he said that steel rebar has been used in the past! So, it is class legal to use carbon fiber for reinforcement on the inside of the hull.

KC

[GreenLake]

The class rules are pretty liberal regarding stiffening the inside of the hull.....steel rebar has been used in the past!

If I had any doubts about having bought the right boat, knowing that the class association can accommodate the steel rebar faction, has entirely removed them.

Thanks K.C. for relaying that nugget - it's made my day!

[Phill]

GreenLake,
The class didnt have to accomodate the " Steel Rebar Faction".

O'Day built many DS 1's with steel rebar glassed into the upper aft inside edge of the cuddy top. If you have a 1000 to about 3000 boat (maybe more) run your fingers on the inside of that edge. You should feel the rounded glassed over approximately 3/8" rebar reinforcment that supports your weight when you stand on the rear edge of the cuddy.

Many of us have rust oozing out of cracks in that glass tape that let water into and feed the oxidization of that rebar. Mine got so bad it was no longer really attached to the boat. I removed it, discovered how weak that area is without it, and foam cored about 12" of the aft inside edge of the cuddy roof to replace the lost structure. (must have boat upside down for this project)

The class could not outlaw an original O'Day structural feature. So I'm sure that is why Bob said it would be Ok...... if you really want to.

[K.C. Walker]

GreenLake,
The class didnt have to accomodate the " Steel Rebar Faction".

O'Day built many DS 1's with steel rebar glassed into the upper aft inside edge of the cuddy top. If you have a 1000 to about 3000 boat (maybe more) run your fingers on the inside of that edge. You should feel the rounded glassed over approximately 3/8" rebar reinforcment that supports your weight when you stand on the rear edge of the cuddy.

Now this is interesting. Reaching up under at the edge of the cuddy on my boat, I'm pretty sure that I've got the rebar. Fortunately, I don't see any rust stains yet. I wonder if I should sand that area lightly and give it a coat of epoxy as a preventative sealant. It makes me look at that gelcoat crazing a little more critically. And, my boat is 7002.

KC

[GreenLake]

One never stops learning. In a way, I'm glad I made my flippant remark - or I would have never learned why magnets stick to my cuddy rim :-0
(Like K.C., I didn't have any telltale rust to alert me).

[K.C. Walker]

One never stops learning. In a way, I'm glad I made my flippant remark - or I would have never leared why magnets stick to my cuddy rim :-0
(Like K.C., I didn't have any telltale rust to alert me).

Humm... probably not a good place to put a compass!

KC