Hull flex and soft fiberglass

[kkearns]

I have a 1965 DS I which seems to have significant hull flex at the forward end of the centerboard trunk, both starboard and port side. I'm not inclined to attempt an internal liner to stiffen the hull because a) I dont' race and am not terribly concerned about performance and b) it seems to be a job in which a LOT can go wrong if you don't know what you are doing and don't have the proper work environment in which to make the modification.

My real concern is structural failure of the hull and I am wondering if the flex could produce a "soft" hull or structural failure on a boat this age.

So, I'm interested in the experiences of others who own boats of this vintage (40-50 years old). Can the fiberglass go soft and, if so, what has been the experience of others with the maximum life span of this hull? Is there a chance of "catastrophic" structural failure?

Thanks for your thoughts on this topic.

Kevin
DS #1827

[algonquin]

Are you noticing the “hull flex” when the boat is in the water ?

I have a 1961 DSI and notice some hull flex in the area you describe when the boat is on the trailer but not when it is in the water.

I don’t consider this flex to be a problem with the fiberglass going “soft” on my boat. Softening of the fiberglass could result from a chemical type spill and possibly from the boat being stored in extreme heat conditions. As old as these boats are it may be hard to determine what may have happened to them over the years. In general I believe that flex in these hulls is normal. Brad

[GreenLake]

I don’t consider this flex to be a problem with the fiberglass going “soft” on my boat. Softening of the fiberglass could result from a chemical type spill and possibly from the boat being stored in extreme heat conditions. As old as these boats are it may be hard to determine what may have happened to them over the years. In general I believe that flex in these hulls is normal.

The hulls are single layers of laminate (not cored) in these (and many other boats of their vintage). Keeping the weight reasonable forces you to keep the laminate thin (in any case much thinner than a core). That automatically means that any flat hull panels will flex - the stiffness of a panel increases dramatically with thickness.

So, you are correct when you say flex is expected. However, because of the nature of the polyester/glass matrix, this is not necessarily benign. The problem with polyester is that it is weaker in tensile strength than the glass (only 1/3 as strong). This leads to internal softening and ultimately delamination whenever the flexing stress locally exceed the limit for the resin.

This process is mechanical (not chemical, as you assumed) and cumulative. In some boat classes the issue is severe enough, I'm told, that old hulls are not competitive. Your DS is now 50 years old. In that time, who knows what stresses it's been subjected to from trailering to walking on the hull w/o supports to waves.

I didn't use to notice the flex in my DS while sailing, but that's because I had the floorboards installed (and significant waves are few and far between on the lake). Having read K.C.'s discussion on his core project and other sources, I've started looking harder. I now see the "oil-canning" while sailing.

I've also turned my DS upside down, and while working on the bottom of the hull noticed that the hull on the outside of the seat-tanks also has a lot of "give" to it (flex). The latter surprised me because I had hoped for a stiffening effect of the curved panel.

My DS, a 63, has no reinforcements on the hull - later boats have stringers there, but those (not being epoxied) often will have rotted by now.

I remember K.C. writing that he could twist his boat by lifting a corner 1-3 inches before the mast would move. That at least is something I cannot reproduce (at least not with the boat on the trailer). So I'm hoping that means that my boat saw a bit less severe use than his. (I've also never observed the severe deflection on trailering that he reported).

What I would be interested in is finding a way to measure and compare the degree to which unmodified DS's of this vintage do deflect. This might give each of us an idea where we stand along the process of gradually softening panels.

For any serious racer, the need for extensive hull stiffening would seem inescapable - unless somebody can enlighten us that they are winning races with an unmodified hull.

[Baysailer]

I read through KC's endeavors and decided it's to much work and money for me. I do plan a minor siffening measure though making ribs made of pipe insulation cut in half and covered in glass tape. Seen it from a site updating a Thistle but can't find it now. Should be cheap and easy and hopefully help some. I do think it will make it stiffer but probably not quieter, you can hear the resonation from the waves.

May also lay down a layer of biaxial glass to restore some lost rigidity.

Greenlakes, I thought of the area under the seats, not sure if pour foam would help any there but it would be a hard area to tighen up.

Fred B
Daysailer #1351

[K.C. Walker]

I suppose that "catastrophic" failure is always possible. Though, I think it is unlikely. The softening of the polyester/fiberglass matrix is ongoing and inevitable with flexing. The boat hull is more than a quarter of an inch thick which would have been pretty stiff when new. Each time the panel flexes there is micro fracturing, the more deviation that the panel experiences the larger the fracturing. I speculate that most damage happens to a hull when it is being trailered. And of course, as the boat ages and becomes softer it's easier to deflect the panels with the same amount of force.

There is another chemical means of softening polyester/fiberglass composites which is osmosis that can lead to hydrolyzed polyester. The main symptom of this is blisters.

I don't know if I would go to the extremes I did again. I don't think that my boat was in danger of catastrophic failure. I'm sure I could have patched it for years and sailed it. I definitely would have reinforced it in some way, though. On the other hand, I sure do like the way the boat sails now. I seriously doubt a new hull is as stiff or as fair.

[GreenLake]

A couple of additions.

A good description of the properties of a polyester/glass matrix can be found on page 129 of Frank Bethwaite's book "Higher Performance Sailing", the follow-on book to his "High Performance Sailing". Both books are excellent reads, by the way.

I just rechecked, and the reason why epoxy resin is a better match for glass fiber is that its "elastic limit" is similar. Polyester resin will only stretch 2% before failing, much sooner than the glass, which leads to the separation of resin and glass inside the matrix at whatever point exceed that limit locally.

(You can bend a similar panel laminated with epoxy three times as far, at which point it will fail catastrophically all at once, but you should be able bend it twice as far many times safely).

If you build a structure that is so strong that it doesn't flex (under normal load peaks) then there would be no weakening. But despite what appears a bit of conservative scantling for the DS (after all, they are quite heavy), there may be some hull areas where that is not enough.

By the way, the fact that to my knowldedge no-one has reported on actual delamination of a Daysailer gives hope that leaving these hulls unmodified doesn't immediately risk dire consequences of complete hull failure, just reduced performance. However, I would definitely address any spots that seem suspicously soft.

Some stiffening would be desirable to alleviate some of the deterioration of sailing quality compared to a new hull, the problem is that you don't want to make things worse by creating hard spots (where local stresses really exceed the elastic limit).

That's the main allure of a complete "core job". Essentially, the entire structure stiffens, with the transitions at places that can handle the extra stress (keelson, CB trunk, seat edge, bulk-head, etc.).

That said, I'm still mulling over the question what a good minimal stiffening would look like - preferably one that would let me sail with floorboards in the original position for family daysailing and perhaps without for racing.

[K.C. Walker]

Adding to what GreenLake said, the polyester fiberglass bond is broken by the flexing. Panels that are made up of fiberglass cloth are more susceptible to this than ones made up of unidirectional or biaxial fiberglass. The reason is that on the tension side, in particular, is under more stress because the cloth weave tries to straighten and puts micro-stresses on the fiberglass to polyester bond, whereas unidirectional is already straight.

Fred, my experience from putting down the biaxial before putting down the core was that it did add noticeable stiffness. I had probably already removed 1/4 inch of hull thickness before laminating the biaxial. I would suspect adding a layer of biaxial to the full thickness hull would have even more stiffening effect.

My speculation is that the inside of the hull has more micro fracturing/delaminating than the exterior. My reasoning is that most of the damage to our boats comes from tailoring where the flexing is towards the interior. I think walking around inside the boat is a minor factor. Therefore, adding stiffening to the interior of the boat has more effect than one might suspect.

If I was going to stiffen up a boat that was not really gone, stiffness wise, I would add a good stringer. I would put down the biaxial first maybe 8-10 inches wide. I would put down a marine grade STRUCTURAL FOAM stringer 1/2 inch thick by 2-3 inches with unidirectional tape on both top and bottom and cover that with biaxial. I would use epoxy here is why http://www.masepoxies.com/How_To/About_ ... sters.html . I think this would add major stiffness to a somewhat soft hull.

To do even this amount of repair is going to be a time commitment. It's messy work and doing the prep work correctly is vitally important to success. The amount of money spent on materials is a minor part of the equation. Your sweat equity is where the value is. Of course, fiberglass boats are repaired all over the world so you can find someone locally to do the work for you.

[K.C. Walker]

For any serious racer, the need for extensive hull stiffening would seem inescapable - unless somebody can enlighten us that they are winning races with an unmodified hull.

Mike Gillum's boat does not have any additional stiffening to the hull and he is winning races. He does have a column connecting the cuddy roof to the keelson to give the boat overall stiffness, though. He says his boat has good "bones" and is not soft. Dave Keran added a number of stringers to #316 to stiffen it up and also uses the column. Even with the addition of stringers Dave decided that #316 was starting to get soft after a number of years of trailering and competition. He decided to overhaul a Rebel balsa cored boat for the added hull stiffness.

[kkearns]

Everyone,

Thanks for your excellent input on this. KC I hate to ask a dumb question, but what is "unidirectional tape"? And when you say use epoxy, are you saying that I should not use resin with the fiberglass but epoxy instead? Sorry. I know these are really basic issues, but I'm not familiar with this type of job. I'm seriously thinking of finding someone to do it for me in the fall. Would a typical marina have this type of expertise? I live in Pittsburgh and, unfortunately, sailor-friendly marinas are not very plentiful here. Power boating is king on our rivers and the state park where I sail has almost no professional support facilities for sailors.

Thanks,
Kevin

[K.C. Walker]

Kevin,

For the unidirectional tape http://www.jamestowndistributors.com/us ... Glass+Tape . The reference link above to MAS epoxy is the reasoning as to why to use epoxy instead of polyester for repair work. You can find the same rationale on the West System website and repair manual. http://www.westsystem.com/ss/assets/How ... enance.pdf
Both polyester and epoxy are resins.

I think having someone else do the repair is a good alternative. Not everyone likes to get down and dirty doing this kind of work. I do think that having an idea of how you want the repair done helps to talk to the people that will do the repair for you, though. Powerboats rule in most places but fiberglass work is pretty universal. If you ask at the marinas I'm sure you can find someone to do the work. And, fall is the best time of year to lineup this kind of work.

[GreenLake]

In a unidirectional tape most fibers go in one direction and, importantly, they lie flat in that direction. As tension is applied in that direction, they don't have to straighten out to take up the full load (as fibers in an over-and-under weave must do).

This makes the laminate stronger - provided you know which direction the load is coming from (or are able to lay up several layers in different directions). I tend to think that these are overkill - the reason being that the stiffening effect comes not primarily from really strong laminate, but from the separation of two layers of laminate by a spacer (usually foam).

On a hull plate, the stiffness of such a composite goes with the cube of the thickness. If you had an apple peeler with which you could separate your existing laminate in two, and insert the width in foam between, you would double the total thickness, but increase the stiffness of the panel by a factor 8. Or, if you replace the original 1/2 inch laminate by two skins of 1/4 inch separated by 1" of foam you would increase the stiffness of the panel by a factor of 27!

It seems to me, using stronger laminate is simply icing on the cake, given these factors. However, this is an argument I've been having with K.C. for a while now.

To summarize: the standard reasons why you use epoxy are

1. It's a better glue, so it works better on cured fiberglass.
2. It has a higher elastic limit, so the laminate you create is less subject to softening (see earlier posts)
3. it provides a better water barrier, so you can use it to encase wood, etc.

There's a downside to epoxy for repairing the outside of a hull, and that is that gelcoat may not adhere as well to cured epoxy (however some people claim the got even that to work).

There you have it.

[Alan]

GreenLake,

"There's a downside to epoxy for repairing the outside of a hull, and that is that gelcoat may not adhere as well to cured epoxy (however some people claim the got even that to work)."

Can you refresh my memory as to whether it works the other way around? In other words, does epoxy adhere to cured gelcoat? I'm pretty sure you've discussed the topic before, but I can't remember the general topic of that thread, and if I go looking I'll be up all night reading interesting stuff that has nothing to do with my question.

[GreenLake]

Alan,

epoxies are pretty good glues, and yes, they should stick to gel coat. They are often used as barrier coats on underwater portions of hulls that are kept in the water to cut down on osmosis.

While we are back-filling details: epoxies need to be painted, as they are not UV stable. Again, something that works well for repairs inside the hull, or on painted (portions of) hulls.

[GreenLake]

I've always found working with epoxies rather easy. You need to measure them meticulously and stir them really well, but the instructions available really cover all these points well. For example the "Epoxy Book" from SystemThree (google for a link).

Getting the glass tape, cloth or mat to "behave" when it's wet, is something that needs a little practice. Fortunately much can be learned by doing a small sample project. An easy one would be to make a fiberglass tub or bucket, using a suitable plastic one as a plug. This will give you confidence and a good feel for how many layers of laminate you will need to get a desired thickness or strength.

(I repaired a kid's wheel-barrow by building a tub using the old, brittle one as a plug. With only three layers of cloth/mat/cloth it's stronger, if a bit more elastic, than the original. This was not a cost effective repair, for that epoxy is too pricey, and the toy too cheap, but it helped me fine-tune technique.)

With a bit of preparation (and a general "hands-on" attitude) adding stringers would be something any DS owner could tackle. Just be sure to taper all edges so that they don't become stress points.

Perhaps, turn your boat upside down and add a stringer or two under your deck first, for added experience. For those you can use cardboard half-rounds (no need to use foam) such as paper towel rolls. (The stringers would be hollow.) You would use thickened epoxy (e.g. EZ Fillet from SystemThree) along the edges to provide a gentle curve for the glass to the deck. Many DS decks (or foredecks) could use a bit of stiffening and doing a project like that should tell you definitely whether you're cut out for doing this kind of work on your boat.

[K.C. Walker]

It seems to me, using stronger laminate is simply icing on the cake, given these factors. However, this is an argument I've been having with K.C. for a while now.

And... He is most likely correct. My tendency is to go straight to overkill when good enough would do. In my earlier post I did say that it is the way that I would do the reinforcement, not the way GreenLake would do it.