Hull Stiffness

[Mike Seibert]

Hello again - Now that my traditionally cedar-planked Lightning is finished, I am thinking my next one might be a plywood Lightning sailboat.

At present I am thinking about a couple of options for the bottom of the boat:

1. Three layers of 4mm Okoume 1088 marine plywood, glued together with epoxy.

2. Two layers of 4 mm Okoume 1088 marine plywood with 6 oz fiberglass on both the inside and outside.

I am trying to determine which option would be stiffer. Haven't had much luck finding anything helpful when I try to google this question.

Thanks in advance,

Mike Seibert

[Ian McColgin]

Build according to the class specs. The Lightening does not benefit from added weight on the bottom.

If you contemplate the physics of it all, a heavier bottom will not contribute to stiffness much at all, especially not compared to parking your butt to weather.

Edited to add: I took the question to mean resistance to heeling. If you mean resistance to flexing, again I see no reason to depart from the class specs.

[Woxbox]

Stiffness comes with thickness. Thickness rules over whatever material you might use. Maximum stiffness and lowest weight involves strong skins with light filler between. But I agree with Ian -- just do what the class rules call for.

[Mike Seibert]

Ian and Dave - I appreciate the quick responses. I probably should have given you some of the reasons that I asked my original question.

In my research, I found that there was a stringer Lightning build from all plywood that was 40 lbs under the minimum weight of 700 lbs. And there was another home built plywood boat that came in 15 lbs light.

The Class Specs for the boat hull establish a minimum thickness of 5/8" for solid wood, but 5/16" for plywood. I have done estimates on the weight of wood as well as the weight of epoxy (with the help of West tech), and it looks like a plywood bottom built to the minimum thickness would likely weigh 20-40 lbs less than a cedar planked hull.

With respect to the topsides, my preliminary research indicates that using plywood could save 10-20 lbs compared to cedar.

When Nickels & Holman built wooden Lightnings back in the 1950s & 1960s, their Lightnings were cedar planked and most of them came in 15-20 lbs light.

So, while a cedar-planked Lightning can be built to a little less than the minimum Class weight, an epoxy glued plywood Lightning built to the Class minimums would be likely to come in, 30-40 lbs light.

One option would be to build the plywood boat to the Class mins and just have the Class measurers add the weight to bring it up to 700 lbs. It would be comforting to work on the build, knowing that you had so much weight to work with.

But I am trying to evaluate another option . . . to try to build a boat much closer to 700 lbs that would be stiffer than the lighter boat. For example, the keel could be built larger, which would add weight low in the middle of the hull, and also stiffen the backbone. And the frames could be built larger, which would spread out the additional stiffness in the boat. And one other option may be to add thickness to the bottom to spread out the added stiffness even more.

I believe that a Lightning that the beefed up 700 lbs Lightning could be a lot stiffer than the one built 30-40 lbs lighter. If, as they say, stiffer is faster, then the beefed up Lightning should be faster. Also, I believe its additional size and strength should make it a more durable boat.

So that is why I asked my original question. I am thinking about different ways that you could make the bottom a little heavier but also stiffer. The West guys told me fiberglass on the inside and outside of the bottom would make it stiffer.

But a 3rd layer of 4mm plywood would also make it stiffer, and would probably save a lot of work.

So, anyway, that is the method to my madness. I am trying to figure out which option would make better sense.

Thanks again for the feedback.

Mike

[Ian McColgin]

When I raced Lightenings, the ability to stand up to weather was a combination of form stability and live ballast. Strong hull good. But no need to get carried away with weight distribution beyond the class specs.

[Woxbox]

Mike -- If you want to add stiffness to improve performance, the additional material would have to be added in very specific areas to limit the flex the boats normally experience. Adding material throughout the hull may or may not help, and there is of course a risk of pushing the weight above the minimum. I don't know about the Lightning, but most sloop rigged boats benefit most from whatever can be done to create and maintain good headstay tension. But again, it's such a huge class with so much history, there's nothing new to be figured out. I'm sure the best answers to your questions are to be found in the Lightning archives.

[Gib Etheridge]

It could be that the only way to get an accurate answer will be to build a couple of test panels. My guess is that your second option, 2 layers with glass both faces, will be the stiffer and lighter. I've never used it but have heard that Okoume is very flexible. Meranti is stiffer by far as well as heavier and more rot resistant and less expensive.

[Mike Seibert]

Gib - I really appreciate your thoughts . . . hadn't considered Meranti, but using it instead of Okoume may be a very viable option.

Dave - The challenge is that, at this point, there are only a small number of Class members interested in wooden boats, and an even smaller number of folks interested in designing or building them. Dave Nickels is the only professional builder I know of who actually built wooden Lightnings, and his advice was generally to stiffen the keel and the frames in the area of the mast and the centerboard pin. Tom Allen never actually built wooden Lightnings, but he was very helpful during my build. I will shoot an email to him and see what he says.

Mike

[John Meachen]

I have never seen a Lightning,but have spent a long time racing dinghies of several types.The general principle of keeping the hull down to weight is always sound and the Finn class demonstrated a long time ago that keeping weight out of the ends is a good policy.You will definitely benefit from making sure the amount of flexing under rig loads is minimised but I doubt that the centreboard pin needs too much care.Mostly because it ought to be doing little more than allowing the board to move.Its a common misconception that the pin acts as a rigid pivot in the hull,when it does little more than locate the board.The lateral rigidity is ensured by having a capping along the top of the case that is strong enough to resist the loads and the keel and bottom panels resist the loads at the lower end.If there is a thwart that is secured to a robust frame or two it gets better.As you get up to planing speeds you need stiff bottom panels in the forward part of the boat as this is where the lift is generated-as you move aft the bottom panels just serve to keep the water out.

[neil.henderson]

Hi All,
I used to race dinghies but have not encountered a Lightning, but boats are boats so a couple of comments:
a) Plywood varies enormously in weight across exactly the same dimensions - I use okoume plywood made by Joubert in France and shipped to Australia - bends well and is light
b) I make glued clinker hulls and find that what starts as a wobbly hull is transformed after I have added : Knees, breasthook, floors and inwale - that framework really stiffens the hull
c) I never fibreglass my hulls
each of us has our ways of building - good lick with you Lightning :-)

[SHClark]

“Stiffness” is pretty subjective. Everything bends the question is what is the correct amount. Racing sailors have an unquenchable appetite for stiff boats. Boats that absolutely do not flex in waves and which can sustain; high rig tension without deformation. Empirically, it seems that if you are on weight, stiffer is almost always faster.

For establishing panel design you should establish some kind of baseline, maybe by analyzing the lull laminate of the best new boats, building a test sample and loading it to get a stress/strain curve. You can then build some test panels to compare. It seems like you could save weight in the general structure and add it back as additional thickness ( stiffness & weight) modern glues make it possible to reduce the size of chine logs, gunwales and keelsons, so you can make an effort to optimize the internal structure.
Chasing this a bit, didn’t Woodenboat publish plans for a West System wood Lightning? A good place to start.
SHC

[Bob (oh, THAT Bob)]

Beam bending stiffness is proportional to the cube (^^3) of thickness, I can't recall the relationship for flat sheets in multi-axis bending. Putting a higher stiffness-modulus material like fiberglass on the outsides, can further increase stiffness, but that also depends greatly on the orientation of the glass fibers, and adhesion to the inner core. This stuff can be calculated. Alas, not by me currently, I've burned some circuits, and I don't have handy, Roark's Formulas for Stress and Strain. And stiffness of flat sheets, I might also need Timoshenko. But I'm sure online, someone has a calculator to do it for you, but again, orientation of the materials makes a difference.

[Mike Seibert]

Thanks again for the additional responses.

John - My prior email should have been more clear about Dave's reference to stiffening the centerboard pin area. As you have indicated, the area of the hull near the centerboard pin is one of the stress areas because of the lateral forces the centerboard creates on the centerboard trunk. The forward thwart seats are just aft of the pin. The supporting framework under those seats, which are attached to the frames, brace against that stress. So I understood Dave's advice to be to beef up the bottom and side frames in that area. On my cedar-planked Lightning that I finished last Summer, I used beefy mahogany thwart seat supports, and would plan to do the same on the plywood Lightning.

SHC - Woodenboat Magazine did a project in the late 1990s to build the modern Lightning. Architectural plans were created for the building of a plywood Lightning with a cold molded bottom. I have those plans. In the year 2000, the magazine did a three-part series of articles describing that build in considerable detail. I have those articles and have been reviewing them closely.

In general, that Lightning was built to the Class minimum specs for the hull, frames, etc. Aside from spruce bottom frames and mahogany trim for the splash rails and cockpit trim, it was an all plywood epoxy-glued boat. There was no reference to what that Lightning weighed, but my guess is that it was a good bit lighter than 700 lbs. I have asked the Class office about the weight of that boat, but haven't heard back.

Those articles didn't discuss beefing up the boat to make it stiffer.

Here are a couple of pictures to show you what a Lightning looks like. The one with the shiny varnish is the one I built. The dark blue one is my restored 1956 Lightning - without its cockpit trim.



DSCN2073.jpg
fullsizeoutput_1d.jpg

fullsizeoutput_c7.jpg

[xkdrolt]

If you know all the material properties, you can calculate the bending and torsional stiffness --- if you have a good book on Strength of Materials for the math. That said, it is much easier to get some samples built of each method, then clamp them to a bench and perform some load testing. These don't have to be destructive tests, just simple load + deflection measurements. Small deflections can be magnified by clever use of an affixed laser pointer and a distant wall target plus rule. My guess is that 2 layers plus dual thin-skin epoxy-fiberglass will be stiffer, because the inner and outer glass layers raise the EI product immensely (E=material modulus, I=geometry moment of inertia). This assumes you aren't dealing with class rules. HTH.

[P.I. Stazzer-Newt]

Why glass? or why not carbon or kevlar (other aramid are available)?

[Mike Seibert]

Good question on the carbon and kevlar.

The Lightning Class is pretty traditional, and mostly limits the building materials to wood and fiberglass. Carbon fiber can only be used for the tiller extension and to reinforce the rudder blade, but nowhere else. Similarly, kevlar isn't allowed.

[P.I. Stazzer-Newt]

That's reasonable.

You can try using maths and the young's modulus to figure out panel stiffness for a proposed laminate schedule - but - there can be large differences between theoretical values and any you can actually achieve in yoour own workshop - a couple or three test panels should tell you all you need to know.

[John Meachen]

Could you use S-glass rather than E-glass?It isn't a huge advantage, but better than the more common version.

[Mike Seibert]

John - Not completely sure, but I don't recall any restrictions the Lightning Class has on the use of different kinds of fiberglass. So I think S-glass would probably be OK.

[Russell Brown]

I have used S-glass a bit and really like it. It is not as strong as carbon and it costs a lot more than E-glass, but there is a noticeable difference in stiffness and strength over E-glass and it is so much easier to use than carbon. It seems like it is mostly available in narrower widths as it is used a lot in surfboard and sailboard construction. I used it to build a new mast head float for my catamaran because carbon would have blocked the effectiveness of the internal radar reflector. I did a blog post about it and not sure why the link isn't live.
https://gougeon32.blogspot.com/2017/02/blimpy.html

[xkdrolt]

Why glass? or why not carbon or kevlar (other aramid are available)?

Any of them will work. The ones you mention have larger moduli, nearly the same density for purposes of this use, but they do tend to be more $$ and might not be as easy to obtain.

[SHClark]

I am familiar with the Lightning but not intimate with all the details of the construction and how much variation is permitted in the Class Rules. If I remember correctly, there have been quite a bit of tolerance testing in hull shape and more than one breakthroughs. I do not know what can be done in building, whether you need to have x many frames of x dimensions etc. I have built similar boats ( International 14 and Rhodes Bantams) in wood as well as any number of International Canoes and numerous prototype dinghies. I have tweaked more than a few dinghies designed to make them stiffer.
Obvious opportunities I see. Increase the thickness of the bottom and topsides panels to eliminate the need for ribs. Or stated another way, use the weight of the ribbing to increase the global thickness (stiffness) of the panels. I would prioritize the bottom over the topsides and the deck. I would carefully took at using Balsa or foam core in between plies of 3 mm plywood. I would look for the most efficient structural method for stem, sheer clamp, and chine logs. This is likely a tapered biaxial glass laminate.
I would maximize the bulk heading between the mast step, chain plates and centerboard trunk. I would look for some way within the rules to minimize the possibility of twisting between the chain plates and the stern. I would look at some way to have the seats play a part in stiffening the hull. If legal, I would put a full depth web between the foredeck and keel for headstay bending loads.
I would minimize the weight of everything, like keelson, chine logs, centerboard trunk and bed logs to have have the weight necessary to add this material where it will do the most good. The only way I know how to do this is to build a speed sheet that details every part of the build and assigns target weights for each and every piece. If there is uncertainty, build test samples.
SHC

[John Meachen]

Any of them will work. The ones you mention have larger moduli, nearly the same density for purposes of this use, but they do tend to be more $$ and might not be as easy to obtain.

Doesn't post #16 make that look unlikely? Re SHClark's points,all excellent incidentally,I have added bracing wires below deck before now to hold down parts that might be lifted by rig loads as wire doesn't weigh too much.

[xkdrolt]

It depends on the intended use. For class racing, you obviously can't do it. Otherwise, it's the owner's choice. I was answering the engineering question first posted by the OP.