The Core Project

[GreenLake]

Looking nice. I also like the way you've faired your CB wall reeinforcements.

[K.C. Walker]

In repairing the soft bottom/bilge of my DS1 I have an observation regarding carbon fiber reinforcement. In my repair I decided to make some carbon fiber "ribs" epoxing on carbon fiber to the inside of the hull and then gluing foam core and more carbon fiber on the other side of that. At this point I only have the carbon fiber glued to the hull with the foam core glued on top of that BUT no reinforcing on the other side of foam.

So, my observation regarding 12 oz. carbon fiber cloth glued to the inside of the hull. It makes a significant difference to the stiffness of the hull skin with out any other reinforcement. I also epoxied 12 oz. bias ply biaxial e-glass to the inside of the hull at the same time with far less effect.

I just thought it was interesting.

KC

[GreenLake]

Very interesting.

How were you determining that "stiffness"? By pushing at the hull from below? Because that would put tensile stress on the inside, having a carbon layer there should help.

BTW, in my reading on out of plane loads on hull panels, I came across something interesting: that such panels can act like membranes - in other words, both surfaces are under tension. Unlike a beam, where the inner surface (in relation to the bending curvature) would be compressed.

Changing such a membrane by adding a layer that's significantly better at resisting tension ought to be noticeable - you are saying it is.

Your carbon fabric should stretch, what, 2-5 times as little for the same load compared to the glass? If we assume that section of the hull acts as a membrane where the entire laminate is under tension, the existing glass contributes. If you add more glass, then the effect is simply proportional to the increase in cross section.

If, let's assume, one layer amounts to 25% of the total laminate, i.e. 4 layers in total, you've added perhaps 33% by using glass. By using carbon you added perhaps 3x33% (or more, depending on the factor) which would be a doubling (or more) of the existing membrane strength.

Now, all of this changes when you add a sandwich. I don't know whether the membrane or the beam model gives a better description of your sandwiched hull, but in a beam style situation adding carbon near the middle (between skin and core) should be least effective, because of geometry.

But, to the degree that the hull acts like a stretched (deflected) membrane, then adding tensile strength to either skin should have some incremental effect.

Unfortunately, I expect that your foam core (even with only glass) is going to be so stiff that you could not readily observe the difference to carbon, as you did with the single added laminate. Would be really interesting.

Keep building, keep observing!

[K.C. Walker]

Green Lake,

The "scientific" method that I used for determining the "stiffness" was by walking around inside the hull and pushing on both the inside and the outside. Because the foam is razor cut into squares for contour ability I can also watch the slight gaps in the foam open and close while pushing with thumb pressure.

That acting like a membrane thing would explain what I've observed. It is somewhat like tightening up a banjo head as far as feel goes.

I suspect that you are correct and that I will not be able to observe any differences in stiffness of the skin once I glass the inside of the foam, at least I hope that's the case.

Thank you for your thoughts.

KC

[GreenLake]

Right. Thanks for the added detail.

[K.C. Walker]

Well, I just put the last fiberglass on and have finished the core project except for paint! I'm glad that's over. I can't say that this was a fun project and it sure did take a lot of time. I can say that the boat feels really solid now. Here are a few photos of work in progress from a few days ago. I still had not done the fillets or the edge fiberglass tape at that point. I finished that this evening. Yay! http://www.walkerguitars.com/photos/daysailer/foam2/

Phill, thanks again for all your help! I had to think about it a little bit but your idea of the "guitar kerfing" worked perfectly on the edges. I sliced up a spruce 2 x 4 into trapezoidal pieces about a foot long and varying in size to fill the odd space at the edge of the seats and keelson. Encapsulated them in epoxy and bedded them in epoxy putty.

Hopefully, I'll get sailing before too long.

KC

[persephone]

Impressive work. Time to enjoy that boat KC!

[K.C. Walker]

Thanks Geoff!

[GreenLake]

Great job!. I note, the photos nicely show where you experimented with carbon for the skin...

Now, we are all waiting for your reports of how it feels on the water!

[K.C. Walker]

So, it seems I need a conclusion or a follow-up to finish off this thread. That is, how did the boat sail?

The 1st season I sailed this boat I felt that it was fun but it definitely felt old. Hitting a boat wake made the boat shudder and I could feel chop through the floorboards to my feet. The boat looked to be in very good condition so it was a bit of a mystery to me as to whether the boat would have felt this way when it was new. Without having any prior experience with Daysailers I just didn't know how stiff they should be. Well… it turns out that they should be stiffer than what my boat was.

I am totally satisfied with the stiffness of the boat now. It really has the feel of a new boat. It's much quieter in the water, somewhat like sailing a stiff wooden boat. When it pounds through a wake it feels much more like it's cutting through than bouncing through. The shuddering and flexing are totally gone. It really has a great feel when hiking out. I know that's kind of a subtle thing but it really does make a difference sitting out on the rail with the boat being stiff.

I feel much more confident in the boat now. I probably shouldn't get overconfident because I'd probably end up breaking stuff but I did test it out in some heavy weather. A friend of mine likes to sail fast and also is a fan of high wind conditions. We took the boat out in what I normally would not have gone out in (20 with gusts to 30+) some of the gusts were really making the rigging whistle. We did come close to capsizing the boat even with the main reefed, but the boat felt really solid.

I'd started to restore the floorboards but got anxious about the season slipping away so I started sailing the boat without the floorboards. There is zero flex in the floor/hull. Basically, it feels like a much more solid surface than the floorboards did, so I really don't feel the need for floorboards anymore. It certainly doesn't hurt to leave 50 pounds out of the boat if it's not needed. Also, the foamcore insulates not only for sound but also for temperature so even with bare feet in very cold water it feels comfortable. Another reason to leave them out is that it does give about 1 inch more legroom.

So, with the pain of the actual work having subsided and a fun season of sailing, I considered the project to be a great success.

KC

[kkearns]

I found this old thread because I am contemplating stiffening my hull next spring. Mine is a 1965 (hull #1827) and like Greenlake I have no stringers. I'm wondering ... would long thin planks of balsa, saturated in resin do the trick? Why choose foam? Honestly, I'm not sure I want to get into this project. Yes, there is flex, but I'm not racing and if the hull hasn't failed in 45 years, not likely it will fail any time soon. Your thoughts?

Kevin

[K.C. Walker]

It kind of seems like you're talking yourself out of it before you get started which might not be a bad thing. It is a lot of work and unless you really know you want to stiffen up the boat you might be better off ignoring it. Though I think if you added some stringers you would get some added stiffness.

I love the way my boat feels now. It really transformed it. In hindsight I might have been better off searching out a stiffer hull. If I ever do decide to sell my boat I will search out somebody who would want to race it because I think that's where it would really make a difference.

Why foam core? It's proven to be the latest and greatest material for the job and won't rot. Balsa wood core is so last century .

KC

[GreenLake]

KC mentioned somewhere that his boat could be lifted 2.5" at the stern before the front would move. My DS is perhaps even older than Kevin's, and when my boat is on the trailer, I can see any movement at the transom telegraphed to the mast pretty much right away.

When I put mine on its side recently I pounded on various spots on the hull to see how much they would flex. The biggest surprise to me was how much the area under the seats would flex - that's the area where adding strength is not easily done. (I had naively assumed that the curved chine would lend some strength to this area).

Altogether, I think different boats have differences in how strong they were when new, and differences in how they were sailed and stored. That should mean that doing the same core project could well result in a less noticeable change relative to the original state.

One thing I've been curious is how much of the beneficial effect of a stiff hull shows in flat water, or whether most of the benefits are found in chop.

Another thing I have been curious about is what a minimal stiffening program would be. Currently, my thinking is that two long thinnish stringers plus some wider cross members that go in front of the seats and connect to the cuddy opening might be a minimal design. (Minimal from the amount of material, not necessarily the amount of work).

As was extensively discussed earlier in this thread, the task of the core is to separate the inner skin from the floor, and that separation gives the strength. Balsa should never be used as "planks" but as end-grain, so it can't wick water from one end to the other. At that point, you might as well use foam and be sure that your stringers are waterproof.

I'm not done optimizing other aspects of my boat that promise to be lower hanging fruit, so hull stiffening is still low on my list, but I keep thinking about it.

[Bob Hunkins]

Another alternative to using foam core that I've heard of is to use biaxial fiberglass cloth. I heard this from Bill Donnelly of MA a few years ago. I've never tried it, but it might be a cheaper alternative than foam. I had foamed a hull a few years back, but it was disappeared in Hurricane Ike... Sigh.

[K.C. Walker]

Bob,

Being that I also used 45/45 biaxial glass laminated against the hull before putting the foam core in, it did make the hull skin much stiffer. However, adding the foam core and another layer of biaxial glass made a far more significant change in stiffness.

I would think that using biaxial glass and stringers could make a significant difference in stiffness without having to go to a full foam core, especially if the boat is only marginally soft. Dave Keran's #316 had a number of additional stringers added for stiffness.

Without having to remove old stringers, I think adding stringers and some biaxial glass would not take a huge amount of time and would probably add plenty of stiffness.

KC